Digital signal recording and reproducing apparatus.

This invention relates to digital signal recording and reproducing apparatus.

Digital signal recording apparatus has been developed for recording digital signals representative of different types of information. One particular application thereof is to record a video signal in digital form on a moving record medium, such as a magnetic tape, by using a rotary head assembly. In this application, the digital video signals may be of different types, each type having a different transmission rate, such as an extended play video signal, a normal play video signal having a transmission rate higher than the extended play video signal, and a high definition television (HDTV) video signal having a still higher transmission rate. Notwithstanding these different transmission rates, it is desirable to record such digital video signals at a substantially constant, uniform data rate, thereby simplifying the mechanical assemblies and components, as well as the control electronics used with such recorders. One example of a digital signal recording system operable with video signals having different transmission rates is described in Japanese laid open patent specification 1/258255.

As described in the aforementioned patent specification, the transmission rates of the extended play, normal play and HDTV signals are in a predetermined ratio, such as the ratio 1:2:4; and the apparatus is operable in an extended play mode, a normal play mode or an HDTV mode to record either the extended play, the normal play or the HDTV signals. To record these different signals, which are selectively supplied, a rotary head assembly comprising at least two 180° spaced apart sub-assemblies is used, with each sub-assembly having two heads displaced from each other in the direction parallel to the pitch of the tracks recorded by those heads. The video signals which are selectively supplied to the rotary head assembly are timebase-compressed to produce recording signals of the same data rate. For example, the extended play digital video signals are timebase compressed to reduce the period thereof to ¼, the normal play digital video signals are timebase compressed to reduce the period thereof to ½ and the HDTV digital video signals are not timebase compressed. This has the effect of increasing the effective transmission rate of the extended play digital video signals by a factor of four and increasing the effective transmission rate of the normal play digital video signals by a factor of two, so that the effective transmission rates of the extended play, normal play and HDTV digital video signals all are equal to the transmission rate of the HDTV signals.

In the aforementioned patent specification, the extended play digital video signals are recorded in two tracks substantially simultaneously by one head sub-assembly while the rotary head assembly makes two complete rotations. The normal play digital video signals are recorded in two tracks substantially simultaneously by one head sub-assembly while the rotary head assembly makes only one complete rotation. Finally, the HDTV digital video signals are recorded in four tracks by both head sub-assemblies (one sub-assembly at a time) while the rotary head assembly makes one complete rotation. To achieve this, the magnetic tape on which these tracks are recorded are transported at different speeds during the extended play, normal play and HDTV modes, these different speeds being in the ratio 1:2:4.

In this apparatus, since the signals that are recorded are processed to have the same data rate for the extended play, normal play and HDTV modes, even although the original transmission rates of these signals differ from each other, separate recording electronics for each mode need not be provided. This permits the recording circuitry to be relatively simplified; and the overall reliability of the apparatus is improved. Furthermore, since the rotational speed of the rotary head assembly is constant and need not be changed from one mode to another, the mechanical elements and construction of the assembly likewise may be simplified, thus providing better overall reliability. Moreover, the video signals which are recorded on the magnetic medium are recorded with a substantially constant recording density and, therefore, the recording medium may be used more effectively.

To increase the number of tracks recorded on the magnetic medium, it is desirable to reduce the width of each track which, of course, increases the recording density and, thus, increases the amount of information that may be recorded on a magnetic tape of given length. However, it has been believed heretofore that, if the track width is reduced, special tracking systems must be employed to ensure accurate tracking of the individual heads during a playback mode. Thus, it had been thought that ATF, DTF (with a pilot signal) and other tracking techniques must be used. Consistent with this believe that special tracking systems must be provided, it also had been thought that the linearity of each track being recorded must be strictly maintained. This, in turn, requires the rotary head assembly and drive mechanism to be manufactured with limited tolerances and high accuracies. However, the requirement of special tracking control circuits and precise rotary mechanisms results in high manufacturing costs and, in addition, presents a serious risk that the reliability and operating stability of the overall apparatus may face substantial degradation.

According to the present invention there is provided digital signal recording and reproducing apparatus for recording digital signals in successive tracks of given pitch on a record medium and for reproducing digital signals therefrom, the apparatus comprising:    rotary head means including first and second head assemblies angularly separated by approximately 180° and each comprising a pair of heads displaced from each other in a direction parallel to the track pitch;    head drive means for rotating said rotary head means at a predetermined rotation speed such that each head assembly makes a pass across said record medium during one rotation of said rotary head means;    input means for selectively supplying digital signals at respectively different original transmission rates having a predetermined ratio;    timebase processing means for selectively modifying the timebase of the supplied digital signals to produce timebase-modified digital signals having substantially the same data rate even although the original transmission rates of the selectively supplied digital signals may differ from each other;    coupling means for coupling the timebase-modified digital signals to said rotary head means;    medium drive means operable during a recording mode for driving said record medium at respective different speeds corresponding to the original transmission rate of the digital signal selectively supplied by said input means, whereby the number of tracks recorded on said record medium during a unit time period TP (where TP is the time for said rotary head means to make a predetermined number of rotations) corresponds to said original transmission rate, said medium drive means being operable during a reproducing mode for driving said record medium at respective different speeds corresponding to said original transmission rate of the digital signals to be reproduced, whereby during said time period TP, said rotary head means reproduces digital signals from twice the number of tracks than had been recorded during said time period TP;    memory means for storing the digital signals reproduced by said rotary head means; and    recovery means coupled to said memory means for recovering therefrom the digital signals at the original transmission rate.

