Note: Descriptions are shown in the official language in which they were submitted.
1097~19
BACKGROUND OF THE I~IVENTION
Field of the Invention
This invention relates to the processing of digital sig-
nals in such a way that there is a high probability of detecting
and correcting errors in certain stages of the processing. In
particular, the invention relates to deriving primary pulse code
modulation (PCM) multi-bit digital signals from an analog sig-
nal, deriving secondary PCM signals from the same values of the
analog signal as were included into the primary PCM signals,
delaying either the primary or secondary signals, and combining
groups of the delayed signals in interleaved sequence with groups
of the relatively undelayed signals.
The Prior Art
It has been proposed in companion Patent No. 1,067,622,
issued December 4, 1979 and patent application 272,468, filed
February 23, 1977, both assigned to the assignee of the present
application, to process analog audio signals on a video tape
recorder (VTR). These companion applications, along with a
third companion application 292,740, filed December 9, 1977,
describe techniques for encoding the audio signals by means of
PCM techniques and arranging the resultant pulse signals to
correspond to a video format so as to be handled easily by a
VTR.
The analog signals are sampled at a frequency at least
substantially twice as high as the highest information frequency
embodied in such signals. A convenient sampling rate is three
times the horizontal line frequency rate of the video signal,
because this makes the resultant sampled signal commensurate
with video horizontal synchronizing signals incorporated into
the psuedo-video format.
VTRs are capable of handling audio signals with excellent
fidelity, but there is still an occassional problem of loss of
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signal due to a drop out or to a noise burst. Since the quality
of reproduction possible w~ith equipment of the type described
hereinafter is of such high quality as to be consistent with
commercial standards, it is important to avoid even occassional
errors in the processed signals.
OBJECTS AND SUMMARY OF THE INVENTION
It is one of the objects of this invention to minimize
the error in processing digital signals by providing time dis-
placement of groups of such signals with respect to groups of
signals containing substantially the same information.
It is a further object of this invention to incorporate
code signals along with groups of primary and secondary multi-
bit digital signals to make it possible to determine whether
one, both, or neither of the processed signals is free of error
and to arrange for further processing of a selected one of the
signals chosen because of its minimum error.
Further object will be apparent from the following de-
scription and the related drawings.
In accordance with this invention a multi-bit digital sig-
nal, referred to as a primary signal and a secondary multi-bit
digital signal on which the same information is encoded in the
same PCM arrangement, except that the lowest bit orders of the
primary signal may be omitted from the secondary signal are
arranged in corresponding, or related, groups, each of which may
comprise a digital word signal. Either the primary signal is
delayed relative to the secondary signal or the secondary signal
is delayed relative to the primary signal, and groups are then
interleaved to form a sequential multi-bit signal. After this
sequential has been processed, related groups may be brought
back into coincidence, and one of the signals selected for
further processing. This selection is made on the basis of
which signal is error free at that point. If both of the sig-
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nals are error free, a predetermined one of them is selectedfor further processing. If neither signal is error free, a pre-
ceding, temporarily stored signal may be processed a second time
instead of processing either of the erroneous signals.
In order to determine whlch signal is error free, cyclic
redundancy check code signals are obtained for both the primary
and secondary PCM signals prior to processing, and these cyclic
redundancy check code signals (CRC) are interleaved with the
primary and secondary signals to make up a complete sequential
signal. In determining whether there ls an error in either of
the processed signals, the primary and secondary signals may be
measured for coincidence, and if they do coincide, the present
invention provides that one of them, usually the primary signal,
be chosen for further processing. Simultaneously, the CRC sig-
nals for the respective primary and secondary signals may be
used to determine whether either the primary or secondary sig-
nal is exror free. If only one of these signals is error free,
which means that the two signals would not be found to be in ,
coincidence, the error free signal is automatically selected
for further processing. Each signal selected for further pro-
cessing is retained in what may amount to a temporary memory
circuit to be used yet again if neither of the next succeeding
pair of primary and secondary signals is error free. Continued
utilization of the same signal a second time is consistent with
the fact that it is likely to be less distracting if the output
signal does not vary at all for an additional increment of time
than if it varies sharply.
In order to provide time for inserting the secondary sig-
nal grc~ups and the CRC signals in sequence with the primary sig-
nal groups, both the primary and secondary signals are subjectedto time compression. This may be effected by applying these
signals to memory circuits at one clocking rate and reading them
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out of the memory circuits at a different clocking rate. ~or
ease of handling the signals in subsequent circuits, such as the
coincidence circuit, it is preferable that the time compression
of the primary signals be exactly equal to the time compression
of the secondary signals. Since both of these signals contain
substantially the same information, except, possibly, for inform-
ation encoded in the least significant bits of the primary sig-
nal, the time compression should be approximately 2:1. In fact,
by omitting enough of the least significant bits of the primary
signal to correspond to the bits that are required to transmit
the CRC signals, the sum of the truncated secondary signal groups
plus the two sets of CRC signals may be made exactly equal to
the number of bits of the primary signal groups so that time
compression of exactly 2:1 will result in a uniformly dense
sequenti.al pulse signal.
A further time compression is required to allow space
between certain of the pulses to insert horizontal and vertical
synchronizing and equalizing signals. In addition, since it is
desirable to provide an integral number of complete, or compo-
site, groups in each horizontal video interval, it is also de-
sirable to include word synchronizing pulses between each pair
of composite groups. The term "composite" is used to indicate
that such a group includes a primary signal digital word, the
CRC signal for that word, a secondary digital word spaced in
time from the primary digital word, and the CRC signal for the
secondary signal.
More particularly, there is provided:
A method of reducing errors in processing digital sig-
nals grouped into digital words and which comprise first multi-
bit signals, said method comprising the steps of: selecting apredetermined group of each digital word group of said first
multi-bit signals to comprise second multi-bit signals by select-
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~0~78~9
ing only part of the digital signals of each of said digital
words, each digital word group and said predetermined group
thereof comprising related signal groups; delaying one of said
signal groups relative to another of said groups; and combining
the delayed signal groups in interleaved sequence with sub-
sequent, relatively undelayed signal groups to form composite,
sequential signals.
There is also provided:
The method of minimizing errors in a composite signal
comprising groups of first multi bit digital word signals inter-
leaved with groups of second multi-bit digital signals in which
each of said groups of second multi-bit signals is a predetermined
group of one of said digital word signals and each of said sig-
nals of one of said groups is delayed relative to the signals
of the other of said groups, said composite signal comprising a
plurality of first multi-digit cyclic redundancy check code
binary signals, each related to one of said digital word signals
and each following a group of said first signals and preceding
the next group of said second signals and a plurality of second
multi-digit cyclic redundancy check code binary signals, each
related to one of said groups of second multi-bit digital sig-
nals and each following a group of said second signals and pre-
ceding the next group of said first signals, said method compris-
ing comparing the delayed and relatively undelayed signals and
choosing for further processing the one of said compared signals
having minimum errors.
