Note: Descriptions are shown in the official language in which they were submitted.
3~05~
RCA 69,812
This invention relates to a signal defect
compensator which may be used in a system for reproducing
recorded television image information.
In the reproduction of television video signals
from information recorded, for example, on magnetic tape,
magnetic discs or other types of recording media, a
decrease or loss of the recorded signal information may
be encountered, which is apparent to the viewer as a -
degraded picture. A principle form of such a defect is
known as a dropout which may occur due to imperfections
in the recording media or an accumulation of dirt or
other debris at the junction where the playback/recording
heads and the media meet. When such a dropout occurs,
the signal recovered from the media generally suffers a
sharp reduction in amplitude which appears in the picture
seen by the viewer as random black and white streaks or
flashes on the screen of the television receiver or other
means utilized as a reproducer. In severe cases, such a
dropout may occur for one or more entire lines of the
television picture.
Since it is known that television image
information is to some extent redundant from line to line,
dropout compensators (DOC) have generally operated on the
principle that since the image information is redundant
from line to line, it is possible to compensate for a
dropout by substituting information from a previous image
line during the duration of the dropout. Known defect
compensators, as shown in U.S. Patent No. 2,996,576,
3~ therefore, store image information from a previous line
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RCA 69,812
1 and insert the stored previous line information into the
output video signal when a signal dropout occurs. Such
a direct replacement is generally satisfactory for a
monochrome video signal; however, in the case of a color
video signal, a direct replacement from the previous line
is not satisfactory since the phase relationship between
scanning sync and color sync (burst in the NTSC system)
differs on adjacent lines of a television picture. The
scanning sync pulses are in-phase from line to line, but
the color sync (burst) is interlaced or 180~ out-of-phase
from line to line. If a direc-t replacement signal from
the previous line storage is attempted, the color signals
will be inverted, and the replacement appears on the
reproduced picture as complementary to the true colors.
This affect is particularly disturbing to the viewer if
the dropout exists for any appreciable portion of a
television line. Therefore, in a color dropout compensating
circuit, as shown in U.S. Patent No. 3,463,874, the stored
image information is separated by a low pass filter into
a luminance portion containing substantially all the
frequencies below 2.5 MHz and by a bandpass filter into
a chrominance portion containing substantially all the
frequencies from 2 to 4 ~Hz. The chrominance portion is
passed through an inverting signal processor and then
recombined with the luminance portion so that the color
phasing is correct when the stored video is utilized as
replacement video during a dropout.
:'
A video signal processing system is provided
for dropout compensation of a color video signal by
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RCA 69,812
1 substituting a replacement color signal for the color
video signal during the period of the dropout. The
replacement signal consists of video information from
the television lines before and after said dropout. The
system comprises first delay means responsive to a
source of video signals for delaying the video signals
by at least one television line to provide a first
information signal. Second delay means coupled to the
first delay means so that the first and second delay
means are responsive to the video signals for delaying
the video signals by at least two television lines to
provide a second information signal. First combining
means are responsive to the video signals and the
second information signal to provide a third information
signal at a line time intermediate the video signals
and the second information signals. Signal processing
means responsive to the third information signal for
developing a replacement signal equal in subcarrier
phasing to the first information signal. Switch means
coupled to the first information signal and the second
video output signal being responsive to a dropout signal
for substituting one of the first information signal
or the second video output signals for the other in the
presence of the dropout signal.
FIGURE 1 is a block diagram of an analog
system embodying the present invention;
FIGURE 2 is a graphical representation of the
operation of the system of FIGURE l;
FIGURE 3 is a block diagram of a digital
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RCA 69,812
1 system embodying the present invention;
FIGURE 4 is a graphical representation of the
operation of the system of FIGURE 3;
FIGURE 5 is a block diagram of an alternate
embodiment of an analog system embodying the present
invention; and
FIGURE 6 is a graphical representation of the
operation of the system of FIGURE 5.
