Video disc system having true and complemented digital auxiliary information codes

DAXI information, recorded in the vertical interval of a video disc signal, is augmented by program data encoded in the DAXI format but inverted prior to recording and interleaved between a blank video line and a line containing a masking signal to form a triad of lines. In a video disc player the data is recovered by comb filtering the video signal. A decoder processes the DAXI signal normally for controlling operation of the player but decodes the triad of lines as a data error when the DAXI signal is anticipated or most likely to occur. Program data in each triad is recovered when the DAXI data is not anticipated or least likely to occur by inverting the program data applied to the decoder thereby eliminating the need for a separate decoder in the player for decoding the program data and providing output data for use by an external computer in an interactive application of the video disc player.

FIELD OF THE INVENTION 
This invention relates to television systems generally and particularly to 
video disc systems of the type wherein digital auxillary information is 
recorded with video information and recovered upon playback of a disc or 
providing certain player control functions. 
BACKGROUND OF THE INVENTION 
U.S. Pat. No. 4,308,557 of C. B. Dieterich entitled "VIDEO DISC SYSTEM" 
which issued Dec. 29, 1981 incorporated herein by reference, describes 
video disc recording and playback apparatus wherein video fields on a disc 
are identified by digital information recorded during a selected line of 
the vertical interval during each field of the recorded video signal. This 
"auxillary" digital information, commonly known as the system "DAXI" code, 
is utilized to control a number of video disc player operating functions 
during playback of the disc such as calculation and display of elapsed 
playing time, lifting the pick-up stylus at the end of the program 
material, detection and correction of locked grooves, etc. 
As proposed by Dieterich, the recorded DAXI code includes, in the order 
named, a Barker start code sequence, an error detection check code and a 
plurality of information bits which include a field number corresponding 
to the recorded video field. The DAXI code format desirably provides 
improvements in noise immunity and simplifies the decoding hardware 
requirements for the player. 
Further improvements for decoding the DAXI data of the Dieterich format are 
described by Christopher in U.S. Pat. No. 4,309,721 entitled "ERROR CODING 
FOR VIDEO DISC SYSTEM" which issued Jan. 5, 1981. In Christopher's system 
the error code portion of the DAXI data is chosen so that the error code 
check register in the video disc player begins with the system start code 
in the register and, if no errors are detected after the full data message 
is received, also ends with the start code in the check register. This 
desirably simplifies the DAXI decoding logic in the video disc player. 
Other U.S. Patents relating to use of DAXI code in a video disc player are 
U.S. Pat. No. 4,307,418 entitled "VIDEO DISC PLAYER SYSTEM FOR CORRELATING 
STYLUS POSITION WITH INFORMATION PREVIOUSLY DETECTED FROM DISC" which 
issued Jan. 26, 1982, and U.S. Pat. No. 4,313,134 entitled "TRACK ERROR 
CORRECTION SYSTEM AS FOR VIDEO DISC PLAYER" which issued Jan. 26, 1982, to 
Rustman et al. 
Video disc records employing the DAXI encoding format are commercially 
available, for example, from RCA Corporation and CBS, Incorporated. In 
such records, chrominance information is recorded in the "buried 
subcarrier" (BSC) format proposed by D. Pritchard in U.S. Pat. No. 
3,872,498. The DAXI code is recorded by pulse code modulation (PCM) of the 
luminance signal level during line 17 of odd fields and line 280 of even 
fields. The DAXI data comprises a 77 bit PCM word synchronized with the 
"buried" color subcarrier frequency (about 1.53 MHz for NTSC compatible 
players) to facilitate subsequent detection in the player by comb 
filtering. Each DAXI word comprises a 13 bit start code (a Barker sequence 
for data framing) followed by a 13 bit CRC (cyclic redundancy check) error 
check code and ending with a 51 bit information code. Of the 51 bits in 
the information code, 6 provide a record band number, 18 provide a video 
field identification number and the remaining 27 are currently not 
assigned but are included to provide information capacity for future 
expansion or other uses of the DAXI code. 
In video disc players for use with DAXI encoded discs, the PCM signal is 
obtained from the subtractive output tap of a 1-H delay comb filter in the 
video signal path. Each line of DAXI code is preceeded by a blank video 
line. As a result of the subtraction of the current and previous video 
lines, the resultant PCM signal is "self-referenced" and is therefore 
relatively unaffected by D.C. drift. This simplifies subsequent PCM 
detection and reduces potential bit errors which might other wise occur 
without such drift compensation. Examples of video disc players wherein 
DAXI information is comb filtered prior to PCM detection are given in U.S. 