There is also provided:
Apparatus for processing a primary multi-bit digital
signal, said apparatus comprising:
first means to receive said primary signal into said
apparatus;
second means to generate a secondary multi-bit digital
78~9
signal comprising at least ~he high order bits o~ said primary
signal;
delay means connected to one of said first and second
means to delay the signal therein relative to the signal in the
other of said fixst and second means by a predetermined interval
of time;
first yating means connected to said delay means and to
said other of said first and second means to conduct, alternately,
the delayed signal and the signal in said other of said first
and second means;
first time-compression means connected to said first means
to receive and store said primary signal at one rate and to ex-
tract said primary signal from storage at a higher rate in first
groups spaced apart in time;
second time-compression means connected to said second
means to receive and store said secondary signal at said one
rate and to extract said secondary signals from storage at said
higher rate in second groups spaced apart in time; and
second yating means connected to said first and second
time compression means to extract said groups of said primary
signal in alternation wi~h said groups oE said secondary signal.
There is further provided:
Apparatus for processing a primary multi-bit digital
signal, said apparatus comprising:
first means to receive said primary signal into said
apparatus;
second means to generate a secondary multi-bit digital
signal comprising at least the higher order bits of said primary
signal;
delay means connected to one of said first and second
means to delay the signal therein relative to the signal in the
other of said first and second means by a predetermined interval
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of time;
first gating means connected to said delay means and to
said other of said first and second means to conduct, alter-
nately, the delayed signal and said signal in the other of said
first and second means;
first time-compression means connected to said first
means to receive and store said primary signal at one rate and
to extract said primary signal from storage at a higher rate in
first groups spaced apart in time;
second time-compression means connected to said second
means to receive and store said secondary signal at said one
rate and to extract said secondary signals from storage at said
higher rate in second groups spaced apart in time;
second gating means connected to said first and second
time compression means to extract said groups of said primary
signal in alternation with said groups of said secondary signal;
first check code generating means connected to said first
means to receive said primary signal to generate a first check
code signal based on each of said first signal groups;
second check code generating means connected to said
second means to receive said secondary signal to generate a
second check code signal ~ased on said secondary signal groups;
and
third gating means connected to said first and second
check code generating means to transmit, in sequence, an interval
of predetermined length of said primary signal, an interval of
predetermined length of said first check code signal; an interval
of predetermined length of said secondary signal, and an interval
of predetermined length of said second check code signal as a
composite digital word signal.
There is further provided:
Apparatus for processing a digital signal divided
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into composite digital word portions, each of said composite
word portions comprising a primary digital word, a first check
code signal for said primary digital word, a secondary digital
word related in information content to a primary digital word in
a different composite digital word portion occurring at a dif-
ferent time in said digital signal, and a second check code sig-
nal for said secondary digital word, said apparatus comprising:
a source of said digital signal;
delay means selectively connected to said source to bring
each said secondary digital word and the primary digital word
related thereto in information content substantially into time
coincidence;
comparison means connected to said delay means and to
said source to compare each said secondary digital word to the
primary digital word related thereto in information content and
to generate a first logic signal having a predetermined value if
at least predetermined portions of the compared digital signals
are identical;
selection means connected to said comparison means to
select said primary digi.tal signal if the compared digital sig-
nals are identical;
first decoding means connected to said source to generate
a second logic signal having a predetermined value if said
primary digital word is error-free;
logic circuit means connected to said decoding means and
said selection means and being actuated by said second logic sig-
nal to select said primary digital word if said primary digital
word is error-free; and
second decoding means connected to said source to generate
a third logic signal having a predeterm,ined value if said second-
ary digital word is error-free, said logic circuit means being
connected to said second decoding means to cause said selection
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1t~97~3~L9
means to be actuated by said third logic signal to select said
secondary digital word if said primary digital word has an error
and said secondary digital word is error-free.
BRIEF DESCRIPTION OF THE DRAWINGS
.. .. _ . .
Fig. 1 is a block diagram of a signal processing system
according to the present invention.
Fig. 2 is a block diagram of an encoder section of the
circuit in Fig. 1.
Figs. 3A-3F are symbolic representations of digital sig-
nals illustrating the operation of the circuit in Fig. 1. in
accordance with the present invention.
Fig. 4 is a block diagram of a time compressor for use in
the circuit in Fig. 2.
Fig. 5 is a waveform diagram of signals obtained in the
operation of the circuit in Fig. 4.
Fig. 6 is a schematic diagram in block form of a CRC en-
coder and gate suitable for use in the circui-t in Fig. 2.
Fig. 7A-7D are waveforms illustrating the operation of
two embodiments of the circuit in Fig. 6.
Fig. 8 is a block diagram of a decoder suitable for use
in the circuit in Fig. 1.
Fig. 9 is a schematic and symbolic circuit representation
of CRC decoder circuits as used in the circuit in Fig. 8.
Figs. lOA-lOC are waveforms obtained in the operation of
the circuit in Fig. 9.
Fig. 11 is a schematic and symbolic diagram of a logic
circuit in Fig. 8.
Fig. 12 is a symbolic representation of digital signals
as applied to the coincidence detector in Fig. 8.
Fig. 13 is a truth table representing logic conditions in
the operation of the circuit in Fig. 8.
Fig. 14 is a schematic diagram of a gate and memory cir-
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cuit for use in the circuit in Fig. 8.
Fig. 15 is a symbolic representation of signals as pro-
cessed according to this invention to illustrate error reduction.
One of the coding concepts to be used in the following
disclosure is known as the cyclic redundancy check code (CRC).
The mathematical aspects of the CRC will be described first in
terms applicable to the embodiment that follows.
Cyclic Redundancy Check Code
The CRC code is generally expressed by a polynominal F(x)
with indeterminant x and coefficients from an n bit code (an 1
an-2' ~~~ al~ aO) as follows~
F(x)~ an lxn ~ a 2xn 2 ~ ____ ~ a
For example, if the 5 bit code (10011) is expressed by the poly-
nomial F(x), then
F(x) - x ~ x + 1
This polynomial is called the polynomial over Galois field of 2.
The encoding and decoding of the CRC code is essentially
characterized by a division algorithm such that the code poly-
nomial F(x) is divided by the generator polynomial G(x).
