APPARATUS FOR REPRODUCING A DIGITAL SIGNAL

Abstract: In an apparatus for reproducing a digital signal wherein a signal is read by a plurality of reproducing rotary heads (3A, 3B) which trace a track with an interval narrower than the interval of tracks formed on a tape (MT), a wrap angle which a tape (MT) is wrapped around the periphery of a rotary drum (2) to which the rotary heads (3A, 3B) are attached is selected to be less than 360°/n where n is the number of rotary heads (3A, 3B), the rotary heads (3A, 3B) are rotated at a speed twice the normal rotational speed of the heads (3A, 3B), the demodulated digital signal is stored in a memory (25) corresponding to a frame address and a block address detected from the reproduced digital signal, and then the digital signal stored in the memory (25) is read out therefrom in a predetermined order, whereby an apparatus for correcting the tracking of the tape (MT) by the rotary heads (3A, 3B) can be eliminated.

Description: This invention relates to apparatus for reproducing a digital signal recorded on a tape by a rotary head. Recently, tape recorders which convert an audio signal into a pulse code modulated (PCM) signal and record the PCM signal on a tape using a rotary head have been proposed. These tape recorders are referred to, for example, as R-DAT players. An apparatus for reproducing the PCM signal needs a tracking controller which reproduces a record tracking signal, and controls the running of the tape so that the rotary head exactly scans tracks on which the signals are recorded. To scan tracks formed obliquely on the tape by a plurality of rotary heads, without employing an ordinary tracking method of the type which detects a pilot signal for the tracking control, there has been proposed a so-called no-tracking system in which a head scans a tape at an interval narrower than the track interval recorded on the tape to read a signal at a speed more than twice the normal speed. In the above-mentioned no-tracking system, wavering of a bearing of the rotary shaft of a drum can make not only adjacent tracks different in curvature from each other, but can also vary the track width. If the variation in the track width becomes large, mistracking can occur. When a tape in which the track width varies or mistracking has occurred is reproduced, the output level of the reproduced signal is lowered so that the signal-to-noise ratio is also deteriorated. To eliminate such defects, as shown in Figure 1 of the accompanying drawings, two rotary heads 3A and 3B are respectively mounted on the periphery of a rotary drum 2 connected to a rotation shaft 1 through a head board (not shown) with a sufficiently small angular spacing 0, for example 1°, relative to the rotating direction thereof, whereby, as shown in Figure 2 of the accompanying drawings, the heads 3A and 3B are controlled to record signals on tracks Ta and Tb obliquely formed on a magnetic tape MT with an azimuth angle, in a manner such that the tracks Ta and Tb are parallel to each other and the track width thereof is constant. This method is described in Japanese laid-open patent publications nos 59/2205 and 59/177712. A problem in the above no-tracking method is that since the heads 3A and 3B contact the tape MT substantially at the same time, signals reproduced by the heads 3A and 3B must be simultaneously processed at the reproducing side. It is therefore required to provide at the reproducing side two sets of signal processing circuits, such as demodulating circuits, corresponding to the two heads 3A and 3B, thereby rendering the reproducing circuit arrangement complicated. Our European patent specification EP-A1-0 115 699 discloses a rotary head type reproducing apparatus having features similar to those of the pre-characterizing part of claim 1. According to the present invention there is provided helical scan type apparatus exclusively for reproducing a digital data signal recorded in blocks with address signals in oblique tracks formed on a tape, the apparatus comprising: a rotary drum; n rotary heads, where n is an integer greater than one, mounted with equal spacings around the rotary drum; and guiding means for wrapping the tape on the periphery of the rotary drum with a wrap angle of 360°/n; characterized by: driving means for rotating said n rotary heads at a speed which is sufficient, without tracking control, to scan each track more than once and reproduce each said block at least once; and demodulating means supplied with all the digital signals reproduced by said n rotary heads and for extracting therefrom the reproduced digital data signal. 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: Figure 1 is a representation of a rotary head assembly of a previously proposed apparatus; Figure 2 is a representation of a track pattern on a magnetic tape formed by the rotary head assembly of Figure 1; Figure 3 is a representation of a rotary head assembly of an embodiment of the present invention; Figures 4 and 5 are graphs showing the characteristic of a non-tracking system; Figure 6 is a circuit block diagram showing an embodiment of the present invention; Figure 7 is a diagram showing a track format of one segment data formed by the embodiment; Figure 8 is a diagram showing a PCM block format thereof; Figure 9 is a table showing formats of symbols W1 and W2 in a PCM block; and Figure 10 is a circuit block diagram showing a practical example of a reproducing signal processing circuit in Figure 6. First, the principle of the present invention will