According to the present invention there is also provided digital signal reproducing apparatus for reproducing from successive tracks of given track pitch on a record medium digital signals that had different original transmission rates and that have been recorded in a number of tracks during a unit time period TP, said number differing with the original transmission rate of the digital signals, said apparatus comprising: rotary head means including first and second head assemblies angularly separated by approximately 180° and each comprising a pair of heads displaced from each other in a direction parallel to the track pitch for reproducing digital signals from said tracks; head drive means for rotating said rotary head means at a predetermined rotary speed such that said unit time period TP is the time for said rotary head means to make a predetermined number of rotations; medium drive means for driving said record medium at respectively different speeds corresponding to the original transmission rate of the digital signals to be reproduced, whereby during said time period TP, said rotary head means reproduces digital signals from twice the number of tracks than had been recorded during said time period TP; memory means for storing the digital signals reproduced by said rotary head means; and recovery means coupled to said memory means for recovering therefrom the digital signals at the original transmission rate.

An embodiment of the invention can then provide digital signal recording and reproducing apparatus for use with digital signals selectively of differing transmission rates, wherein those signals are recorded in tracks of reduced width and wherein a tracking servo operation is not needed for good reproduction.

An embodiment of the invention may comprise digital signal recording and reproducing apparatus for recording digital signals in successive tracks of given pitch on a record medium and for reproducing digital signals from those tracks. The apparatus includes a rotary head assembly comprising first and second head sub-assemblies which are angularly separated from each other by about 180°, each sub-assembly including a pair of heads displaced from each other in a direction parallel to the track pitch, with the rotary head assembly being rotated at a predetermined rotation speed. Digital signals having respectively different original transmission rates which have a predetermined ratio are selectively supplied to the apparatus. A timebase processor modifies the timebase of the supplied digital signals to produce timebase-modified digital signals having substantially the same data rate even although the original transmission rates of the selectively supplied digital signals may differ from each other. These timebase-modified digital signals are coupled to the rotary head assembly for recording on the record medium which is driven at respectively different speeds depending upon the original transmission rate of the digital signal that is supplied. During a recording operation, the number of tracks recorded on the record medium during a unit time period TP (where TP is time for the rotary head assembly to make a predetermined number of rotations) is a function of the original transmission rate. During a reproducing mode, the record medium is driven at respectively different speeds corresponding to those used for recording the digital signals, whereby during the time period TP, the rotary head assembly reproduces digital signals from twice the number of tracks that were recorded during the time period TP. The reproduced digital signals are recovered, by way of a memory, to minimize defects and reproduce the original transmission rate.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: <ul id="ul0001" list-style="none"><li>Figure 1, which is formed of Figures 1A and 1B, is a block diagram of recording and reproducing apparatus which embodies the present invention;</li><li>Figure 2 is a schematic representation of a head sub-assembly used in the apparatus of Figure 1;</li><li>Figures 3A to 3C are schematic representations of track patterns which are recorded with input digital signals having differing original transmission rates; and</li><li>Figures 4A and 4B are schematic representations of the positions of the playback heads relative to the tracks in which are recorded different types of digital signals.</li></ul>

Referring now to the drawings, Figure 1A is a block diagram of digital signal recording apparatus with which the present invention finds ready application. Input terminals 1N and 1H receive video signals, such as analogue NTSC signals, HDTV signals, or the like. In the preferred embodiment, normal video signals, such as NTSC video signals, are supplied to the input terminal IN and HDTV video signals are supplied to the input terminal 1H. The input terminal 1N and 1H are selectively coupled to an analogue-to-digital (A/D) converter 3 by way of a selector switch 2. Depending upon the operating mode of the illustrated apparatus, either the input terminal 1N or the input terminal 1H is coupled to the A/D converter 3. A selector control signal is coupled to the switch 2 from a mode switching control circuit 4, which may comprise a microprocessor or other controlling circuit, to establish the condition of the switch 2. It will be appreciated from the forthcoming description that the illustrated apparatus operates in a normal play recording mode, wherein normal video signals (such as an NTSC signal) are received and recorded in a "normal" recording mode, in an extended play recording mode wherein a normal video signal, such as an NTSC signal, is recorded in a reduced length of record medium such that a smaller amount of record medium is needed to record a video signal of given duration, and in an HDTV recording mode wherein an HDTV video signal is received and recorded on the record medium. As already described, the preferred record medium is magnetic tape.