In FIGURE 1, a source of video signals, for
example from a video tape recorder (VTR), are coupled
to the input terminal of a lH delay line 10 and to one
input terminal of a summing amplifier 40. The lH delay
lines 10, 20 and the 140 and 280 nanosecond delay lines
15 30 and 50 may be constructed in the form known as a
glass delay line of conventional design and capable of
providing a delay, for example, in delay line 10 of
63.5 microseconds which is equivalent to a signal input
delay of one horizontal television line in the ~JTSC
20 system standards. The output terminal of lH delay line
10 is coupled, in turn, to the input terminal of a second
delay line 20 and to the input terminal of a 140 nanosecond
delay line 30. The output terminal of delay line 30 is
coupled to one input terminal of a video switch 70.
The output terminal of delay line 20 is coupled
to the other input of summing amplifier 40. The output
terminal of amplifier 40 is coupled, in turn, to one
input of a second summing amplifier 60 and to the input .
terminal of a 280 nanosecond delay line 50, the output of
30 which is coupled to the other input of summing amplifier 60.
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RCA 69,812
1 The output of amplifier 60 is coupled to the other input
of video switch 70. Video switch 70 is of conventional
design and operates, i.e., couples either input SN from
delay line 30 or input SR from amplifier 60 to a
common output terminal (Video Out), in response to a
control signal from signal dropout detector 80. Signal
dropout detector 80, also of conventional design,
monitors the R. F . output signal (signal IN) of the VTR,
which is generally in the form of a frequency modulated
signal bearing the image or video information. The
detector 80 is responsive to a loss of R.F~ rather than
video in~ormation so as to distinguish between a true
dropout and the varying video signal.
The operation of the circuit of FIGURE 1 is as
follows. Referring to the graphical illustration of
FIGURE 2, which illustrates any three successive lines of
a television field identified as line 0, 1, 2, the
incoming video signal shown in FIGURE 2, as of an arbitrary
length S, is delayed in delay line 10 by one television
line which, in effect, displaces the signal segment to
a position Sl. Although -the video signals S, Sl, SN, etc.,
are shown as segments of a TV line, it is understood that
the video signal is a continuous signal and that FIGURE 2
represents a moment of stopped time during which a
dropout of duration equal to line segment S has occurred.
The video signal is further delayed by delay line 30 by
an additional 140 nanoseconds, which is equivalent to
one-half cycle a-t a burst carrier frequency of 3.58 MHz
in the NTSC system. The incoming video signal S delayed
by one line (Sl) plus 140 nanoseconds yields video signal
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RCA 69,812
SN, which is coupled through video switch 70 to the output
terminal and, as such, is utilized as the normal video
signal.
Simultaneously with the processing of the
input video signal S to provide the video output signal
from video switch 70, the once-delayed output of delay
10 is delayed yet another line, that is, from time
position Sl on line 1 to position S2 on line 2. The
original input signal S and twice-delayed input signal
S2 are coupled to summing amplifier 40, the output of
which is equal to Sl or the combined and weighted output
of line segments Sl and S2. The signal segment S1 is
again summed with Sl and S2, delayed by 280 nanoseconds
in delay line 50, so as to provide a combined and
weighted line segment S5 at the output of amplifier 60
which, as is illustrated, represents line segments S
and S2 delayed by 280 nanoseconds or one full cycle of
subcarrier at 3.58 MHz. Therefore, the output of
summing amplifier 60, which provides the replacement ~;
video segment to video switch 70, comprises one-quarter
of S, S2, S3 and S4 delayed by one full line plus 140
nanoseconds. This weighted and delayed input video signal
SR, coupled to video switch 70, is identical in time and
amplitude to the normal video signal input to video
switch 70 and further represents a weighted average of
the image information surrounding the point of interest
(the dropout occurring during the line segment SN) rather
than a direct replacement of more or less redundant
video image information from a previous line.
