Note: Descriptions are shown in the official language in which they were submitted.
1149947
- I - RCA 74, 436
lMPROVED DIGITAL ON VIDEO
` RECORDING AND PLAYBACK SYSTEM
This invention relates to a means for encoding a digital
- 5 signal on a video signal, and to a means for detecting the digital
` signal thereirom. This invention is particularly useful in video disc
systems.
A video disc player converts recorded parameters on a disc
record into a standard television signal for application to the antenna
terminals of a television receiver. Before recording, in the preferred
embodiment of the video disc system discussed herein, the video
signal is processed in accordance with the buried subcarrier
technique as taught by D. H. Pritchard, in U.S. Patent 3,872,498, .-
entitled COLOR INFORMATION TRANSLATION SYSTEMS, issued
March 18, 1975. In the player, video processing circuitry detects
the recorded FM modulated video signal and separates respective
chrominance and luminance information. This separation is
accomplished by comb filtering as described in U.S. Patent 3,996,606
entitled "Comb Filter for Video Processing", issued December 7, 1976
to D. H. Pritchard. Then the chrominance signal is translated in
the frequency spectrum by heterodyning, up to a standard color
subcarrier frequency and combined with the luminance signal. The
resulting composite video signal is further translated in the
frequency spectrum to a convenient channel frequency for viewing
26 on a standard television receiver.
In addition to vldeo signal processing, it is desirable
to provide the video disc player with advanced features, such as
detecting and automatically skipping over locked grooves, automatically
detecting the end of program play, and displaying program playing
ao time. As used herein, the term "locked groove" refers to a condition
wherein things such as a defect in the record or contaminants on the
surface of the record cause the stylus to skip backward in the
program material.
Detecting locked grooves is particularly important, since
36 locked grooves retard program progress unless the viewer or some
other mechanism advances the pickup device past the locked groove
every time such defect is encountered. Even in systems where the
pickup device is a stylus riding in a driven carriage and the
carriage drive eventually causes the stylus to break out of the
.
, ~
:
: ` :
:
''" " ' 'li~994'7 `
-2- RCA 74, 436
locked groove, program interruption is significant. With loclsed
groove detection, a stylus "kicker" may be used to automatically
advance the stylus past the disc defect. In fact, if the system
5 detects a locked groove early enough, the stylus can be rapidly
moved past the defective area without any noticeable interruption
- to the viewer.
- At the end of the program, a positive detection of end-
of-program is preferred, as compared to a mere loss of signal. In
10 this way, thé viewer gets an immediate response from the player to
change or flip the video disc. An indicator, displaying program
playing time is useful to the viewer, particularly for returning
to a desired point in the program. ~ -
The abovementioned features, as well as other useful
-15 features, are implemented in the preferred embodiment disclosed
herein, by recording digital information on the video signal. In
particular, a digital number is assigned to each picture field (or
other indicia of location such as each groove convolution).
Program play time is computed by dividing field number by a
20 constant. Basically, a locked groove is then detected by noting
at least two non-sequential field numbers. See United States '
patent 4,313,134, TRACK ~RROR CORRECTION SYSTEM, AS FOR
VIDEO DISC PLAYER, issued January 26, 1982, for a more
detailed treatment of the locked groove problem.
, ,
Another digital number, referred to herein as a band
number, is recorded in addition to field number to indicate the type
of program material. For example, end-of-program detection is
afforded by detection of, a unique end-of-program band number.
30 The digital format has spare, unassigned information bits to allow
for later expansion to other features in the future.
From the foregoing discussion, one can appreciate the
desirability o~ developing a system for recording digital numbers
in some form on the video signal. It is known from other
35 digital-on-video systems used with video disc recording that
prerecorded digital information can be provided during one or more
horizontal lines occurring in the vertical blanking interval,
Previously known systems use vertical and/or horizontal sync to
synchronize the player's data subsystem to the prerecorded data
40 interval. Since a video disc signal has time base variations during
~3
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,.......... `~
. ....... ;
`
`.
1~49947
-3- RCA 74, 436
playback, some of these systems use self-clocking waveforms to
sample individual bits. A typical example of a self-clocking waveform
used in a prior art video disc system is Miller encoding. In a Miller
5 format, signal transitions at regular intervals indicate the beginning
of a clock period, and transitions (or the lack thereof) in between
clock transitions indicate the data bits. ;;
A disadvantage of these prior systems is that the video
piayer requires special circuitry to separate the digital bit stream.
For example, if the data is synchronous with vertical sync, then
vèrffcal sync is detected. Since vertical sync is not normally used
in a video disc player, a separate detector is required. A further
disadvantage of prior systems is that sync pulse edges are not
sufficiently accurate as a time reference for sampling digital data.
Yideo disc noise tends to resemble sync signals. Therefore, it is
desirable to completely avoid using video sync signals to synchronize
the digital subsystem to the video signal. Also, self-clocking
waveforms require a more complex detector, and generally have a
reduced data rate, as compared to a non-self-clocking waveform.
In the present invention, the video carrier is modulated
by a digital data stream in synchronism with the color subcarrier
signal during a horizontal lin~e. Data is represented as a luminance
Içvel. Therefore, to interface the digital system to the video
system, the video signal is sampled using the color subcarrier
25 oscillator as a clock.
The horizontal line adjacent to the line containing data is a
constant luminance level (black). Such arrangement permits the use
of a signal already available in the video disc player -- the
chrominance related output of the comb filter (referred to herein as
30 processed video) -- as the data signal. Processed video is regarded
as data, and continuously sampled at the clock rate of the color
subcarrier signal. Since the comb filter subtracts one line from an
adjacent line, the processed video signal is self-referenced,
eliminating a source of data errors caused by shifts in the d-c
36 level of the video signal.
To synchronize the digital system to the recorded data,
a start code is used. The digital system samples continuously
to find the first start code. Thereafter, the digital system stores
the rest of the digital message and stops sampling data. A
40 microprocessor control is used to calculate the approximate time of
,.
,
1149947
-4- RCA 74, 436
occurrence of the next start code. The dig;tal system begins to
sample incoming data again shortly before the expected time of
arrival of the next horizontal line containing data. In this manner
5 the digital system resynchronizes itself to the incoming data in each
field. At the same time, the insensitivity of the digital system to
input signals during times when no data is expected, enhances the
noise immunity of the system.