The selector control signal produced by the mode switching control circuit 4 disposes the switch 2 in a condition to couple the input terminal 1N to the A/D converter 3 during the normal play recording and extended play recording modes, whereas the switch 2 couples the input terminal 1H to the A/D converter 3 during the HDTV recording mode. It will be recognized that the transmission rates of the respective extended play, normal play and HDTV video signals differ from each other. Typically, these transmission rates are in the ratio 1:2:4, wherein the transmission rate of the extended play video signal is less than the transmission rate of the normal play video signal which, in turn, is less than the transmission rate of the HDTV video signal. The A/D converter 3 samples the input video signal selectively supplied thereto by the switch 2 at respectively different sampling rates, depending upon the transmission rate of the input video signal, that is, depending upon the particular recording mode of the apparatus. The sampling rate of the A/D converter 3 is established by the mode switching control circuit 4 to be compatible with the transmission rate of the video signal supplied thereto, or the type of recording mode to be carried out. Thus, the A/D converter 3 digitizes the input video signal to produce digital video signals of respectively different transmission rates. For the purpose of the present discussion, the A/D converter 3 operates to produce extended play, normal play and HDTV digital video signals.

It is preferred to perform digital compression on the digitized video signal produced by the A/D converter 3. Accordingly, a bit reduction circuit 5 is coupled to the A/D converter 3 to reduce the number of bits included in the digital video signal. The bit-reduced digital signals are coupled, in turn, to an error correction encoder 6 which, for example, adds a parity bit to the bit-reduced digital signal. Other error correction encoding techniques may be used, if desired. Nevertheless, the transmission rate of the encoded digital signal depends upon the recording mode of the apparatus, as represented by the control signal produced by the mode switching control circuit 4. In the preferred embodiment, the extended play, normal play and HDTV digital video signals supplied from the encoder 6 have transmission rates in the ratio 1:2:4. Thus, the transmission rate of the HDTV digital video signal is four times the transmission rate of the extended play digital video signal, and the transmission rate of the normal play digital video signal is twice that of the extended play digital video signal.

The encoded digital signal produced by the encoder 6, and having a respective transmission rate as aforementioned, is timebase compressed by a timebase processing circuit 7 coupled to the encoder 6. Thus, the period of the particular digital signal supplied to the timebase processing circuit 7 is reduced by a factor determined by the original transmission rate thereof, thereby producing digital signals of substantially the same, constant data rate. In the preferred embodiment, the period of the extended play digital video signal is timebase compressed to a period ¼ of its original period (it is compressed by a factor of four), the normal play digital video signal is timebase compressed to a period that is ½ its original period (it is timebase compressed by a factor of two) and the period of the HDTV digital video signal is not reduced. As a result, the effective data rate of the extended play digital video signal is increased from an original transmission rate of ¼ the HDTV transmission rate so as to be equal to the HDTV transmission rate; and, similarly, the effective data rate of the normal play digital video signal is increased from ½ the transmission rate of the HDTV digital video signal to a rate that also is equal to the HDTV transmission rate. Consequently, the timebase processing circuit 7 functions to provide digital video signals of substantially uniform, constant data rate. Accordingly, notwithstanding the particular recording mode, the digital signals produced by the timebase processing circuit 7 all have substantially the same data rate.

The timebase-compressed digital signals produced by the timebase processing circuit 7 are supplied to recording circuits 8a and 8b, alternately by, for example, field or other interval to be recorded in a record track. Each recording circuit 8a and 8b may be of substantially identical construction, and each converts the digital signal supplied thereto to a recording code used in digital signal recorders for recording on, for example, a magnetic medium, such as magnetic tape T. The signals produced by the recording circuit 8a and 8b are referred to as recording signals a and b, respectively, and the recording signals a and b are amplified by recording amplifiers 9a and 9b, respectively, from which they are supplied to a rotary head assembly 11 by way of recording/reproducing switches 10 for recording on the tape T. The rotary head assembly 11 is illustrated as comprising a pair of head sub-assemblies, each sub-assembly being shown more particularly in Figure 2. In one embodiment, each sub-assembly comprises a pair of recording/reproducing heads Ha and Hb having different azimuth angles. The two sub-assemblies are angularly disposed from each other by 180° and are rotated by a drive motor 12 coupled thereto.

As shown in Figure 2, the heads Ha and Hb are displaced from each other in a direction parallel to the pitch of P of the tracks recorded thereby. Stated otherwise, the heads Ha and Hb have a track pitch offset P. In addition, the heads Ha and Hb are displaced from each other in the track scanning direction by an amount GL. This displacement GL enables both the heads Ha and Hb to commence the scanning of adjacent tracks substantially simultaneously, even although the tracks themselves are longitudinally displaced from each other, as shown in Figures 3A to 3C. In any event, it will be appreciated that each pair of heads Ha and Hb makes a pass across the tape T during one rotation of the rotary head assembly 11. For convenience, the heads Ha and Hb included in one sub-assembly are identified as heads Ha₁ and Hb₁, and the heads included in the other sub-assembly are identified as heads Ha₂ and Hb₂.

The rotary head assembly 11 is rotatably driven by the drive motor 12 at a substantially constant rate, notwithstanding the different recording modes with which the apparatus may be used and the different original transmission rates of the input digital signals supplied by the A/D converter 3. However, the tape T is driven about a guide drum by a capstan 13 which, in turn, is driven by a tape drive motor 14 whose speed is determined by the mode switching control circuit 4. More particularly, the tape drive motor 14 drives the tape T at speeds in the ratio 1:2:4 during extended play, normal play and HDTV recording modes, respectively. Although the data rates of the recording signals a and b produced by the recording circuits 8a and 8b are substantially the same for all recording modes, the density with which the digital video signals are recorded varies as a function of the original transmission rate thereof. Thus, during an HDTV recording mode, the tape T is driven at four times the speed used to record an extended play digital video signal; and during a normal play recording mode, the tape T is driven at twice the speed used to record the extended play digital video signals. It is seen, then, that the transport speed of the tape T varies in the ratio 1:2:4 to record extended play, normal play and HDTV digital video signals, respectively.

