Note: Descriptions are shown in the official language in which they were submitted.
7~S~.
The present invention .relates -to a method and
appara-tus for dropout compensa-tion of color televi~ion
signals where both -the luminance and chrominance compon-
ents forming the dropout compensation signal are delayed
during consecutive horizontal line intervals for a time
different from one horizontal line period by less than
one eyele of the eolor subearrier eonponent which is
suitable for utilization .in both. analog and digital signal
systems.
; . 10 As it is well known in the art! television signal
dropout eompensators are utilized to .replaee a missing or
degraded portion of the television sic3nal whieh has
"dropped out" due to an unpredietable instantaneous mal-
funetion of th.e system. For examplef when the teIevision
signal is reeorded and subsequently played bac~ from a
recording medium, a dropout may oecur due to diminutive
defeets of the reeording medium. When such dropouts in the
television signal oeeur, they introduce subjective dis-
tort-ion in the displayed televisi.on picture~ Dropout
'eompensators are utilized to eliminate the subjective dis-
tortion of dropouts from the television pieture displayed
to the viewer.
A eomprehensive survey of prior art teleyision
dropout eompensators is contained in copending applieation
serial no. 362,7S4 by B. Yeshwan-t Kamath, filed on October 20,
1980, which applieation is assigned to the assignee of this
applieation, whieh deseribes a digital color television
signal dropout compensatorO Generally, prior art dropout
eompensators utilize an RF envelope level de-tec-tor that
monitors the amplitude level of the modula-ted television
signal carrier waveform to detec-t dropouts in -the television
signal. A compensa-table dropout is manifested in a television
pc~ 3 ~
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signal as a momentary substantial reduction in the
amplitude or loss of the television signal. A delay
line is utilized to continuously delay the original
television signal. When a dropout ih the original
signal is de-tected, the delayed signal is applied as
a dropout compensation signal to replace the dropou-t
portion of the television signal information. More
specifically, a switch in the color television signal
path is controlled to apply the incoming color tele-
vision signal, or ~he delayed dropout compensation
signal, respectively, in response to a control si~nal
from the dropout detector. The color teleYision signal
is delayed by one horizontal line period and the chrom-
inance component is processed to have its phase sele~tively
altered on consecutive television lines. For e~ample~
in NTSC television signal systems phase adjustment of the
separated chrominance component of the dropout compensation
signal is provided by delaying the chrominance component
by two television line periods, or alternatively, the
separated chrominance component is xeversed on consecutive
television lines in various ways well known in the art.
~ However, there is a significant disadvantage
characterizing the aforedescribed prior art techniques of
dropout compensation. The original color television signal
is separated into a luminance and chrominance component
and each component is respec-tively delayed and processed
in a separate signal pa-th. Then, the separately processed
signal components are recor~ined for use as a dropout com-
pensation signalO When two or more consecutive horizontal
lines contain a dropout, the previously processed and re-
combined composite signal is again separa-ted and processed,
as above described.
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This re-separation and re-processing is repeated for each one
of the consecutive lines containing a dropout. The re-
separa-tion and re-processing causes line-to-line distortion and
progressive degeneration of the dropout compensation signal
results. Thus, when utilizing the above described prior art
dropou-t compensation techniques, the obtained dropout compen-
sation signal may become unacceptable whenever a few con-
secutive lines of the color television signal contain dropouts.
SU~MARY OF THE INVENTION
Accordingly, it is an object of the present inven-
tion to provide a method and apparatus for dropou-t compensation
of composite color television signals, in which the above-
mentioned disadvantages are eliminated.
It is a further object of the present invention to
provide a method and apparatus for color television signal drop-
out compensation in which consecutive lines of the dropout com-
pensation signal are derived from the same portion of the
original color television information signal without dropout
immediately preceding the dropout.
It is another object of the invention to provide a
dropout compensation method and apparatus in which -the same
portion of the original composite color television signal is
stored in a memory, from which signal portion consecutive lines
of a dropout compensation signal are derived ahd which stored
original signal remains unchanged while the dropout compen-
sation takes place.
It is still another objec-t of the invention to
provide a color television signal dropout compensation method
and apparatus in which both the luminance and chrominance
component of -the original signal are delayed during consecutive
horizontal line intervals for a time differing Erom one tele-
vision line interval by less than one cycle of the color sub-
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carrier component.
It is still a further object of the present
invention to provide a method and apparatus for color
television signal dropout compensation, suitable for
use in both analog and digital color television signal
systems.
Yet another object of the present invention is
to provide a method and apparatus for dropout compensation
of composite color television signals suitable for util- -
ization, in known television signal systems, such as
NTSC, PAL, PAL-M, etc.
It is still a further object of the present in-
vention to provide a method and apparatus for dropout com-
;~ pensation of composite color television signals suitable
for use in embodiments in which the signal is sampled at
a frequency equal to an integral multiple grea-ter than two
of the color suocarrier signal frequency.
The foregoing and other objects are accomplished
by the method of the present invention, according to which
both the chrominance and luminance components of the com-
posite color television signal are stored for a period of
time equal to substantially one horizontal line period,
and the stored information is circulated in response to a
dropout signal. The actual length of delay of the chrom-
inance component is controlled in accordance with the phase
of the color burst component of the television signal line to
be compensated for dropouts -to provide a delayed chrominance
component, which is in phase with the color burst componen-t
contained in the incoming composite color -television signal
line to be compensated during consecu-tive horizontal line
periods. The actual length of delay of the luminance
component is controlled to provide a luminance
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eomponent whieh is in synchronism with the ineoming
horizontal line synchronizing componen-t of the composite
eolor television signal during conseeutive horizontal
line periods. The respec-tive]y delayed chrominance and
luminance components are combined to form a eomposite
dropout compensation signal, which, in turn is combined
with the original eomposite eolor television signal to
replace a dropout portion when it is detee-ted.
The eolor television signal dropout eompen-
sator of the present invention has a eircula-ting storage
means for delaying both the luminance and chrominance
eomponents of an ineoming eomposite eolor television
to be used in forming a dropout eompensation signal. A
first eontrol means, which is responsive to the color burst
eomponent, is utilized to control the actual delay of the
ehrominanee eomponent to provide a delayed ehrominance
eomponent which is in phase with the eolor burst eompon-
ent contained in the incoming composite eolor television
signal during eonseeutive horizontal line periods. A
second eontrol means, which is responsive to ~he horizontal
line synchronizing component of the ineoming eomposite
eolor television signal is utilized to eontrol the aetual
delay of the luminance component to provide a delayed
luminance eomponent whieh is in synehronism with the hori-
zontal line synehronizing eomponent of -the incoming com-
posite color television signal. The delays of the stored
luminance and chrominance components during consecutive
horizontal line intervals differ from one horizontal line
period by less -than one eyele of the color subcarrier
componen-t of the color television signal. A signal com-
bining means is utilized to eombine the respeetively
delayed luminanee and ehrominance components to provide
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a composite dropout compensation signal. A switching
means responsive to the dropout control signal, selectively
applies the incoming color -television signal or -the com-
posite dropou-t compensation signal to an outpu-t of the
dropout compensator.
Other objects, advantages and features of the
present invention will become apparent from the following
detailed description taken with reference to the accompany-
ing drawings.
BRIEF DESCRIPTION OF T~E DRAWINGS
FIGURES la and lb lllustrate a preferred method
of providing a dropout compensatlon signal in accordance
with the invention.
FIGURE 2 is a functional block diagram of a
preferred embodiment of the invention.
FIGURES 3a and 3b illustrate another preferred
method in accordance with the invention.
FIGURE 4 illus-trates the operation of the
apperatus of FIG. 2.
FIGURE 5 is a functional block diagram of
- another preferred embodiment of -the inven-tion.
FIGURE 6 illustrates the operation of the
apparatus of FIG. 5.
FIGURE 7 is a functional block diagram of a
still further preferred embodiment of the invention.
FIGURES 8a to 8h are consecu-tive parts of a
detailed electrical schematic diagram corresponding to the
block diagram of FIG. 7.
_TAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An example of a preferred dropou-t compensation
method of -the present invention will be described now with
reference to Figs. la and lb. :[n the following description
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"dropout compensation" is sometimes referred to as
"DOC" and "dropout" as "DO". In Fig. la, a sine
wave W1 is shown representing a color subcarrier signal
waveform typically found in the chrominance component
of a composite color television signal. Waveform Wl
is in the form of consecutive samples lA, 2A! 3A, etc.,
at regularly spaced inter~als of the incominy color
television signal, using any color television signal
sampling techinque well known in the art. In this
particular example, -the samples are obtained by
sampling an NTSC composite color television signal at
a frequency equal to three times the color subcarrier
signal frequency, that is, at 3 x 3.58 ~Iz or 10.~4 ~z.