Accordingly, the present invention is embodied in a video
10 disc digital recording and playback system wherein the video-to-
digital interface is arranged to eliminate reliance on vertical or
horizontal sync signals and uses readily available signals, generated
during video signal processing, to detect recorded digital information.
In accordance with one aspect of the present invention, a
15 video disc recording method is provided for recording a horizontal
line of a video signal modulated in accordance with digital information
adjacent to a constant luminance horizontal line of said video signal.
In accordance with a further aspect of the present
invention a video disc recording method is provided wherein
20 individual bits of the digital informationarerecorded synchronously
with the color subcarrier of the video signal.
In accordance with yet another aspect of the present
invention a video disc playback method and apparatus is provided
wherein the digital information signal is detected by storing a
25 horizontal line of the video signal and subtracting the stored
horizontal line from an adjacent line of the video signal.
In accordance with another aspect of the present invention
a video disc playback method and apparatus is provided wherein
individual bits of the digital informationaredetected by sampling
30 the digital information signal synchronously with the color subcarrier
of the video signal.
In accordance with still another aspect of the present
invention a method and means are provided for synchronizing the
digital system to a recorded video signal, the me-thod comprising
35 the steps of: detecting a first digital message by continuously
sampling individual bits encoded on the recorded video signal until
a first digital message is received; timing a gating time interval
approximately equal to one video field -time interval after the start
of each horizontal line containing a digital message; timing a data
40 window time interval beginning before and extending after the end
'
.
1~49947
-5- ~CA 74, 436
of the gating time interval; and determining if a subsequent digital
message is detected within the data window time interval.
IN THE DRAWING:
FIGURE 1 is a graphical representation of a television signal
including the vertical blanking interval between odd and even fields;
FIGURE 2 is a graphical representation of the digital data
format used in the disclosed recording method;
FIGURE 3 is a block diagram of a video disc encoder;
FIGURE 4 is a bloc~ diagram of a video disc player;
FIGURE 5 is a block diagram showing more detail of the
digital data generator of the video disc encoder of FIGURE 3;
FIGURE 6 is a block diagram showing more detail of the
information buffer for the video disc player of FIGURE 4;
FIGURE 7 is a schematic diagram of a means for generating
an error check code from the information bits for the video disc
encoder of FIGURE 5;
FIGURE 8 is a schematic diagram, shown partially in block
form, of the information buffer for the video disc player of FIGURE 4;
FIGURE 9 is an embodiment of a receiver control counter for
the information buffer shown in FIGURE 8;
FIGURE 10 is a state transition diagram for the
25 microprocessor control means of FIGURE 4; and
FIGURE 11 is a flow chart representing a program
algorithm for the microprocessor control means of FIGURE 4.
SIGNAL FORMAT
Particular details of an NTSC type television signal
30 formatted in accordance with the buried subcarrier technique as
described in US Patent 3,872,498, "Color information translating
systems", to D. Pritchard, are shown in FIGURE 1. A vertical
blanking interval separates the interlaced odd and even fields.
Those skilled in the television arts will readily recognize the
35 standard vertical blanking interval containing a first equalizing
pulse interval, a vertical sync interval, a second equalizing pulse
interval, followed by a number of horizontal line intervals at the
start of each new field. As shown in FIGURE 1, the video signal
information begins on line 22' of field 1, and on line 284' of field 2.
- ~i499~7
-6- RCA 74, 436
The digital information representative of the field num~er
appears at line 17' of field 1, and line 280' of field 2. Digital
information could, as well, be inserted in other lines of the vertical
blanking interval. To show the details of the digital siynal format,
FIGURE 2 expands the time scale during the horizontal line containing
data (line 17' or line 280').
Data are represented in terms of luminance level: 100 IRE
units is a logical "one'i and 0 IRE units (blank) is a logical "zero".
The first data bit follows the standard horizontal sync pulse 140
and color burst 142. The frequency of the burst 142 is about
1.53 MHz, the frequency of the buried subcarrier. Each data bit
is transmitted synchronously with the 1.53 MHz buried subcarrier
signal. As shown in FIGURE 2, the digital message comprises a
13-bit start code termed B(x), a 13-bit redundant error check code
termed C(x), and 51 information bits termed I(x). The beginning
of the next horizontal line is indicated by the next horizontal sync
pulse 140a and color burst 142a. Thus, the individual data bits are
synchronous with the color subcarrier, and the overall digital
message is synchronous with the vertical sync pulse. Note that the
data rate can be a multiple or submultiple of any convenient
subcarrier frequency. AISOr other values of luminance may be
assigned to logic one and zero, or more than one bit may be
associated with a given luminance level.
26 A start code is used in the present system to synchronize
the data system to the digital message thereby avoiding the need to
detect the edge of horizontal or vertical sync. Synchronizing errors
in a serial digital data system result in framing errors, i.e. where
received data is shifted by one or more bits from its proper position.
30 Previously known systems for recording digital data on a video disc
encoded signal have shown that the edges of sync signals are not
reliable as a time reference and have resulted in framing errors.
Start codes have proven to be more reliable.
The specific start code chosen, 1111100110101, is one of
35 the Barker codes known in radar and sonar technology. See "Group
Synchronization of Binary Digital Systems", by R. H. Barker,
published 1953 by Academic Press, New York, N.Y. Barker codes
are designed such that the auto-correlation function, of a signal
containing a Barker code shifted with respect to itself, is maximized
40 when coincidence occurs, and minimized elsewhere. That is, if one
:,
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~14~
-7- RCA 74 ,436
assigns a value of +1 or -1 to each bit in the start code and computes
the sum of the respective bit products for each shifted position of
the start code with respect -to itself, such auto-correlation function
5 will produce a sharp maximum when coincidence occurs. Specifically,
a Barker code shifted any odd number of places with respect to itself
produces an auto-correlation of 0. A Barker code shifted any even
number of places with respect to itself produces an auto-correlation
of -1. However, when there is coincidence, the auto-correlation is N,
10 where N is the number of bits in the Barker code. In other words,
a Barker code shifted any number of places with respect to itself
differs in a maximum number of bit positions. In the presence of
noise, this characteristic reduces the probability of a false start code
detection, as compared to an arbitrarily chosen start code.
lS The information bits, I(x), include a field number, a band
number, and spare information bits for future expansion. Field
numbers identify each field of the video signal by a unique 18-hit
binary number. At the beginning of the video disc, the first field
of the video program is fie]d "zero". Thereafter, each field is
20 consecutively numbered in ascending order. Band numbers refer
to recorded video signal in a group of adjacent convolutions of the
spiral groovès which form a band-like shape. All of the material
in such band of grooves is identified by having a common band
number. As an example of band number utility, the video signal
25 after the end of the video program material is recorded having band
number "sixty-three". The video disc player senses band sixty-three
as the end of program and responds by lifting the stylus from the
record .