If a unit time period TP represents the time for the rotary head assembly 11 to make two complete rotations, during the extended play recording mode two tracks of the digital video signals are recorded by, for example, the heads Ha₁ and Hb₁ during only one pass across the tape T during the unit time period TP. During the normal play recording mode, two tracks of digital video signals are recorded by, for example, the Ha₁ and Hb₁ during each rotation of the rotary head assemblies, resulting in the recording of four tracks during the unit time period TP; and during the HDTV recording mode, two tracks are recorded by the heads Ha₁ and Hb₁, followed by two tracks recorded by the heads Ha₂ and Hb₂ during one rotation of the head assembly, resulting in the recording of eight tracks during the unit time period TP. That is, during the extended play recording mode, only one pair of heads records digital video signals during alternate rotations of the rotary head assembly; during the normal play recording mode, only one pair of heads records digital video signals, but these are recorded during each rotation of the rotary head assembly; and during the HDTV recording mode, each pair of heads records a respective pair of tracks of digital video signals during each rotation of the rotary head assembly. Hence, during the unit time period TP (during two rotations of the rotary head assembly), two tracks of extended play digital video signals, four tracks of normal play digital video signals, or eight tracks of HDTV digital video signals are recorded. It is appreciated, then, that during a recording mode, the number of tracks which are recorded during the unit time period TP corresponds to the original transmission rate of the digital signals, and this number of tracks has the ratio 1:2:4.

In the preferred embodiment, the tape T is wrapped about a rotary head drum over an angular extent of about 180°. In this arrangement, only one pair of heads at a time is in magnetic contact with the tape T and, thus, only one pair of heads at a time operates to record digital signals.

The track forming operation in the different recording modes is best appreciated by referring to Figures 3A to 3C. Figure 3A represents the tracks which are recorded during the extended play recording mode. In this mode, the tape drive motor 14 operates to transport the tape T at a speed such that it is advanced by an amount equal to one track pitch P during the time that the rotary head assembly makes one complete rotation. Accordingly, if it is assumed that the head Ha₁ records track a₁ and the head Hb₁ records track b₁, then, during the first pass of these heads across the tape T, the tracks a₁ and b₁ are recorded. As mentioned above, no signals are supplied to the heads Ha₂ and Hb₂ when these heads pass across the tape T. Now, when the heads Ha₁ and Hb₁ rotate to make the next pass across the tape T, the head Ha₁ is aligned with the track b₁. To prevent the digital signals recorded in the track b₁ from being overwritten, digital signals are inhibited (not shown) from being supplied to these heads during this rotation. Hence, digital signals are supplied to only one pair of heads (assumed herein to be heads Ha₁ and Hb₁) only during alternate rotations of the rotary head assembly 11. Therefore, only two tracks a₁ and b₁ are recorded (with track pitch P) during the unit time period TP (where TP is the time for the rotary head assembly 11 to make two complete rotations).

During the normal play recording mode, let it be assumed that the heads Ha₁ and Hb₁ record the tracks a₁ and b₁, respectively, on the tape T, as shown in Figure 3B. The mode switching control circuit 4 controls the tape drive motor 14 to drive the tape T at twice the extended play transport speed. Accordingly, during one pass of the heads Ha₁ and Hb₁ across the tape T, the tracks a₁ and b₁ are respectively recorded thereby. At the end of this pass, the heads Ha₂ and Hb₂ begin to pass across the tape T and the recording of digital signals by these heads is inhibited. Since the tape T is transported at twice the extended play transport speed, the tape T is advanced by a distance equal to 2P during the time for one complete rotation of the rotary head assembly 11. Hence, when the heads Ha₁ and Hb₁ commence their next pass across the tape T, it is sufficiently advanced to permit the next pair of tracks to be recorded. As shown in Figure 3B, the heads Ha₁ and Hb₁ record successive pairs of tracks a₁ and b₁, respectively, at each rotation of the rotary head assembly 11. Accordingly, during the unit time period TP (equal to two rotations of the rotary head assembly), four tracks a₁, b₁, a₁, b₁ are recorded.

During the HDTV recording mode, the mode switching control circuit 4 controls the tape drive motor 14 to advance the magnetic tape at four times the extended play transport speed. Hence, it is transported a distance equal to 4P for each complete rotation of the rotary head assembly 11. Accordingly, when the heads Ha₁ and Hb₁ make one pass across the tape T, the tracks a₁ and b₁ are respectively recorded thereby. At the end of this pass, the heads Ha₂ and Hb₂ commence a pass across the tape T, and these heads record the tracks a₂ and b₂, respectively. Since the tape T is transported at a speed which moves it at a distance 4P, these four tracks are recorded in succession, as shown in Figure 3C. Therefore, for each complete rotation of the rotary head assembly 11, the tracks a₁, b₁, a₂ and b₂ are recorded by the heads Ha₁, Hb₁, Ha₂ and Hb₂, respectively. This operation is repeated for each rotation of the rotary head assembly 11. It will, therefore, be seen from Figure 3C that during the unit time period TP (where TP is the time for the rotary head assembly to make two complete rotations), eight successive record tracks are recorded.