Thus, a replacement video signal is generated
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RCA 69,812
1 which more nearly represents the original video signal
in terms of luminance and chroma content. This is of a
particular performance advantage in regard to the
luminance detail of the replacement video signal, as
compared with the original luminance video signal detail
since the replacement video signal comprises luminance
detail information, which is the combination of the line
before and the line after the line of interest. This
averaging of luminance detail yields an improved video
display in all modern television systems which utilize
an interlaced television image field sequence since
adjacent lines in any particular field are really
displaced by two lines on the television display as a
result of the field interlace.
FIGURES 5 and 6 illustrate an alternate
embodiment to the system of FIGURES 1 and 2 wherein the
correct phasing of the chrominance portion of the :-
replacement video signal is obtained by chroma signal
inversion of one of the two signals making up the
replacement video signal rather than by means of the
delay devices 30 and 50 of the circuit of FIGURE 1.
The operation of the circuit of FIGURE 5 is as
follows. Referring to the graphical illustration of .
FIGURE 6, which illustrates any three successive lines of
a television field identified as line 0, 1, 2, the
incoming video signal, shown in FIGURE 2, as of an arbitrary
length S, is delayed in delay line 10 by one television
line which, in effect, displaces the signal segment
to a position SN on line 1. Video signal SN is coupl.ed
through video switch 70 to the output terminal and, as
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RCA 69,812
1 such, is utilized as the normal video signal.
Simultaneously with the processing of the
input video signal S to provide the video output slgnal
from video switch 70, the once-delayed output of delay
line 10 is delayed yet another line in delay means 20
to provide a signal S2. The original input signal S and
twice-delayed signal S2, which are in the correct
subcarrier phase since they are on alternate television
lines, are coupled to summing amplifier 40 wherein the
signals Sl and S2 are combined and weighted to yield at
the output of amplifier 40 a signal which is equal to
the required replacement for signal segment SN in
amplitude, but not in chroma phase since adjacent lines
have a subcarrier burst which, in the NTSC system, differs
from line to line by lB0 of subcarrier frequency. To
obtain the correct chroma phase, the output signal from
summing amplifier 40 is coupled to a low pass filter 110
wherein the luminance portion of the signal from 0 to 2 MHz
is separated into a luminance signal component; and to a
bandpass filter 90 wherein the chroma portion of the
signal from 2 to 4 MHz is separated into a chroma signal
component. The chroma signal component is, in turn, ~ -
coupled to an inverter 105, the output of which is the
chroma signal component inverted in phase. The luminance
signal component output of low pass filter 110 and the
inverted chroma signa]. component output of inverter 105
are coupled to the input terminals of a summing
amplifier 60, the output of which provides the replacement .
video segment to video switch 70 in the form of one-half
of S and S2 with the chroma portion inverted. This
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~ 33709
RCA 69,812
weighted and inverted input video signal SR, coupled to
video switch 70, is identical in time and amplitude
to the normal video signal input to video switch 70 and
represents a weighted average of the image information
both before and after -the point of interest (the dropout
occurring during the line segment SN) rather than a
direct replacement of more or less redundant video
information from the previous line above.
FIGURE 3 further illustrates the present
invention in a digital television signal processing
system.which utilizes conventional shift registers to
obtain the signal delays afforded by the glass delay lines
of FIGURE 1. In FIGURE 3, the reference numerals
generally correspond to the reference numerals of
FIGURE 1, wherein, for example, the lH delay shift
register 100 of FIGURE 3 corresponds to the lH glass delay
line 10 of FIGURE 1. Similarly, the 200 to 800
reference numerals identify corresponding functions of
FIGURE 1 identified by reference numerals 20 to 80.