Pat. No. 4,275,416 of Dieterich entitled "PCM DETECTOR" and U.S. Pat. No. 
4,278,992 of Christopher entitled "PCM DETECTOR FOR VIDEO REPRODUCER 
APATUS". 
SUMMARY OF THE INVENTION 
In systems of the type described, it would be desirable to provide a source 
of digital information for use by a device external to the player (e.g., a 
computer, a game accessory, etc.) to facilitate interactive uses of the 
video disc system. Since there are currently a total of 27 "unassigned" 
information bits in the DAXI code, it would appear that these bits might 
be used to convey "interactive" program control information to the 
external device. Such an approach has the virtue of simplicity (no 
additional PCM detectors or error check decoders are needed) but, as is 
herein recognized, suffers from numerous technical and performance 
disadvantages as will now be explained. 
A first problem is that the DAXI code occurs on only one line per field. 
This corresponds to a data transfer or "through-put" rate of only 1620 
bits per second (60 fields/second times 27 bits/field). A relatively short 
interactive program of, say, 4000 eight-bit bytes would thus require 
nearly twenty seconds to read (down load) assuming no redundancy and no 
error correction. Taking these factors into consideration, it might take a 
minute or more to transfer even such a simple program to the external 
computer or other device. 
As a second example, it is recognized herein that one cannot avoid the data 
transfer rate problem by simply increasing the number of DAXI code lines 
in a given field. This is because conventional video disc players are 
designed to recognize repeated field numbers as a locked groove condition 
and to exit the locked groove condition by effecting radial translation of 
the player pickup stylus (known as stylus "kicking" or skipping"). 
Accordingly, to preserve compatibility with existing records and players 
it is required that no video field contain more than one DAXI code line. 
The alternative of recording additional auxillary digital information in a 
format other than the DAXI format is unattractive from a cost standpoint 
since additional decoding and error checking circuits would be necessary. 
It is an object of the present invention to provide a method of recording 
additional data on a video disc record which is compatible with existing 
and future video disc players which exploy DAXI or similar control systems 
which provides a relatively high data density. 
It is a further object of the invention to record the data in a format 
which may be detected and error checked by means of conventional PCM 
detectors and error check decoder but which does not interfere with normal 
operation of the player control system in existing or future video disc 
players. 
In accordance with the invention, a method for encoding data on a composite 
video signal for recording on a video disc comprises generating first and 
second data words, the first data word including video disc player control 
information, the second data word including supplementary information. The 
first data word is encoded in a format including a start code, an error 
check code and an information code to provide a first line of encoded 
video signal. The second data word is encoded in a format complementary to 
the predetermined format to provide a second line of encoded video signal. 
A line of masking data is encoded in a format different from either of the 
first and second lines of encoded video signal. The first data word is 
recorded as a pair of adjacent lines comprising a blank line and the first 
line of encoded video signal and the second data word is recorded as a 
triad of adjacent lines, the triad comprising a blank video line, the 
second line of encoded video signal and the line of masking data in the 
order named. 
A video disc player embodying the invention includes a signal recovery 
means for playing a video disc record and providing a video output signal 
having an encoded data component. A comb filter means subtractively 
combines adjacent lines of each field of said video output signal to 
separate the data component from the video signal to provide a first line 
of separated data for each pair of adjacent lines encoded in a first 
format and a second line of separated data for each triad of adjacent 
lines encoded in a second format. A logic means provides non-inverting 
signal coupling when in a first operating state and provides inverting 
signal coupling when in a second operating state. An error check decoder 
means is coupled to the comb filter means via the logic means for 
receiving the lines of data and has a first output for providing an error 
check decoded data output signal and a second ouput for providing a data 
status signal indicative of an error check result for the decoded data. A 
control means places the logic means in the first operating state for 
enabling error check decoding of each pair of adjacent lines encoded in 
the first format and places the logic means in the second operating state 
for enabling the error check decoding of each triad of adjacent lines 
encoded in the second format.

