Digital signal reproducing apparatus

Abstract: A digital signal reproducing apparatus comprising a magnetic head (4) for reading a video signal from a magnetic tape (2), a rotary transformer (6), a reproduction amplifier (8), a reproduction equalizer (10), a phase-locked loop (12), a threshold discriminator (14), and an amplitude detector (16). The amplitude detector detects a dropout by threshold-discriminating the nonequalized reproduced signal, and the frequency characteristic of the equalizer is changed in response to detection of any dropout so as to compensate for the dropout. The threshold discriminator detects the reproduced digital signal from the equalized reproduced signal in accordance with a clock signal extracted by the phase-locked loop. The apparatus is capable of reproducing an errorless video signal by compensating for any dropout on the magnetic tape. There is also disclosed a digital signal reproducing method which detects the existence of any dropout when the amplitude of the read signal has become smaller than a predetermined value, and changes the characteristic of the equalizer to compensate for the dropout.

Description: The present invention relates to an apparatus for reproducing a digital signal. JP-A-3207003, used to delimit the two-part form of claim 1, discloses a magnetic reproducing circuit which has the purpose to realize a magnetic tape reproduction circuit not making the error rate worse even if S/N is deteriorated through a simple circuit configuration by switching the equalizing characteristic of an equalizer circuit to the characteristic of high band enhancement when the deterioration of the S/N of the reproduced signal is detected from the change of AGC control voltage. After a reproduced signal read from a magnetic tape by a magnetic head is amplified by an AGC amplifier, it is given equalizing correction by the equalizer circuit. A control voltage VAGC from a control voltage generator controls the gain of the AGC amplifier so that the amplified signal level can be put in a prescribed range. If the control voltage VAGC is below a prescribed threshold voltage, the deterioration of the signal to noise ratio of a head reproduced signal is discriminated, and the equalizer circuit is switched by a switching circuit from a usual first characteristic to a second characteristic to enhance a high band and the signal to noise ratio is improved. SMPTE Journal, Volume 97, Number 1, Pages 8 to 12 Seiichi Mita et al "Adaptive Equalization Techniques For Digital Video Recording Systems" proposes an adaptive automatic equaliser applicable to a digital video tape recorder. It proposes the use of a pre-equalizer comprising an analog transversal filter followed by an adaptive equaliser comprising a linear canceller. In a digital magnetic recording/reproducing apparatus such as a digital video tape recorder (VTR), there is employed a filter known as a reproduction equalizer for obtaining, from the reproduced signal, data equal to the recorded digital data composed of "1" and "0" bits. Fig. 5 shows an exemplary constitution of a reproducing section in a conventional VTR. The reproduced signal obtained by reading out the recorded data from a magnetic tape 2 by means of a magnetic head 4 is amplified by a reproduction amplifier 8 after being processed through a rotary transformer 6, and then its waveform is shaped to eliminate the intercode interference by a reproduction equalizer 10. A clock signal S12 synchronized with a nonequalized reproduced signal S8 from the amplifier 8 is extracted by a PLL (phase-locked loop) circuit 12, and an equalized reproduced signal S10 is supplied to a threshold discriminator 14 where the signal S10 is threshold-discriminated by the use of a clock signal S12, so that the original binary digital data is resumed. The equalizer 10 is so adjusted as to minimize the errors after the discrimination with respect to a normal reproduced signal. In the magnetic recording medium such as the magnetic tape 2 used here, it is likely that extremely small defects are included in the surface of the magnetic layer. And there may occur a disadvantageous occasion that minute dust or the like is deposited on the magnetic tape 2. Such defects or deposition of dust may cause a phenomenon of dropout that brings about temporary diminution of the amplitude of the reproduced signal. Since occurrence of any dropout sharply deteriorates the characteristic of the reproduced signal, errors after the discrimination are generated concentratively in the vicinity of the dropout. And it is impossible to completely avert such dropout. Even if complete quality control of the magnetic recording medium is realized in the manufacturing process and no defect is existent to cause dropout at the time of delivery or shipment, it is still impossible to eliminate occurrence of small flaws on the magnetic tape 2 or deposition of minute dust or the like thereon due to, e.g., contact of the magnetic head 4 with the magnetic tape 2. However, no adequate method has been proposed heretofore for reducing the errors derived from the dropout, and no proper means for solving the problem has been devised either. Prior to contriving the present