The obtained samples may be encoded into a digital form by
an analog-to-digital encoder, utilizing, for example,
pulse code modulation (PCM), where each individual code
element represents a particular signal amplitude value.
; As an example, the subcarrier waveform Wl is indicated as
occurring during a particular television line interval Al.
It is a well known feature of the NTSC television signal
system that the subcarrier waveform W2 in a next sub-
sequent television line interval Bl will have an opposite
phase with respect -to subcarrier waveform Wl. Therefore,
if a dropout occurs, for e~ample, after the original
television line Al has been received and delayed by one
television line period for subsequent utilization as a
dropout compensation signal to replace a dropout occurring
in line Bl, the delayed line Al would exhibit an undesired
180 phase shift with respect to Bl. The undesired 180
phase shift is reflected in -the samples lA, 2A, 3A, etc.,
of the delayed Wl signal being displaced by one and one-
half sampling interva]s relative to the phase locations
of the corresponding samples 2B, 3B, etc., of waveform W2.
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To replace a dropout in television line interval Bl
with a dropout compensation slgnal derived from the
immediately preceding television line interYal Al,
sample lA has to be displaced with respect -to corres-
ponding sample 2B, and similarly, sample 2A with
respect to sample 3B, etc., by one and one-half
sampling intervals in order to maintain the proper
line-to-line chrominance phase relationship.
By the preferred method of the present
inven-tion, the above-indicated phase shift oE the color
subcarrier signal on consecutive lines is obtained by
storing the color television signal and alternatively
increasing or decreasing the storage deIay of the chrom-
inance component of the television signal by a delay
corresponding to one`and one-half sample periods. This
latter feature i5 illustrated in Fig. la ! showing sample
3B of line sl as being obtained by delaying the chrominance
component of sample 2A of the immediately~preceding original
, television line Al by one horizontal line period~ increased
by one and one-half sample periods. A dashed line CHl
connecting corresponding samples 3B and 2A illustrates
this feature. Fig. lb shows the subcarrier waveform W2 of
line Bl, followed by waveEorm W3 of a next consecutive line
A2. Fig. lb illus-trates an example where line sl is the
last received original television signal line and line A2
is the first line containing a dropout requiring compen-
sation. As Fig. lb reveals, sample lA of the subcarrier
signal waveform W3 is obtained by delaying the chrominance
component of sample 2B of waveform W2 by one horizontal
line period decreased by a time interval corresponding
to one and one-half sample periods. Dashed line CH2,
connecting corresponding samples lA and 2B, indicates
the latter feature.
..,
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However, when the above-described dropout
compensation method is utilized, the luminance component
of the dropout compensation signal will introduce sub-
jective distortions in the display of the compensated
color television signal if it is also displaced by +
one and one-half sample periods with respect to its
phase position in the original color te]evision signal
line. A phase displacement of the chrominance component
of + one and one-half sample periods is not objectionahle
because chrominance informa-tion chan~es in a color tele-
vision signal are ordinaxily at a low rate, whereas to
obtain a high quality color si~nal for dropout compen-
sation it is essential to maintain the line-to-line phase
of the chrominance component as close to the required
;` phase as possible. On the other hand, such phase dis-
placement of -the luminance component is quite objection-
able because it introduces too large a line-to-line
~ horizontal displacement of samples in the television
; signal with respect to the samples of the original signal.; 20 To eliminate the above-discussed drawback,
the preferred dropout compensation method of the inven-tion
provides line-to-line adjustment of the luminance com-
ponent displacement as follows.
When the chrominance component of the stored
television signal is delayed one horizontal line plus
one and one-half sample periods for dropout compensation,
as depicted by Fig. la, the luminance component of the
stored television slgnal is delayed by one horizontal
line plus one-half sample period. When a horizontal
line of a television line is dropout compensated with
information from an immediately preceding horizontal line,
the horizontal displacement of the luminance component
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of the dropout compensation signal relative to -the
luminance component of the -television signal being
compensated is reduced from an objectionable one and
one-half sample periods of an acceptable one-half
sample period. The foregoing is illustrated in Fig. la
b~ dashed line Ll connecting samples 3B and 3A.
Analogously, for obtaining the above reduc-tion in the
luminance component displacement when the stored
television signal is delayed one horizontal line less
one and one-half sample periods, as depicted by Fig. lb,
the luminance component of the stored television signal
is delayed one horizontal line less one-half sample
period. This is illustrated in Fib. lb by dashed line
L2 connecting samples lA and lB. As will be appreciated
I from the foregoing, the horizontal displacement of the
luminance component of the dropout compensation signal
relative to the television signal being compensated is
reduced to an acceptable one-half sample period, which
is one-third of a c~vcle of the color subcarrier in the
example in all cases when a horizontal line of a tele-
` vision signal is compensated for dropouts with a dropout
compensation signal derived from the immediately pre-
ceding horizontal line of the -televisi.on signal. In
addition in both such cases the delay of the respective
- chrominance components of the dropout compensation signal
remains displaced by the desired one and one-half sample
periods.
More specifically, the above features of the pre-
ferred method of -the present invention can be conveniently
realized through the practice of the method as follows.
One horizontal line of the incomin~ original composite color
television sianal is con-tinuously stored in a memory.
.,
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The memory is updated with a new horizontal line of
the incoming -television signal each time a received
horizontal line contains no dropouts. The length of
delay of the stored composite signal is con-trolled in
accordance with the line-to-line phase of the color
burst componen-t of the original color television signal.
The exact delays of the chrominance component and
luminance component are achieved by separating the
composite signal into its luminance and chrominance
components so that the length of delay of the lumin-
ance component can be modulated about a length of one
horizontal line a different amount rela-tive to that of
the chrominance component. The difference is an
integral number of sample periods. With respect to the
above-described particular example illustrated in Figs.
la and lb, the s-tored chrominance component of the com-
posite color television signal is delayed an interval
equal to one horizontal line period modulated by + one
and one-half sample periods on consecutive lines. Be-
sides that, the length of delay of the stored luminance
; component is modulated line-to-line by + one-half sample
period, in the same sense as the stored chrominance com-
ponent, to reduce the line--to-line luminance component
displacemen-t.
To illustrate the foregoing, a preferred
embodiment of the apparatus of -the present invention
will be described with reference to the func-tional block
diagram of Fig. 2 of the attached drawings. A composite
color television signal is received at inpu-t terminal 12
and is applied to firs-t input 20 of a first two-way
switch 1. In this particular example, the television
signal is received in digital form obtained , for example,
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by encoding the color television signal in-to a well-~nown
NRZ PCM code, as previously mentioned with referenct to
Figs. la and lb. A control signal, DO, indicating
presence of a dropout is received by a control terminal
23, for example, from a conventional dropout de-tector
(not shownl~
A suitable dropou-t detector may be of a con-
ventional carrler monitor type which provides a control
signal when the RF envelope of the modulated television
signal dropout is below a predetermined level, such as
desired, for example, in AVR-2 Videotape Recorder, Theory
of Operation, Catalog No. 1809179-01, published November,
1977, by Ampex Corporation, payes 5-31 to 5-33. The control
terminal 23 is coupled to a control input 24 of first
switch 1, and, also, to control input 29 of a second
switch 2. Output 25 of switch l is coupled to an output
terminal 13 of the dropout compensator circuit, as well as
to first input 26 of switch 2. The output 27 of switch 2
~ is coupled via a controlled delay line 3 and a compensating
j 20 fixed delay line 4 to a second input 28 of switch 2.
Elements 2, 3 and 4 form together a circulating memory
` circuit 44, which will be described hereinafter in more
;::
detail. A control terminal 30 which receives a color
burs-t synchronous control signal, CB, that controls the
length of the controlled delay line 3 in the manner to be
described in detail hereinafter, is coupled to a control
input 31 of the controlled delay line 3. The output 32 of
delay line 3 is coupled to an input 33 of a filter 5 and
via a compensating fixed delay line 6 to a first input 35
of a differencing circuit 7. The compensating delay lines
4 and 6 are respectively utilized to compensate for circuit
delays in the color television signal path, which will be
described hereinaEter. An ou-tpu-t 34 of filter 5 is
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coupled to a second input 36 of differencing circuit 7,
as well as to a first input 37 of a third two-way switch
10. The filter's output 34 is also coupled via a fixed
delay line 9 to a second input 38 of switch 10. A control
terminal 43, receiving a control signal HS/2, is coupled
to a eontrol input 44 of ~hird switch 10. An output 39
of switeh 10 is coupled to a firs-t input 40 of a signal
combining circuit 11. An output 45 of signal differencing
eircuit 7 is coupled via a fixed delay line 8 to a seeond
input 41 of eireuit 11. An output 42 of signal eombining
cireuit 11 is eoupled to a seeond input 21 of the first
two-way switeh 1.