DETAILED DESCRIPTION 
The video disc mastering system of FIG. 1 includes a video tape recorder 10 
for providing a source of audio signal S1 and video signal S2 to be 
recorded. Recorder 10 is preferrably a slow motion recorded provided with 
automatic scan tracking (AST) to facilitate mastering at half rate (one 
half normal playback speed) using the skip field mastering technique 
described in detail in U.S. Pat. No. 4,277,796 of M. Ross entitled "SLOW 
DOWN COLOR PROCESSOR FOR VIDEO DISC MASTERING USING A SPECIAL MODE VTR". 
The video signal S1 is applied to a buried subcarrier (BSC) encoder 12 
which, preferably, is of the type described in the aforementioned 
Pritchard U.S. Pat. No. 3,872,498. In the BSC format chrominance 
information is represented by a color subcarrier of the general form 
employed in the well known NTSC format but is buried in a lower portion of 
the video band rather than being located in the high end of the luminance 
signal band. An illustrative subcarrier frequency choice is in the 
vicinity of 1.53 MHz with the color subcarrier sidebands extending plus or 
minus 500 KHz thereabout and with the luminance signal band extending well 
above the highest color subcarrier frequency (to 3 MHz, for example). 
The BSC encoded video signal is applied via a summing circuit 14 to a video 
slow down processor 16 which preferably is of the type described in the 
Ross patent. Briefly, in the Ross system selected fields of the redundant 
field (slow motion) video signal S3 are stored in a memory in processor 16 
at a standard clock rate and recovered at a lower clock rate that is 
inversely proportional to the number of times each field is repeated so as 
to produce a non-redundant video output signal (S4 here) of reduced 
bandwidth. The bandwidth of the audio signal S1 is reduced by the same 
factor as the video signal (one half) during operation of recorder 10 in 
the slow motion mode. The audio signal S1 and the processed video signal 
S2 are applied to respective frequency modulators 18 and 20 and the 
resultant frequency modulated signals S5 and S6, respectively, are 
combined in a summing circuit 22 to form a resultant composite recording 
signal S7 processing the full informational content of the original audio 
and video signals. 
The recording signal S7 is applied via an amplifier/equalizer 24 to the 
cutterhead 46 of a recording lathe 26. The turntable 28 of lathe 26 is 
rotated by a drive motor 30 which receives half-rate drive signals S8 
produced by processor 16 whereby full bandwidth master recordings may be 
made with a cutterhead of lesser bandwidth. Synchronization of recorder 
10, encoder 12, processor 16 and remaining elements of the mastering 
system is provided by means of a timing bus 32 supplied with timing 
signals (e.g., vertical, horizontal, color subcarrier, etc.) produced by a 
timing signal generator 34. A generator suitable for producing NTSC format 
standard video timing signal is the Tektroniz Model 146 NTSC Test Signal 
Generator. (For mastering in other formats such as or SECAM an 
appropriate generator should be used). 
The remaining elements of the mastering system encode digital data for 
recording along with the video signal S3 on lathe 26. Three data sources 
are provided, namely, a DAXI data source 50, a "program" data source 52 
and a "masking" data source 54. The sources receive timing signals from 
bus 32 and are selectively enable by control signals produced by a control 
logic unit 56 which also receives timing signals from bus 32. DAXI data 
source 50, illustratively, is of the type described in the aforementioned 
Dieterich and other patents relating to DAXI coding and provides video 
field and band numbers and other suitable information relating to the 
video signal produced by recorder 10 (e.g., bits may be assigned to 
identify monophonic or stereophonic or bilingual material, etc.). As an 
illustration, field identification numbers may be produced by source 50 by 
counting vertical field pulses provided by bus 32. Alternatively, source 
50 may be coupled to receive a SMPTE time code signal provided by recorder 
10 and "compute" the corresponding field numbers therefrom. 
Program data source 52 may comprise, illustratively, a disc memory or other 
suitable source of digital data to be recorded on the video disc for 
ultimate use by an external device coupled to a disc player. For purposes 
of illustration and explanation it will be assumed that source 52 provides 
data (when enabled by logic unit 56) in word lengths of 51 bits which 
corresponds to the number of information bits per word provided by DAXI 
data source 50. 
The masking data source 54 produces, when activated by control logic unit 
56, a 77 bit data sequence selected to correspond to a framing error 
and/or a CRC error check error when ultimately comb filtered and decoded 
in either normal or complemented (inverted) form by a standard DAXI 
decoding algorithmn. As an example, the masking data may include a start 
code which is different from and not the complement of the bit sequence 
1111100110101 which is the standard DAXI start code. In addition, or in 
the alternative, the masking data may be produced by polynomial division 
using a generator polynomial different from that described by Dieterich. 