Now assuming that the code polynomial of degree (k-l) for
a k bit code is expressed as M(x) and the generator polynomial
of degree (n-k) as G(x), the division algorithm is as follows,
M(x)x - G(x) Q(x) ~ R(x)
in which Q(x) is the quotient polynomial and R(x) is the remain-
der polynomial having a greatest degree of (n-k-l). It should
be noted that the encoded code polynomial V(x) comprises the
code polynomial M(x) xn k and the remainder polynomial R(k)
added to the former polynomial. Therefore, the encoded poly-
nomial V(x) has degree (n-l) and is given as follows:
~8C~78~9
V(x) = ~(x)xn 1 ~ R(x)
= G(x)Q(x)
This means that the encoded polynomial V(x) is divisible by the
generator polynomial G(x).
Next, if a noise signal, which is expressed by a poly-
nomial E(x), is introduced into the code polynomial V(x) during
transfer, the code polynomial V'(x) at the decoding side is ex-
pressed as follows;
V'(x) ~ V(x) ~ E(x)
If no error is introduced therein, E(x) = 0. Then, V'(x)=V(x)
and hence the polynomial V'(x) is divisible by the polynomial
G(x).
However, if the polynomial V'(x) is not divisible by the
generator polynomial G(x) in the decoder, causing a remainder
polynomial R'(x) to be generator, the polynomial V'(x) is re-
garded as having an error bit. Then, the polynomial V'(x) is
given as follows;
V'(x) = G(x) Q'(x) ~ R'(x)
The polynomial V(x) should be divisible by the generator poly-
nomial G(x), so that the remainder polynomial R'(x) must be theremainder in the dividing algorithm of dividing the polynomial
E(x) by the generator G(x). Accordingly, it is apparent that
the remainder polynomial R'(x) is a factor showiny whether or
not the code polynomial V'(x) contains the error bits. Such a
remainder R'(x) is called a syndrome.
One example will be shown in the condition of n=7, k=4
and the generator polynomial G(x) ~ x3 ~ x ~ 1,
(1) M(x) - x + 1-(1001)
M( x)x3 = x6 + x3
M(x)x = G(x)Q(x) + R(x)
R(x) = x + x
(2) V(x) = M(x)x + R(x) = x6 ~ x3 ~ x2 + (100
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(3) E(x) ~ x5_(0100000)
V (x~ v(x) + E(x) = x6 t x5~ x3 ~ x2 t (11
V'(x) ~ G(x)Q'(x) ~ R'(x)
(5) R'(x) ~ x2 + x + 1-(111)
The basic circuit of the CRC code encoder and decoder
comprises a dividing circuit with the divisor G(x) which gener-
ates the remainder, not the quotient. The dividing circuit is
essentially formed by a shift register, each stage of which is
preceded by a modulo 2 adder that adds, on a modulo 2 basis
(which means counting to the base 2 without carry), the output
of the preceding stage and output of the shift register according
to whether the appropriate element of the polynomial is gi-l or
gi-0 in the divisor G(x) = g xr~ ~ gb lxn 1 + gn 2xn 2 + _______
------- ~ g2x + glx + gO.
Now, the generator polynomial G(x) in the above example is
given as follows:
G(x) s x + x ~ 1
Accordingly, the dividing circuit of the polynomial G(x) includes
a three-stage shlft register with feedback loops fro~ the output
to modulo 2 adders at the input and between the first and second
stages. The clocking conditions in each shift register stage
and the calculation example are shown;
(i) E(x) - 0
V(x) _ x6 x3 1 x2
x3 ~ x
X3 ~ x ~ 1 x6 ~ x3 + x2 ~ x
x6 ~ x4 t x3
. ~
x4 ~ X2
x4 + X2 +
.
0
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, ;, .................................................................... .
.i.
1~7~
T~BLE I
Conditions in shift registers
clock input Do Dl D
(initial condition) 0 0 0
tl 1 1 0 0
t2 0 0 1 0
t3 0 0 0
t4 1 0 1 0
t 1 1 0
t5 1 0 0 0
t67 0 0 0 0 ~ I
. . ~
The remainder --
(ii) E(x) - x5
V'(x) = x6 ~ x5 ~ x3 + x2 +
x3 ~ x2 +
r _ _ __
x3 + x ~ 1 x6 ~ x5 ~ x3 t x2 t x
x6 ~x4 ~ x3
5 4 2
x~x ~ x + x
x5 + x + x
. . . _
x4 ~ x3 ~ x
x4 ~ x ~ x
-
x3 ~ x2
x3 + x + 1
. _
the remainder x ~ x ~ 1
TABLE 2
. . _ . . _ . . _ _ _ _ . . . ~ _ _
Conditions in shift registers
Clock input Do D1 2
(initial condition) 0 0 0
tl 1 1 0 0
t2 1 1 0 0
t3 0 0
t4 1 0
t5 1 0
t 1 0 1- 1
t67 0 1 1 1 ~- i
_ _ . _ . . _ _ . . _ _ _ ,
The remainder - '
Accordingly, the contents of the shift registers show whether or
not the transferred code contains error bits.
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The circuit in Fig. 1 includes a VTR 1 which may be, for
example, of the type ~e~erred to in the companion applications,
supra. The VTR has an input terminal li and an output terminal
o .
The system is arranged for use with stereophonic audio
signals, although it can be used with other types of signals.
As set up for stereophonic audio signals, it includes two input
terminals 2L and 2R, to which the left and right audio signal
channels may be applied, respectively. The input terminal 2L is
connected to a low pass filter 3L which is connected, in turn,
to a sample-and-hold circuit 4L. The output signals of the
sample-and-hold circuit is connected to an analog-to-digital
(A-D) circuit 5L, the output of which is connected to a parallel-
to-serial converter 6.
The input terminal 2R is connected to the parallel-to-
serial converter 6 through an identical circuit including a low
pass filter 3R, a sample-and-hold circuit 4R and a A-D circuit
5R .
The output of the convert 6 is connected to an encoder 7
which will be described in greater detail hereinafter in con-
junction with Figs. 2-7. Following the encoder 7 is a time
compressor 8 that provides additional time compression of the
output signal of the encoder to allow for insertion of syn-
chronizing signals in a synchronizing signal adder circuit 9
that follows the time compressor 8. The output of the syn-
chronizing signal adder circuit 9 is connected to the input
terminal li of the VTR 1.
The circuit to this point includes the elements used in
recording a signal in the VTR 1. For playing back signals
previously recorded, the output terminal lo of the VTR 1 is con-
nected to a synchronizing signal eliminator circuit 10 that ex-
tracts the synchronizing signals and utilizes them in the control
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of the VTR in the normal manner. The output of the synchroniz-
ing signal eliminator circuit 10 is connected to a time expansion
circuit that returns the spacing between successive pulse sig-
nals to an even amount and closes the gaps that were provided for
insertion of the synchronizing signals. The output of the time
expansion circult 11 is connected to a decoder 12 that performs
the converse of the function of the encoder 7 and will be des-
cribed hereinafter in greater detail, particularly in connec-
tion with the circuits in Figs 8-14.