To represent the different tape transport speeds, Figures 3A to 3C illustrate, in broken line, the trace of a head sub-assembly when the tape T is stopped, thereby recording and reproducing a still image. This trace is referred to as the "still angle", and it is seen that the angle at which the tracks are recorded relative to the "still angle" increases as the tape transport speed increases. It is also seen that as the tape transport speed increases while the rotary head speed remains constant, the number of tracks which may be recorded during the unit time period TP also increases.

Turning now to Figure 1B, there is illustrated a block diagram of digital signal reproducing apparatus. For convenience, it is assumed that the same rotary head assembly 11 that was used to record the digital signals on the tape T is used to reproduce those signals. It will be appreciated, however, that a different assembly, such as that found in a different machine, may be used to reproduce these previously recorded digital signals. Accordingly, the sub-assemblies comprising the heads Ha₁, Hb₁ and Ha₂, Hb₂ are rotatably driven by the drive motor 12 to scan the tracks having the pattern shown, for example, in Figure 3A or 3B or 3C. The signals reproduced by these heads are coupled by way of the recording/reproducing switches 10 to playback amplifiers 15a₁, 15a₂, 15b₁ and 15b₂, respectively. As shown in Figure 1A, when the recording/reproducing switches 10 are disposed in the reproducing mode, digital signals picked up from the tape T by the head Ha₁ are coupled by the switch 10a₁ to the playback amplifier 15a₁. Similarly, digital signals picked up by the head Hb₁ are coupled by the switch 10b₁ to the playback amplifier 15b₁, digital signals picked up by the head Ha₂ are coupled by the switch 10a₂ to the playback amplifier 15a₂, and digital signals picked up by the head Hb₂ are coupled by the switch 10b₂ to the playback amplifier 15b₂. Thus, the digital signals picked up by the respective heads are amplified by individual playback amplifiers. Preferably, each amplifier exhibits an equalizing characteristic to provide equalization to the reproduced digital signals.

The amplifiers 15a₁ and 15a₂ are coupled to a memory 16a and the amplifiers 15b₁ and 15b₂ are coupled to a memory 16b. The capacity of each of the memories 16a and 16b is sufficient to store a field interval of digital video signals and, preferably, at least one frame of digital video signals is stored thereby. In the event that portions of the reproduced digital video signals are defective, as may be caused by drop-out, interference, or the like, such defective portions may be replaced by corresponding portions in the next scan of the heads across the tape T, or in the next field or frame reproduced from the tape T. Thus, defects are compensated by information recovered during the next scan or the next field or frame. The inherent redundancy of television signals makes such compensation from the next field or frame substantially unnoticeable.

The digital video signals stored in the memories 16a and 16b, including the compensated digital signals, are read therefrom to signal detecting circuits 17a and 17b, respectively. The signal detecting circuits 17a and 17b operate to detect drop-outs and defects and cooperate with the memories 16a and 16b to effect the aforementioned compensation. The outputs from the signal detecting circuits 17a and 17b are seen to be similar to the digital video signals a and b produced by the reproducing circuits 8a and 8b of Figure 1A, and these digital video signals are supplied to reproducing circuits 18a and 18b, respectively, whereat the recording code that had been used to record the digital signals on the tape T is decoded. Thus, these signals produced by the reproducing circuits 18a and 18b are in a form similar to that which were supplied to the recording circuits 8a and 8b during the recording operation.

From the description of Figure 1A, it will be appreciated that the signals reproduced by the reproducing circuit 18a are timebase-compressed and, depending upon whether extended play, normal play or HDTV digital video signals are being reproduced, such signals are recovered from the tracks a₁ (as shown in Figures 3A and 3B) or from the tracks a₁ and a₂ (as shown in Figure 3C). Similarly, the signals reproduced by the reproducing circuit 18b are timebase-compressed and, depending upon whether the apparatus is operated in an extended play, normal play or HDTV reproducing mode, such signals are recovered from tracks b₁ (as shown in Figures 3A and 3B) or from the tracks b₁ and b₂ (as shown in Figure 3C). These digital signals are supplied by the reproducing circuits 18a and 18b to a timebase processing circuit 19 which operates to restore the original period of the digital signals that had been recorded. For example, the period of the extended play digital video signals is expanded by a factor of four, the period of the normal play digital video signals is expanded by a factor of two, and the period of the HDTV digital video signals is expanded by a factor of one (it is not modified). Hence, whereas the original transmission rates had the ratio 1:2:4, the timebase-expansion so these digital signals is in the ratio 4:2:1.

In addition, the timebase processing circuit 19 serves to multiplex the digital signals recovered by the heads Ha₁ and Hb₁ as well as those recovered by the heads Ha₂ and Hb₂ to produce a single channel of digital signals. It is seen that the timebase processing circuit 19 is coupled to the mode switching control circuit 4 to have the timebase-expansion factor thereof determined by the operating mode selected by the mode switching control circuit 4. Hence, when the reproducing apparatus operates in the extended play reproducing mode, the timebase processing circuit 19 is controlled to expand the period of the digital signals supplied thereto by a factor of four. Analogously, when the reproducing apparatus operates in the normal play mode, the timebase processing circuit 19 is controlled to expand the period of the digital signals supplied thereto by a factor of two. Preferably, when the apparatus operates in the HDTV mode, the timebase processing circuit 19 does not modify the period of the digital signals.