It is to be realized that the above-described
preferred embodiment of Fig. 2 as well as -the respeetive
embodiments of Figs. 5 and 7 which will be described later,
all represent respective digital dropout compensators in
aeeordance with the present invention in which high speed
digital data is processed. Consequently, the various
elemen-ts shown in these respective functional block dia-
grams may be constructed uslng conventional digital circuitsin which the high speed data is precisely clocked at the
sampling frequency, equal to an integral number greater
; than two times the color subcarrier signal frequency,
the various embodiments described herein being arranged
to operate with a sampling frequency of the three times
the NTSC color subcarrier signal frequency or 3 x 3.58
10.74 MHz, which elock signal is frequency and phase-
locked to the color subearrier eomponent of the sampled
television signal. In the further deseripti.on, the terms
sampling frequeney and clock frequency will be used inter-
changeably. For simplici-ty of representa-tion, the cloek
signal pa-th is now shown in the above-indicated Fig~lres,
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however, it is shown in the detailed electrical schematic
diagram of Figs. 8a to 8h, corresponding to -the block
diagram of Fig. 7, which detailed diagram will be described
hereinafter.
Now the opera-tion of the preferred embodiment of
the invention shown in the block diagram of Fig. 2 will be
described. A digital NTSC color television signal in the
form of discrete data representing consecutive samples is
continuously received at input terminal 12 and fed to
10 first input 20 of switch 1. When the dropout compensator
system is in normal operation, that is, no dropout in the
incoming signal is detected, swi-tch 1 is in its first
position as shown, receiving the input signal a-t 20 and
simultaneously applying it via output 25 to output terminal
13 of the dropout compensator and via input 26 and output
27 of switch 2 -to the controlled delay line 3. Delay line 3
has a length that is controllable + one and one-half sample
periods abou-t a nominal delay equal to one horizontal line
period less circuit delays in the color television signal
20 path, as will be disclosed hereinafter.
The delayed composite signal from delay line 3 is
fed -to filter 5 which may be, for example, of the type,
disclosed in the above-indicated copending patent application
SN 362,754, or alternatively, it may be a well known digital
comb filter. Filter 5 separates the luminance component
from the stored composite color television signal, in a
manner well known in the art. The separated luminance
component ob-tained at the output 34 of filter 5 is applied
to input 36 of differencing circuit 7. The composite color
30 television signal from output 32 of delay line 3 is applied
via compensating fixed delay line 6 to the other input 35
of circuit 7. Differencing circui-t 7 provides at its
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output 45 a difference signal of the two signals received
at its respective inputs. The resulting difference signal
represents the separated chrominance component of the stored
color television signal. Delay line 6 compensates for the
circuit delay provided by filter 5, in a manner well known
in the art, to eliminate undesirable relative phase shifts
in the respective signal paths of the signals subsequently
processed in differencing circuit 7.
In the particular embodiment of the invention
illustrated in Fig. 2 a non-integral number of samples
equal to 3 ~ 227.5 = 682.5 clock cycles is obtained within
one horizontal line period, the color subcarrier frequency
being equal to 227.5 times the horizontal line frequency
in NTSC color television signals. The con-trolled delay
line 3 has a length of delay equal to one horizontal line
period represented by 682.5 clock cycles, less one clock
cycle plus the following circuit delay, which is alterable
by - one and one-half sample periods. Loading an~ unloading
of the samples into and from delay line 3, respec-tively, is
controlled by the control signal CB received a-t terminal 30,
which signal is derived from the color burst component of
the incoming color television signal duriny each horizontal
line in a manner well known in -the art so that it is
synchronous with such color burst component. The CB
con-trol signal is applied to control inpu-t 31 of delay
line 3. As men-tioned earlier, in this particular embodiment
one cycle of the color burst corresponds -to three sample
periods defined by three clock cycles. Consequently, the
relative line-to-line phase shift by 180 oE the color burst
component represents a delay of plus or minus 12 sample
periods, that is plus or minus 1 12 clock cycles. As will
become apparent from the following description, the controlled
delay line 3 is operated to provide alterna-tive delays of the
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composite color television signal equal to 682 1 + 1 2 = 684
clock cycles and 682 1 - 1 12 = 681 clock cycles on consecutive
lines, less the aforementioned one clock cycle and circuit
compensation delays. The latter operation may be described
as modulating the fixed length of delay of the composite
color television sisnal stored in delay line 3 during
consecutive television line periods by the phase of the
color burst component. It will be understood that the
average signal delay during several consecu-tive lines
approaches 682 2 clock cycles, which is e~ual to the
original one horizontal line period.
From the foregoing description it follows that
delay line 3 of Fig. 2 is controlled hy a control signal CB
synchronous with the color burst component, which signal is
received at terminal 30 to provide a delayed composite
color television signal of one horizontal line whose color
subcarrier component is in phase with the color burst
component of a following horizontal line, as previously
described in detail wi-th respect to Figs. la and lb.
As previously disclosed with reference to Figs.
la and lb, it is desirable that the luminance component
samples of the dropout compensation signal be processed to
minimize the horizontal displacement of respective sample
positions on consecutive lines of the displayed television
signal. Now, the line-to-line adjustment of the luminance
component delay provided by the apparatus of Fig. 2 to
achieve the foregoing will be described. As above described,
in this preferred embodiment the luminance component is
separated in fil-ter 5 from the composite signal having a
modulated delay. The length of delay of -the delayed
separated luminance componen-t is further modulated by -
1 clock cycle in -the opposi-te sense with respec-t to the
chrominance component that is to be combined with the
,mg/ ~ - 18 -
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luminance component to form the dropout compensation composite
color television signal. To this effec-t, the delayed
separated chrominance component at the output 45 of
differencing circuit 7 is delayed by additional one clock
cycle by fixed delay line 8. At the same time, the delayed
separated luminance component from filter 5 is directly
applied to the first input 37 of two-way switch lO while it
is also applied via additional delay line 9 of -two clock
cycles to the second input 33 of switch lO. Switch lO is
controlled by the control signal HS/2 received at terminal
43, having a frequency equal to one-half horizontal line
frequency. The switch lO is responsive to the HS/2 signal to
connect first and second inputs 37 and 38 to the output 39
on alternate horizontal lines, thereby alternating the
delay of the luminance component between zero and two
clock cycles on alternate horizontal lines. Signal HS/2
is derived from the horizontal line synchronizin~ component
contained in the incoming color -television signal received
at input terminal 12. In the respective preferred
embodiments of the invention described herein, -the control
signal XS/2 is frequency and phase-locked to the horizontal
line synchronizing component and the color burst component
of the incoming color television signal. During dropout
compensation, when the composite television signal is not
received, the respective control signals cs, HS or HS/2 are
generated synchronously with the respective synchronizing
componen-ts of the missing television signal, utilizing a
flywheel signal generator (not shown) frequency and phase-
locked to the color television signal, in a manner well
known in the art. As will be understood Erom the foregoing
description, the resulting delay of the luminance component
is modulated by - one clock cycle delay relative to the
chrominance component delay. By this latter feature the
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length of delay of the luminance component oE the dropout
compensation signal is adjusted on consecutive horizontal
lines in response to the horizontal line synchronizing
component to minimize line-to-line horizontal displacement
of samples inserted in the outpu-t color television signal
for dropout compensation.
The respectively delayed luminance and chrominance
components are combined in adder 11 and the combined
signals at the output 42 of adder 11 represent the composite
dropout compensation signal applied to input 21 of first
switch 1. The dropout compensation signal is continuously
available to be utilized for dropout compensation in response
to a dropout compensation control signal DO received at
control terminal 23, as has been described previously.
Upon receiving a control signal DO at terminal 23,
indicating that a dropout has occurred in the incoming
television signal a-t terminal 12, switch 1 is controlled to
switch from input 20 to 21, and switch 2 to switch from
input 26 to 28. Consequently, switch 1 will apply to output
terminal 13, the dropout compensation signal received at its
input 21, which is provided by the circuit of Fig. 2 as
above disclosed. On the other hand, switch 2 will close
the circulating memory circuit 44, and, thus, effect
circulation of the last received line of the television
signal information from a previous horizontal line or set
of previous horizontal lines without dropouts immediately
preceding the dropout, which signal has been stored in delay
lines 3 and 4. Consequently, the above-indicated original
color television signal line will circulate in circulating
memory 44 until the dropout is eliminated and the con-trol
signal DO at terminal 23 is discontined. During the dropout
compensation, the output signal at outpu-t 32 from the
circulating memory 44 is processed by the dropout compensator
mg/ - 20 -
L5'~L
circuit of Fig. 2 in the previously disclosed manner,
instead of the incoming color television signal.