For reasons explained in detail subsequently, the principal characteristic 
of the "masking" data is that it be unrecognizable as a valid DAXI code 
sequence in either its true (non-inverted) form or in its complemented 
(inverted) form in a video disc player having a standard DAXI decoder. 
The DAXI data (S10), program data (S11) and masking data (S12) are applied 
to a gate 58 controlled by control logic unit 56. During video lines 16 
and 278 all data sources are disabled. During video lines 17 and 279 of 
odd and even fields, respectively, gate 58 couples the DAXI signal S10 to 
a data encoder 60 which adds the Barker start sequence and error check 
code bits to the information code (S10) to provide an encoded output 
signal S13 in the standard DAXI format (77 bits as described by 
Dieterich). The DAXI encoded data S13 is coupled via an exclusive-OR gate 
62 to a level shifter 64 which translates the logic level signal S13 to a 
video voltage level to provide a PCM encoded video signal S14 to summing 
circuit 14. Gate 62 receives a low level control signal (logic zero) from 
control logic unit 56 during lines 17 and 279 whereby the DAXI code is 
recorded in its normal polarity. 
The table of FIG. 3 summarizes the recording of the DAXI code. 
Specifically, the DAXI code is recorded as a pair of video lines with each 
line of DAXI code preceeded by a blank line and with one "DAXI Pair" of 
lines recorded in each field. The reason for the blank line preceeding 
each DAXI line is that in video disc players which utilize DAXI coding, 
the code is separated from the recovered video signal by means of a comb 
filter which subtracts adjacent lines of video signal. This self 
references the recovered PCM signal as previously mentioned but requires 
that the delay line of the comb filter be "empty", so to speak, when the 
DAXI code is being read (detected) to prevent interference with previous 
contents of the delay line. 
Recording of the "program" data differs from that of the DAXI information 
in two important respects. Firstly, the fully encoded program data is 
complemented (inverted) by gate 62 prior to being applied to summing 
circuit 14. Secondly, every line of program data is immediately followed 
by a line of masking data and always preceeded by a blank line forming a 
triad of lines rather than a pair of lines as in the DAXI recording. 
In more detail, during lines 18 and 280 of the odd and even fields, 
respectively, control logic unit 56 disables sources 50, 52 and 54. These 
lines are blank lines in the vertical interval, accordingly, each 
"program" data triad beings with a blank line. During lines 19 and 281 
unit 56 enables gate 58 to couple signal S11 to encoder 60 which encodes 
the program data in the standard 77 bit DAXI format. At the same time, 
unit 56 supplies a high level control signal (S14) to exclusive-OR gate 62 
which complements the encoded program data prior to recording. 
It is instructive to note at this point that the first two lines of a 
program data triad are similar to a DAXI line pair except for the 
inversion of the encoded program data line. The program data is thus in a 
format line an "upside down" DAXI code and it may be detected and error 
checked in a standard DAXI decoder if it is reinverted prior to decoding. 
This must be done, as will be explained subsequently in connection with a 
video disc player embodying the invention, only when the true DAXI code is 
not anticipated or least likely to occur. There remains a problem, 
however, with regard to the player comb filter. Recall that the DAXI code 
is obtained from a subtractive output of the player comb filter. If the 
program data is followed by a blank line, then the player comb filter will 
itself invert the program data. If this were allowed to happen, then the 
player microprocessor would interpret the program data as DAXI data and 
would attempt to process it as if it represented a valid field number. 
Such an occurrence (misreading program data for DAXI data) would in all 
likelihood substantially impede proper player performance since field 
tracking information might be lost. 
Erroneous decoding of the complemented program data as DAXI information is 
avoided by following each line of program data with a line of masking 
data. Specifically, control logic unit 56 enables gate 58 during lines 20 
and 282 to couple the masking data signal S12 to level shifter 54. The 
masking data is thus recorded after each line of program data in a program 
data triad as illustrated in FIG. 3 (two program data triads are shown in 
the table). Since the masking data does not resemble a valid start code or 
error check code it cannot be recognized by a DAXI decoder even when 
followed or preceeded by a blank line. Also, when the program data is 
present in the comb filter delay line, the appearance of the masking data 
"scrambles" the output signal of the comb filter to insure that the 
resultant signal is undetectable by a standard DAXI decoder. 