be explained with reference to Figures 3 to 5. A known system, as disclosed in Japanese laid-open patent publication no 59/2205, requires the rotary heads 3A and 3B to be mounted substantially at the same position on the rotary drum 2 so as to make the width of tracks on which signals are recorded constant. However, when an apparatus which is used exclusively for the reproduction of the signals is involved, that is an R-DAT player, the heads 3A and 3B can be mounted at more preferred positions on the periphery of the drum 2 since the difference between the traces by the heads 3A and 3B does not cause any problem. Therefore, in embodiments of the present invention, as shown in Figure 3, the heads 3A and 3B are mounted on the periphery of the drum 2 with an angular spacing of 180°, so that the two heads 3A and 3B are diametrically opposed. The normal tracking system has an error rate to tracking error characteristic as shown in Figure 4. On the other hand, as shown in Figure 5, the no-tracking system can always reduce the error rate to below f(0) in total, regardless of the head mounting position, if the rotary heads are rotated at a speed twice the ordinary recording speed to scan tracks at more than twice the normal recording speed, and read signals at more than twice the normal recording speed. In the case where n rotary heads are provided, the condition that the n rotary heads are not simultaneously in contact with the tape will be satisfied if the tape wrap angle is below 360°/n. More specifically, assuming that the number n of rotary heads for recording signals with plus and minus azimuth angles is 2m (n=2m), and a normal rotational speed of the drum is k, the drum rotational speed should be selected to be 2k/m. For example, with m=1 and k=2000rpm, the drum rotational speed is calculated to be 4000rpm. In this case, if there are two heads mounted on the periphery of the drum, these heads are separated by an angular spacing of 180°, that is, the two heads are respectively mounted on the periphery of the drum at positions diametrically opposite to each other and the tape wrap angle is set to be below 180° (360°/2). Next, the circuit arrangement of an embodiment of the present invention and the operation thereof will be explained with reference to Figure 6. Figure 6 shows the circuit arrangement of an apparatus which reproduces a signal recorded on a tape by an ordinary R-DAT. In Figure 6, a rotary drum 2, the diameter of which is half that of the ordinary R-DAT drum, for example, 15mm, is rotated at a speed twice the normal speed of the R-DAT drum, for example, 4000rpm. A pair of rotary heads 3A and 3B are mounted on the periphery of the drum 2 with an angular spacing of 180°. A magnetic tape MT is obliquely wrapped around the outer periphery of the drum 2 with a tape wrap angle of 180°. Another embodiment of the apparatus may also be considered in which, if the diameter of the drum 2 is 30mm, the tape wrap angle is selected to be 90°, and the drum 2 is rotated at a speed twice the normal speed of the R-DAT, that is, 4000rpm. The magnetic tape MT is stretched between reel hubs 4 and 5 provided in a tape cassette, and is transported at the predetermined normal speed of the R-DAT by a capstan 6 and a pinch roller 7. In the aforementioned ordinary R-DAT, the rotary drum having a pair of heads mounted with an angular spacing of 180° on the periphery thereof, has a diameter of 30mm and is rotated at 2000rpm. The magnetic tape is wrapped around the outer periphery of the rotary drum with a wrap angle of 90°. With this R-DAT, two oblique tracks are formed on the magnetic tape MT by the rotary heads which are alternately in contact with the magnetic tape MT. The directions of the magnetic gap of the magnetic heads are respectively chosen to be +α (=20°) and -α relative to the direction perpendicular to the direction of the track. The angles of the magnetic gaps of the respective rotary heads are referred to as a 'plus azimuth' and a 'minus azimuth', respectively. Reference is next made to a reproducing apparatus of the present invention which reproduces a digital signal recorded on a magnetic tape by the above-mentioned ordinary R-DAT. As shown in Figure 6, the heads 3A and 3B are alternately selected by a change-over switch 8. Signals reproduced by the respective heads 3A and 3B are supplied to a reproducing amplifier 9 through a rotary transformer and a head amplifier (both not shown). A control system for controlling the switch 8 is generated by a timing control circuit 10. The reproduced signals are derived at a speed twice the transmission speed of the ordinary R-DAT since the rotary heads are rotated at 4000 rpm, that is, a rotational speed twice the rotational speed of the normal R-DAT. The respective tracks formed on the tape are also scanned at twice the scanning rate of the normal R-DAT, therefore, the rotary heads trace the tracks with an interval half that of the track recorded on the tape MT. An output signal from the reproducing amplifier 9 is supplied to a phase locked loop (PLL) circuit 11 wherein a clock signal in synchronism with the reproduced signal is extracted. Signal processing such as error correction, interpolation and so on are effected for the reproduced signal in a reproduced signal processing circuit 12. Then, the reproduced and processed digital audio signal is supplied to a digital to analogue (D/A) converter 13. the reproduced audio signal from the D/A converter 13 is supplied to an output terminal 15 through a low-pass filter 14. Also