As above mentioned, the leng-th of delay of controlled
delay line 3 is decreased from the previously considered
one horizontal line period to compensate for circuit delays
in the processed composite signal path. Thus, if the delay
provided by filter 5 in the separated luminance signal path
is designated ~ and the additional one cloc~ cycle delay
provided in both the separated luminance and chrominance
signal paths is designated ~, then it will become apparent
from the block diagram of Fig. 2 that to obtain an overall
length of delay equal to one horizontal line period of the
composite DOC signal, while not considering the above-
described modulation of the delay by the respective control
signals, delay line 3 must have its delay decreased by a + ~.
Similarly, it will become apparent from the above disclosure
that to provide a circulating memory 44 which has a length
of delay equal -to exactly one horizontal line period, it is
necessary to re-insert the combined delays ~ + ~ in form of
a fixed delay line 4 as shown between the output 32 and
input 27 of delay line 3.
As it is seen from the foregoing description, it
is a significant advantage of the inven-tion that consecutive
lines of the dropout compensation signal are derived from
the same portion of the original color television signal
without a dropout immediately preceding the dropout.
It is a further important advantage, that the
composite signal is stored in the circula-ting memory ln
its original form, and remains unchanged by -the line-to-line
processing of the signal components throughout the dropout
compensation. Consequently, no deterioration of the
resulting dropout compensation signal occurs, as takes
place in the known prior art dropout compensators. At the
mg/J - 21 -
5'~
same -time, each sample of the dropout compensation signal
chrominance component is processed to have the same phase
as that of a corresponding sample of the incoming television
signal, for which the dropout signal is formed, while -the
dropout compensation signal luminance component is
processed to introduce a minimum horizontal displacement
of samples on consecutive lines of the dropout compensa-ted
composite color television signal. Since in the above
described example related to the operation of the block
diagram of Fig. 2, a sampling rate of three times the color
subcarrier signal frequency is considered, there is a
line-to-line displacement of the dropout compensation signal
sample equal to 1 12 sample periods for the chrominance
component and just 1 sample period for the luminance
component. It is understood from the foregoing description
that by the above-indicated respective line-to-line
displacement of samples, the previously described objectives
related to a high quality dropout compensation signal are
achieved.
The operation of the apparatus of Fig. 2 for
dropout compensation of a series of consecutive horizontal
lines containing dropouts is illustrated in Figs. 3 and 4.
These figures illustrate the line-to-line displacement of
the luminance and chrominance components combined to form
a series of dropout compensation signals from one stored
horizontal line of a composite television signal without a
dropout for use in consecutive lines of the DOC television
siynal~ As seen in Fig. 4 a last received original
television signal line Al, immediately preceding the series
of dropouts, comprises consecutive samples 2A, 3A, 4A, 5A,
etc. It is seen from Fig. 4 that the apparatus of Fig. 2
operates to delay the chrominance component forming each of
the samples 2B, 3B, 4B, etc., of -the first DOC line Bl,
mg/ - 22 -
s~
as well of all subsequent odd DOC lines B2, B3, etc.,
relative to the phase position of such each chrominance
component in the original line by an amount equal to an
odd multiple of the interval of one horizontal line plus
one and one-half sample periods. However, it is seen from
Fig. 4 that the apparatus of Fig. 2 operates to delay such
chrominance component again after circulation in circulating
memory 44 on the second DOC line ~2 and all subsequent even
DOC lines A3, A4, etc., by an amount equal to the interval
of one horizontal line less one and one-half sample periods.
AS a result of the operation of the circulating memory 44,
each chrominance component forming each sample of the
dropout compensation signal for even DOC line A2, A3, etc.,
is delayed even multiples of the interval of one horizontal
line. Consequently, the chrominance components of samples
3A, 4A, etc., on even DOC lines A2, A3, etc., are in phase
with those of original line Al, however, displaced by 1 12
sample periods with respect to an immediately preceding
odd DOC line. On the other hand, the operation of the
apparatus results in the displacement of the luminance
component forming each sample of even DOC lines A2, A3, etc.,
by one sample period with respect -to the oriyinal line Al,
but each is displaced only by 1 sample period with respect
to the luminance component of the same sample appearing in
an immediately preceding odd DOC line. During the first
odd DOC line Bl, the apparatus operates to delay -the
luminance component forming each sample of the dropout
compensation signal by an amount equal to the interval of
one hori~ontal line plus one-half sample period. The
opera-tion of the circula-ting memory in cooperation with
the operation of -the third -two-way swi-tch 10 in alternately
inserting a two clock cycle delay and no delay in the
luminance component path on consecutive horizontal lines
" .
ir ~ mg/ ~ - 23 -
S;~
results in delaying the circulated luminance component of
each sample forming the dropout compensation signal for even
DOC lines A2, A3, etc., relative to the phase position of
such luminance component in the original line ~y an amount
equal the lnterval of one horizontal line less one-half
sample period, whereby such luminance component is positioned
in phase within one-half sample period of its phase position
in the original line Al. ~s a result of the above-described
dropout compensation method shown in Fig. 4, a line-to-line
displacement pattern of sample components is formed in the
display of the compensated color television signal in which
the displacements of chrominance and luminance components
forming a dropout compensated sample point, represented by
lines CH2, L2; CH3; CH4, L4; etc., connecting corresponding
chrominance and luminance component samples of adjacent DOC
lines, are in opposite directions horizontally from line to
line. By this pattern, the desired respective line-to-line
horizontal displacement of - 1 12 sample periods of the
chrominance component and of ~ 12 sample period of the
luminance component is achieved.
The foregoing preferred me-thod of the invention
with respect to providing a delayed luminance component of
the composite dropou-t compensation signal on consecutive
television lines is illustrated in Figs. 3a and 3b. In
Fig. 3a a waveform Ml represents a portion of -the luminance
component of an incoming original color television signal
line Al received at terminal 12 by the dropout compensation
circui-t of Fig. 2, immediately preceding a line containing
a dropout. Samples lA, 2A, 3A, etc., of waveform Ml are
obtained by sampling the color television signal a-t a
frequency equal to three times the color subcarrier signal
frequency, as described previously. When considering the
above~described method of providing a dropout compensation
.,
~ ~ mg~ - 24 -
S~21
luminance component employing the apparatus of Fig. 2,
the luminance component of the first dropout compensation
signal line Bl, following the original television signal
line without a dropout, is represented by M2 in Fig. 3A,
which illustrates the dropout compensation signal lines
as it appears in a raster display of the television signal.
It is seen that waveform M2 is a replica of the original
waveform Ml with the exception of samples lB, 2B, 3B, etc.,
of M2 being displaced by only one-half sample period in
one direction, for example, to the right with respect to
samples lA, 2A, 3A, etc., of waveform Ml. However, waveform
M3 of the next consecutive line A2 which waveform is a
replica of the original waveform Ml, is displaced from M2
of line Bl by one-half sample period, but it is displaced
from the original waveform Ml of line lA by one sample
period, as illustrated by Figs. 3a and 3b by the location
of samples 2A, 3A, 4A, etc., on M3 with respect to the
samples of the previous lines Bl and Al, respectively.
The lat-ter effect is not objectionable to the viewer, since
only a one-half sample period displacement in the horizontal
direction of the luminance component of a given DOC sample
occurs from line-to-line in the displayed dropout compensated
color television signal when a single horizontal line is
used to compensate a series of consecutive horizontal lines
containing dropouts. The above me-thod allows forming a
composite dropout compensation signal having its composite
samples composed of chrominance component sample portions
horizontally displaced in one direction and luminance
component sample portions horizontally displaced in the
opposite direction with respect to the composite samples
of the previous color television signa] line. This pattern
alternates on consecutive dropout compensa-tion lines by
changing the above directions -to achieve an overall
mg/ ~ - 25 -
~.~ll7~.S~l
compensating effect, as has been described above with
respect to Fig. 4.
Fig. 3b depicts the luminance component of an
incoming oriyinal color television line Bl immediately
preceding a dropout. Waveform Nl comprising samples lB,
2B, 3s, e-tc., is identical to waveform Ml pertaining to
line Al of Fig. 3a, with the exception of having its samples
lB, 2B, etc., displaced by 1 sample period with respect
to waveform Ml. It will be seen from Fig. 3b, that N2
depicts respective waveforms of all the "odd" DOC lines
A2, A3,...AN following the original line Bl. Similarly,
N3 depicts all the even DOC lines s2, B3,...BN following
the original line Bl. There is a difference between forming
DOC luminance components on consecutive television lines,
as shown in Figs. 3a and 3b in that the luminance components
of the samples forming the dropout compensation signal for
the "odd" lines are~displaced with respect to the phase
locations in the original line by one-half sample period
and those forming the dropout compensation signal for the
"even" lines are displaced wi-th respect to the phase
locations in the original line by one whole sample period
in respectively opposite directions. The reason for the
difference in the direction of the displacement of the
samples forming the dropout compensation signal resides in
the arbitrary nature of the control of the delay lines
exercised by the control signals CB and HS/2. Should the
incoming color television signal start with the control
signals causing the delay line 3 to increase the delay by
one and one-half sample periods while the third two-way
switch 10 is conditioned to connect its output 39 to its
input 37, the displacement of the samples follows the
pattern depicted in Fig. 3a. However, should the dropout
compensa-tion sequence start with the control signals causing
mg/ - 26 -
1~7~S~
the delay line 3 to decrease the delay by one and one half
sample periods while the third two-way switch 10 is
conditioned to connect its output 39 to its input 37, the
displacement of the samples follows the pa-ttern depicted
in Fig~ 3b.