The error check code C(x) is computed from I(x) in the
30 video disc recording apparatus. To this end, I(x) is multiplied by
a constant, H(x). The resulting product is divided by another
constant g(x). After such division, the remainder (the quotient
is unused) is added to a third constant M(x). *he result is C(x).
In the video disc player, the received message is checked
36 for errors by dividing the entire message, including the start code,
by the constant g(x) mentioned above. If the remainder is equal to
the start code, B(x), then the message is considered error free.
The constants H(x) and M(x) are chosen so that the remainder of
the entire message will in fact be the start code. The constant
40 g(x), used in both the video disc recording apparatus and the
,~
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1149g~
-8- RCA 74, 436
video disc player is called the generator polynomial of the code. A
specific g(x) is chosen which generates a code having error detection
properties particularly advantageous as applied to the video disc
5 medium. In the system described herein, the addition, multiplication,
and division operations referred to above are performed according to
special rules to accomodate the hardware available for carrying them
out. The error coding will be discussed in greater detail hereinafter
in conjunction with the encoding and decoding hardware.
A block diagram of a video disc encoder is shown in
FIGURE 3. A composite video signal from source 30 is linearly
combined in adder 36 with a digital data bit stream on conductor 37
supplied by the digital data generator 38. Synchronizing means 32
supplies a color subcarrier and synchronizing pulses so that the
15 data bits generated by the digital data generator 38 are synchronous
with the color subcarrier appearing at terminal 31a and so that the
digital message is encoded on the proper horizontal line in the
vertical blanking interval. Information bits, appearing at data bus
39 and representing the video field number and band number, are
20 provided by apparatus 34. The use of field number and band
number information will be discussed in conjunction with the
microprocessor program (FIG, 10 and 11). The digital data and
the video signal are combined in the adder 36. Further signal
processing means 40 conditions the composite video for the recording
26 medium. The composite video signal is of the buried subcarrier type
and is recorded using FM modulation techniques.
In the video disc player of FIGIJRE 4, the FM signal is
detected using pickup transducer and stylus assembly 20 and
converted in video processing circuitry 18 to a standard television
30 signal for viewing on an ordinary television receiver. Video
processing circu}try 18 includes means responsive to the color
burst signal to phase lock a 1.53 MHz local color oscillator to the
color subcarrier. The color oscillator, in addition to its usual us0
for demodulating -the buried subcarrier wave, is also used to provide
35 the digital clock signal and this signal appears on conductor 72. The
video processing circuitry 18 further includes means for demodulating
the video carrier and comb filtering the recovered video signal.
Comb filter 19 subtracts two adjacent field lines, which result appears
on conductor 70 as processed video. Since line 16', which is at the
,,
.
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1149~7
-9- RCA 74,436
black level, is subtracted from line 17', which is modulated with
digital data, the processed video on conductor 70 is the recovered
digital data. Naturally, line 16' may be any constant luminance
5 level. Note that if the subsequent line 18' to the data line 17' is
a constant luminance line (also black) the subsequent output of the
comb filter during line 18' is again recovered digital data, bu-t the
data is inverted. By subtracting one line fror.n a constant luminance
adjacent line, the recovered digital signal is self-referenced, there~y
10 eliminating data errors due to shifts in the c-~-c level of the video
signal. ~f it were desired to place data on conl:ecutive lines, as
compared to placing data adjacent to constant l- ninance lines, then
means for referencing the video signal to a pre(letermined luminance
level, or a d-c reference level would be necessc y in order to
15 separate the digital data stream from the video il,ignal.
As shown in FIGURE 4, the information~buffer 16 is
responsive to processed video on conductor 70 and the l.53 MHz
clock signal on conductor 72 to extract digital data from the video
signal. The buffer 16 is controlled by a digital binary control
20 signal on conductor 71 from the microprocessor 10. In one binary
staté, the control signal on conductor 71 causes the information
buffer 16 to acquire data. IJ1 the other binary state, the control
signal on conductor 71 conditions the information buffer 16 to
transfer the received data to the microprocessor 10. In particular,
25 when the control signal on conductor 71 is high, the information
buffer 16 opens to sample incoming data on the processed video
signal conductor 70 using the 1.53 MHz signal on conductor 72 as
a clock. After a complete message is received, the status signal on
conductor 75 furnishes an indication that a message is complete. To
30 transfer the message to the microprocessor memory, the control
signal on conductor 71 is set low. This action closes the information
buffer 16, resets the internal control circuits, and gates the results
of the message error code check onto status conductor 75. If the
status signal indicates the message is valid (i.e. error code check
35 indicates validity), the microprocessor 10 is programmed to transfer
the data in the information buffer 16 to the microprocessor 10. The
microprocessor supplies an external cJock signal on conductor 73 to
transfer data from the information buffer 16. For each clock pulse,
one bit of data on conductor 74 is shifted out of the information
40 buffer and into the microprocessor 10. When all the data is
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~i499~7
- I0- RCA 74, 436
transferred to the microprocessor 10, and the program is ready for
another digital message, control conductor 71 is again returned to a
high state and the process is repeated.
The microprocessor 10, via the information buffer 16,
controls the gating of line 17' ~or line 280') out of the video signal.
The first digital message is obtained by continuously searchiny the
video signal for a start code. Thereafter, the information buffer 16
is closed. Then, based on the time of arrival of the first digital
message, the information buffer is opened approximately six lines
before the next digital message is expected. If no valid message is
found, the information buffer 16 is closed approximately six lines
after such expected time of arrival. If a valid digital message is
found, the information buffer 16 is closed and a new time of arrival
for the next digital message is calculated based on the time of arrival
of the current digital message. In such manner, the microprocessor
10 opens a gate, or "data window", approximately twelve lines wide
and centered about the expected data. The time interval from the
center of one data window to the next is approximately one video
field time interval. The width of the data window is chosen so that
under worst case timing conditions the expected data will fall within
the data window. Sources c~f timing error, as explained below,
are: finite resolution of the digital timer; the drift rate of the
timer; program uncertainty in determining time of arrival of present
data; and timing differences between odd and even interlaced fields.
