SPECIFICATIONMethod of recording video signalsThis invention concerns a method of recordinga digitally coded video signal on magnetic tape, inparticular of the kind in which the signal isrecorded on a relatively slowly moving tape in aplurality of tracks extending at an angle to thelongitudinal direction of the tape by at least onerecording head carried at the periphery of ascanning device, referred to as a headwheel, which rotates at a high speed compared to the tape transport speed. Such a method will bereferred to hereinafter as "of the kind defined".
Patent Specification GB 2 058 433 A discloses a method of the above kind using a plurality of recording/piayback heads mounted equiangularly on the headwheel, the information content of a frame being recorded in several tracks and optimum utilization of the storage surface of the magnetic tape being achieved by providing a nonintegral number of headwheel revolutions per frame. According to one embodiment of this process there is a storage density of 25 lines per track, so that when recording television signals with 625 lines per frame the headwheel performs12.5 revolutions, whereas in the case of television signals with 525 lines per frame it performs 10.5 revolutions.
For larger bandwidth recordings, particularly digitally coded colour television signals, it is known instead of increasing the relative speed between the magnetic head or heads and the magnetic tape (which would otherwise be necessary), to subdivide the data stream to be recorded into a plurality of parallel channels and to record the information of the individual channels substantially simultaneously on a plurality of sloping, parallel tracks using a corresponding number of magnetic heads arranged on the headwheel (seeGB 2 039 126 A).
In general, the switching over between magnetic heads of a channel, particularly in the case of reproduction, takes place at the end of one track or at the beginning of the next track.
Heretofore, it was sought to put the exact switchover point in the time period between two active lines so that any switchover disturbances that might occur would not appear in the reproduced picture. For this purpose, the looping angle of the magnetic tape about the rotary body carrying the magnetic heads on its periphery was made somewhat greater than what was just necessary theoretically and in the overlap interval thus obtained, the signals of any one channel were simultaneously recorded. Both in recording and also in reproduction, a favourable moment for switchover could be obtained in this region.
For the simultaneous use of both of the methods above mentioned, namely the provision of an identical positional layout of line information in each track and the distribution of the information stream over two or more parallel channels, an equiangular disposition of the magnetic heads around the periphery of the headwheel is desirable for obtaining the simplest possible recording apparatus. Although in principle it is conceivable that an arrangement of multiple heads or of individual heads in an arrangement other than equi-angular could bring the line beginnings and the ends in the individual tracks always into the same relative position with respect to the edge of the tape, these solutions lead to undesirably complicated magnetic head or headwheel constructions.Likewise, it is conceivable, in the case of simultaneous signal processing in two or more channels and an equiangular arrangement of the magnetic heads on the headwheel, to obtain the desired track pattern with identical appearance of the individual tracks by electronic delay of the signal in at least one of the parallel channels. However, the different handling of the signals in the individual channels makes the signal processing more difficult and also introduces the danger of interference. Only when the number of heads is impractically great is it possible to use equiangularly disposed multiple magnetic heads in combination with identical signal processing in each channel.
British Patent No. 2 002 1 54 discloses a recording scheme for digital signals on magnetic tape in which individual data words are grouped together in data blocks and each block is separated from the preceding data word by a start word. In the case of television signals, a data block can include an entire television line or merely a part of one. Each block should advantageously be of the same size, i.e. it should contain the same number of data words. A similar recording scheme is disclosed in "Digital Video Recording-Some Experiments and FutureConsiderations" in SMPTE Journal, September 1980, Vol. 89, Pages 658 to 662.
According to the present invention there is provided a method of the kind defined for recording a digitally coded video signal on magnetic tape, the method comprising recording each television field in a plurality of the said tracks by retiming the video signals of the television lines of each field to subdivide the information content of each active line into at least two line segments separated by a gap, inserting a synchronizing word between the successive segments of each line, and recording the video signal thus processed on the tape in such manner at each track contains the same number of line segments whose positional relationship relative to the edges of the tape is substantially identical in each track.