Ano-ther preferred embodiment of the dropout
compensator in accordance with the present invention is
shown in Fig. 5. In Fig. 5, circuit elements similar to
those previously described with respect to Fig. 2 are
designated by like reference numerals. Description of these
elements wi-th respect to Fig. 5 will be omitted to avoid
undue repetition. In the dropout compensation circuit of
Fig. 5, filter 5, which separates the luminance component
from the composite color television signal, is coupled to
the output 25 of the first two-way switch 1. The separated
luminance component at the output 34 of filter 5 is fed to
input 36 of differencing circuit 7, whose other input 35
is coupled to receive the color television signal from
output 25 of switch 1, via compensa-ting fixed delay line 6.
The resulting separated chrominance component at the output
45 of differencing circuit 7 is coupled to a first input 47
of a second two-way switch 14. The output 50 of switch 14
is coupled via a first controlled chrominance delay line 16,
which has its output coupled to a second input 49 of switch
14. The delay line 16 and switch 14 thus represent a first
circulating memory circuit 81 for the separated chrominance
component. The delay line 16 is controlled by the
previously described control signal indicated CB received
at control terminal 30 and applied to the control input 84
of the delay line. The separated luminance component at
the outpu-t 34 of filter 5 is further coupled to a first
input 46 of a third two-way switch 15, and it is coupled by
output 51 of switch 15 via a second controlled luminance
mg/ ~ - 27 -
delay line 17 to a second input 48 of swi-tch 15. It is
seen that delay line 17 and switch 15 form a second
circulating memory circuit 84 for the separated luminance
component. The delay line 17 is controlled by a control
signal indicated HS received at control terminal 43 and
applied to the control input ~4 of the delay line. A
dropout control signal DO received at control terminal 23
is coupled to respective control inputs 24, 85 and 86 of
switches 1, 15 and 14, respectively.
In operation, a dropout control signal DO, which
indicates the presence of dropout in the incoming composite
color television signal received at 12, activates all three
switches 1, 14 and 15, respec-tively. In response to the
receipt of a dropout control signal DO switch 1 is conditioned
to couple a dropout compensation signal from its input 21
to its outpu-t 25, which signal is provided by the circuit
of Fig. 5. The above-indicated dropou-t compensation signal
is formed as follows. During times when no dropout is
de-tected, filter 5 continuously receives the digital color
television signal present at input terminal 12 composed of
samples of an analog television signal taken at a frequency
equal to three times the color subcarrier signal frequency.
The separated l~ninance component at the output 34 of
filter 5 is fed via switch 15 to the luminance delay line 17.
Delay line 17 has a nominal length of delay equal to one
horizontal line period and stores one horizontal line of
the separated luminance component ob-tained from a horizon-tal
line preceding the horizontal line of the color television
signal then being received at terminal 12. The loading and
unloading of the signal stored in delay line 17 is
controlled by the control signal HS received a-t terminal 43.
Signal HS is derived from the horizontal synchronizing
component of the received color television signal and is
mg/~. - 28 -
~7~L5~
synehronous therewi-th. Wi-th respect to the ahove feature,
a separated luminance component is obtained whieh is delayed
by one horizontal line period and, at the same time, which
is coherent with the horizontal line synehronizing eomponent
during eonseeutive television lines.
Similarly, the ehrominance delay line 16, whieh
also has a length equal to one horizontal line period,
eontinuously reeeives the separated chrominance component
from differencing circuit 7, via switch 14. Thus, one
horizontal line of the separated chrominance component will
be stored in the chrominance delay line 16, which chrominance
component is obtained from the same portion of the color
television signal as the separated luminance component
stored simultaneously in the luminance delay line 17.
Loading and unloading of the chrominance component stored
in delay line 16 is controlled by the control signal CB
received at terminal 30, which signal is derived from the
color burst synchronizing component of the color television
signal received at terminal 12. sy controlling the
chrominance delay line 16 by the control signal CB, a
separated chrominance component is obtained which is delayed
by one horizontal line period, and at the same time, which
is in phase wi-th the color burst component during consecutive
television lines.
As it is shown in Fig. 5, the respectively delayed
separated luminance and chrominance components are combined
in the signal combining circuit 11 and the combined signal
at the output 42 of circui-t 11 is fed to the second input 21
of first switch 1. The signal at inpu-t 21 represents the
dropout compensation signal which is utilized by the
apparatus of Fig. 5 when a dropout control signal DO is
received.
mg/` - 29 -
117~5'~
~ hen a dropout is deteeted, switches 14 and 15,
controlled by signal DO, elose the respective circulating
memory eircuits 81, 82. Consequently, the separated
luminanee and ehrominance components circulate in their
respective circulating memory circuits, synchronously,
eontrolled by a elock signal at a frequeney equal to three
times the color subcarrier signal frequency in the manner
deseribed hereinbefore with reference to the dropout
eompensation embodiment illustrated in Fig. 2.
It follows from the above description, that the
cireuit of the preferred embodiment of Fig. S has an
advantage with respect to the cireuit of Fig. 2 in that it
does not require addi-tional modulation of the delayed
separated luminance component with respect to the delayed
separated chrominance component to reduee the line-to-line
horizon-tal displaeement o-f the luminance component samples.
Ins-tead, in the embodiment of Fig. 5, two separate eireulating
delay lines are provided, one delay line for the separated
luminanee eomponent and another one for the separated
chrominanee eomponent, respectively. Each delay line 16,
17 is controlled by a separate control signal cs or HS,
respeetively, to provide a signal component in synchronization
therewith.
The operation of the embodiment of Fig. 5 is
illustrated in Fig. 6, where the respective horizontal
displacements between the luminanee and ehrominanee component
samples on consecutive dropout compensation lines is shown.
To facilitate comparison with the previously described
diagram of Fig. 4 and the rela-ted circuit of Fig. 2, the
last received original color television si.gnal line without
a dropou-t, -the eonseeutive dropout compensation IDOC) lines,
as well as conseeutive samples of these respeetive lines
are designated by like reference charaeters in the diagrams
. mg/' - 30 -
lS~
of Figs. 4 and 6. It is seen in the diagram of Fig. 6 that
the luminance component of each DOC sample 2B, 3B, 4B, etc.,
on odd DOC lines Bl, B2, etc., is obtained by delaying the
luminance component of a corresponding sample 2A, 3A, 4A,
etc., of the original line Al by one horizontal line period
increased by one-half sample period. At the same time, the
chrominance component of each above-indicated DOC sample
3B, 4B, etc., on odd DOC lines is obtained by delaying the
chrominance component of a corresponding sample 2A, 3A of
-the original line Al by one horizontal line period increased
by one and one-half sample period. On the other hand,
the luminance component of each DOC sample 3A, 4A, etc.,
on even DOC lines A2, A3, etc., is obtained by delaying
the luminance component by an interval equal to one
horizontal line period less one sample period and the
chrominance component of the corresponding samples by an
interval equal to one horizontal line period less one and
one-half sample periods. Thus, in the operation of the
preferred embodiment of the invention of Fig. 5, the
relative line-to-line horizontal displacement of the DOC
signal samples is ~ 12 sample period for the luminance
component and - 1 12 sample periods for the chrominance
component. However, it i5 an advantage of the preferred
method of the invention depicted by Fig. 6 when comparing
it to the method depicted by Fig. 4 -that, when a sequence
of consecutive horizontal lines are compensated for dropouts
from a single horizontal line provided by the dropout
compensa-tor, every other DOC line is in phase wi-th the
original television signal line wi-th respect to both the
luminance and chrominance components, and no horizontal
displacement of samples takes place on these lines with
respect to both luminance and chrominance components.
'' ; mg/ - 31 -
1~7~S~
In the embodiment of the inven-tion illus-trated in
Fig. 5 it is not necessary to have the color television
signal sampled at a frequency equal to an in-tegral multiple
of the color subcarrier signal frequency, as is the case
in the embodiment of Fig. 2. In the embodiment of Fig. 5,
a sampling frequency equal to a rational number multiple
of the subcarrier frequency may be utilized.
