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Recording digital signals

Info

Publication number
CA1134941A
CA1134941A CA 293748 CA293748A CA1134941A CA 1134941 A CA1134941 A CA 1134941A CA 293748 CA293748 CA 293748 CA 293748 A CA293748 A CA 293748A CA 1134941 A CA1134941 A CA 1134941A
Authority
CA
Grant status
Grant
Patent type
Prior art keywords
words
word
error
recording
signals
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA 293748
Other languages
French (fr)
Inventor
John L.E. Baldwin
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Independent Broadcasting Authority
Original Assignee
Independent Broadcasting Authority
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Grant date

Links

Classifications

    • GPHYSICS
    • G11INFORMATION STORAGE
    • G11BINFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B20/00Signal processing not specific to the method of recording or reproducing; Circuits therefor
    • G11B20/10Digital recording or reproducing
    • G11B20/18Error detection or correction; Testing, e.g. of drop-outs
    • GPHYSICS
    • G11INFORMATION STORAGE
    • G11BINFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B20/00Signal processing not specific to the method of recording or reproducing; Circuits therefor
    • G11B20/10Digital recording or reproducing
    • G11B20/18Error detection or correction; Testing, e.g. of drop-outs
    • G11B20/1806Pulse code modulation systems for audio signals
    • G11B20/1809Pulse code modulation systems for audio signals by interleaving
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N9/00Details of colour television systems
    • H04N9/79Processing of colour television signals in connection with recording
    • H04N9/80Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback
    • H04N9/808Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback involving pulse code modulation of the composite colour video-signal
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N9/00Details of colour television systems
    • H04N9/79Processing of colour television signals in connection with recording
    • H04N9/87Regeneration of colour television signals
    • H04N9/88Signal drop-out compensation
    • H04N9/888Signal drop-out compensation for signals recorded by pulse code modulation

Abstract

ABSTRACT OF THE DISCLOSURE

A method of recording digital video signals, particu-larly colour television signals, in such a manner that tape drop-outs and other errors can be corrected, comprises ar-ranging the words in a manner on a recording medium such that the information used to replace an error comes from a part of the recording medium which is statistically well separated from the error. This is achieved using a plurality of recording tracks. The signals can be transmitted as an alternative to recording. Apparatus for carrying out the method comprises a plurality of shift registers for storing the digital signals and means for reading out the signals in the desired manner.
The preferred method of error correction is to arrange signals from the same line of video raster.

Description

~:~3~

The present invention relates to transmitting or recording digital signals, and in particular to a method of and apparatus for transmitting or recording digital signals in such a manner that errors in the signals can be concealed~
When digital video signals are recorded on magnetic tape, errors such as tape "drop-outs" can occur. Drop-outs usually arise due to a speck of dust or a blemish on the tape causing the recording or play back head to separate from the tape. Due to the relative head-to-tape speeds, which are of the order of 1500 inches per second, the fre~uency of oc-currence of drop-outs, of average duration 3 microseconds, would be 10 to 100 per minute. Errors can also occur due to spurious signals.
The present invention provides a method of trans-mitting or record;ng digital signals, comprising the steps of sampling an analogue signal, converting said samples into digital words, and transmitting or recording the words such that adjacent words or word groups are well separated. The -~
present invention also provides an apparatus for carrying the -said method.
In one preferred embodiment of the present invention, there is provided a method of recording television video signals which comprises the steps of sampling an analogue signal representative of a line of video raster at a sampling frequency which is a multiple of the video subcarrier frequen-cy, converting said samples into digital words, and recording the words on a recording medium such that words representing ~-adjacent samples of the same subcarrier phase are well sepa-rated on a recording medium.
Features and advantages of the present invention will become apparent from the following description of an embodiment 3~

thereof given by way of example with reference to the accompa-nying drawings, in which:-Figure 1 shows in block diagram form apparatus for recording digital signals;
Figure 2 shows in block diagram form apparatus for recovering an analogue signal from a digita:L signal recorded using the apparatus of Figure l; and Figure 3 shows in block diagram, Eorm a portion of the apparatus of Figure 2.
A preferred embodiment of the present invention, will now be described in relation to its use as a television video signal recorder using magnetic tape. However, it should be understood that the present invention may also be used as a signal transmitter.
The digital signals are recorded in the form of words which consist of a plurality of bits, preferably 8 bits.
The words are reconstituted on play back and are checked for errors. If an error occurs, the word in question is rejected and replacement word inserted in its place. The words are re-corded in such a manner that the information used to generate a replacement word comes from parts of the tape sufficiently remove so that there is negligible correlation between the incidence of drop-outs on the parts of the tape considered.
This is what is meant by the term "well separated" which is used in the following description. For example at least two tracks may be used simultaneously to record the words, whole words being recorded on each track. Alternatively one track -~
may be used together with addition buffer storage to simulate the effect of two or more tracks by using two or more channels 30 as is disclosed in our co-pending application 307,317. The present arrangement concentrates the errors intorelatively few