It will be seen that the single channel digital signal produced by the timebase processing circuit 19 has a transmission rate substantially equal to the original transmission rate at which that signal had been supplied for recording. In the example discussed herein, the original transmission rates has the ratio 1:2:4, wherein the transmission rate of the normal play digital signal is twice that of the extended play digital video signal, and the transmission rate of the HDTV digital video signal is four times that of the extended play digital video signal. The digital signals having restored original transmission rates are supplied from the timebase processing circuit 19 to an error correction decoder 20 which carries out an error correction decoder 20 which carries out an error correction operation based upon the parity bit added to the digital signal by the error correction encoder 6 (Figure 1A). More generally, the decoder 20 is compatible with the error correction encoding technique used by the encoder 6 and operates to correct, or remove, errors that may be present in the reproduced digital signal.

The error corrected digital signal produced by the encoder 20 is coupled to a bit restoration circuit 21 which is compatible with the bit reduction circuit 5. It is recalled that the bit reduction circuit 5 functioned to perform digital compression by reducing the number of bits included in the digital video signal. The bit restoration circuit 21 operates in a reciprocal manner to restore those bits which had been reduced. Thus, the bit restoration circuit 21 may be thought of as performing digital expansion; and this expansion is controlled by the mode switching control circuit 4 as a function of the particular reproducing mode in which the apparatus operates. In one embodiment, the extended play, normal play and HDTV digital signals are uniquely digitally expanded by the bit restoration circuit.

The bit-expanded digital signals are converted from digital form to analogue form by a digital-to-analogue (D/A) converter 22. The rate at which the D/A converter 22 operates is controlled by the switching control circuit 4 and varies as a function of the reproducing mode of the illustrated apparatus. For example, each of the expanded play, normal play and HDTV digital video signals is converted to analogue form at a respectively different rate, depending upon the original transmission rate of those signals. The resultant analogue signals are supplied to one of two output terminals 24N and 24H by a selector switch 23. The selector switch 23 is controlled by the mode switching control circuit 4 and operates to couple the analogue output from the D/A converter 22 to the output terminal 24N when the reproducing apparatus operates in the extended play or normal play mode; whereas the analogue signals produced by the D/A converter 22 are coupled by the selector switch 23 to the output terminal 24H when the apparatus operates in the HDTV mode. Thus, normal video signals, such as an NTSC video signal, whether recorded in normal or extended play modes, are coupled to the output terminal 24N; whereas HDTV video signals are coupled to the output terminal 24H. Stated more generally, video signals having a relatively lower transmission rate are coupled by the selector switch 23 to the output terminal 24N, whereas video signals having a higher transmission rate are coupled to the output terminal 24H. This selection in the output terminal to which the video signals are coupled is intended to be compatible with the additional apparatus (not shown) coupled to these terminals.

By using the reproducing apparatus illustrated in Figure 1B, and by recording the digital signals in the manner discussed above to produce the track patterns shown in Figures 3A, 3B and 3C, the playback heads recover the recorded digital signals without requiring sophisticated tracking control arrangements. This is because multiple tracks are reproduced during the unit time period TP (as before, TP is the time for the rotary head assembly to make two complete rotations), thus providing multiple scans of the same track to assure good reproduction therefrom. That such accurate reproduction of digital signals may be obtained without requiring a complicated tracking control arrangement is describee in US patents specifications US-A-4 774 605 and US-A-4 875 111. The techniques described in these specifications may be adopted herein, yet it will be appreciated with the present embodiment, there is no need to rotate the rotary head assembly during a reproducing mode at twice the rotary speed used during a recording mode. Rather, the rotary head assembly is rotated at the same, constant rate for both modes.

The advantages attained by the embodiment now will be described in conjunction with Figures 4A and 4B which are schematic representations of the scanning of the playback heads across the tracks recorded on the tape T as the tape T is driven at different speeds for different reproducing modes. Initially, reference is made to Figure 4A wherein the tracks a₁, b₁, a₂, b₂, etc had been recorded in the extended play recording mode and now are reproduced in the extended play reproducing mode by the two head sub-assemblies. Figure 4A also illustrates the heads Ha₁ and Hb₁ as they scan successive tracks during successive rotations of the rotary head assembly. From the discussions set forth hereinabove with respect to Figure 3A, it is appreciated that the tracks a₁, b₁, a₂, b₂ are recorded by the heads Ha₁ and Hb₁ during alternate rotations of the rotary head assembly; and the heads Ha₂ and Hb₂ are inhibited from recording signals when they pass across the tape T.