The output of the decoder 12 is connected to a serial-to-
parallel circuit 13, which has two output terminals. One of the
output terminals is connected to a digital-to-analog (D-A)
circuit 14L and the other to a D-A circuit 14R for the left and
right audio channels respectively. The output of the D-A cir-
cuit 14L is connected through a low pass filter 15L to an output
terminal 16L, and the output of the D-A converter l~R is similarly
connected through a low pass filter lSR to an output terminal
16R.
The conventional reference oscillator circuits and clock-
ing, synchronizing, and gating signal circuits utilized in con-
junction with the sample-and-hold circuits, the A-D and D-A con-
verters, the parallel-to-serial and serial-to parallel converters,
the encoder and decoder, the time compressor and expander cir-
cuits, the synchronizing signal adder and eliminator circuits,
and the VTR are all standard and need not be described in detail.
In the operation of the circuit in Fig. 1, audio signals
to be processed are sampled at a suitably high rate in the
sample-and-hold circuits 4L and 4R. It is convenient and satis-
factory for that rate to be three times the repetition rate of a
horizontal video synchronizing signal, or about 47.25KHz. At
each sampling, the respective A-D circuits 5L and 5R provide
sixteen-bit PCM signals to the converter 6. The latter may be a
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1~a7~9
32 position shift register clocked at a su~ficiently high speed
to read out all 32 bits of information applied to the converter
from the two A-D converters 5L and 5R. The resulting multi-bit
digital signal is represented in Fig. 3A as including a left
channel signal with sixteen bits ranging from a most significant
bit M to a least significant bit L at a right channel signal
that also includes sixteen bits and ranges from a most signif-
icant bit M to a le~t significant bit L. The interval of time required to
extract the digital signal represented in Fig. 3A from the converter 6 is e~ual
to the sampling time and therefore, in this embodiment, is 3,
where H is the horizontal line interval of a video signal. The
entire multi-bit digital signal represented in Fig. 3~ may be
considered to be a digital word signal, or digital word group.
Digital word signals like that in Fig. 3A are generated
at a constant rate so that three such digital words substan~
tially completely fill up a horizontal line interval. In order
to allow time for a comparison signal, the constant flow of sig-
nals similar to that in Fig. 3A from the converter 6 to the en-
coder 7 must be subjected to time compression. This is
illustrated in the circuit in Fig. 2.
Fig. 2 includes an input terminal 21 connected to a Eirst
time compression circuit 22. The output oE the time compression
circuit 22 is connected to a CRC encoder 23 and to a gate cir-
cuit 24 to which the output of the CRC encoder 23 is also
connected.
The input terminal 21 is likewise connected to a gate
circuit 25, the output of which is connected to a second time
compression circuit 26. The output of the latter is connected
to a second CRC encoder 27 and to another gate circuit 28. The
output of the encoder 27 is also connected to the latter gate
circuit. The output of the gate circuit 28 is connected through
a delay means 29 to a gate circuit 30 to which the output of the
~. .
a7~19
gate circuit 24 is also connected. The gate circuit 30 has an
output terminal 31.
The time compression circuit, or time compressor, 22 re-
duces the length of time required to transmit the digital word
signal sh~wn in Fig. 3A. A circuit -to accomplish this is shown
in Fig. 4 as comprising a double pole input switch 32 connected
to inputs of two 32 bit shift registers 33 and 34. The outputs
of each of the shift registers 33 and 34 are connected to the
terminals of another switch 36. The shift register 33 has a
write clock input terminal 33W and read clock input 33~, and
the shift register 34 has similar write and read terminals 34W
and 34R, respectively.
The bits of the signal shown in Fig. 3A are applied con-
tinuously to the switch 32 and are connected, one word at a time,
to the shift registers 33 and 34 alternately. To accomplish
this, the arm of the switch 32 is switched from one of its out-
put terminals to the other at the end of each 32 incoming bits.
The information is clocked in to the shift register to which the
arm of the switch 32 happens to be connected at any instant by
the write clock signal at a frequency equal to the frequency at
which the bits are generated in the converter 6 in Fig. 1, but
they are read out by the read clock signal at a rate twice the
rate at which they are written in. The arm of the switch 36 is
connected to whichever one of the two shift registers 33 and 34
is being operated in its "read" mode at any given time. Fig. 5
shows that the 32 bit PCM signal applied to the arm of the switch
32 is, by virtue of being read out at twice the speed that it is
written in, compressed by a factor of 2:1. Thus instead of
having the 32 bits occupy the ful] interval of time from one
sample to the next, the bits are bunched together as shown in
Fig. 5 leaviny unused intervals of time each of which is one half
the total interval occupied by the signal shown in Fig. 3A. The
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~7~3~9
second, or comparison, signal, and two sets o~ CRC signals can
be inserted into the resulting unused intervals.
Fig. 6 shows the CRC encoder which may be used either as
the encoder 23 or as the encoder 27. The circuit for each is
the same and is connected in accordance with the CRC equation
G(x) ~ x4 ~ x ~ 1. The encoder has a signal input terminal 37
connected to one input terminal of an exclusive-OR gate 38. The
output of the gate 38 is connected to the D input terminal of a
D-type flip-flop 39. The output of this flip-flop is connected
to one input terminal of another exclusive-OR gate 41, the out-
put terminal of which is connected to the D input terminal of a
D-type flip-flop circuit 42. The latter is connected in sequence
to two other D-type flip-flop circuits 43 and 44. A clock input
terminal 46 is connected to the clock terminals of all four of
the flip-flops 39, 42, 43, and 44. The output terminal of the
flip-flop 44 is connected through an AND gate 47 back to second
input terminals of each of the exclusive-OR gates 38 and 41. The
output terminal of the flip-flop 44 is also connected through
another AND gate 48 to an output terminal 49. Two input termin-
als 51 and 52 are gating signal input terminals to control the
AND gates 47 and 48, which make up the gate circuit 24 connected
to the encoder 23 or the gate circuit 28 connected to the en-
coder 27.
me operation of the circuit in Fig. 6 will be discussed
in connection with the gating signals shown in Figs. 7A-7D. The
signals in Figs. 7A and 7~ are those that are applied to the gate
circuit 24 while the signals in Figs. 7C and 7D are those that
are applied to the gate circuit 28.