The video disc player of FIG. 2 comprises a player mechanism 200 having a 
turntable for rotating a video disc record 202 and a pick-up transducer 
204 for recovering video signal from the disc by sensing capacitance 
variations between a stylus in transducer 204 and the disc being played. 
Data is assumed to be recorded on the disc in the format previously 
described. The output of transducer 204 is coupled to a 
capacitance-to-voltage converter 206 which provides an output signal to 
audio and video demodulators 208 and 210, respectively. The demodulated 
audio signal is applied via an audio processing unit 212 to one input of a 
TV modulator 214 and the demodulated video signal is applied to the other 
input of modulator 214 via a comb filter 216 and a video processing unit 
218. Modulator 214 provides an RF output signal inclusive of picture and 
sound components on a selected TV channel to a television receiver 222. 
Comb filter 216 comprises a 1-H delay line for delaying the demodulated 
video signal by one horizontal line, an adder 226 for adding adjacent 
lines to provide a luminance output signal Y to processing unit 218 and a 
subtractor 228 for subtracting adjacent lines and providing a chrominance 
output signal C to unit 218. The subtractor output tap also contains 
vertical detail information and the digital data encoded in the format 
previously described. 
The data is detected by means of a PCM detector 230 which receives 1.53 MHz 
clock signals (the BSC reference frequency) from an oscillator in unit 218 
and PCM data from the subtractive output of comb filter 216. Detector 230 
supplies a retimed clock signal CL to one input of an information buffer 
232 and supplies detected PCM data to another input of buffer 232 via an 
exclusive-OR gate 234. Buffer 232 includes logic circuits (as described by 
Dieterich, for example) for detectng valid start codes and for providing 
error check decoding the PCM data. When error free data is detected with 
the correct start code buffer 232 supplies a "valid data" status signal 
S17 and the data S18 to a control unit 236 (a microprocessor) which 
normally processes the data to supply an elapsed time signal to a time 
display unit 238 and stylus tracking and control signals S17 to player 
mechanism 200. Unit 236 also receives player mode control signals from a 
mode control key unit 240. 
To facilitate use of the player in interactive video disc applications, an 
output port of unit 236 is coupled via a peripherial interface adapter 
(PIA) 242 to an input/output terminal 244 coupled to an external computer 
246. Computer 246 is suitably programmed to accept the program data 
recovered from disc 202, as will be described, and to supply control data 
to the player control microprocessor for controlling selection of player 
operating modes, stylus tracking, etc. DAXI data is distinguished from the 
recorded program data with the aid of a DAXI window signal S16 produced in 
microprocessor 236 which controls the state (inverting or non-inverting) 
of exclusive-OR gate 234 as will now be described. 
Referring to FIG. 3, during lines 16-18 the window signal S16 is low. This 
corresponds to a period of time when the DAXI signal is expected and 
during this period gate 234 is placed in its non-inverting mode. 
Accordingly, the status flag is set during line 17 when the DAXI signal 
appears and the player microprocessor processes the DAXI signal normally 
to compute the elapsed playing time and field number sequence. The 
"window" signal S16, it will be noted is "wider", so to speak, than the 
DAXI signal. When the player is first placed in the play mode the window 
may be "open" for a complete field to assure acquisition of the DAXI code 
and then it may be narrowed to a few lines when the code is acquired to 
avoid false DAXI "reads". Dieterich, and others, describe suitable 
programs for providing variable data windows for DAXI code acquisition. 
In the present invention the DAXI data window controls gate 234 and 
effectively "scrambles" the program data when the player control 
microprocessor is in the DAXI acquire mode. Once the DAXI data is 
"acquired", the data window is closed (signified by a logic one in FIG. 3) 
and the exclusive-OR gate then inverts the program data on line 19, for 
example, which places the program data in the correct DAXI format. This 
done, information buffer 232 checks the validity of the program data and 
sets the status flag (S17) whereupon microprocessor 236 then outputs the 
program data to computer 246 via PIA 242. The masking data accompanying 
each triad of program data lines functions as previously described to 
prevent the comb filter from inverting the program data stored therein 
during line 19 and thus prevents the program data from being interpreted 
as valid DAXI data after DAXI acquisition.