invention, an experimental apparatus of the constitution shown in Fig. 6 was produced to evaluate dropouts. The test evaluation was executed by first equalizing a reproduced signal obtained from a digital apparatus of the D2 format, then converting the reproduced signal by an analog-to-digital converter 26 in accordance with a clock signal extracted by a PLL 12, and feeding such reproduced digital signal data into a computer 40. The successive reproduced signal data was composed approximately of 750 Kbits corresponding to 1 track. The data obtained by threshold discrimination of the reproduced signal was compared with the recorded data, and it was found that 53 errors were existent to signify an error rate of 7.1E-5 (7 x 10 -5 ). According to the result of the detailed inspection, the aforementioned dropouts were included in the reproduced signal data. Fig. 7 graphically shows the envelope of the reproduced signal with the dropouts existent therein, where the abscissa and the ordinate denote the time and the amplitude respectively. In this example, it is observed that the amplitude is deteriorated approximately in a range of 2000 bits and is diminished to be nearly zero at the center of the range. Fig. 8 is an enlarged graphic diagram showing the dropout portions of the reproduced signal shown in Fig. 7, wherein 53 bits recognized as errors after the discrimination are represented by vertical lines. It was found by examining such error bits that the entire errors were approximately within a range of 500 bits in the vicinity of the center of the dropouts. It is evident from the above evaluation that, in a digital VTR having a sufficiently high signal-to-noise ratio and equipped with a proper equalizing function, errors derived from dropouts are dominant usually. Therefore, if such errors can be reduced by some appropriate means, the reliability of the system will be widely enhanced and the reference level in quality control of recording media will be lowered with alleviation of the requisite condition to eventually curtail the production cost of magnetic tape and so forth. In view of the circumstances mentioned above, the present invention is concerned with the problem of reducing or eliminating dropouts to thereby reduce the errors derived from such dropouts. According to the present invention, there is provided a digital signal reproducing apparatus comprising: means for reproducing a digital signal recorded on a recording medium; means for equalising the signal thus reproduced; a drop-out detector for detecting the amplitude of either the non-equalized signal or the equalized signal; a linear canceller coupled to the equalizing means for compensating for the drop-out component; means for extracting a clock signal from the equalized signal; a threshold-discriminator for threshold-discriminating the equalized signal in accordance with the clock signal; and a switch responsive to the drop-out detector for coupling the linear canceller to the threshold-discriminator in response to detection of any drop-out by said drop-out detector and for coupling the equalizing means to the threshold-discriminator otherwise. It therefore becomes possible to enhance the reliability of the digital signal reproducing apparatus as a result of the dropout compensation. In view of the invention where dropout is compensatable, the requisite condition relative to a recording medium such as a magnetic tape can be alleviated to consequently facilitate quality control of the recording medium with another advantage of curtailing the production cost. Specific embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a block diagram of fundamental component circuits in a digital signal reproducing apparatus; Fig. 2 is a block diagram of a digital VTR as a first embodiment of a digital signal reproducing apparatus of the invention; Fig. 3 is a block diagram of a digital VTR as a second embodiment of a digital signal reproducing apparatus of the invention; Fig. 4 graphically shows the result of an exemplary experiment executed in the digital VTR of Fig. 3; Fig. 5 is a first block diagram of a conventional digital VTR; Fig. 6 is a second block diagram of another conventional digital VTR; Fig. 7 graphically shows the result of an exemplary experiment executed in the digital VTR of Fig. 6; and Fig. 8 is an enlarged partial graphic diagram of Fig. 7. Fig. 1 is a fundamental block diagram of a digital VTR representing a digital signal reproducing apparatus. In comparison with the constitution of the aforementioned conventional digital VTR shown in Fig. 5, an amplitude detector 16 is additionally employed in the digital VTR of Fig. 1. More specifically, the digital VTR shown in Fig. 1 comprises a magnetic head 4 for reading recorded data from a magnetic tape 2, a rotary transformer 6, a reproduction amplifier 8 for amplifying the output signal of the rotary transformer 6 and delivering a nonequalized reproduced signal S8, a reproduction equalizer 10 for equalizing the nonequalized reproduced signal S8, a PLL 12 for extracting a clock signal S12 from the equalized reproduced signal S10 by phase lock; a threshold discriminator 14 for extracting reproduced data S14 by threshold-discriminating