The clusters of 32 bit PCM signals are applied to the in-
put terminal 37 of the encoder 23. During the time this 32 bit
signal is being applied, the signal in Fig. 7A is applied to the
input terminal 51 to enable the AND circuit 47 to act as a dir-
-- 19 --
-
1~97~9
ect connection from the output of the flip-flop 44 back to the
second input terminal of each of the exclusive-OR gates 38 and
41. This enables the encoder 23 to operate as a shift register
with feedback in accordance with the equation G(x)= x4 ~ x ~ 1,
as described previously in connection with the cyclic redundancy
check code. After the 32 bit interval, which may be considered
a digital word, the AND gate 47 is disabled to prevent any
further signal feedback from the output of the flip-flop 44 to
the exclusive-OR gates 38 and 41. At the same time no further
input signals are applied to the terminal 37. During the next
four bit intervals, the gating signal in Fig. 7B is applied to
enable the AND gate 48, and during this interval, the flip-flops
39 and 42-44 unload through the output terminal 49. As shown
the input terminal 37 is directly connected to the output termin-
al 49 so that, during the 32 bit intervals while the AND gate 48
is di8abled, the PCM input signal consisting of 32 bits is trans-
mitted through the short circuit to the output terminal 49. The
CRC pulses are added se~uentially in the immediately ensuing four
bit interval. These bits are clocked at the same rate as the in-
put signal applied to the terminal 37. This is also the sameclocking rate as the read clock applied to the terminals 33R and
34R in the time compressor in Fig. 4. Thus, all of the pulses
at the output terminal 49 of the circuit in Fig. 6 are at the
same repetition rate, and they occur over a 36 bit interval,
which is the sum of the two intervals shown in Figs. 7A and 7B.
The complete signal is represented in Fig. 3B as comprising a
time-compressed 32 bit portion that corresponds to the signal in
Fig. 3A, and a four bit portion that includes the CRC signal.
Since the signal in Fig. 3A has been referred to as a digital
word, the signal in Fig. 3B may be referred to as an expanded
digital word.
The total time difference between the signal in Fig. 3A
- 20 -
1~7~19
and that in Fig. 3B is the time required for 28 bits. As sug-
gested previously, this interval can be uni~ormly filled by a
28 bit signal comprising 24 bits of information and another four
bit CRC signal. The 24 bit signal corresponds to the most sig-
nificant bits of the original information signal applied to the
input terminal 21 in Fig. 2, and this siynal is illustrated sym-
bolically in Fig. 3C as comprising 12 bits for the left channel
and 12 bits for the right channel. This is a truncation of the
original signal shown in Fig. 3A, but all that it represents in
physical terms is a relatively small reduction in the dynamic
range of operation of the system. Since the range of operation
represented by even a 12 bit signal is still quite good, the
loss of additional range is almost unnoticeable.
The derivation of the truncated signal from the original
input signal at the terminal 21 is made by the gate circuit 25.
This truncated signal is then compressed by a time compressor 26
identical with the compressor 22, and it is actually the compres-
sed signal that is represented in Fig. 3C. The CRC signal for
the compressed truncated signal is generated in the encoder 27
and the gate circuit 28 by applying the gating signals shown in
Figs. 7C and 7D to the circuit in Fig. 6.
While the signal with a reduced number of bits is, under
almost all circumstances, perfectly satisfactory as a comparison
signal to compare with the primary signal, which is the signal
at the output terminal of the gate 24, it is also possible to
use a non-truncated, or full range, comparison signal. This
simply means compressing the signals dif:Eerently. For example
the time compressor 26 could be operated to compres.s a full 32
bit signal by the ratio of 8:3 instead of 2:1. However the bits
would no longer have the same repetition rate. It is prefer-
able to keep the same bit repetition rate and to truncate the
signal that passes through the gate 25.
- 21 -
~\7~9
The output signal of the gate 28 as illustrated in Fig.
3B iS related to the output si~nal of the gate 24 in that it
represents, or duplicates, the most significant bits of that
signal. It can there~ore be used as a comparison signal to de-
tect errors that occur in further processing of the primary sig-
nal Ai that emerges from the gate 24. In order to make better
use of the error-reduction potential of the secondary signal,
which now may be referred to as the signal Bi at the output term-
inal of the gate 28, this signal si is delayed by an interval
time long enough to make it unlikely that any signal that inter-
ferred with the transmission of the signal Ai would also effect
the secondary signal Bi. This delay is obtained in the delay
means 29, and a delay period of approximately six horizontal
line intervals has been found sufficient to separate signals Ai
and Bi that were generated from the same sampling information.
Naturally, since the signals represent substantially the same
information and thus start virtually at the same time, it is
necessary that the system include sufficient delay of -the second-
ary signals Bi to place them in sequence with the primary sig-
nals Ai. This delay is equal to the duration of the signals Aias shown in Fig. 3E. As a result, the delay to offset the re-
lated signals by six horizontal line intervals, which is the
equivalent of 18 digital words, there being three digital words
per line interval, should be 6H ~ Ai. If a shorter delay is
sufficient, the integer 6H may be replaced by N where N is one
composite digital word shown in Fig. 3E to comprise 64 bits.
Actually, Fig. 3E has been labeled to indicate that the signal
Ai is part of a composite digital word with the signal Bi 18
that is related to the primary signal that occurred 18 digital
words previously. Fig. 3F shows one horizontal line interval
consisting of three composite digital words illustrating the
fixed 18 word deIay. Fig. 3F also illustrates the synchronizing
- 22 -
-
1~7~9
signal at three times the horizontal repetition ~re~uency used
as word synchronizing signals HD in ~urther processing of the
signals.
It should be noted that instead of delaying the signals
Bi, the signals Ai could be delayed. What is important is that
there be a separation in related signal groups, that is, groups
derived from the same sampling interval.
The advantage of seIecting a predetermined group of each
digital word group of the primary multi-bit digital signals Ai
to comprise the secondary multi-bit signals Bi, delaying one of
the signals groups relative to the other, and combining the de-
layed signal groups in interleaved sequence with subsequent, rela-
tively undelayed signals is only obtained after the signals thus
modified have been further processed. In the present embodiment,
such further processing includes recording the signals as shown
in Fig. 3F on magnetic tape in the VPR 1. As described in de-
tail in the companion applications, su~ra, this requires add-
itional time compression to insert synchronizing signals before
recording and then requires that the synchronizing signals be
removed and that the relative timing of the pulses be restored
to the original value.