The invention has the advantage that the storage surface of the tape may be used efficiently while the magnetic recording and reproduction apparatus may at the same time have a simple mechanical and electrical construction. This is because the track switchover can take place at other times than in the horizontal blanking interval, allowing the switchover points to lie essentially on an imaginary line parallel to the tape edge. Thus some tracks may begin or end at the gap between two segments of the same line, while others begin or end at the gap between complete lines.
The invention further comprehends as an embodiment thereof a system in which digital television signals are distributed in processing between, for example, two recording channels such that the signals of both channels can run through identical circuit arrangements and thereby have equal propagation times therethrough, thus practically avoiding delay equalization problems.
The invention is further described by way of illustrative example with reference to the accompanying drawings, in which:Fig. 1 is a diagram of the disposition of line segments in tracks recorded on a magnetic tape in accordance with a first embodiment of the method of the invention;Fig. 2 is a diagram of the disposition of line segments in tracks recorded on a tape in accordance with a second embodiment of the method of the invention;Fig. 3 is a block diagram of a two-channel recording system for performing the method according to the invention;Fig. 4 is a block diagram of a two-channel reproduction system for reproducing signals recorded by the system of Fig. 3;Fig. 5 is a diagram of a changeover switch circuit for receiving signals alternately from two heads picking up consecutive tracks of the same channel;Fig. 6 is a circuit diagram of a parallel-to-serial converter;;Fig. 7 is a circuit block diagram of a storing stage for time-compressing segments of a television line and inserting synchronizing words between them; andFig. 8 is a pulse diagram of the storing stage ofFig. 7.
Fig. 1 shows the beginnings of a plurality of tracks in its lower portion, and the ends of these same tracks in its upper portion, the tracks being recorded obliquely on a magnetic tape A by a method according to the invention. For spacesaving in the drawing the middle portion of the tape has been omitted and the line beginnings have been shifted to the right relative to the line ends to make the drawing more compact. In Fig.
1, seven tracks are shown on the tape A running diagonally from one longitudinal tape edge to the other. The first, second and third pairs of tracks each represent the information content of twentyfive television lines divided into two channels, and the seventh track shown represents the content, for one channel, of another twenty-five television lines. The first track of each pair of tracks comprises the first channel and is made up of segments identified by line number and a Roman numeral, while the second track of each pair comprises the second channel and is made up of segments identified by line numbers to which a prime (') has been affixed and also by a Roman numeral, a diagonal slash being used to separate the line numbers, with or without prime ('), from the Roman numerals.Between the identified segments, which are all of equal duration, are gaps referred to further below. In this example the lines in each channel are each divided into two equal time-compressed segments for protection of the integrity of the recorded information in the event of signal dropouts, as well as for purposes further described below. In Fig. 1 , for example, the content of the first television line 1 is divided into two segments separated by a gap B in which a synchronizing word is inserted, the two segments of the line 1 being respectively designated 1/l and 1/it. At the end of the first television line and before the beginning of the second is another gap in which another synchronizing word C is provided.
For handling the large quantity of information involved in the recording of digital television signals, the information flow is, as mentioned above, distributed to two channels in this example. The content of a single active television line is therefore subdivided into four track segments, each pair separated by a synchronizing word in its channel.The information storage efficiency of the track layout shown, in which each track contains the same number of complete line segments whose positions relative to the longitudinal edges of the tape are the same in each track, may be achieved using recording equipment in which a plurality of magnetic heads are spaced equiangularly around the periphery of the headwheel by an appropriate choice of relevant parameters such as the diameter of the recording and reproducing device (headwheel), the tape looping angle, the width of the tape and the tape transport speed.
Actual parameters of a recording and reproducing apparatus by which the track layout of Fig. 1 was obtained were as follows: diameter of the rotatingtransducer assembly(headwheel): 50 mm looping angle of the magnetictape about the headwheel: 1 9or (note 10 for running in and running out) width of magnetic tape: 1 inch tape transport velocity: 0.243 m/s headwheel speed: 312.5 r.p.m.
tape scan velocity (vector sum ofperipheral speed and tapetransport speed): 50 m/sThe headwheel was equipped with four equiangularly spaced magnetic heads serving both for recording and for reproduction, each diametrically opposite pair corresponding to a respective one of the two channels.