the equalized reproduced signal S10 in accordance with the clock signal S12; and an amplitude detector 16 for generating an equalization characteristic control signal S16 by detecting the amplitude of the nonequalized reproduced signal S8 inputted thereto. In the equalizer 10, the nonequalized reproduced signal S8 is equalized on the basis of the equalization characteristic control signal S16. Thus, in the digital VTR, the characteristic of the equalizer 10 is changed in conformity with the amplitude of the nonequalized reproduced signal S8 by the control signal S16 outputted from the amplitude detector 16. Although the amplitude detector 16 employed in this circuit constitution is so contrived as to detect the amplitude of the nonequalized reproduced signal S8, there is no difference if the detector 16 is so connected as to detect the amplitude of the equalized reproduced signal S10. Namely, the amplitude detector 16 may detect either the amplitude of the nonequalized reproduced signal S8 or that of the equalized reproduced signal S10. The equalization characteristic control signal S16 outputted from the amplitude detector 16 may be a voltage or current proportional to the amplitude of the reproduced signal S8 (or S10), or may be a voltage or current of a staircase waveform. The simplest method comprises the procedure of first detecting that the amplitude of the reproduced signal has become smaller than a preset value continuously for a predetermined period of time, then regarding it as a dropout, and outputting a flag indicative of such dropout. The characteristic of the reproduction equalizer may be continuously changed in accordance with the equalization characteristic control signal S16 along a predetermined curve, or may be selectively switched stepwise. The simplest method comprises the procedure of switching the reproduction equalizer 10 temporarily to another equalizer of different characteristic in response to a dropout-indicating flag outputted from the amplitude detector 16. The constitution of the digital VTR shown in Fig. 1 is fairly similar to that of an automatic gain controller (AGC) with respect to the function of controlling the circuit on the basis of detection of the signal amplitude. However, the object of the AGC is to keep the output signal amplitude constant by controlling the entire gain while maintaining the fixed frequency characteristic. Meanwhile in the digital VTR of Figure 1, there exists a great difference that the frequency characteristic of the equalizer 10 is changed in accordance with the amplitude, and it is not exactly necessary to keep the output signal amplitude constant. A first embodiment of the invention having a circuit constitution of Fig. 2 is contrived by combining the simplest of the fundamental circuits mentioned above. The digital VTR of Fig. 2 comprises a magnetic head 4, a rotary transformer 6, a reproduction amplifier 8 (all not shown), a reproduction equalizer 10, a PLL 12 and a threshold discriminator 14. In addition, the digital VTR further includes a linear canceller 18, a dropout detector 20 and a switch 24 which are connected in parallel to the PLL 12. The linear canceller 18 is disposed next to the equalizer 10 in the digital VTR, and the output of the equalizer 10 is discriminated either directly in the absence of any dropout or after being passed through the linear canceller 18 in the presence of any dropout, whereby errors derived from the dropout are reduced. It is supposed that, upon occurrence of any dropout, the spacing between the magnetic tape 2 and the magnetic head 4 is increased in the vicinity of the dropout to consequently deteriorate the higher frequency component. However, using the linear canceller 18 is effective in a simple circuit configuration to compensate for the higher frequency component without emphasizing the noise. Therefore the linear canceller serves as an additional equalizer sufficiently to compensate for the characteristic deterioration in the vicinity of the dropout. The dropout detector 20 has a function of selectively changing the switch 24 by detecting that the amplitude of the equalized reproduced signal S10 has become smaller continuously than a preset value for a predetermined period of time. Fig. 3 shows a digital VTR as a second embodiment of the present invention. This digital VTR comprises a magnetic head 4, a rotary transformer 6, a reproduction amplifier 8 (all not shown), a reproduction equalizer 10, a PLL 12, an A-D converter 26, a digital linear canceller 28, a digital dropout detector 30, a switch 32 and a threshold discriminator 14. The linear canceller 18 can be realized with extreme facility by the use of a digital circuit. Therefore, the circuit configuration is so arranged that the equalized reproduced signal S10 obtained from the equalizer 10 is digitized by the A-D converter 26 in accordance with the sampling clock signal S12 outputted from the PLL 12, and each of the digital linear canceller 28 functionally equivalent to the linear canceller 18 and the digital dropout detector 30 functionally equivalent to the dropout detector 20 is composed of a digital circuit. In the digital VTR of Fig. 3 employing the dropout detector 