Use of an alternate microprocessor and/or timer may be accommodated
by adjusting the data window width accordingly. The microprocessor
program which controls the logic for searching for data and centering
the data window is discussed hereinafter in conjunction with
FIGURES 10 and 11.
The microprocessor 10 is also responsive to the player
panel controls 14 (load, pause, and scan) to operate the player
mechanism 12 and drive the player display 22 in accordance with a
predetermined program, as discussed hereinafter. The player
36 mechanism is further provided with at least one stylus "kicker"
operable by the microprocessor 10. A kicker is a means,
piezoelectric, electromagnetic, or otherwise, for impulsively
moving the signal pickup means to adjacent grooves or signal
tracks on the video disc medium. The use of the kicker to break
-' 114g~47
-11- RCA 74,436
out of locked grooves will be discussed hereinafter in conjunction
with the flow diagrams of FIGURES 10 and 11.
ERROR CODE
As mentioned above, the video disc recording apparatus
uses the information bits I(x) to compute C(x). Because of the
large number of potential combinations - I(x) and C(x) together are
64 bits long - and the desire to determine the error detection and
correction characteristics of a given code without resorting to
enumeration, error codes are treated mathematically. A general
mathematical development of ring theory and Galois Fields GF(2m),
applicable to error codes in general, can be found in "Error
Correcting Codes" by W. Wesley Peterson, published by MIT Press,
Cambridge, Massachusetts. For present purposes, the error codin~
in the video disc may be best understood in terms of a few simple
definitions .
A digital message, comprising ones and zeros, can be
considered as representing an algebraic polynomial comprising
powers of x. The coefficients of the respective powers of x are
the individual bits of the message. For example, the 4 bit message
1011, can be represented by the polynomial P(x), where
P(X) = 1~X3+O~X2+1~X+1~XO
= X3+x+1
Applying this notation to the start code, 1111100110101,
then
B(x) = x12+Xll+xl0+x9+x8+x5+x4+x2+l
The highest power of x is called the degree of the polynomial. In
the above example, B(x) is a polynomial of degree 12.
Polynomials may be added, subtracted, multiplied, and
divided using the ordinary rules of algebra except for expressing
coeficients in modulo 2 terms. A shorthand notation for the
remainder of a polynomial after division by another polynomial is
indicated by brackets. That is, if
g(x) = Q(x) + ~
where the remainder, r(x), has a degree less than the divisor, g(x),
then
[P(x)] = r(x)
J
~114~S4~
-12- RCA 74,436
In the video disc recording apparatus, the total message
recorded on the video disc is represented by a polynomial,
T(x). From FIGURE 2,
T(x) = B(x)x64+C(x)x51+I(x) (1)
The term X64 shifts B(x) by 64 bits, because B(x)
is at the beginning of the data format. Similarly, the term x5
shifts C(x) 51 bits to represent that C(x) is recorded before
I(x). In accordance with the apparatus being described, the
recording apparatus computes a value for C(x) so that the
total message, T(x), has a remainder equal to B(x) after heing
divided by g(x). That is, assuming C(x) to be of the form
C(x) = [I(x) H(x)~ + M(x), (2)
then H(x) and M(x) are constant polynomials chosen so that
LT(x)] = B(x) (3)
It can be shown that equations (1), (2), and (3), when
solved for the constant polynomials H(x) and M(x), yield
H(x) = lx
M(x) = [B(x)xl3+B(x)xl27]
FIGURE 7 includes a table enumerating the chosen values
for B(x) and g(x), as well as the derived values for H(x) and M(x).
Note that the table in FIGURE 7 shows high order bits on the right,
so that they are in the same order as the flip flop storage elements
appear in the logic diagram of the same figure.
ln the video disc player, the recorded digital message
is read by the player electronics. The data recorded on the video
disc is T(x). The data read by the player is R(x). If no errors
are generated between recording and playback then T(x) = R(x).
The received message, R(x), is checked for errors by dividing R(x)
by g(x). If the remainder is equal to B(x), the start code, then
the message is considered error-free. On the other hand, if the
remainder does not equal B(x), then an error is thereby indicated.
The characteristics of a code generated in the above
manner depend on the choice of g(x), which is called the generator
polynomial. The particular g(x) chosen for the video disc medium
is one from the computer generated codes demonstrated by Tadao
Kasami in "Optimum Shortened Cyclic Codes for Burst Error
_;
114S9~7
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-13- RCA 74,436
Correction" published in IEEE Transations on Information Theory
1963. A burst error in a digital system is a type of error where
adjacent bits in the digital message are lost. Burst errors are
6 considered a likely type of transmission error in the video disc
medium. As shown by Kasami in the aforementioned reference, a
code which can correct single burst errors of 6 bits or less, can
be implemented using a generator polynomial given by
( ) = xl3+xl2~xll+xlo+x7+x6+x5+x4+x2+l
~0 Furthermore, it may be shown that for the g(x) given above, all
single burst errors of 13 bits or less will be detected, and 99.988
percent of all single burst errors longer than 13 bits will be detected
as well. The video disc player, as described herein, uses only the
error detection capabilities of the chosen code.
As a specific example of error code generation, consider
the ca,se where the field number is 25,000, the band number is 17,
and the spare bits are 0. Since 25,000 in binary representation is
000 110 000 110 101 000, and 17 in binary representation is 010 001
(high order bits are on the left), the 51 information bits are
000 000 000 000 000 000 000 000 000 000 110 000 110 101 000 010 001.
The order of transmission is spare bits first, followed by field
number, and then band number, wherein the most significant bit
ls transmitted first. The error code for the above specific I(x),
computed as the remainder of I(x) times H(x), plus M(x), is
represented by 0111100100010. The next video field is 25,001 or
000 110 000 110 101 001 in binary representation. For the
corresponding information bits, 000 000 000 000 000 000 000 000 000
000 110 000 110 101 001 010 001, the proper error code is
1000101101110. The complete digital message for field 25,001
ao including the start code is therefore, 1111100110101 1000101101110
000 000 000 000 000 000 000 000 000 000 110 000 110 101 001, 010
001, shown in order of transmission. The start code is the first
13 bits, the error code is the next 13 bits, and the 51 information
bits are last. In the video disc player, the above digital message is
checked for errors by dividing the received message by g(x). If
no errors are detected, the remainder is 1111100110101, which is
exactly the start code.