During the extended play reproducing mode, the tape T is transported at the same speed as was usd to record these signals. Accordingly, the tape T is transported by a distance equal to one track pitch P during the interval that the rotary head assembly makes one complete rotation. Let it be assumed that the heads Ha₁ and Hb₁ initially are positioned to reproduce the digital signals recorded in the tracks a₁ and b₁, respectively. After these heads pass across the tape T, the rotary head assembly will have completed one-half of a complete rotation and the tape T will have been advanced by P/2. Thus, the head Ha₂ now is positioned to reproduce the signals recorded in the tracks a₁ or b₁ and the head Hb₂ is positioned to reproduce the signals recorded in the tracks b₁ or a₂. By reason of azimuth loss, the signals recorded in the track br are not reproduced by the heads Ha₂ and, similarly, the signals recorded in the track a₂ are not reproduced by the head Hb₂. Connsequently, during this pass of the heads Ha₂ and Hb₂ across the tape T, the signals recorded in the tracks a₁ and b₁ are reproduced thereby, respectively. Thus, during this one complete rotation of the rotary head assembly, the heads Ha₁ and Hb₁ reproduce the digital signals recorded in the tracks a₁ and b₁; and the heads Ha₂ and Hb₂ likewise reproduce the digital signals recorded in the tracks a₁ and b₁, respectively. The tracks a₁ and b₁ thus are effectively scanned twice during this rotation of the rotary head assembly.

Now, at the beginning of the next rotation of the rotary head assembly, the tape T will have been advanced sufficiently to position the head Ha₁ over the tracks b₁ and a₂; and the head Hb₁ will be positioned over the tracks a₂ and b₂. Because of azimuth loss, the head Ha₁ reproduces the signals from the track a₂ only and the head Hb₁ reproduces the signals from the track b₂ only. Hence, during this half rotation of the rotary head assembly, the digital signals recorded in the tracks a₂ and b₂ are reproduced by the heads Ha₁ and Hb₁. As the heads Ha₂ and Hb₂ are positioned to commence their scanning of the tape T, the head Ha₂ reproduces digital signals from the track a₂ and the head Hb₂ reproduces digital signals from the track b₂. Thus, during this second complete rotation of the rotary head assembly, the digital signals recorded in the tracks a₂ and b₂ are reproduced by the heads Ha₁ and Hb₁, respectively, and also by the heads Ha₂ and Hb₂, respectively.

It is recalled that, when extended play digital signals are recorded, only two tracks (a₁ and b₁) are recorded during the unit time period TP (where TP is the time for the rotary head assembly to make two complete rotations). However, during this same time period TP when the apparatus operates to reproduce the extended play digital signals, twice the number of tracks (a₁, b₁, a₂ and b₂) are reproduced. Moreover, each pair of tracks is reproduced twice (once by the heads Ha₁ and Hb₁ and once again by the heads Ha₂ and Hb₂) during each complete rotation of the rotary head assembly. This pemits accurate reproduction of the digital signals without requiring a complex and sophisticated track following arrangement.

It is appreciated that, as each head sub-assembly reproduces digital signals from respective tracks, thorn digital signals are written into the memories 16a and 16b. If any such digital signals are defective, as may be caused by drop-out or the like, during one pass of one head sub-assembly, those defective portions may be replaced by non-defective portions recovered from the same, or next adjacent racks, during the pass of the other head sub-assembly.

The foregoing discussion has assumed that both sub-assemblies reproduce digital signals from the tracks scanned thereby. If desired, only one sub-assembly can be used to reproduce digital signals and the other can be inhibited, as is done during a recording mode. For example, only the heads Ha₁ and Hb₁ say be used to reproduce digital signals from the tracks a₁ and b₁ during one rotation of the rotary head assembly and then, during the next rotation, those same heads may be used to reproduce signals from the tracks a₂ and b₂. Still, during the unit time period TP, digital signals will be reproduced from twice the number of tracks than had been recorded during this unit time period TP. This is because, during the recording operation, only the tracks a₁ and b₁ are recorded in the unit time period TP; whereas during reproduction, digital signals from the tracks a₁, b₁, a₂ and b₂ are reproduced during the unit time period TP, even if the heads Ha₂ and Hb₂ are disabled.

Turning now to Figure 4B, it is assumed that digital signals are recorded in the tracks a₁, b₁, a₂, b₂, a₃, b₃, etc by, for example, the heads Ha₁ and Hb₁ only, during a normal play recording mode. That is, and consistent with the example discussed above in conjunction with Figure 3B, it is assumed that only the heads Ha₁ and Hb₁ record digital signals in the tracks a₁ and b₁ during one rotation of the rotary head assembly, whereby the tape T is advanced by a distance equal to twice the track pitch 2P, and these same heads Ha₁ and Hb₁ next record digital signals in the tracks a₂ and b₂ during the next following rotation of the rotary head assembly. Thus, during a normal play recording mode, the heads Ha₂ and Hb₂ are not used to record digital signals.