In FIGURE 3, the video input signal S represents
video image information, which has been processed into a
digital format, for example, by means of an analog to
digital (A/D) conversion (not shown) at a typical clock
rate of 14.32 MHz (four times the subcarrier frequency
of 3.58 MHz) or approximately 70 nanosecond sample
time using an 8 bit binary digital representation of the
magnitude of each sample taken at the clock rate. ~-
Therefore, the video input signal S to lH delay 100
is an 8 bit digital word; and since clock 900, which is
coupled to shift register 100, is operated at four times
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RCA 69,812
the subcarrier frequency, the lH delay line shift
register 100 comprises 910 stages, times 8 bits per word,
so as to provide a signal delay between the input and
output terminals of shift register 100 of
910 X 4 X Subcarrier nanoseconds for a total delay time
of 63.5 microseconds, which is equivalent to one
horizontal television line of delay. In similar fashion,
four stage shift register 500 and two stage shift
register 300 clocked at the clock rate of 14.32 MHz
yield, respectively, 280 (4 X 70 nanoseconds) and 140
(2 X 70 nanoseconds) of delay to a signal between their
input and output terminals.
The operation of the circuit of FIGURE 3 is
as follows. Referring to FIGURE 4, which is a graphical
representation of any three successive television lines
of a given field identified by lines 0, 1, 2, the dots
representing digital samples of the video image
information taken at the clock rate of four times
subcarrier or, as illustrated in FIGURE 4, at a sampling
time interval of 70 nanoseconds. Although the video
signal in FIGURE 4 is shown as comprising fixed samples
at a moment of stopped time, it is understood that the
video signal is a continuous stream of digitized data
points under influence of clock 900 of FIGURE 3.
The digitlzed input video signal S coupled to the
input terminal of lH delay 100, which delays the input
slgnal by one line, that is, from location S on line 0
to the same location on line 1. This signal is further
delayed 140 nanoseconds by shift register 300, which
places -the input signal two clock pulses to the right at
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RCA 69,812
1 location SN on line l; thus, the input video signal S
has been delayed one line plus 140 nanoseconds, as i.n the
apparatus of FIGURE 1, to yield the normal video output
signal SN coupled to one input of video switch 700. In
the absence of a dropout control signal to video switch
700, video signal SN appears at the video output terminal
of video switch 700, still in digital form for any further
desi.red processing prior to reconverting (D/A) the signal
to analog form for reproduction in a television receiver
or other reproducing means.
Simultaneously with the processing of the input
signal to provide the normal video output signal SN from
video switch 700, the once-delayed signal from the output
terminal of delay 100 is coupled to delay 200 to obtain
a twice-delayed signal sample, which is shown in FIGURE 4
as being located at position S2. The original signal S
and twice-delayed S2 are combined in an 8 bit summer/
divider 400, where the signals are combined and divided
to yield a weighted average of the signal at the line
before (0) and the line after (2) the line of interest (1).
As in FIGURE 3, the combined signal Sl, delayed by 280
nanoseconds in shift register delay 500 to provide S5 .
is recombined and weighted with Sl in 8 bit summer/divider
600 to yield a replacement video signal SR. The.refore,
the output signal SR of 8 bit summer/divider 600 comprises
one-quarter of S, S2, S3, and S5 delayed by one full
line plus 140 nanoseconds. This weighted and delayed
input video signal SR, coupled to video switch 700, is
identical in time and amplitude to the normal video si~nal
input SN, coupled to video switch 70, and further
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RCA 69, 812
1 represents a weighted average of the image information
surrounding the point of interest (the dropout occurrlng
at a time corresponding to sample SN) rather than a direct
replacement of a more or less redundant sample of video
from the previous line. Therefore, a replacernent video
signal is generated in digital format having all the
advantages of the analog format described in conjunction
with FIGURES 1 and 3.
Although the present invention has been
described in terms of an NTSC television system standard,
the principles of the invention are equally applicable to
other television standards. These other standards do
contain differences from the NTSC system which require
modification to the delay times and clock rates described
in conjunction with FIGURES 1 and 4 among these are:
the clock frequency which determines the number of samples
per line, i.e., 4.33 MHz in PAL, versus 3.58 MHz in
NTSC as well as the delay time for the one-half and full
cycle of burst subcarrier. Similarly, in the PAL system,
the phase alternation that occurs from line to line
requires that the image information for generating the
replacement video be taken two lines before and two lines
after the line of interest where the dropout may occur.
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