HARDWARE
A block diagram of a means for generating T(x~ is shown
in FIGURE 5. Under the control of the transmitter control means 50,
_,
,
` 11499~7
-14- RCA 74,436
24 information bits are loaded via data bus 39, and 27 spare
information bits are loaded via data hus 39a into a 51 bit shift
register 44. I(x), which comprises these 51 bits, is then shifted
into another 51 bit shift register 52.
At the same time, during the 51 shift pulses, an encoder
45 computes C(x) in the following way. Polynomial dividing and
multiplying means 46 is responsive to the 51 bit serial transmission
of I(x) to compute the remainder of l(x) times H(x) divided by
g(x). M(x) is then added in parallel in polynomial adder 48. The
resulting code C(x) is loaded into a 13-bit shift register 54, and
B(x), the start code, is loaded via data bus 49 into another 13 bit
shift register 47. Since the start code is a constant digital value,
such loading is preferrably accomplished by fixed connections to the
parallel load inputs of shift register 47 as opposed to a software
implementation. In positive logic notation, the corresponding parallel
inputs to shift register 47 are connected to ground potential wherever
the start code has a zero, and to a positive potential wherever the
start code has a one. Transmitter control means 50 controls the total
message T(x), contained in the three shift registers 52, 54, 47,
being shifted out serially in synchronism with the color subcarrier
on conductor 31a. A video synchronizing pulse applied on conductor
33 provides transmitter control means 50 with a time reference so
that the digital message is transmitted at the proper time with
26 respect to the video signal.
A specific embodiment of the encoder (45 of FIGURE 5)
is shown in FIGURE 7. Clocked flip flops having output terminals
QO through Q12 form a remainder register. Multiplication by H(x)
and division by g(x) is performed simultaneously in bit serial fashion.
Afterwards, the remainder is held in the remainder register outputs
QO through Q12' See Chapter 7, pages 107-114 of the
above-mentioned Peterson reference for a general treatment of such
circuits. To appreciate the simplicity of the circuit in FIGURE 7 for
multiplying and dividing polynomials, it is noted that both addition
36 and subtraction (of coefficients of terms of like power) is performed
by an exclusive OR gate. ~qultiplication of I(x) by H(x) is
performed by appropriate connections to one or more exc]usive OR
gates 80 through 91. In particular, wherever a coefficient of H(x),
but not g(x), is equal to 1, (bit positions 1, 3, and 8) input I(x)
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~i49947
1 -15- RCA 74,436
is connected to an input of an exclusive OR gate 80, 82, and 87,
respectively. Division of T(x) by g(x) is performed by multiplying
the output of Q12 by g(x), and subtracting the resulting product
5 from the contents of register Q0 through Q12- In particular,
wherever a coefficient of g(x), but not H(x), is equal to 1, (bit
positions 4, 7 and 11) the output of Q12 is connected to an input
of exclusive OR gate 83, 86, and 89, respectively. Where H(x) and
g(x~ are both equal to 1 (bit positions 0, 2, 5, 6, 10 and 12) the
10 output of exclusive OR gate 91 is connected to an input of exclusive
OR gates 81, 84, 85, 88, and 90, respectively. After 51 clock pulses,
one for each bit of I(x), the contents of register Qo through Q12 is
the remainder of I(x) H(x) after division by g(x).
Note how M(x) is added to the contents of the remainder
15 register. Addition of coefficients is in modulo 2 arithmetic performed
as the exclusive OR function. Wherever M(x) has coefficients of ~1,
the complement output, Q, of the corresponding flip flop is used;
wherever M(x) has coefficients of 0, the uncomplemented output, Q,
is used.
ao A block diagram of a means for decoding the received
message, R(x), appears in FIGURE 6, which is an embodiment of the
information buffer 16 of FIGURE 4, discussed above. Control signal
on conductor 71, an input, conditions the receiver decoder of
FIGURE 6 either to receive data from the video signal, or transfer
data to the microprocessor.
In the receive state, each bit is simultaneously shifted
into two separate registers. One such register 60 is for data, and
the other 62 is for error checking. The error check register 62 is
a polynomial divider. However, when acquiring new data, the divider
feedback path is disabled so it functions as a straight shift register.
The operation of divider register 62 will be discussed subsequently
in greater detail in conjunction with FIGURE 8. For present purposes,
register 62 is responsive to the receiver control means 64 to either
shift in successive bits of R(x), or divide successive bits of R(x)
36 by g(x). In either case, the contents of register 62 is available on
data bus 78 and provided to the start code and valid data detector
66.
The receive operation begins with register 62 conditioned
to operate as a shift register. After B(x) is detected by detector
,
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1 -16- RCA 74,436
66, control means 64 conditions register 62 to operate as a polynomial
divider. Thus, polynomial division by g(x) begins with B(x) in the
divider register 62. The receiver control means 64 is further
5 responsive to the detection of B(x) to time out a period equal to the
remaining message bits (64 clock pulses). After the time out period,
the divider 62 contains the remainder of R(x) modulo g(x), which
should be B(x) if the message is valid. During the error check
process, data register 60 has been shifting in data bits. At the end
10 of the time out period, the data register 60 stores only the last 24
bits. However, since the 24 information bits are placed at the end
of the message, register 60 will contain the assigned infarmation bits.
If it is desired to utilize the spare information bits, additional shift
register stages may be added.
Interpretation of the output status signal on conductor 75
depends on the state of control signal on conductor 71. When the
control signal on conductor 71 conditions the receiver for acquiring
data (receive state) the status signal on conductor 75 is defined as
"message received". When the control signal on conductor 71
20 conditions the receiver for transferring data (transfer state), status
signal conductor 75 indicates "data valid". The control signal on
col~ductor 71 also resets the ~eceiver control means 64 and gates the
results of the remainder check onto the status signal on conductor
75,
26 The received information is transferred out of shift
register fi0 in response to external clocks supplied by the
microprocessor on conductor 73. After the data is shifted out,
the control signal on conductor 71 may be returned to its previous
state which will again condition the receiver-decoder to continuously
8earch for another start code.