For the purpose of a more complete disclosure
of the apparatus and method of the present invention, a
further embodiment of the present invention is shown in
the block diagram of Fig. 7. A corresponding detailed
electrical schematic diagram is shown in consecutive
Figs.8a to ~h. The embodiment of Figs. 7 and 8a to 8h has
been designed for a specific DOC apparatus in which a two
horizontal line delay of the original color -television
signal is required. First, the block diagram of Fig. 7
will be described, followed by the description of the
corresponding detailed circuit diagram illustrated in
Figs. 8a to 8h. In the embodiment of Fig. 7, a color
television signal is received at an input terminal 90,
in the form of consecutive digital samples at a frequency
equal to three times the color subcarrier signal frequency.
The received signal is applied to input 91 of a two-way
switch 52 and via the switch's output 93 to a first
controlled delay line 54, providing one horizontal line
delay less circuit delays designated ~ . The delayed
color television signal V from output 115 of delay line 54
is applied via a second compensa-ting fixed delay line 56
providing a delay equal to ~ to a second input 92 of
switch 52. Thus, a circulating memory circui-t 94 is formed
by elements 52, 54 and 56, similarly as previously descrihed
with respect to the embodiments of Figs. 2 and 5, respectively.
mg/` - 32 -
s~
A control signal CB is applied to a control input 125 of
the delay line 54, which is received at control terminal 124.
The CB control signal is similar to that described previously
with respect to Figs. 2 and 5, respectively. The color
television signal V from delay line 54 is also applied via
a third compensating fixed delay line 113 having a delay
designated T to a first input 101 of a signal differencing
circuit 64, as a delayed signal V'. Signal V from the
output of delay line 54 is also applied to input 104 of a
digital filter 60. The digital comb filter described in
the above-mentioned copending patent application SN 362,754
may be employed as filter 60. Fil-ter 60 separa-tes the
luminance component from the color television signal, in a
manner described in the copending application. The separated
luminance component L' obtained at the output 105 of
filter 60 is applied to a first input 107 of a second two-way
switch 62 and it is also applied via a fixed two-clock cycle
delay line 67 to a second input 108 of switch 62. An output
109 of switch 62 is connected to a first inpu-t 98 of a
signal combining circuit 66. Thus, the delayed separated
luminance component L is applied by switch 62 to signal
combining circuit 66 either directly or delayed by additional
two clock cycles indicated 2 ~. Switch 62 is controlled at
its con-trol input 112 by a control signal designated HS/2
received at a control terminal 111. An inverted separated
luminance signal component L' obtained at an inverting
output 106 of filter 60 is applied to a second input 102
of the signal differencing circuit 64. Circuit 64 provides
at output 103 a difference signal C' representing the
separa-ted chrominance component obtained as a difference
between the delayed composite color television signal V'
and the delayed separated luminance component L', respectively
applied to the first and second inputs of the signal
.g/ ~ - 33 -
1~7~S~
differencing circuit 64. In this preferred embodiment, -the
differencing circuit 64 is implemented as a signal adder and
the separated chrominance componen-t C' is obtained by adding
the delayed composite signal V' and inverted separated
luminance component L'.
The separated chrominance component Cl provided at
output 103 of differencing circuit 64 is applied via a
one-clock-cycle delay line 68 to one input 99 of signal
adder 66. As previously mentioned, the other input 98 of
signal adder 66 receives the separated luminance component
L from the second swith 62. Signal adder 66 combines the
received chrominance and luminance components into a
composite color television signal at its output 100, which
signal represents the color television dropout compensation
signal. The dropout compensation signal is applied to a
first input 95 of a third two-way switch 58. A second
input 96 of switch 56 is coupled to receive the composite
color television signal present at input terminal 90 via
first switch 52, first controlled delay line 54 and second
compénsating delay line 56. A control input 97 of switch
58 is coupled to receive a dropout compensation control
signal DO via a delay line 53 providing a one horizontal
line delay. The latter control signal is received at
input terminal 110 when a dropout in the incoming color
television signal at terminal 90 is detected, for example,
by a conventional dropout detector (not shown) of the kind
previously described. The dropout control signal at 110
is also applied directly to a control input 114 of the
first switch 52. An ou-tput 127 of switch 58 is coupled
via a further controlled delay line 126 -to an output
terminal 70 of the dropout compensation circuit of Fig. 7.
Delay line 126 provides a one horizontal line delay and is
controlled by signal CB applied to i-ts control input 128,
mg/ - 34 -
:~.7~
simultaneously with and in the same manner as previously
described with respec-t to controlled delay line 54.
Now the operation of -the dropou-t compensa-tion
circuit of Fig. 7 will be described. When a dropout in
the composite color television signal is detected, for
example, by a conventional dropout detector (not shown),
a dropout control signal DO received at terminal 110 is
applied to switch 52 and to switch 58 via delay line 53
providing a one horizontal line delay. Switch 52 closes
the circulating memory circuit 94 and, consequently, the
last received horizontal line of the color television signal
immediately preceding the dropout, which has been stored
in delay lines 54 and 56, circulates in the memory circuit
94 via input 92 and ouput 93 of switch 52 at a clock signal
frequency equal to 3 x 3.58 MHz~10.74 ~Iz. The clock sianal
which is utilized for clocking the respective circuit elements
of Fig~ 7 is not shown in the block diagram for better
clarity of representation, but it is shown in the correspond-
ing detailed schematic diagram of Figs. 8a to 8h, which
will be described hereinafter. Fil-ter 60 receives the
composite signal V, delayed by one horizontal line period
less circuit delay compensation, from the circulating
memory 94 and separates the luminance component L' -therefrom
in a manner described in the above referenced copending
patent application SN 362,754. The separated luminance
component is applied via switch 62 to signal combining
circuit 66 which is implemented in the preferred embodiment
as a signal adder. Switch 62, in response to -the control
signal HS/2, alternatively applies the separated luminance
component L', directly or delayed by two additional clock
cycles in delay line 67, to signal combining circuit 66.
The control signal ~S/2 has a frequency of one-half of the
horizontal line frequency. It is derived from the
mg/ - 35 -
5'~
horizontal synchronizing component of the color television
signal coupled to terminal 90. The phase of signal HS/2
is con-trolled by signal CB received at terminal 124 to
assure that the same phase-relationship of these two control
signals will be maintained during the entire operation.
Thus, switch 62 is controlled by the signal HS/2 to
alternatively apply, during consecutive horizontal lines,
the luminance component signal L', undelayed and delayed
by two clock cycles 2 ~, respectively, to input 98 of
adder 66. On the other hand, as previously described,
signal adder 64 receives both the delayed composite signal
V' and the inverted luminance component L' and provides a
separated chrominance component C' at its output 103. To
compensate for the circuit delay ~ effected by processing
-the separated luminance component in filter 60, signal V'
is delayed by ~ in the third fixed compensating delay line
113. The separated chrominance component C' is delayed by
one clock cycle ~ in delay line 68 and the delayed
chrominance component C is combined with the above-described
luminance component L in signal adder 66.
As will be appreciated from the foregoing description
with respect to Figs. 3 and 4, alternately delaying and not
delaying the separated luminance component L' by additional
two clock cycles on the occurrence of alternate horizontal
line periods, and delaying -the separated chrominance
component by an additional one clock cycle during consecutive
horizontal line periods, and combining such components in
adder 66, results in the genera-tion of a composite dropout
compensation signal, whose luminance component delay is
modulated by - 1 clock cycle with respect to the chrominance
component delay during consecutive horizontal line periods.
Thus, the previously mentioned feature of the invention
related to decreasing the luminance component delay with
~,~
~f mg/~ - 36 -
1~7~S~
respect -to the chrominance component delay of the resulting
dropout compensation signal is achieved in the preferred
embodiment of Fig. 7 by the above-described combination of
circuit elements.
It will become apparent from -the above-description
of -the block diagram of Fig. 7, that delay I compensates
for the delay in the filter circuit 60 and -that delay
for the combined delays ~ and delay ~ provided by delay
line 68; thus, ~ = T + ~, when considering that circuits
64 and 66 have no significant circuit delays.
It will also become apparent with respect to
Fig. 7 that in this particular preferred embodiment the
controlled delay line 54 is coupled in the main composite
color television signal path. Therefore, when no dropout
compensation takes place, the incoming composite signal
received at 90 is continuously delayed in delay lines 54
and 56 by one television line period, which delay is
controlled by a clock signal that is synchronous with the
color burst component contained in the incoming television
signal. Thus, the original composite signal at the output
127 of switch 58 exhibits an undesired - 180 phase shift
on alternative lines relative to the reference timing
associated with the main composite color television signal
path. To compensate for this effect, an additional delay
line 126 providing a one horizontal line delay is coupled
in the main color television signal path at the output 127
of switch 58. The loading and unloading of delay line 126
is controlled by the above~described con-trol signal CB
received at control terminal 124 and applied to control
input 128 of delay line 126. Since the color television
signal received by delay line 126 has been previously
delayed by one horizontal line period in delay lines 54
and 56, and both delay lines 54 and 126, respectively are
c ~ mg/ - 37 -
~17~5~
controlled by the same signal CB, respective phase shifts
of - 180 in opposite sense are effected on respective
output signals of these controlled delay lines 54 and 126.