in the reproduced signal processing circuit 12, a sub-code and a sub-code ID are separated from the reproduced signal, and are supplied to an output terminal 16. To the output terminal 16 there is connected a sub-code decoder (not shown) which generates control data and so on from the sub-code. The timing control circuit 10 generates a clock signal and a timing signal which are required by the reproducing signal processing circuit 12. Reference is next made to the format of the data recorded on the tape by the ordinary R-DAT. All of the data recorded in one track is called 'one segment'. Figure 7 shows the format of the data recorded in one segment by one of the rotary heads 3A or 3B. If a unit amount of the recorded data is determined as 'one block', one segment includes the data of 196 blocks (corresponding to 7500 µsec). At each end portion of one segment corresponding to each end portion of the track there are respectively provided margins (eleven blocks). Sub-codes (eight blocks) are respectively recorded in portions adjacent to the margins. The contents of the two sub-codes are the same, namely, the same data is double-recorded. The sub-codes may be a programme number, a time code, a start identifying signal and so on. At both sides of each of the eight block sub-code recording area, a PLL run-in interval or portion (two blocks) and a postamble interval or portion (one block) are respectively located. There are also provided inter-block gaps (three blocks) in which no data is recorded. A pilot signal for the ATF is recorded in two ATF regions of five blocks which are respectively located at portions between two inter-block gaps. In a central region having a length of 130 blocks in one segment except for the PLL run-in portion of two blocks, that is, in a 128 block length region, the processed PCM signals are recorded. The PCM signals of two segments correspond to audio signals read by the rotary heads during one rotation thereof. the PCM signal comprises two-channel stereo PCM signals consisting of a L (left) channel and a R (right) channel signals and parity data consisting of error detection and error correction codes. When one segment as shown in Figure 7 is recorded and/or reproduced by the head 3A, data Le is recorded in the left half of the PCM signal recording region and data Ro in the right half of the same. The data Le comprises even-numbered data of the L channel and a parity code for correcting possible errors occurring in the data Le, while the data Ro comprises odd-numbered data of the R channel and a parity code for correcting possible errors occurring in the data Ro. Each number of the odd- and even-number indicates a number counted from the head of the interleave block. One segment of data formed in the same manner as described above is recorded in a track formed by the other rotary head 3A or 3B. The data region of one segment in one track formed by the other rotary head 3A or 3B is divided into the left half portion for recording data Re and the right half portion for recording data Lo. The data Re comprises even numbered data of the R channel and a parity code for correcting possible errors occurring in the data Re. In the same manner, the data Lo comprises odd-numbered data of the L channel and a parity code for correcting possible errors occurring in the data Lo. The reasons why the even-numbered data are recorded in the same track is to prevent data errors from occurring in the successive data of the same channel due to drop-out and so on. Figure 8 shows the data format of one block of the PCM signal. At the head of one block, a block synchronizing signal having an 8-bit length (eight bits are referred to as 'one symbol') is allocated, which is followed by a PCM-ID (referred to as 'W1') of eight bits and a block address portion (referred to as 'W2') of eight bits. An error correction coding by a simple parity is effected with regard to the two symbols of the PCM-ID (W1) and the block address portion (W2) and a parity cod of eight bits is added after the block address. Referring to Figure 9, the block address portion is formed by seven bits except that the most significant bit (MSB). The MSB is set to '0' to indicate that the block is a PCM block. The block address indicated by seven bits changes sequentially from (00) to (7F) in hexidecimal. the PCM-ID indicative of data recorded in each block is recorded in locations, the block addresses of which are (000), (010), (100) or (110) in the lower three bits. Optional codes of the PCM-ID can be recorded in locations, the block addresses of which are (001), (011), (101) or (111) in the lower three bits. The PCM-ID includes two bits of ID-1 to ID-8 and four bits of frame address. The respective ID-1 to ID-7 respectively show identifying information. A pack or packet is formed of thirty-two ID-8s (two bits). For example, ID-1, which is a format ID, indicates whether recorded data is for audio or for any other purposes. ID-2 indicates on or off of pre-emphasis and the characteristics of the pre-emphasis, and ID-3 a sampling frequency. The above-mentioned ID-1 to ID-7 and frame address are recorded equally in two segments forming an interleave pair. In other words, the frame address is incremented by one at every two segments. Next, a practical example of the reproduced signal processing circuit 12 in Figure 6 and its operations will be described with reference to Figure 10. The reproduced signal is supplied by way of a terminal 21 to a demodulating circuit 22 wherein one symbol formed of