Due to the staggered engagement of successive magnetic heads with the tape, with successive heads recording in alternate channels, the information flow is so distributed between the track pairs that every other track begins with a I line segment, whereas the remainder, in this case those with the primed line number designations, are so matched to the first-mentioned alternate tracks that they begin with a II line segment.
Accordingly, the tracks designated by unprimed line numbers end with a ll line segment, while the complementary tracks end with a I line segment.
It can also be seen from Fig. 1 that the first tracks of each pair contain the information of twenty-five complete lines, while the second tracks of each pair begin and end with half a line (one line segment) of which the other half is on the next but one preceding track in the former case and on the next but one succeeding track in the latter case. As already mentioned, the relative position of the two tracks of each pair is provided by the relative position on the headwheel of the magnetic heads that record the substantially contemporary portions of the respective channels, whereas the relative position of successive tracks of the same channel is of course determined by the diametrically opposite position of the two magnetic heads of the same channel on the headwheel.
Fig. 2 is a diagram similar to Fig. 1 of the disposition of recorded track segments on a tape according to a second embodiment of the method of the invention. In this case, the digital information content of each active television line is recorded on the tape, simultaneously in each of two channels, in four successive timecompressed segments of equal duration identified by the Roman numerals I, II, Ill and IV. Here again the line segments of the second track of each pair, corresponding to the second channels, are provided with primed line numberdesignations.
As can be seen from the example, each television picture begins at the lower edge of the magnetic tape with the recording in one track of the line segments l/l and l/ll of one channel, the corresponding line segments l'/l and l'/ll of the other channel being simultaneously recorded in the middle of the immediately preceding parallel track, as can be deduced from Fig. 2. As the recording progresses, there are now simultaneously recorded in the parallel tracks the subsequent line segments I/Ill, I/IV and l'/llI, I'/IV respectively, the recording continuing in this manner with the corresponding line segments of the two tracks being recorded in staggered fashion.
The individual line segments are separated from each other by gaps in a manner similar toFig. 1 in which respective synchronizing words are inserted. In this manner it is assured that in reproduction only the remaining content of a line segment that precedes the next synchronizing word will be lost if individual bits or data words fail to come through.
For attaining the object of the invention in the present case,.i.e. that the available storage surface on the magnetic tape should be utilized as fully as possible, the tracks are so laid down that the track beginnings of pairs of tracks belonging together differ by the content of 50 quarter-line segments. The beginnings of all tracks start at the same distance from the lower edge of the magnetic tape and also end at the same distance from the 'opposite upper edge of the tape.
Furthermore, as in Fig. 1 each track begins at the beginning of a line segment and ends at the end of a line segment and all tracks contain the same number of whole line segments. As in the Fig. 1 embodiment, the efficient use of tape area exhibited by the track layout shown in Fig. 2 can be obtained using a headwheel with equiangularly disposed heads. In both cases this is made possible by the provision of the gaps between the line segments (e.g. gaps B in Fig. 1) in addition to the gaps C between the complete lines. Thus head switchover is not restricted to the horizontal or line blanking intervals, but can occur during the recording of a line in an appropriate gap such as B without disturbing the picture information.