Consequently, these opposite phase shifts cancel, and no
adverse phase shift is present in the output signal at
output 70 of the dropout compensation circuit of Fig. 7.
In -the dropout compensator embodiment of Fig. 7,
the dropout compensation signal at the output terminal 70
is delayed by three horizon-tal line perlods from the line
it appeared at input terminal 90 to the time it appears
at the output terminal 70 as the dropout compensation
signal. This delay consists of the one horizontal line
period delay resulting from the ini-tial passage through the
memory circuit 9~ before the dropout being compensated is
detected. After -the dropou-t is detected, the horizontal
line is again circulated through the memory circuit 94 before
the s~itch 58 receives and responds to the delayed control
~signal DO to couple the dropout compensation signal formed
from the recirculated horizontal line to the output 127.
This adds a second one horizontal line period delay to the
dropout compensation signal. The third one horizontal line
period delay results form the presence of the delay line 126.
The three horizontal line delay is needed to satisfy the-
specific requirements of a particular appara-tus in which
this par-ticular embodiment of the invention is u-tilized.
However, the actual dropout compensation signal obtained at
output 127 of siwtch 58 is still formed from -the last
received original television signal line at input 90,
substantially in the same manner as that previously described
with respect to Figs. 2 and 5.
It is noted, however, tha-t the previously described
respective embodiments of Figs. 2 and 5 do not have their
respective controlled delay lines coupled in the main
mg/ - 38 -
5~
composite color television path and, -therefore, they do
not exhibit an undesired phase shift such as provided by
the embodiment of Fig. 7 during normal system operation
when no dropout compensation is provided.
The detailed circuit diagram shown in consecutive
Figs. 8a to 8h essentially corresponds to the previously
described simplified block diagram of Fig. 7. To facilitate
comparison, individual circuits in the detailed diagram
corresponding to elements of the b]ock diagram are delineated
by dashed lines and designa-ted by like reference numerals.
Similarly, connecting lines between -the circuits of the
detailed diagram are designated by reference numerals
corresponding to input/output designations of corresponding
blocks of Fig. 7. For the purpose of complete disclosure,
the integrated circui-t components shown in the preferred
embodiment of Figs. 8a to 8h are designated by respective
part numbers commonly used by manufacturers.
Specifically, the filter circuit 60, which is
utilized to separate the luminance component from the
composite color television signal and which is shown in
consecutive Figs. 8d and 8e is similar to that disclosed
in the above-indicated copending application SN 362,754.
It is understood, however, that a well known digi-tal comb
filter may be utilized instead. As it will be seen ~rom
the detailed circuit diagram of Figs. 8a to 8h, it is an
advantage of the digital dropout compensation circuit of
the present invention that all signal processing is provided
in real time utilizing standard TTL (transistor~to-transistor
logic) circuitry. The circuit of the above-indicated
Figures is designed for dropout compensation in a color
television signal recording and reproducing system where
an NTSC color television signal is encoded in digi-tal form
by sampling at a frequency equal to three times -the color
` mg/` - 39 -
subcarrier frequency of the television signal, -that is,
fsampl = 3 x 3.58 ~Iz ~ 10.74 ~z. The sampling signal
is phase locked to the color burst component of -the
subcarrier signal in a manner well known in the art. The
sampling frequency is equal to the clock frequency as
previously mentioned with respect to the description of
Fig. 2.
Generally, for operation of the dropout compensator
of the invention, the sampling frequency utilized to encode
the composite analog color television signal, does not have
to be -the same as the clock signal frequency utilized to
synchronize the various elements of the dropout compensation
circuit. The samples may be received and stored, for
example, at the sampling frequency, and subsequently
recovered at the clock frequency, while the latter frequency
is utilized for synchronization of the circuit.
Now the preferred embodiment of the invention
shown in the detailed circuit diagram of Figs. 8a to 8h
will be described. In Fig. 8a, consecutive samples Sl,
S2, S3, etc., of the digital color television signal are
received at input 90 of the dropout compensator as 8-bit
parallel data by a first set of inputs 91 of two data
selector/multiplexers A24 and A34 of two-way switch 52.
These multiplexers also receive data at second input 92
from output 116 of delay line 56, shown in Fig. 8c. A
dropout control signal DO from a conventional RF envelope
level dropout detector circuit (no-t shown) is received by
the multiplexers at 114 via input 110 and flip-flop A16.
Flip-flop A16 is utilized for the common purpose to precisely
clock the dropout con-trol signal. In normal operation,
the multiplexers apply the input data 90 via flip-flop A23
to output 93. Generally, flip-flops similar to A23 are
utilized throughout the entire circuit -to delay the
mg/~ - 40 -
s~
processed data by one clock cycle to assure precise data
clocking. When the DO control signal at 110 is received,
the output of multiplexers is switched from input 91 to
input 92. The data from output 93 is fed to output 70 of
the dropout compensator circuit via first controlled
delay line 54 of Fig. 8b; second compensating delay line
56 of Fig. 8c; switch 58 of Fig. 8g; and a fur-ther controlled
delay line 126 of Fig. 8g.
Delay line 54 of Fig. 8b comprises eight identical
4 x 256 bit random access memories of which six memories
designated Bl, Bll, B31, B51, B61 and B81 are shown. Two
groups of four memories each are utilized for receiving
higher and lower order bits, respectively. The controlled
delay line 54 has a length of delay equal to one horizontal
line period of the color television signal less the
compensating delay ~ provided by fixed delay line 56 of
Fig. 8c coupled in the delayed color television signal path.
Delay line 56 is implemented by eight shift registers, of
which four registers A51, A61, A52 and A82 are shown.
Flip-Flop ~41 at the output of delay line 54 is utilized
to assure proper timing of the output data to achieve the
desired delay. The delayed composite signal at output 115
is applied via fixed compensating delay line 56 to input
96 of switch 58 of Fig. 8g.
As shown in Fig. 8d, the data S1, S2, S3, etc.,
from output 115 of delay line 54, representing signal V,
is also applied to one set of inputs 104 of filter 60.
As described in -the above-iden-tified copending application,
filter 60 continuously provides an average value of three
consecutive samples Sl, S2, S3. As shown in Fig. 8c,
a sample Sl, applied via a portion of delay line 56, is
obtained at output 117 thereof, where it has been delayed
by one clock signal period to assure its proper timing for
mg~ - 41 -
~7~C~
addition with a sample S2 received one clock signal later.
The delayed sample, S2, is applied to a second se-t of
inputs 104 of filter 60. Samples Sl and S2 are added in
two 4-bit binary adders A33, A43 of Fig. 8d and -their sum
Sl + S2 is delayed in flip-flop A53 by one clock signal in
preparation for addition with the subsequently received
sam~le S3 from output 115 of 54. The lat-ter summation
is performed by two 4-bit binary adders A54, A44 and an
output signal therefrom represents the sum S=Sl+S2~S3.
Signal S is fed through flip-flop A55 to assure proper
timing for further processing. In this particular
embodiment of the invention, an average sample value is
obtained by dividing signal S by 3. The division by 3
is performed with a 0.13% accuracy by an approximation
algorithm:
S ~ S + S + S
3 4 16 256
For the particular application of averaging the
samples in the presently described preferred embodiment,
the approximation algorithm is implemented in two steps
as follows:
PS = 4 + 16
3 ~ PS ~ P16
These two steps are performed in the remaining
portions of the circuit diagram of filter 60 shown in
Fig. 8e as described below.
Two 4-bit binary adders A64, A65 of Fig. 8e receive
the signal S at two se-ts of inputs, they shift the signal S
in the well known manner to effect a division and generate
4 at one of the sets of inputs and they provide a sum of
mg~ - 42 -
~ 7~S~
(S + 4). The output signal from the lat-ter adders is
further shifted to obtain a divided ou-tput signal
corresponding to (S + S4)/2. The latter output signal
represents twice the partial sum PS defined above. The
signal 2PS is applied to flip-flop A75 which supplies
signal 2PS to two sets of inpu-ts of two 4-bi.-t binary
adders A66 and A76. The latter adders shift the signal
2PS to obtain 2P6S at one set of inputs and they provide
an output signal corresponding to (2PS + 216S)/2. This
output signal represents S3 of the approximation algorithm
indicated above. The obtained signal 3 corresponds to
the average value output signal provided by filter 60 in
accordance with the disclosure of the above-referenced
copending application. The output signal of adders A66,
A76 thus represents the chrominance-less color television
signal, that is, the separated luminance component
described hereinbefore with reference to Fig. 7. Signal
S3 is applied to flip-flops A67 and A77, which provide both
an output S/3, indicated L' at output 105, to input 107 of
switch 62 in Fig. 8f and an inverted output signal (-S/3),
indica-ted -L' at output 106 and applied to a first set of
inputs 102 of adder 64 of Fig. 8f~ Adder 64 is implemented
by -two 4-bit binary adders, A36, A46.