30 and the 3-tap linear canceller 28, the following operation is performed with 8-bit analog-to-digital conversion. Dropout detector 30 When the amplitude of successive 3-bits is smaller than a preset value, it is detected and regarded as a dropout, if (¦X[k-1]-T¦<DR)AND(¦X[k]-T¦<DR)AND(¦X[k+1]-T¦<DR) where X[k]: amplitude of kth reproduced signal T: threshold value DR: range of amplitude at dropout Linear canceller 28 A preset value is added to the reproduced signal in accordance with the result of discriminating the preceding and following bits, and another discrimination is executed. X' [k] = X[k] - (2* (A[k+1]-1) *Cn) - (2* (A[k-1]-1) *Cb). if X' [k] > T, A' [k] = 1 if X' [k] < T, A' [k] = 0 where A[k]: result of kth discrimination (1 or 0) Cn: coefficient to following bit Cb: coefficient to preceding bit A'[k]: result of kth rediscrimination (1 or 0) Fig. 4 graphically shows the result of verifying the operation in the digital VTR of Fig. 3 by using the data obtained through a computer 40 (not shown) which is disposed next to a threshold discriminator 14 (not shown) in the digital VTR of Fig. 3 as in the aforementioned constitution of Fig. 6. This experiment was executed under the conditions where the threshold value T was set to 127 which is the middle of values 0 and 255 obtained by the 8-bit analog-to-digital conversion, and Cb = -3.5, Cn = -0.5, and DR = 32. In this case, 53 errors derived originally from the dropout were reduced to 30 errors. Vertical lines in the upper half of the diagram denote positions of the errors generated originally, and vertical lines in the lower half denote positions of the errors generated in the second embodiment. Although it is impossible as shown to eliminate the errors in the range corresponding to the center portion of the dropout where the amplitude is nearly zero, some errors in the vicinities of the beginning and the end of the dropout where the amplitude still remains partially can be reduced to a certain extent by changing the characteristic of the equalizer. The embodiments mentioned above represent examples of using a magnetic tape 2 as a recording medium. However, it is to be understood that in the digital signal reproducing apparatus of the present invention, the recording medium is not limited to such magnetic tape alone, and any other recording medium such as a magnetic disk is applicable as well. Furthermore, although a video signal is recorded as an exemplary digital signal on the recording medium in the description, the apparatus of the invention is usable for reproduction of any other digital signal in addition to the video signal mentioned.

Claims: A digital signal reproducing apparatus comprising: means (468) for reproducing a digital signal recorded on a recording medium; means (10) for equalising the signal thus reproduced; a drop-out detector (20, 30) for detecting the amplitude of either the non-equalized signal or the equalized signal; characterised in that it further comprises: a linear canceller (18, 28) coupled to the equalizing means for compensating for the drop-out component; means (12) for extracting a clock signal from the equalized signal; a threshold-discriminator (14) for threshold-discriminating the equalized signal in accordance with the clock signal; and a switch (24, 32) responsive to the drop-out detector (20, 30) for coupling the linear canceller to the threshold-discriminator in response to detection of any drop-out by said drop-out detector (20, 30) and for coupling the equalizing means to the threshold-discriminator otherwise. Apparatus according to claim 1, wherein the equalized (S10) signal is an analogue signal, the linear canceller (18), the drop-out detector (20) and the threshold-discriminator (14) operating on the equalized analogue signal (S10). Apparatus according to claim 2, wherein the clock signal extracting means (12) supplies the clock signal (S12) to the threshold-discriminator (14) threshold-discriminating the equalized signal in accordance with the clock signal. An apparatus according to claim 1, further comprising an A-D converter (26) for converting the equalized signal (S10) to digital form, wherein said linear canceller (28), said drop-out detector (30) and the threshold-discriminator operate on the digital equalized signal (S26). Apparatus according to claim 4, wherein the clock signal extracting means (12) supplies the clock signal to the A-D converter. Apparatus according to any one of claims 1 to 5, wherein the drop-out detector (20, 30) detects the existence of a drop-out when the amplitude of the reproduced signal has become smaller than a predetermined threshold value. Apparatus according to any one of claims 1 to 6, wherein said means for extracting the clock signal includes a phase-locked loop (12). Apparatus according to any preceding claim, wherein the means for reproducing the digital signal includes a magnetic head (4). A digital VTR comprising a digital signal reproducing apparatus according to any preceding claim.

Inventors: HARA MASAAKI, HARA, MASAAKI, Hara, Masaaki, c/o Intellectual Property Division
Applicants: SONY CORP, SONY CORPORATION
Application year: 1993
Main classification: G11B20
IPC codes: G11B20, H04N5, G11B5