The decoder 12 in Fig. 1 is shown in greater detail in
Fig. 8 and is the circuit in which the discrepancy between the
signals Ai and Bi is utilized to effect error correction. The
decoder circuit includes an input terminal 51 connected to a
gate 52 that separates the Ai portion of each composite word
from the Bi 18 portion. The Ai portion is applied to a delay
means 53 that achieves the same delay as the delay means 29 in
Fig- 2 to bring the Ai 18 signal into coincidence with the Bi 18
signal. The Ai ~ 18 signal from the delay means 53 is connected
to the input of a CRC decoder 5~, and the Bi 18 signal from the
gate 52 is applied to a similar CRC decoder 55. The output signal
- 23 -
~7~3~9
of the delay means 53 is also applied to the a gate 56~ and the
output of the latter circuit is connected to a time expansion
circuit 57. In a similar manner, the Bi 18 signal from the gate
52 is applied to a gate 58, and the output of the latter gate is
connected to a time expansion circuit 59. The output of the time
expansion circuit 57 is connected through a gate 60 that trun-
cates the signal to the same degree that the Bi signal was
previously truncated. In addition the output of the time expan-
sion circuit 57 is connected to an output gate 61. In a similar
manner the output of the time expansion circuit 59 is connected
to another input terminal of the gate 61 and to a coincidence
detector 62 that also receives the truncated output signal of
the gate 60. Output signals from the CRC decoders 54 and 55 and
from the coincidence detector 62 are applied to a logic circuit
63 to produce an output gate control signal connected -to the
gate 61 and to a "hold" signal.
The CRC decoders 54 and 55 in the circuit in Fig. 8 are
shown in greater detail in Fig. 9. Each of these circuits is
quite similar to the CRC encoder circuit 23 shown in Fig. 6.
~0 The decoder 54 includes an input terminal 66 to which the Ai sig-
nals are applied. This terminal is connected to one of the in-
put terminals of an exclusive-OR gate 67 the output of which is
connected to the D input terminal of a D-type flip-flop 68~ The
output of the D flip-flop is connected to one of the input term-
inals of another exclusive-OR gate 69, and the output of the
latter is connected to the D input terminal of the first of three
successive d-type flip-flops 71-73. The output of the last flip-
flop stage 73 is connected directly back to a second input term-
inal of the exclusive-OR gate 67 and to the second input termin-
al of the exclusive-OR gate 69. The decoder 54 also has a gate
signal input terminal 74 connected to an AND circuit 76, the
output of which is connected to the clock signal input terminals
- 24 -
~97~3~9
of each of the flip-~flops 68 and 71-73. The clock ~ignal~ it-
self, is applied by way of a clock signal input terminal 77 to
the other input terminal o$ the AND gate 76. The output term-
inals of the four flip-flops 68 and 71-73 are connected to a
common OR circuit 79, the output terminal of which is identified
by reference numeral 81.
The CRC decoder 55 includes an input terminal 82 connected
to one input terminal of an exclusive-OR gate 83, the output of
which is connected to the D terminal of a D-type flip-flop 84.
The output of the flip-flop 84 is connected to one input term-
inal of an exclusive-OR gate 86, and the output of the exclusive-
OR gate 86 is connected to the D input terminal of the first of
three D-type flip-flops 87-89 connected in sequence. The out-
put of the flip-flop 89 is connected back to second input term-
inals of the exclusive-OR gates 83 and 86. A gating signal in-
put terminal 91 is connected to a second input terminal of an
AND gate 92, the other input terminal of which is connected to
the clock signal input terminal 77. The output of the AND gate
92 is connected to the clock terminals of each of the flip-flops
84 and 87-89. The output terminals of all four of the flip~flops
84 and 87-89 are connected to input terminals of an OR gate 93
that has an output terminal 94,
The input signal Ai applied to the input terminal 66
should be, unless it contains an error, exactly like the Ai sig-
nal of the composite digital word represented in Fig. 3E. This
section of the digital word is 36 bits long and so the gate sig-
nal illustrated in Fig. 10A is applied to the gate signal input
terminal 74 to enable the AND gate 76 to allow clock signals
applied to the input terminal 77 to clock 36 bits of information
into the decoder 54, starting just after each word synchronizing
HD shown in Fig. 10C. If there are no errors in the signal Ai,
the signal ~ at the output terminal 81 will be "0", but if there
- 25 -
i .
~781g
are any errors, the si~nal ~ will be "1".
In a similar manner, the gates signal shown in Fig. lOB
is applied to the gating signal input terminal 91 to allow 28
bits of information to be clocked into the decoder 55 starting
one bit after the word synchronizing pulse HD. If there are no
errors in the ~i signal applied to the input terminal 82, the
output signal ~ at the output terminal 94 will be "0", but if
there are errors, the signal ~ will be "1".
g Ai_l8 and Bi_l8, which may be referred to as
extended word signa]s because they include CRC components, are
applied to the gate circuits 56 and 58, respectively. These
gate circuits allow only the basic information signals to pass
through and delete the respective CRC signals appended to the
basic information signals. Thus, at the output of the gate cir-
cuit 56, the signal should correspond exactly to the first 32
bits of the signal illustrated symbolically in Fig. 3B and at
the output of the gate circuit 58, the signals should correspond
to the signal illustrated symbolically in Fig. 3C, assuming, in
each instance, that no error has entered either of the informa-
tion signals.
The respective information signals, striped of the CRCpulses, are re-expanded by the respective time expansion cir-
cuits 57 and 59. Thus, at the output of the time expansion cir-
cuit 57~ the signal should be exactly like the signals symbolic-
ally illustrated in Fig. 3A, if there is no error in the proces-
sed signal. The signal at the output terminal of the time ex-
pansion circuit 59 should be similar to the signal in Fig. 3A
except that it has a total of 24 bits instead of 32 bits.
The output signals of the time expansion circuit 57 and
59 are compared for coincidence in the coincidence detector 62.
However since the output signal of the expansion circuit 59 has
only the 24 most significant bits of the 32 bit output signal of
- 26 -
1~'a7~9
the expansion circuit 57, the latter signal may be truncated in
the gate 60 to delete its ei~ht least significant bits so that
the two signals applied to the coincidence detector 62 will each
having the same number of bits of the highest order. These two
signals must h~ve the same number of bits, if they contain in-
formation for two stereophonic signals, because such signals
contain two MSB signals and each of these MSB signals in the
primary signal must be compared with the corresponding two MSB
signals of the secondary signal.
The two signals to be compared in the coincidence detect-
ing circuit are illustrated in Figs. 12A and 12B. The signal in
Fig. 12A is the secondary signal, which was originally generated
in truncated form to include only the highest orders of bits
from M to L' of the primary signal. Since this signal, as sym-
bolically represented in Fig. 12A, contains only information
bits and no CRC bits, it is simply labeled as signal B. The
truncated primary signal, which now has the same number of bits
as the secondary signal B is referred to as the signal A' to
distinguish it from the full range, or non-truncated, primary
signal A. If the two signals B and A' coincide bit for bit, the
coincidence detector 62 will produce an output signal ~ having a
value of "1". However, if the two signals B and A' do not coin-
cide, the value of the output signal ~ will be "0".