In this preferred embodiment, the compensating
fixed delay line 113 shown in the block diagram of Fig. 7,
is implemented by a portion of the fixed compensating
delay line 56 at ou-tput 130 thereof, as i-t is shown in
Fig. 8c. The thusly delayed composite color television
signal from output 130 of delay line 56, indicated V',
is applied to a second set of inputs 101 of two 4-bit binary
adders A36, A46 of adder 64 in Fig. 8f. Adder 64 provides
at output 103 an output signal indica-ted C', which represents
the separated chrominance component, as it has been described
, ---i~
.? mg/'. _ 43 _
~.7~5~
before with reference to Fig. 7. Signal C' is applied via
flip-flop A56 and A5 representing the compensating fixed
delay line 68 to a first set of inputs 99 of two 4-bit binary
adders A84, A85 of adder 66 in Fig. 8g. The separated
delayed luminance component L is applied from the outputs
109 of multiplexers A87, A86, representing switch 62 in
Fig. 8f, to the other set of inpu-ts 98 of adder 66. As
described above, the separated luminance component L' from
output 105 of filter 60 in Fig. 8 is applied to one set of
inputs 107 of multiplexers A87, A86. The luminance
component L', which has been delayed by delay line 67,
implemented by flip-flops A68, A78 shown in Fig. 8f, is
applied to the other set of inputs 108 of the multiplexers
representing switch 62. Multiplexers A87, A86 of Fig. 8f
are controlled at input 112 by the control signal HS/2
of 7.8 kHz, received at 111, which signal has been
described with reference to Fig. 7. As it has been
previously disclosed with respect to block diagrams of
; Fig. 7, in response to control signal HS/2, switch 62
applies to input 98 of signal adder 66 the separated
luminance component L directly from output 105 of filter
60, or via the two-clock-cycle delay line 67, alternatively,
on consecutive horizontal lines. Adder 66 of Fig. 8g
combines the respective luminance and chrominance components
received at its respective inputs 98, 99 into a composite
color television signal, representing the dropout compensa-
tion signal. Tha-t signal is applied, via re-timing flip-
flop A89, to a first set of inputs 95 of switch 58 in
Fig. 8g, which is implemented by multiplexers A73, A74.
- 30 On the other set of inputs 96 of mul-tiplexers A73, A74,
an output signal from delay line 56 of Fig. 8c is received.
The outpu-t signal at 127 from swi-tch 58 is applied to delay
line 126 which is shown as a bloc]c. This delay line is
mg/J~ - 44 -
~ ~ 7~S'2~
utilized in the specific embodiment o Figs. 8a -to 8h to
eliminate the undesired 180 phase shift as previously
explained with reference to Fig. 7. Delay line 126
provides a one horizontal line delay and is controlled
by the control signal CB, simultaneously with and ln the
same manner as the controlled delay line 54. Delay line
126 may be constructed utilizing random access memories
similar to those of delay line 54 in Fig. 8b. The output
signal of delay line 126 thus represents the output signal
of the dropout compensation circuit of Figs. 8a to 8h
provided at output terminal 70.
Switch 58 of Fig. 8g is controlled by the dropout
control signal DO received at input terminal 110 in
Fig. 8a and applied via output 55 of delay line 53 of
Fig. 8a to control input 97 of switch 58.
Fig. 8h shows a memory address generator circuit
150 providing output signals at memory address lines Ao
to A7 of a memory address bus AB, coupled to control the
data flow through the respective delay lines 53, 54 shown
in Figs. 8a and 8b, respec-tively. In Fig. 8h counters
A2, A12, A22, B2, B12 and B22 are utilized to count the
respective clock cycles corresponding to the actual delay
provided by these delay lines. The diagram of Fig. 8h
reveals the memory address generator 150 in sufficient
detail, consequently, no further disclosure is necessary.
To provide a complete disclosure of -the detailed
circuit, respective clock signals indicated 10.7 MHz and
inverted clock signals indicated 10.7MHz utilized for
clocking various circuit elements are shown in the detailed
circuit diagram of Figs. 8a to 8h. These signals are
derived in a well known manner, as described, for example,
in the above-indlcated copending applica-tion.
mg~ ~ - 45 -
In the foregoing specification, the preferred
embodiments of the invention have been described with
respect to a digital dropout compensation circuit, in
which an NTSC composite color television signal is
sampled at a frequency equal to three times the color
subcarrier signal frequency, fsampl ~ 3 x 3.58 MHz = 10.74
MHz. As it will become apparent to those skilled in the
art, the respective block diagram of Figs. 2, 5 and 7 could
also be utilized for other digital as well as analog
color -television signal systems, such as NTSC, PAL, PAL-M,
etc. For example, if the dropout compensation apparatus
of the invention is utilized in an analog color television
signal system, it may be preferable to implement the
controlled delay lines, for example, delay line 3 of Fig. 2,
by conventional charge-coupled devices (CCD). As it is
known in the art, such devices could be utilized as delay
lines for delaying signals in analog form, while the
information to be delayed may be clocked in and out of the
delay line in response to a control signal, similarly,
as it has been disclosed with respect to the above-described
embodiments. At the same time, delay lines, such as 8 and 9
of Fig. 2, may be implemented by conventional analog delay
lines. In the later case, delay line 8 would provide a
desired amount of the analog signal delay substantially
corresponding to the line-to-line offset of the color
subcarrier signal, and advantageously delay line 9 would
provide substantially twice that amount of delay to achieve
a desired luminance component delay modulation with respect
to the chrominance component delay, àS described above.
Generally, the invention wi-th respect to Fig. 2
may be utilized ln any digital color -television signal
system where the sampling frequency is equal to an integral
multiple of the color subcarrier signal frequency. For
mg/` - ~6 -
5i~
example, if an NTSC color television signal is sampled
at a frequency equal to four -times the suhcarrier signal
frequency, then, to achieve a desired dropout compensation
signal, having a proper line-to-line phase of the chrominance
component, for example, by the embodiment of Fig. 2,
the original signal chrominance component samples would
have to be displaced on consecutive television lines by
plus or minus two sampling periods. Consequent]y, to reduce
the luminance component displacement on consecutive lines,
the delay line 9 of Fig. 2 would have to provide a four
clock cycle delay 4 ~, while the delay line 8 would have
to provide a two cycle delay 2 ~.
The above-disclosed invention is applicable
also to PAL or PAL-M signal systems, since the respective
con-trol signals, such as signal CB, HS and HS/2, respectively,
in Figs. 2, 5 or 7 are frequency and phase locked to the
incoming color television signal. It will be apprecia-ted
by those skilled in the art that the so-called one-quarter
cycle offset corresponding to the 90 degree phase shift
occurring in the PAL subcarrier component during consecutive
television lines may be compensated for in the dropout
compensator of the present invention by applying the
above control signals to the respective controlled delay
lines of the above-indicated embodiments, whlch will be
loaded and unloaded on consecu-tive lines synchronously with
the control signals of the particular signal system utilized.
As it will become apparent to those skilled in
the art, alternative e~bodiments similar to the disclosed
detailed circuit diagrams of Figs. 8a to 8g, as well as
alternative circuit elements in these embodiments, may be
utilized to obtain the disclosed opera-tion oE the dropout
compensator in accordance with the method of the present
invention. For example, the differencing circuit 64 may be
mg/~. - 47 -
~7~
implemented as a subtracting circuit to which respective
signals L', V' of the same polarity are applied. Similarly,
known alternative circuit elements in -the summing circuit
66 may be utilized to obtain -the desired combination of
the chrominance and luminance components. As an alternative,
different means of obtaining the delay in delay lines ~4
may be utilized, such as shift registers, instead of the
random access memories. Likewise, to obtaln division by
3 of the samples in filter circuit 60, read only memories
may be utilized instead of the disclosed circuit elements
implementing the approximation algorithm.
The individual circuit elements of all the above
disclosed embodiments of the invention may be implemented
by conventional, standard integrated devices.
While the invention has been shown and described
with particular reference to preferred and alternative
embodiments thereof, it will be understood -that variations
and modifications in form and details may be made therein
without departing from the spirit and scope of the
invention as defined in the appended claims.
~ mg/ c - 48 -