- 2 - ;-~3~

words by ensuring that whole words in serial form are recorded on each track and, in addition, if an error occurs, the infor-mation used to generate a replacement word comes from another track or tracks, or from a part of the same track well sepa-rated from the ~riginal word. The preferred method of gener-ating a replacement word is to average words Df the same phase on either side oF the word in which there is an error.

. .

~34~

Referring now to Figure 1, there is shown an arrangement for recording words on two tracks on a magnetic tape. It is assumed that the sampling rate will be twice the subcarrier frequency of the television video signal although the rate could be three times the subcarrier frequency.
Figure 1 shows, in addition to an embodiment of the present invention, an embodiment of a furlher invention which is the subject of our U.K. patent No. 1,599,155 directed to reducing the d.c. component of the words to improve the clock pulse recovery may be improved. However, only those parts of Fig. 1 relevant to the present application will be described.
A video input is Eed through a comb filter 9 and is converted to a plurality of digital words constituted by a plurality of bits by means of an A/D converter 10 sampling the video signal at twice the frequency of the video subcarrier.
Each of the words is then fed via a code converter circuit 11 to each of four word latches 16a to 16d simultaneously.
However, each of the word latches is clocked at a different time to the others and so the word fed to the latches 16 is only clockea into one of them at any one time. Thus the words are separated for recombination in the desired manner.
The word latches 16 are arranged in two channels each having two latches and the clocking arrangement is such that the first word is stored in the uppermost latch 16a in Fig. l; the second word is stored in the next latch 16b; the third word is stored in the next latch 16c etc.
The two uppermost latches are connected to a serial-ising and interleaving circuit 17a which puts the second word back into series with the first in a particular manner de~
scribed in our U.K. patent mentioned above and feeds the ~rl ~3~

resultant signal to a recording head 18a. A similar procedure takes place with the two lower latches 16c and 16d. This allows the clock to be reconstituted from the data words.
Thus, the first, second, fifth, sixth, ninth, tenth, etc words are recorded using the word latches 16a, 16b and recording head 18a on to one track of a magnetic recording tape while the third, fourth, seventh, eighth, eleventh, twelfth etc words are rec~rded using the word latches 16c, 16d and recording head 18b on to another track of the s~me re-cording tape. Since a sampling rate of twice the subcarrierfre~uency is used, the first, third, fifth etc words are of the same phase of subcarrier. Likewise the second, fourth, sixth etc words are all of the same phase of subcarrier but of different phase to that of the first, third etc. words.
Playback of a signal recorded in the above manner will now be described with reference to Figs. 2 and 3.
Figure 2 shows a block diagram of play back appa-ratus and comprises two identical channels, one for each playback head. One channel deals with one track of the tape while the other channel deals with the other track.
The operation of one channel will now be described briefly and it is to be understood that the operat;on of the other channel is identical.
Locking at the lower portion of Fig. 2, the tape, head 18b produces signals from one track of the magnetic tape and these signals are fed to an equaliser circuit 21. From there the signa:Ls are fed to a clock recovery circuit 23, a start word detector 25 and a drop-out detector 27. The start word detector may be as described in our U.K. patent No.
1,599,157.

.

Assuming that the third and fourth words are picked up by the head 18b (the first and second words having been picked up by the head 18a) they are fed through the start-word detector 25 to circuits for separating the first and second words. These circuits are constituted by a demulti-plexer circuit 26 which feeds two word latches 28a, 28b in parallel. A data selector circuit 29 alternately feeds the words at the output of the latches one after another to a parity detector circuit 30 and also to a fur-ther word latch 31. The individual words are checked Eor parity by the parity detector 30, and, if correct and if no drop-outs have been detected by the drop-out detector 27, the words are stored in a data buffer 32 or 34 which are used on alternate lines of the video scan.
If a drop-out has occurred~ or if the parity check indicates an error, the doubtful word or words are not written into the data buffer thus leaving information for two lines earlier in the buffer. An error signal is stored in the appropriate error buffer 36 or 38.
2Q A data selector 39 alternately connects for a line at a time the upper data and error buffers and the lower data and error buffers on to a channel output route 40 where a further data selector 41 alternately connects for two words at a time the two channels to a further output route 42. The words are re-converted in a converter 43 and then fed to a digital to analogue converter 46, from which is produced the video signal, through a word latch 44 and error concealment circuit 45 which are shown in more detail in Fig. 3.