The full range primary signal A at the output of the time
expansion circuit 57 and the truncated secondary, or comparison,
signal B at output of the time expansion circuit 59 are applied
to separate terminals of the gate circuit 61, which selects one
of these two signals to be passed through the gate circuit to an
output terminal 64 for further processing. Essentially, the gate
61 is like a switching circuit that connects either the expansion
circuit 57 or the expansion circuit 59 to the output terminal 64.
The output signals ~ and ~ from the CRC decoders 54 and
- 27 -
1~7B~9
55 and the output signal ~ from the cvincidence detector 62 are
all applied to the logic circuit 63 to generate therein signals
to control the operation of the gate 61, The logic circuit is
shown in greater detail in Fig. 11. It includes four input term-
inals 96-99 to receive the signals ~ , ~ , and ~ and the word
synchronizing signal HD, respectively. The terminals 96 and 99
are connected to an AND gate 101 the output of which is connected
to an inverter 102 and to one input terminal of each of two AND
gates 103 and 104. The output terminals 97 and 99 are connected
to two input terminals of another AND gate 106, the output term-
inal of which is connected to the other input terminal of the
AND gate 104 and the input of an inverter 107.
The input terminals of the inverter 102 is connected to
one of the input terminals of a NAND gate 108 and to one of the
terminals of a second NAND gate 109. The output terminal of the
AND gate 106 is connected to the other input terminal of the
NAND gate 109. The output terminals of the two NAND gates 108
and 109 are connected to two input terminals of a third NAND
gate 110.
The output terminal of the NAND gate 110 and the input
terminal 98 are connected to the two input terminals of an OR
gate 111, and the input terminal 98 is also connected to the in-
put terminal of an inverter 112. The output terminal of the OR
gate 111 and the input terminal 99 are connected to the two in-
put terminals of another NAND gate 113, the output terminal of
which is connected to the set terminal of a fli.p-flop 114. The
input terminal 99 is connected to the reset terminal of this
flip-flop and to the reset terminals of two other flip-flops 115
and 116 as well as to one of the input terminals of an AND gate
117.
The other input terminal of the AND gate 117 is connected
to the output terminal of the inverter 112, and the output term-
- 28 -
lQ~7~
inal of the AND gate 117 iS connected to one of the input term-
inals of each of two NAND gates 118 and 119. The output term-
inals of the ~ND gates 103 and 104 are connected, respectively,
to second input terminals of the NAND gates 118 and 119 respect-
ively, and the output terminals of these NAND gates are connect-
ed, respectively, to the set input terminals of the flip-flop
115 and 116. The three flip-flops 114-116 have output terminals
121-123, respectively.
The circuit of the gate 61 controlled by the logic cir-
cuit 53 is shown in somewhat more detail in Fig. 14. It includestwo input terminals 126 and 127 connected, respectively, to out~
put terminals of the time expansion circuits 57 and 59, respect-
tively. In series with the leads from the input terminals 126
are two electronic switching circuits 128 and 129, which are
controlled by the flip-flops 114 and 115, respectively, as in- .
dicated by the labels on the arrows adjacent the symbols of the
switching circuit.
The switching circuits 128 and 129 are connected together
to the input terminal of a delay or memory device 131 having
sufficient capacity to store enough signal bits to equal one full
range primary signal A. For example, the device 131 may be a 32
bit shift register. The output of the device is connected to
one terminal of a double-throw switching circuit 132, the arm of
which is connected to the output terminal 64. The other termin-
al of the switching circuit 132 is directly connected to the out-
put terminals of the switching circuits 128 and 129. As indicat-
ed by the arrow adjacent the symbol of the switching circuit 132,
the state of conductivity of that switching circuit is controlled
by the flip-flop 116.
The logical relationship between the signals ~ , ~ , and
y and the conditions of the signals transmitted from the gate
61 to the output terminal 64 is summarized in the truth table in
- 29 -
~,~
1@97~3~9
Fig. 13. When the signal j;l- ha~ the value "1" after comparing
one digital word of the truncated primary signal A' with a cor-
respondlng digital word of the secondary signal B, the truth
table indicates that the signal ~ will be transmitted through
the switching circuit 132 to the output terminal 64, no matter
whether the signals Cand ~Bare "0" (whlch is probable if the
value of signal j~' is "1") or "1".
When ~= 0, and ~= 0, indicating that there are no errors
in the primary signal A, the switching circuit 128 is also closed
10 and the switching circuit 132 connects the primary signal A dir-
ectly to the output terminal 64. This condition exists whether
or not the signal ~8also has the value "0".
On the other hand, if O~= 1, indicating that there are
errors in the primary signal A, but ~g=0, indicating that there
are no errors in the secondary signal, the switching circuit 128
will be open, and the switching circuit 129 will be closed. The
switching circuit 132 will continue to conduct the signal direct-
ly to the output terminal 64. The fact that this will be the
somewhat truncated, secondary signal B instead of the full range
20 signal A only means that the smallest changes in amplitude rep-
resented by the least significant bits will not be present in
the signal at the terminal 64, but the omission of these lowest
order bits for one digital word or even for several words will
be virtually unnoticeable.
If there ar~o errors in both signals A and B,C= ~ = 1.
In such rare instances, both the switching circuits 128 and 129
will be open-circuited until the next digital word ls measured.
However, instead of suddenly dropping the digital signal at the
output terminal 64 to a zero condition for one digital word in-
30 terval, which might cause a sudden high-amplitude negative sig-
nal to be produced by the D-A converters 14L and 14R, the switch-
ing circuit 132 is actuated to connect the output terminal of
-- 30
~''
,g l~
the memory or delayed device 131 to the output terminal 64. The
device 131 has received whichever word signal was carried through
the switching circuit 132, and has replaced each word signal with
the next word signal as long as one of the switching circuits
128 or 129 was conductive. When neither is conductive the de-
vice 131 still has the last-used word signal in it and, can un-
load this signal to the output terminal via the switching circuit
132. This simply means that for a very short interval of time,
the output signal will remain constant. This is less likely to
be objectionable than if the output signal value were allowed to
vary sharply.
As an alternative to providing the storage device 131,
the circuit may be lrranged to interrupt the supply of clock
pulses to the D-A converters in Fig. 2 for one digital word in-
terval if ~ - ~ - 1, thereby effectively holding the output sig-
nal level relatively constant briefly.
The operation of the circuit in Fig. 11 to obtain the
signals to control the switching circuits 128, 129 and 132 in
Fig. 14 will now be described.
Various circuit point within the circuit in Fig. 11 are
labeled alphabetically for ease of description of the operation
of the circuit. The three signalsc~ , ~ , and ~ establish the
operating conditions of the circuit 53, while the word synchron-
izing pulses applied to the terminal 99 initiate each operation.