ten bits is demodulated to form one symbol formed of eight bits. (Eight bits of one symbol have been digitally modulated to a suitable pattern of ten bits upon being recorded on the magnetic tape MT so as to reduce low frequency components.) The reproduced data from the demodulating circuit 22 is supplied symbol by symbol through a data register 23 to a data bus 24. The data bus 24 is connected with a buffer RAM 25 and an error correction circuit 26. The buffer RAM 25 should have a capacity larger than that of the buffer RAM provided in an ordinary R-DAT, which generally has a capacity for storing data recorded in four tracks, since it is required to store in the buffer RAM 25 data reproduced by the rotary heads which possibly overlap tracks due to omission of the tracking control. The block address and the frame address of the reproduced signal from the signal from the demodulating circuit 27 are detected by an address detecting circuit 27 and supplied to an address control circuit 28. The output signal from the address control circuit 28 is supplied to the buffer RAM 25 as an address signal. The reproduced data is stored via the data bus 24 in the buffer RAM 25 in accordance with the address signal from the address control circuit 28. Therefore, the reproduced data is stored in the buffer RAM 25 in accordance with the block address and the frame address without tracking control. In the error correction circuit 26, error correction is effected by a Reed-Solomon code on each track of data stored in the buffer RAM 25. The reproduced data, after being subjected to the error correction, is read out of the buffer RAM 25 in the original order and the error-corrected PCM signal is supplied to an interpolation circuit 29 wherein uncorrectable errors are interpolated. The reproduced PCM signal is then supplied through an output terminal 30 to the D/A converter 13 shown in Figure 6. The sub-code is supplied to a sub-code output terminal after processing, such as error correction, in the sub-code decoder (not shown). Conventionally, two sets of the reproducing amplifier 9, the PLL circuit 11, the reproduced signal processing circuit 12 and so on have been necessary for the rotary heads 3A and 3B. However, with the embodiment of the invention described above, the tape wrap angle is selected to be less than 180° in order that the heads 3A and 3B will not simultaneously be in contact with the tape, and the rotational speed of the drum 2 is selected to be twice the recording speed of the ordinary R-DAT, so that the number of sets of the reproducing amplifier 9, the PLL circuit 11, the reproduced signal processing circuit 12 and so on can be reduced to one set without the necessity of the tracking control. It is therefore possible to provide an apparatus exclusively used for reproducing a digital signal constructed by a simple circuit arrangement. Further, with the present embodiment, in the signal processing apparatus of the non-tracking system, the tape wrap angle is selected to be less than 360°/n with respect to the number of rotary heads mounted on the periphery of the rotary drum, and a plurality of rotary heads will not be in contact with a tape at the same time. Thus, the arrangement of the reproducing circuit can be simplified, and consequently the production cost can be reduced. Moreover, when the diameter of the rotary drum is made small, a compact-sized apparatus exclusively used for reproducing a digital signal can be provided.

Claims: Helical scan type apparatus exclusively for reproducing a digital data signal recorded in blocks with address signals in oblique tracks formed on a tape (MT), the apparatus comprising: a rotary drum (2); n rotary heads (3A, 3B), where n is an integer greater than one, mounted with equal spacings around the rotary drum (2); and guiding means for wrapping the tape (MT) on the periphery of the rotary drum (2) with a wrap angle of 360°/n; characterized by: driving means (1) for rotating said n rotary heads (3A, 3B) at a speed which is sufficient, without tracking control, to scan each track more than once and reproduce each said block at least once; and demodulating means (22 to 29) supplied with all the digital signals reproduced by said n rotary heads (3A, 3B) and for extracting therefrom the reproduced digital data signal. Apparatus according to claim 1 further comprising: detecting means (27) for detecting a block address contained in each block and a frame address contained in a block at every predetermined number of blocks from a plurality of blocks forming the digital signal recorded in the track on the tape (MT); means (25) for storing a demodulated digital signal of each block in a memory (25) corresponding to the block address and the frame address detected by said detecting means (27); and means (23, 24) for reading the stored digital data from said memory (25) in a predetermined order. Apparatus according to claim 1 or claim 2 wherein the number of said rotary heads (3A, 3B) is two, and said guiding means wraps the tape (MT) with a wrap angle of approximately 180° around said rotary drum (2). Apparatus according to claim 1, claim 2 or claim 3 wherein said rotary drum (2) has a diameter of approximately 15mm and is rotated at approximately 4000rpm.

Inventors: FUKAMI, TADASHI PATENTS DIVISION SONY CORPORATION, ODAKA, KENTARO PATENTS DIVISION SONY CORPORATION, OZAKI, SHINYA PATENTS DIVISION SONY CORPORATION
Applicants: SONY CORPORATION
Application year: 1987
Main classification: G11B5
IPC codes: G11B5, G11B15, G11B27, G11B20