~, . . . ... . .

~:~3~

Referring now to Figure 3, it will be appreciated that the words fed to the latch 44 are in series, with the first two words coming from one channel the next two words from the other channel etc. Assume now that the first word is in word latch 45a, the second in latch 45b etc, the fifth word is this in word latch 44 and is simultaneously fed to one side of an averaging circuit 45e where it is averaged with the first word. The output Erom the error concealment circuit 45 to the converter 46 (Fig. 2) is taken from either the output from the word latch 45c, i.e. the third word, or the averaging circuit 45e depending on whether or not the latch 45c contains good information or information from two lines previously if the worcl which should be there contained an error it will be remembered that the data buffers 32,34 are disabled in the presence of an error leaving information from two lines before in the buffer.
The concealment circuit 45 recognises a word or an error by virtue of an output from the word latch 44. An error signal from latch 44 is clocked through two latches and is used to alter the position of a data select circuit 45f so that the output from the latch 45c is not fed to the digital to analogue converter 46 when the word in error would have reached latch 45c, rather the output from the averaging circuit is fed to the converter 46.
For example, assume that the third word has an error. The first and second words thus are clocked through the latches 45d to 45a normally and fed to the converter 46 when they reach the latch 45c. Howeverr when the third word should have reached latch 44, an error signal is present ~L~39~

additionally. At the next clock pulse, the error signal is clocked into latch 45g. The first and second words continue along the line of latches ~5d to 45a normally. On the next clock pulse the word from two lines previously is clocked into word latch 45c where it shou]d be fed via the data select circuit 45f to the converter 46. However, latch 45h is engaged, which switches over the data selector circuit 45f so that it feeds the converter 46 from the averaging circuit 45e. The averaging circuit 45e produces the average of the first word, which is now in latch 44, and the fifth word.
It will be remembered that the words are in groups of two with the first and second words being in one group being taken from one track of the magnetic recording tape while the third and fourth words are in another group, and are taken from the other track, and so on. Thus, the missing third word which was in one track is replaced by the average of the first and fifth words of the same line which were on the other track.
The above arrangement of replacing information missing from one track by information taken from another track can be altered to cater for two tracks and a sampling frequency of three times the video subcarrier fre~uency, or for three tracks and a sampling frequency three times the subcarrier frequency. In general, if sampling occurs at '_' times subcarrier frequency, a system recording 'n' successive words on one track before passing to the next track gives -satisfactory error concealment. However, with two tracks and sampling at three times to subcarrier frequency succeeding words can be recorded on alternate tracks.

i 1~34~

The replacement words are chosen so that they relate to similar positions on the subcarrier signal as this improves the concealment. Thus, when sampling is at twice the sub-carrier frequency, the replacement words are taken from positions two words away on either side of the word in error, and when sampling is at three times the subcarrier frequency, the replacement words are taken from positions three words away on either side of the word in error.
It is also possible to use the immediately adjacent words on either side of the word in error but this requires a different disposition of the words on the tape than is de-scribed above. It would be used for the reproduction of digitally coded audio signals or for digitally coded component signals, i.e. luminance and/or colour difference signals.
:;

~, g ~ '

Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. The method for recording an alternating current analog signal in a manner to alleviate drop-outs, comprising the steps of (a) sampling the analog signal at a sampling frequency thereby to produce a plurality of digital words constituted by a plurality of bits;
(b) introducing into said digital words an error detection signal (P);
(c) separating said digital words into at least two separate channels with adjacent words or, in the case of composite video signals, adjacent words of the same phase being in different channels;
(d) serializing the subgroups of words of each channel, respectively; and (e) recording on a recording medium the serialized channel signals, respectively.
2. A method according to claim 1, wherein the recording step further comprises processing the words by delaying words in one channel with respect to words representing adjacent samples or adjacent samples of the same phase in another channel.
3. A method according to claim 2, wherein the recording step further comprises combining the channels and recording them on a single track on a recording medium.
4. A method according to claim 1, wherein each channel is recorded as a separate track on a recording medium.
5. The method as defined in claim 1, wherein the analog signal represents a line of a video raster.
6. A method according to claim 1 wherein the analog signal has a given subcarrier frequency and the sampling is at 'n' time the subcarrier frequency, and wherein the information for replacing the word in error is derived from words of the same line that are 'n' displaced before and after the word in error.
7. A method according to claim 6, and wherein sampling occurs at two times subcarrier frequency
8. A method according to claim 6, wherein there are two channels each recorded as a separate track on a recording medium and wherein sampling occurs at 'n' times subcarrier frequency and 'n' successive words are recorded on a first track before a further 'n' successive words are recorded on the other track.
9. A method according to claim 6, wherein there are two channels each recorded as a separate track on a recording medium and wherein sampling occurs at three times subcarrier frequency, succeeding words being recorded on alternate tracks.
10. A method of correcting errors in digital video signals recorded by the method according to claim 5, and comprising detecting a word in error and replacing that word using information derived from at least one digital word from the same video line as the word in error.
11. A method according to claim 10, wherein when a word in error is detected a corresponding word from a previous line is retained in a memory.
12. A method according to claim 11, wherein the corresponding word from two lines previously is retained.
13. The method as defined in claim 10, and further including the steps of (f) detecting the individual words of each channel recorded on said recording medium, respectively;
(g) separating the words of each channel, respectively;
(h) examining the error detection signals of the words of each channel to detect an error therein;
(i) storing in data storage means for each channel the correct words that do not contain an error;
(j) storing in error storage means for each channel the words that contain errors; and (k) generating, in response to the correct words stored in said data storage means, a replacement word for a given word in error.
14. A method according to claim 13, wherein said at least one digital word is from the same line of video.
15. A method according to claim 13, wherein said at least one digital word is from a previous line of video.
16. Apparatus for recording an alternating current analog signal in a manner to alleviate drop-outs, said signal having a given subcarrier frequency (fsc), comprising (a) means including an analog to digital converter for sampling the analog signal at a sampling frequency which is a multiple of the sub-carrier frequency (fsc), and for producing a plurality of digital words constituted by a plurality of bits;

(b) means for introducing into said digital words an error detection signal (P);
(c) means for storing groups of said digital words in at least two separate channels, respectively, a first group of words of one channel including first and second subgroups of words of different phase relative to each other, and a second group of words of the other channel including third and fourth sub-groups of words of different phase relative to each other;
(d) at least two parallel to series converter means for serializing the subgroups of words of each channel, respectively; and (e) at least two recording devices for recording on a recording medium -the serialized channel signals from said parallel to series converter means, respectively.
17. Apparatus as defined in claim 16, and further including:
(f) means for detecting the individual words of each channel recorded on said recording medium, respectively;
(g) means for separating the words of each channel, respectively;

(h) error detector means for each channel for examining the error detection signals of the words to detect an error therein;
(i) data storage means for each channel for storing those words that do not contain an error;
(j) error storage means for each channel for storing the words that contain errors; and (k) error concealing means responsive to the correct words stored in said data storage means for generating a replacement word for a given word in error.
18. Apparatus as defined in claim 17, and further including data selector means for supplying to said error concealing means in series the words contained in said storage means, said error concealing means including an averaging means, and word latch means for feeding words successively to said averaging means at a frequency which is the same as the sampling frequency of the analog signals.
CA 293748 1976-12-24 1977-12-22 Recording digital signals Expired CA1134941A (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
GB5416376A GB1599156A (en) 1976-12-24 1976-12-24 Recording digital signals
GB54163/76 1976-12-24
US86400277 true 1977-12-23 1977-12-23
US06046267 US4277807A (en) 1976-12-24 1979-06-07 Recording digital signals

Publications (1)

Publication Number Publication Date
CA1134941A true CA1134941A (en) 1982-11-02

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Family Applications (1)

Application Number Title Priority Date Filing Date
CA 293748 Expired CA1134941A (en) 1976-12-24 1977-12-22 Recording digital signals

Country Status (7)

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US (1) US4277807A (en)
JP (1) JPH0262914B2 (en)
CA (1) CA1134941A (en)
DE (1) DE2757165C2 (en)
FR (1) FR2375779B1 (en)
GB (1) GB1599156A (en)
NL (1) NL7714388A (en)

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Also Published As

Publication number Publication date Type
DE2757165C2 (en) 1986-11-27 grant
JP1648770C (en) grant
CA1134941A1 (en) grant
FR2375779A1 (en) 1978-07-21 application
JPS53114413A (en) 1978-10-05 application
FR2375779B1 (en) 1984-10-26 grant
NL7714388A (en) 1978-06-27 application
JPH0262914B2 (en) 1990-12-26 grant
US4277807A (en) 1981-07-07 grant
DE2757165A1 (en) 1978-07-13 application
GB1599156A (en) 1981-09-30 application

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