That is, it is only when a word synchronizing signal pulse is
applied to the input terminal 99 and from there -to each of the
reset terminals of the flip-flops 114-116 that the output term-
inals 121-123 will be forced to take on the values determined by
the logical "0" or "1" values of the signals c~,~ , and ~ .
If the truncated primary signal A' exactly corresponds to
the secondary signal B in the coincidence detector 62, thereby
making the signal ~ -1, this signal point g at the output of
- 31 -
lQ97~9
the OR gate 111 will have value "1", no matter what the output
value of the NAND ~ate 110 happens to be. As a result, the cir-
cuit point _ at the output of the NAND gate 113 and at the set
terminal of the flip-flop 114 will also be "1" until the word
synchronizing signals applied to the other input terminal of the
NAND gate 113 raises that input terminal to the value of "1",
thereby dropping the output signal at the circuit point _ to "0"
for the duration of the word synchronizing pulse. This causes
the output terminal 121 to have the value "1". As previously
stated, this closes the switching circuit 128 in Fig. 14 and
causes the switching circuit 132 to conduct the full range sig-
nal A through the output terminal 64.
In the foregoing condition, when r= 1 the inverter 112
inverts this to the value "0" thus disabling the AND gate 117
and holding the circuit point _ at "0". This prevents either of
the NAND gates 118 or 119 from responding to a word synchroniz-
ing pulse to set either of the flip-flops 115 and 116. As a
result, both of the output terminals 122 and 123 have the value
"
When ~-0, it indicates that there is an error either in
the truncated primary signal A' or in the secondary signal B com-
pared therewith. If it is assumed that the error is in the sec-
ondary signal B, the signal ~will have the value "1" and the sig-
na ~ will have the value "0". As a result, the circuit point a
will have the value "0" and the voltage level at the circuit
point b at the output of the AND gate 106 will correspond to the
word synchronizing signal applied to the input terminal 99. The
inverter 102 will invert the value "0" at the circuit point a to
the value "1" at the circuit point c. The inverter 107 will in-
vert the word synchronizing pulse signal at the circuit point b
and apply this inverted synchronizing pulse to the NA~D gate 108.
The conditions of the circuit points c and d will cause the out-
- 32 -
-
~ ,,~. .
1~781~
put of the N~ND gate 108 at the circuit point to follow the
word synchronizing pulse. At the same time, the conditions of
the circuit points _ and c at the input terminals to the NAND
gate 109 will cause the output of that NAND at the circuit point ,
f to be the inverse of the word synchronizing pulse signal, and
therefore, opposite to the signal at the point e at all times. -
Thus, one or the other of the input terminals of the NAND gate
110 will always be at the value "0", and therefore the output
terminal of this NAND gate will always be "1" in response to
such input conditions. This will cause the output of the OR gate
111 following the NAND gate 110 to have the value "1" at the cir-
cuit point g. This represents the same condition that prevailed
when the value of the signal r was "1", and so the output term-
inal 121 of the flip-flop 114 will be at the value "1", causing
the switching circuit in the Fig. 14 to be closed so as to trans-
mit the signal A to the output terminal 64.
Since the output of the AND gate at point a is "0", both
of the AND gates 103 and 104 will have "0" values at their out-
put terminals and, hence, at the circuit points 1 and 1. These
conditions keep the output values of the NAND gates 118 and 119
at the points k and 1 at "1" so that the output terminals 122
and 123 of the flip-flops 115 and 116 remain at "0", keeping the
switching circuits 129 and 132 in Fig. 14 in the conditions
shown.
The condition just described in detail in whichCC= 0,
r- , and ~ = 1 is Condition II in the following Table in which
the initials W.S. stand for word synchonizing pulse and the
symbol W.S. stands for the inverted word synchronizing pulse.
The previous condition in which r= 1 is Condition V in the Table.
If there is an error in the primary signal A but none in
the secondary signal B, the value of o~will be "1" and the value
of ~ will be "0". The value of ~ will be "0". This is Condition
1097B19
III in the Table and therefore need not be described in words.
If both the primary and secondar~ signals ~ and B have
errors so thatC= ~ = 1 and ~ =0, the logic circuit will
operate according to Condition IV in the Table, and as previously
described will cause the switching circuit 132 in Fig. 14 to
draw a replacement from the storage device 131. In effect this
makes three signals available to the output terminal 64 with-
out delay; signal A as a first choice, signal B as a second
choice, and the signal stored in the device 131 as a third
choice.
- 34 -
~j,
1~97~g ` .
TABLE
Circuit Points ; Conditions
- I II III IV V
96( input) 0 0
97( input) 0 1 0
99( input) 0 0 0 0
a 0 0 W.S. W.S.
b 0 W.S 0 W.S
c 1 1 W.S. W.S.
d 1 W~S. 1 W.S.
e 0 W.S. W.S. W.S.
f 1 W S.
g 1 1 W.S. W.S.
h W.S. W.S.
O O ~.S. O
O O O W.S.
k 1 1 W.S. ].
W. S .
m W.S. W.S. W.S. W.S. 0
20121 1 1 0 0
122 0 0 1 0 0
123 0 0 0 1 0
Condition I represents a state in which analysis of the
CRC code in each of the decoders 5~ and 55 indicates that there
is no error in either the A signal nor the B signal, so that
~ 0. Yet the statement r= means that the most sig-
nificant bits of the two signals do not coincide so that at
least one of them must be in error. This requires that the
error be such as to make the error cause the information signal
or cause both the CRC signal and information signal to shift
precisely to a new value that will be decoded as a permissible,
and therefore correct, value. Such a situation, while possible,
- 35 -
,. j
lQ~7~3:L9
has a very lo~ probability of occurence. The logic circuit 63
is arranged under this condition to select the primary signal A
to be passed through the gate 61 to the output terminal 6~.
Fig. i5A shows the effect of this invention in overcoming
a noise burst that extends over three composite digital words
including the primary signals Ai through Ai+2 and the secondary
signals si_l8 through si_l5 grouped within this interval. As
indicated in Fig. 3F, this is one horizontal line interval.
As shown in Fig. 15B, after the composite word signals
have been separated and the error-free primary signals Ai 18
through Ai 15 brought into the same time intervals with the
erroneous secondary signals Bi_l8 through Bi_l5, the logic cir-
cuit 53 can easily select the error-free primary signals for
further processing, for example in the serial-to-parallel con-
verter 13 and on beyond.
In the same way, when the erroneous signals Ai through
Ai~3 have been brought into the same time interval with the
error-free related secondary signals Bi through Bil3, the logic
circuit 63 can easily select the error-free secondary signals
for further processing. Thus, the advantage of the invention is
obtained.
- 36 -