Fig. 3 is a block diagram of signal processing circuitry for performing the above-described recording method using the four-head recording apparatus previousiy referred to. At the input 21, digital signals in 8-bit parallel form are available, as the result of the usual processing from analog signals by equipment not shown. The digital television signals first go to a processing storage unit 22 in which the television lines are timecompressed and segmented so that gaps are provided between the segments into which each line is divided and synchronizing words B inserted from a fixed memory 72 into those gaps.Gaps usually exist already between complete lines, corresponding to at least part of the blanking interval, and synchronizing words corresponding to the line synchronizing signals would in some cases already have been provided in the input data stream, but if they have not been provided they may be provided by another fixed memory 73, as the read-out from the processing store 22 goes on. The output of the processing store 22 then goes to a code converter 23 in which the 8-bit words are converted into 10-bit words, for example with the addition of parity bits, for making error detection possible. The 1 O-bit parallel output of the code converter 23 is then distributed by the electronic switch 24 to the parallel-to-serial converters 25 and 26 respectively of the two channels. Distribution is made in alternation so that the bit rate is reduced by half in both channels and of course the gaps and synchronizing words appear in both channels. In the case of the synchronizing words, this may be provided by the insertion of the synchronizing words in duplicate in the processing store 22, so that one synchronizing word will be sure to go to each channel.The serial digital data streams of the two channels are then alternately switched, in each case, at intervals of a half revolution of the headwheel of the recording device, so that in channel 1 the recording of a track by the head 29 will be succeeded without interruption by the recording of a track by the head 30, and in channel 2 the recording of a track by the head 33 between two tracks recorded for channel 1 will be succeeded without interruption by the recording of a track by the head 34. Amplifiers for driving the head and providing the necessary preemphasis for improved recordings are shown at 27 for the head 29, 28 for the head 30, 31 for the head 33 and 32 for the head 34.
Fig. 4 shows the corresponding pick-up and reproduction circuit. The heads 29, 30, 33 and 34 are shown as transducers used both in recording and in pick-up, as is common in magnetic tape apparatus. In pick-up or playback operation these heads are respectively connected to the pick-up amplifiers 40, 41, 50 and 51, which in turn go to the de-emphasis networks 42, 43, 52, 53 respectively, and then to the pulse regenerator circuits 44, 45, 54 and 55 respectively. The switches 46 and 56, respectively in channel 1 and channel 2, then alternately connect the signals of each of the heads of the channel to a serial-toparallel converter, the converter 47 in the case of channel 1 and the converter 57 in the case of channel 2.The outputs of these converters are in 1 0-bit parallel code words and these words are taken from the converters 47 and 57 in alternation for conversion in the code converter 60 into 8-bit coded words. The code converter also contains an error detection circuit of known form based on the presence of the ninth and tenth bit in each word.The 8-bit words are supplied by the code converter 60 to the processing store 61 from which the 8-bit signals are read out at a steady rate omitting the synchronizing signals B and closing up the gaps between the television line segments, and the output of the processing store 61 then goes to the error processor 62, which is designed to mask out picture points affected by error according to one or another of the known methods, for example the method disclosed in our copending patent applicationSerial No. 357,619, filed March 12, 1982. The output from the error processor 62 is then ready for digital-to-analog processing that precedes the further stages necessary for producing television pictures.
The use of the line synchronizing signals and the segment synchronizing signals for restoring proper operation after a signal drop-out and the separation of line and field synchronizing signals for use in controlling the operation of a television picture display is not shown in Figs. 3 and 4, since such circuits and functions are well known and their omission permits the essentials of the system under description to be more intelligibly depicted.
It should be mentioned, further, that the processing store 22 may be combined with the so-called shuffling store described in our copending application Serial No. 381,573, filedMay 24, 1 982, in which the sequence of the successive 8-bit words of a colour television signal is changed in accordance with the distribution pattern such that longer signal dropouts can produce no consistent disturbance. In such a case the converse unshuffling is then provided in the processing store 61 of the reproducing system.
Precise switchover between heads of a channel, particularly in pick-up for picture reproduction, is important for minimizing the size of the gaps necessary between line segments. Fig.
5 shows a suitable switch for service as the switch 46 or the switch 56 of Fig. 4. The integrated circuit 80 is, for example, of the type designation MC 10 1 74 and its numbered terminals are connected as shown, each of the two heads providing a data input and a clock input to the switch chip 80. A square wave switch pulse derived from the headwheel movement is provided to terminal 7 Qf the circuit unit 80. The only external components are the 510 ohm resistors 81 and 82 respectively running from the data and clock outputs to the terminal 8 to which -5 volts is applied.