FIGURE~ 8 shows a logic diagram, partially in ~lock form,
of the receiver decoder of FIGURE 6. The flip flops having output
terminals Q0' through Q12' form a remainder register. Polynomial
division by g(x) is performed by multiplying sucessive register
output terms from Q12' by g(x) and subtracting the product (via
exclusive OR gates 100 through 108) from the contents of the
remainder register. A feedback connector from Q12' (through
NOR gate 109) is made to an exclusive OR gate wherever g(x) has
coefficients of 1, except for bit 13. Since the coefficients of g(x)
are 1 for bit positions 0, 2, 4, 5, 6, 7, 10, 11, 12, an exclusive
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OR gate is placed at the data input of each respective flip flop of
the remainder register as shown. NAND gate 118 detects B(x), which
is both the start code and the valid error check code. The receiver
5 control counter 117, begins counting responsive to a start signal from
AND gate 120, counts 63 clock periods and supplies a stop signal
which is used by NAND gate 111 to stop the clock to all decoder flip
flops. A representative embodiment of the receiver control counter
117 is shown in FIGURE 9 comprising seven flip flops 130 through
136.
The sequence of operations in receiving data is as
follows. When the control signal on conductor 71 is high, data is
gated to divider 62 through AND gate 110. Flip flop 119 has been
previously set, which disables the feedback signals in divider 62 by
blocking NOR gate 109. Register 62 now functions as a shift register.
Upon detecting B(x), the output of NAND gate 118 goes low, and
the Q output of flip flop 119 goes low one clock period later.
Therefore, feedback is enabled for polynomial division by the output
of AND gate 120 via NOR gate 109 when B(x) is detected in the
remainder register. After 63 clock periods, the receiver control
counter 117 stops and the status signal on conductor 75 goes high,
indicating "message received".~ Shift register 60 holds the last 24
bits of I(x). To transfer data, the control signal on conductor
71 is made low. The inverted output of NAND gate 118, which is
low if the remainder after division is B(x), is gated onto -the status
signal on conductor 75. External clock pulses on conductor 73
cause successive shifts of data in register ~0 to the output data
signal on conductor 74. The external clock pulses also c]ear the
remainder register by shifting in zeros.
The above arrangement shows a remainder
register beginning and ending with the same non-zero constant.
However, it will be understood that other arrangements are possible
when using a coset code. For instance, after detecting B(x), the
remainder register can be set to a first arbitrary constant. Then,
after division, the remainder register is checked for a proper second
constant. The first constant, or the second constant, may be zero;
both constants may not be zero.
Observe the simplified hardware which results from the
error code format described herein. By ending with the start code,
40 B(x), as a valid remainder, the start code detector (NAND gate 118)
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-18- RCA 74,436
also serves as a valid code detector. By beginning division with
the start code in the divider, a control step is eliminated in not
having to clear the remainder register.
Typically, error codes are placed at the end of a message.
In accordance with the present invention, by placing the error code
before the information bits, the receiver controller is further
simplified in not having to distinguish information bits from error
code bits with regard to the data storage register 60. In addition,
the receiver controller, as shown in FIGURE 8, is a simple counter 117,
having a start terminal, a stop terminal, and providing a~ timing out
for a single time interval.
MICROPROCESSOR IMPLEMENTATION
Digital information, including band number and field number,
are recorded on the video signal, and utilized by the player to achieve
a variety of features. Band number information is used by the player
to detect end of play (band sixty three). Field number information
in ascending order is used to calculate and display the program
playing time on LED display means 22 in FIGURE 1. If the length
ao of program material is known, field number information can be used
to compute the remaining program playing time. For NTSC type
signals, elapsed program time in minutes may be obtained by
calculating the field number divided by 3600. If desired, the
remaining program time may be derived from the previous calculation.
2$ This feature is useful to the viewer when scanning for a desired
point in the program. A particularly useful feature, derived from
fleld number information, is locked groove correction which will be
discussed hereinafter in conjunction with the more general case,
track error correction.
Field numbers represent actual stylus position. Thus,
whenever the stylus re-enters a groove, whether after jumping tracks
or after the scan mechanism is operated, the actual stylus position
can be determined from the first valid field number read. Both the
track error correction system and the program playing time display
means use field number data, and therefore share the decoding
portion of the video disc digital data system. The particular
embodiment of track error correction system discussed herein after
uses field number data (stylus position) to keep the stylus at or
ahead of its expected position assuming a predetermined stylus/record
40 relative velocity. The program playing time display uses field number
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-19- RCA 74 ,436
data for an indication of playing time, which is actually another
representation of stylus position.
The microprocessor controller has several internal modes.
FIGURE 10 is a state transition diagram indicating the mode logic
performed by the microprocessor program. Each of the circles
represent a machine mode: LOAD, SPINUP, ACQUIRE, PLAY, PAUSE,
PAUSE LATCHED, and END. For each mode, the position of the
stylus and the status of the display is indicated inside each respective
circle. The arrows between modes indicate the logical combination of
signals, supplied by the panel controls (load, pause, scan), that
cause a transition from one mode to another. The load signal indicates
that the player mechanism is in a condition to receive a video disc.
The pause signal is derived from a corresponding control panel switch,
and the scan signal indicates the operation of the scan mechanism.
After power is turned on, the system goes into LOAD mode.
A video disc may be loaded onto the turntable in this mode. After
loading, the player enters SPINUP mode for several seconds, allowing
the turntable to be brought up to the full speed of 450 RPM. At the
end of SPINUP mode, the ACQUIRE mode is entered.
In ACQUIRE mode, the digital subsystem lowers the stylus
and continuously searches for a "good read". In ACQUIRE mode, a
"good read" is defined as a valid start code and a valid error check
remainder, After finding a "good read" the system enters PLAY mode.
26 In PLAY mode, the microprocessor establishes in memory an
expecte,d, or predicted, next field number. The predicted field
number is incremented or updated each field, For all subsequent
reads, the microprocessor uses the predicted field number in
performing two additional checks to further improve the integrity
of the data.
The first additional check is a sector check. The video
disc in the embodiment under consideration contains eight fields in
every revolution, dividing the disc into eight sectors. Since the
relative physical position of the sectors is fixed, the sectors follow
a periodic recurring order as the disc rota-tes, even if the stylus
jumps over a number of grooves. Although the digital information
cannot be read for one or more fields, (sectors) while the stylus is
skipping to a new groove, the microprocessor keeps time, and
increments the predicted field number accordingly. When the stylus
settles in a new groove and picks up a new digital message, the new
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field number is checked by comparison to the predicted field number.