In the normal play reproduction mode, it is assumed that the tape T is advanced by a distance 2P during the interval needed for the rotary head assembly to make one complete rotation. Let it be assumed that, at the beginning of a rotation, the heads Ha₁ and Hb₁ are positioned over the tracks a₁ and b₁, respectively, to reproduce the digital signals recorded therein. By reason of azimuth loss, the digital signals recorded in the track b₁ are not satisfactorily reproduced by the head Ha₁ and, similarly, the digital signals recorded in the track a₂ are not satisfactorily reproduced by the head Hb₁. As the heads Ha₁ and Hb₁ scan the tracks a₁ and b₁ during this rotation of the rotary head assembly, the digital signals recorded in the tracks a₁ and b₁ are respectively reproduced thereby. When the heads Ha₂ and Hb₂ are positioned to pass across the tape T, as represented by the broken lines in Figure 4B, the tape T will have been advanced by the distance P, thus positioning the head Ha₂ on the track b₁, but extending to adjacent track a₂, and thus positioning the head Hb₂ on the track a₂, but extending to the adjacent track b₂. Accordingly, because of azimuth loss, during this pass of the heads Ha₂ and Hb₂ across the tape T, digital signals recorded in the track a₂ are reproduced by the head Ha₂ and digital signals recorded in the track b₂ are reproduced by the head Hb₂. Therefore, at the completion of one rotation of the rotary head assembly, the digital signals recorded in the tracks a₁, b₁, a₂ and b₂ are reproduced. This compares with the digital signals that were recorded only in the tracks a₁ and b₁ during this one complete rotation of the head assembly.

At the beginning of the next full rotation of the rotary head assembly, the heads Ha₁ and Hb₁ are positioned along the tracks a₂ and b₂. Accordingly, the digital signals recorded in these tracks are reproduced. At the completion of this pass of the heads Ha₁ and Hb₁ across the tape T, the tape T will have been advanced by the distance P and the heads Ha₂ and Hb₂ now are positioned along the tracks b₂ and a₃, respectively. However, the head Ha₂ is positioned to pick up signals from the track a₃ and the head Hb₂ is positioned to pick up signals from the track b₃. Accordingly, during this pass of the heads Ha₂ and Hb₂ across the tape T, the digital signals recorded in the tracks a₃ and b₃ are reproduced. Consequently, during this second complete rotation of the rotary head assembly, digital signals are recovered from the tracks a₂, b₂, a₃ and b₃. Here too, it will be appreciated that digital signals are reproduced from twice the number of tracks during the unit time period TP as were recorded. This is because the heads Ha₂ and Hb₂ are used to reproduce signals, whereas during the recording mode, these heads were not used for recording.

In the aforedescribed extended play and normal play reproducing modes, the tape T is transported at the same speed as was used during the respective recording mode; but the rotary head assembly reproduces digital signals from twice the number of tracks than had been recorded during the unit time period TP. For example, during the extended play reproducing mode, digital signals recorded in the tracks a₁ and b₁ are reproduced first by the heads Ha₁ and Hb₁ and then by the heads Ha₂ and Hb₂. During a normal play reproducing mode, digital signals recorded in, for example, the tracks a₂ and b₂ are reproduced first by the heads Ha₂ and Hb₂ and then by the heads Ha₁ and Hb₁. Accordingly, if any defects are present in a portion of the digital signals reproduced by one head, such defects generally may be replaced by the digital signals reproduced the next pass of heads across the tape T. Hence, defects may be avoided, or compensated, without requiring a complex, sophisticated tracking control arrangement. It will be recognized that if defects occur because a record track is not linear, such defects nevertheless are replaced when that track is scanned once again. Since twice the number of tracks are reproduced during the unit time period TP or, stated otherwise, since a given track is subjected to more than one (say two) scans, defective portions present in one scan may be replaced by the same but non-defective portion in the following scan.

Although now shown herein, signals reproduced during an HDTV reproducing mode likewise may be subject to improved reproduction without requiring a complex tracking control arrangement by providing additional sets, or sub-assemblies, or rotary heads. For example, if an additional sub-assembly of heads is disposed between the subassemblies shown in Figure 1A displaced by 90°, and still another sub-assembly is angularly displaced from this additional sub-assembly by 180°, thus resulting in four sub-assemblies of rotary heads, the same effect achieve during extended play and normal play reproducing modes (and described above in conjunction with Figures 4A and 4B) is obtained. For example, when providing these two additional subassemblies of rotary heads, the scanning thereby of the tracks having the pattern shown in Figure 3C is quite similar to the scanning arrangement represented in Figure 4B. That is, during the unit time period TP, twice the number of tracks are reproduced than were recorded, and a given track is scanned twice.

As another implementation, improved signal reproduction during the HDTV reproducing mode may be attained without requiring a tracking control arrangement simply by rotating the rotary head assembly 11 at twice its normal speed only during reproduction. This alternative, however, is not preferred. Rather, one advantage of the system described herein is that the rotary head assembly may be driven at the same, constant speed during both recording and reproduction, even although the original transmission rates of the recorded digital signals vary.

Various changes and modifications may be made. For example, each rotary head sub-assembly has been described in conjunction with a pair of recording/reproducing heads included therein. If desired, each such sub-assembly may be formed of three or more heads. Further, while the original transmission rates of the different digital signals have been described in the ratio 1:2:4, other ratios may be used. Preferably, the speed at which the magnetic tape is transported during recording and also during reproduction varies as a function of the original transmission rate of the digital signal that is recorded. Preferably, such transport speeds have the same ratio to each other as do these transmission rates. Still further, the timebase processing of the respective digital signals during a recording operation preferably is related to the original transmission rate of the digital signal; and during a reproducing operation, the timebase processing is inversely related to that used during recording. As described herein, the digital signals are subjected to timebase correction during recording and to timebase expansion during reproduction. As a result, the data rate at which the digital signals are presented to the rotary head assembly for recording is substantially constant, even although the original transmission rates of these digital signals may have varied.