The switches 35 and 36 of Fig. 2 have one input and two outputs but are otherwise similar to the switch of Fig. 5. The switches 24 of Fig. 3 and 59 of Fig. 4 for 10-bit parallel data can be most conveniently provided with available integrated circuit units by the use of two (or perhaps 3) ICs each.
Fig. 6 is an embodiment of the parallel-to-serial converter 25 or 26 of Fig. 3 which also permits keying-in of the synchronizing words. The 10-bit words are supplied to the parallel inputs 61 of shift register stages 62. If the input S2 of the shift register stages 62 is at L-level then this data is entered into the shift register in parallel. If S2 is atH-level, then the entered data will be read out serially. The two (upper and lower) shift register rows operate alternately. The data of that shift register row which happens to be in the state of being read out is supplied to the output of the multiplexer 63. If the input A of the multiplexer 63 is at L-level, then the data of the input X0 is switched through, and if A is at H-level, then the data of the input X, is supplied to the output.By means of the multiplexer 64 the sync words are keyed in. Their length is 20 bits. These words are programmed by means of a coding switch 65.
Instead of the coding switch 65 a PROM may also be used, which can be reprogrammed in the time between the synchronous words. For the circuit arrangement of Fig. 6 the following circuit components may be used:Shift register 62: M C 1041Multiplexer 63, 64: M C 10174Gate 66: MC 10102The above described circuit arrangement according to Fig. 6 represents an example of an alternative embodiment for keying in the synchronous words to that shown in Fig. 3.
The capacity of the memory 71 is so dimensioned that the information of two television lines can be stored. For this the store71 of a channel is divided into two equal halves 72 and 73. While write-in takes place in one storage half, read-out of the second half is carried out. Write-in is only performed during the active line, that is about 52 Ms in a 625-line television system with a totai duration of the line of 64 Ms.
Consequently only the contents of the active television line are stored and the blanking gap is suppressed. Read-out takes place continuously with the exception of two gaps per line with a duration of two word periods each. One of these two gaps lies at the beginning of a iine, the other in the middle of the line. In these two gaps the sync words are keyed in by the parallel-seriesconverter of Fig. 6. By the suppression of the blanking gap the read-out speed can be lower than the read-in speed, so that an uninterrupted data stream carrying television information arises-only interrupted by short synchronous words.
The addressing for writing in takes place by means of a write-in counter 74, which is re-set by a resetting pulse. The resetting pulse holds the counter 74 at zero for the duration of the blanking gap, that amounts to ca. 12 Ms in a 625-line television system (Fig. 8). The read-out counter 75 is held at zero for a shorter time interval (Clear 2, Fig. 8), for the time necessary for placing additional synchronous information into the signal. In addition the read-out counter 75 is stopped in the middle of each line. During this time the additional synchronous information is also keyed in.
The pulses in block "Sync 2" serve as gaps in the data flow, during which switching can take place. They serve as buffers for possible time base variations of the signals from different magnetic heads. So that the operation of the beat generators is not disturbed, these gaps have to be filled with signals. Any desired word can therefore be repeated, which is transmitted by the system.
It is advantageous, however, to use a word which is not disturbed much by intersymbol interferences and by phase shift. The duration of the block "Sync 2" depends on the time stability of the recorder and can be reduced to zero in case on the reproduction side the switching together of signals occurs not before, but only after removal of the time base variations, in a store. This, however, is connected with an increased storage effort, since in this case no common reproduction store is present for a channel, but only one for each magnetic head. The block "Sync 1" serves for the regeneration of the word synchronization during reproduction.
The keying-in of these signals "Sync 1 " and "Sync 2" occurs by means of octal 2-line-to-1- line multiplexers 76, 77, where eight inputs A are switched through to eight outputs when there is an H-signal at the control input, while the eight inputs B are switched through to eight outputs when an L-signal is present at the control input.
During the interval when pulses of the block "Sync 2" are applied to the reproduction side, switching may take place. This switching period has the duration of two word lengths.
The storage arrangement of Fig. 7 represents a different example of an embodiment for keying-in of the sync words.
Although the invention has been described with reference to a particular illustrative example, it will be understood that variations and modifications are possible within the inventive concept.