If the sector is wrong, the data is considered a "bad read".
Field number is represented by an 18 bit binary number.
5 Sector information may be obtained from field number by finding the
remainder after dividing the field number by eight. However, it is
noted that the three least significant bits of a binary number counts
modulo eight. Therefore the least significant three bits of each new
- field number must equal the least significant three bits of the
10 predicted field number to pass the sector check.
A second check of data integrity is the range check, a
test of the maximum range of stylus movement along the radius of
the disc. No more than 63 grooves are expected to be jumped when
encountering worst case conditions in any mode. Groove numbers are
15 represented by the most significant 15 bits of the field number. The
microprocessor substracts the present groove number from the
predicted groove number. If the difference is greater than the
acceptable range of 63 grooves, then the present data is considered
a "bad read". All other reads are regarded as good reads and are
20 used to update the predicted field number. After fifteen consecutive
bad reads, the system re-enters the ACQUIRE mode. The presence
of a scan signal in certain modes, as shown in FIGURE 10, will also
cause a transition to ACQUIRE mode.
When going from ACQUIRE mode to PLAY mode the
25 microprocessor sets the bad read count to thirteen. This means that
when entering PLAY mode from ACQUIRE mode, one of the next two
fields must supply a good read or the bad read count will reach
fifteen causing a return to ACQUIRE mode.
If the pause button is pressed during PLAY mode, the system
30 enters PAUSE mode. In this mode the stylus is off the record and is
held in its then radial position over the record. When the pause
button is released, PAUSE LATCHED mode is entered and held.
Pressing the pause button again releases the PAUSE LATCHED mode,
causing a transition to ACQUIRE mode. END mode is entered from
35 PLAY mode when band number sixty three is detected.
FIGURE 11 is a flow chart of the program executed by the
microprocessor. The microprocessor hardware includes one interrupt
line and a programmable timer. A commercially available microprocessor
suitable for the present system is the Fairchild Semicondwctor model
40 F8.
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The microprocessor uses the timer to control the window
in time that the information buffer searches for data. This "data
window`' is approximately twelve horizontal lines wide and is centered
5 about the expected data. When no data is found, the timer maintains
internal program synchronization to one field time interval.
The microprocessor interrupt is coupled to the status signal
on conductor 75 (FIGURE 4). Interrupts are enabled only in ACQUIRE
mode when the system continuously searches for data. The program is
10 interrupted when a digital message is received. The interrupt
service routine (not shown) sets an interrupt flag if the error code
check indicates validity. Thereafter, in PLAY mode, the programmable
timer is used to indicate the estimated time of arrival of the next
digital message.
Switch inputs (load, scan, and pause) are conditioned to
prevent switch bounce from causing undesired player response. The
microprocessor program includes logic to debounce switch inputs.
Debounced switch values are stored in memory. A separate debounce
count is maintained for each switch. To check debounce 154 the
20 switches are sampled and compared to the stored switch value. If
the sampled state and the stored state are the same, the debounce
- count for that switch is set to zero. Switch states are sampled as
often as possible. Each field, (every 16 milliseconds for NTSC), all
debounce counts are incremented unconditionally. If the resulting
debounce count is equal to or greater than 2, the stored state is
updated to the new (debounced) value. The new switch state is
then acted upon.
The first programmed step (FIGURE 11), after power is
turned on, is initialization 150 of all program parameters. The
timer is set to time out one video field. Mode is set to LOAD.
The next step 152 is a program to carry out the state
transition logic represented in FIGURE 10. Debounce counts are
normally incremented at this time, and tested to determine whether
a new switch state is fully debounced.
After the mode selection logic 152, the program enters a
tight loop 153 to (1) sample switches setting debounce counts to zero,
if required 154, and (2) check if the timer is close to time out 155,
and (3) check if the interrupt flag has been set 156.
If the interrupt flag is set 156, the program transfers data,
157a, from the information buffer and sets the timer, 157b, to time out
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i4~47
-22- RCA 74,436
a new field interval. When the interrupt service routine sets the
interrupt f]ag, the contents of the timer are saved in memory. The
program now uses the previously stored timer contents to set the
timer, 157b, with a corrected value predicting the approximate time
of occurrence of the next digital message. As previously noted,
even though the data represents the first good read in ACQUIRE
mode, the bad read count is set, 157c, to 13.
If the interrupt flag is not set, the program branches as the
timer gets close to time out, 155. If the machine is not in PLAY mode
159, then the timer is set to time out another field interval, 158. If
the machine is in PLAY mode, 159, then a number of time critical
tasks, 160, are performed. The data window is opened, 160a, (by
setting control signal on conductor 71 in FIGURES 1 and 8 to a
logical one) approximately six horizontal lines before the expected
data. Received data is read and checked as previously described.
After data is received, or if no data is received, the data window is
closed. Timer content, which represents the actual time of arrival
of the digital message, is used as a correction factor to set the
timer again, 160b. The timer is therefore set to center the next data
window over the predicted time of arrival of the next digital
message based on the actual time of arrival of the present digital
message .
Expected field number is updated, 160c, band number is
checked for start (band 0) and end of play (band sixty three), and
the bad read count is incremented, 160g, for a bad read. For valid
field data in the program viewing material, time is calculated and
displayed, 160f. If valid field data indicates that the stylus has
skipped backward, the stylus kicker means is activated, 160e, and
ACQUIRE mode is entered. Also, if the bad read count reaches 15,
ACQUIRE mode is directly entered. Throughout the time utilized for
critical tasks 160, the switch debounce check routine is repeated
periodically so that switches are tested as often as is feasible. The
program unconditionally returns through the mode selection logic 15,7
to the tight loop 153 and waits for the timer test 155, or the interrupt
check 156, to indicate the arrival of the next digital message.
The timer may be set by loading the timer directly via
programmed instructions. However, rather than use a sequence of
instructions, it is best to "set" the timer by establishing a location
in memory (a mark) which corresponds to a time out condition of the
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-23- RCA 74, 436
..
timer. The timer, then, is free running. Time out, or closeness
to the time out, is detected by comparing the contents of the timer to
the mark set in memory. The next desired time out condition is set
5 by adding the next desired time interval to the previous timer
contents and storing the result in memory. The timer is thus "set"
each time valid data is received, or if no data is received within the
data window, by setting a new mark in memory corresponding to the
next time out condition.
The programmable timer in the microprocessor used in the de-
cribed arra~ment is conditioned by the program to divide cycles of
the input 1.53 MHz clock by a factor of 200. The timer thus counts
once for every 200 cycles of the 1.53 MHz clock. One vertical field
(1/60 second for NTSC) is then approximately 128 timer counts. One
15 may alternatively use a timer which counts a different multiple of the
1.53 MHz clock, or one that uses a timing source independent of the
video signal.
The data window is made wide enough to allow f~or several
sources of timing error. Timer uncertainty due to the finite
20 resolution of the timer is equal to one least significant bit, which
corresponds to two horizontal lines. Accumulated drift error,
because 128 timer counts is n~t exactly one vertical field, is
somewhat less than one line after 16 consecutive fields in which
no valid message is found. It is noted that since the 1.53 MHz color
25 subcarrier clock is an odd multiple of one half the line frequèncy,
a timer which counts a corresponding multiple of the color subcarrier
clock would have zero drift rate. In the particular arrangement
described herein, program uncertainty in determining the time of
arrival of the data, is approximately 97 microseconds, or about
30 1.5 lines. Finally, because alternate fields are interlaced, the time
from one digital message to the next is either 262 lines or 263 lines,
depending on whether the present field is odd or even. Although
the program could lceep track of odd and even fields, it is simpler
to just widen the data window by one additional line. Combining
35 the above factors, it can be shown that a data window extending
three timer counts (about 6 lines) both before and after the start
of expected data is adequate to allow for worst case timing conditions.
TRACK ERROR CORRECTION
. _
As mentioned earlier, field number information may be used
40 to detect locked grooves. If the new field number (after sector and
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9~'7 RCA 74, 436
ranye che(:k) i~i less ~han the cxpeclcd îield number, then ~he
stylus has skipped backward and is repeating the tracking of a
previously played convolution(s), i.e. a locked groove has been
5 encountered. If the new field number is greater than the expected
field number, the stylus has skipped forward, i.e. toward record
center. In the present application, skipped grooves are ignored; if
the new field number is greater (but still passes sector and range
check) then the expected field is updated to the new field. In
10 certain other applications, such as where the video disc is used to
record digital information on many horizontal lines, it may be
necessary to detect and correct skipped grooves as well. However,
- for the present video application, a locked groove is corrected by
operating a stylus "kicker" until the stylus is returned ta the
15 expected track. Eventually, the stylus will be advanced past the
locked groove defect.
In a more general sense, t~ie use of field number
information in accordance with the present disclosure provides an
, accurate means for detecting general tracking errors. In any video
~ 20 disc system having spiral or circular tracks, including optical and
grooveless systems, tracking errors due to defects and contaminants
are always possible. The pre~sent system provides a means for
detecting and correcting such tracking errors in a video disc player.
For positive tracking, a bi-directional kicker means is provided for
25 moving the pickup backward or forward in the program material. Thus,
when a tracking error is detected, whether a skipped track or a locked
track, the pickup is moved in such direction so as to correct the
tracking error. Although the regular pickup servo could be used for
track error correction purposes, a separate kicker, or pickup
30 repositioning means, is preferable. The regular servo is generally
adapted for stable tracking of the spiral signal track, and may not
have the proper characteristics to respond to abrupt tracking errors.
A separate kicker, on the other hand, can be specifically adapted to
provide the fast response needed to correct tracking errors. A
35 specific example of a kicker suitable for use with the disclosed
apparatu~s may be found in U.S. patent 4,258,233, of E. Simshauser
entitled "TRACK SKIPPER APPARATUS FOR VIDEO DISC PLA~ER",
issued March 24, 1981.
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1~49947
-25- RCA 74 ,436
Several control algorithms are possible; The pickup
apparatus may be returned directly to the correct track by producing
stylus motion proportional to the magnitude of the detected tracking
5 error. Or, a kicker may be operated in response to a series of
pulses, wherein the number of pulses is proportional to the magnitude
of the detected tracking error. The pickup is moved a given number
of tracks per pulse until the stylus is back on the expected track.
For certain applications (e.g. retrieving digital data stored on the
10 video disc medium) it may be desirable to return the pickup to the
point of departure and attempt a second read, rather than return
the pickup to the expected track. In any event, it is seen that by
the use of a kicker and suitable control logic, successful tracking
can be obtained even though the video disc contains defects or
15 contaminants which would otherwise cause unacceptable tracking
errors .
In a digital track correction system, security against
undetected data errors is particularly important to prevent noisy
signals from advancing or retarding the pickup unnecessarily. The
20 present data system reduces the probability of an undetected read
error to a negligible level.
To a rough approximation, one can estimate the probability
that a random digital input will appear to the data system as a valid
message containing a non-sequential field number, thereby actuating
25 the stylus kicker. The random probability of a good start code is
1 in 213. The random probability of a good error code is also 1 in
213. The random probability of a good field number is calculated as
follows. Field numbers contain 18 bits. Since there are eight sectors
on the disc of the system under consideration, the least significant
30 3 bits of each field number indicate the sector number, which must
match the expected sector number. The remaining fifte~nbits, which
represent groove number, can vary over the allowable range (plus
or minus 63 grooves). Therefore only 126 out of 218 random field
numbers will pass the sector and range checks. Combining all
35 safeguards, the probability of an undetected error is 126 in 244.
The above estimate is based on an assumption of a truly
random input, and it does not take into account several factors which
further reduce the probability of an undetected error.
For example, on a video disc track, burst noise, where
40 the erroneous bits are adjacent to each other, is more likelv than
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other types of noise. A previously noted, the particular error code
chosen detects all single burst errors up to 13 bits, and a high
5 percentage of all longer bursts as well. Also, as previously
explained, the choice of a non-zero remainder for the error check
code (a coset code) further reduces the probability of an undetected
error. Furthermore, the particular start code chosen, a Barker code,
reduces the probability that noise will cause a false start code detection.
10The disclosed data system, as applied to the video disc
system, results in a rate of undetected errors which is relatively low
and false alarms which would otherwise cause unnecessary stylus ;
movement are significantly reduced. The data security provided by
the disclosed system improves the stability of many player functions,
15 such as display of program playing time, which depend on recorded
digital data for proper operation.
