DATA SIGNAL REPRODUCING APPARATUS

Abstract: A data reproducing apparatus for demodulating data recorded on a magnetic tape by a biphase-mark system performs equalization processing using an equalizer (16) adapted so that the eye-opening ratio of an eye-pattern of a picked-up output is maximized, thereby reducing the bit error rate even when different tapes are used as a recording medium. The equalizer incorporates a low-pass filter (18), high-pass filter (19) and switching means for altering the characteristics of one or both of them.

Description: The present invention relates to a data reproducing apparatus and, more particularly, to a data reproducing apparatus suitably adapted for reproducing recorded data from a magnetic tape which is magnetically recorded by a biphase-mark system. In a video tape recorder (VTR) of, eg, an 8 mm video format, an audio signal is PCM-encoded and compressed along the time axis. The signal is then modulated by biphase-mark modulation and recorded on an audio track which is formed adjacent and parallel to a video track. Such a VTR is disclosed in US-A-4551771 assigned to the same assignee as this application. In biphase-system modulation, as shown in fig 9A of the accompanying drawings, frequency signals S₁ and S₂ consisting of sine waves, with a ratio of frequencies f₁ and f₂ of 1:2, are recorded as digital data on a magnetic tape while maintaining a phase relation in which their zero crossing points coincide with each other. For example, when audio data is in logic state "0", the signal S₁ which has a low frequency of f₁ = 2.9 (MHz) is recorded on the magnetic tape, and when the audio signal is in logic state ¨1", the signal S₂ having a high frequency F₂ = 5.8 (MHz) is recorded thereon. In order to reproduce the audio data thus recorded on the audio track, as shown in fig 9B, the zero crossing points of the picked-up signal S₁ or S₂ are detected, and a sampling pulse oscillator using, eg, a PLL (phase locked loop) configuration oscillator produces outputs PLL₀ which maintain a phase interval of 45° with respect to the low frequency signal S₁ and of 90° with respect to the high frequency signal S₂. Of the pulse outputs PLL₀, pulses generated at phases of 45°, 135°, 225° and 315° of the signal S₁ are used as a sampling pulse Ps (fig 9C) to sample the signal level LV₁ corresponding to the peaks of the signal S₁, and the signal level LV₁ is used as reproduced data representing logic state "0". Of the pulse outputs PLL₀, pulses generated at phases of 90° and 270° of the signal S₂ are used as a sampling pulse Ps to sample the signal level LV₂ corresponding to the peaks of signal S₂, and the signal level LV₂ is used as reproduced data representing logic state "1". In a VTR of the 8 mm video format, a recording signal obtained by time divisionally multiplexing audio data as shown in fig 9A, is generated in a recording information forming circuit 1 (fig 10) during operation in a recording mode and a video signal is recorded on a magnetic tape 4 through a recording amplifier 2 and a pair of rotary magnetic heads 3 and 3′. In this case, an audio signal processor in the form of circuit 1 digitally converts an input analog audio signal, encodes it into a PCM signal, forms a serial data string from the encoded data, and then modulates data "0" as a signal of 2.9 MHz frequency and data "1" as a signal of 5.8 MHz. During reproduction, the recording information thus recorded on the magnetic tape 4 is picked up by a pair of rotary magnetic heads 11 and 11′, and input to an intermediate tuning circuit 13 through a reproducing amplifier 12. Of the reproduced signals, high frequency signal components of a video signal are peaking-corrected, and the obtained video signal VD is supplied to a video signal processor 14. Audio data AD is input to an equalizing circuit 16 of the audio signal processor 15. A switching circuit for selecting the video signal and the audio signal has been omitted from fig 10. The equalizing circuit 16 is constituted by connecting a low-pass filter 18 comprising an operational amplifier 17 in series with a high-pass filter 19. An output from the high-pass filter 19 is supplied as an equalizer output ADX to a PCM demodulator 21 through an output amplifier 20, so that an audio signal is demodulated by the PCM demodulator 21. The sampling signal Ps is input to the demodulator 21. The amplitude characteristic of audio data AD obtained from the intermediate tuning circuit 13 is affected by a frequency characteristics of the system from the magnetic tape 4 to the intermediate tuning circuit 13. The signals S₁ and S₂, may be are picked up under conditions in which a level difference, which would clearly distinguish the signal level LV₁ representing logic state ¨0" from the signal level LV₂ representing logic state "1" at the occurrence of the sampling pulse Ps, is not generated or is degraded; the equalizing circuit 16 corrects the signals S₁ and S₂ using the low and high-pass filters 18 and 19. When the level of the picked-up signal S₁ is significantly decreased with respect to that of the signal S₂ as shown in fig 11A, the signal level LV₁ of the signal S₁ at time tps, at which the sampling pulse Ps is generated, is decreased below the signal level LV₂ of the frequency signal S₂. Accordingly, although the frequency signal S₁ representing logic state "0" is intended to be picked up, audio data of logic state ¨0¨ is not reproduced in accordance with the frequency signal S₁. On the contrary, when the signal level of the signal S₂ is significantly decreased, with respect to the signal S₁, as shown in fig 11B, the signal level LV₂ of the frequency signal S₂ at time tps, at which the sampling pulse Ps is generated, becomes very low even if the signal S₂ is picked up. Accordingly, the signal S₂ may not be accurately determined as audio data of logic state "1". Theoretically, when the ratio of the amplitude of the signal S₂ to that of the signal S₁ is about -6 (dB), the level difference between the signal levels LV₁ and LV₂ obtained when the sampling pulse Ps is generated can be practically determined. For this reason, it is assumed that data can be reproduced at a sufficiently low error rate. In consideration of the above problem, the conventional equalizing circuit 16 equalizes such that the amplitude characteristic of the equalizer output ADX is flattened throughout the range of the low frequency F₁ = 2.9 (MHz) to the high frequency f₂ = 5.8 (MHz). Frequency characteristics of the low and high-pass filters 18 and 19 are set to obtain the above characteristic. However, the frequency characteristic of the system from the magnetic tape 4 to the intermediate tuning circuit 13 is actually different for different types of VTRs. Especially when the types of the magnetic tapes 4 used as recording media are different, influences on the characteristics of the picked-up audio data AD are different from each other. For this reason, equalization cannot be performed correctly by the conventional equalizing circuit 16 of fig 10. For example, as shown in fig 12, a coating type magnetic tape 25 formed by coating a magnetic layer obtained by mixing ultrafine particles of an alloy of nickel and cobalt together with a binder into iron and having a thickness of about 3µ on a base 25B is often used as the magnetic tape 4. As shown by the curve MP in fig 13, the frequency characteristic of the tape is such that a substantially uniform signal level can be obtained in the low frequency range but the signal level is abruptly decreased toward the high frequency range, and an amplitude decrease of about ΔG₁₂ = 2 to 3 (dB) occurs near 5.8 (MHz). On the contrary, as shown in fig 14, when a vapour metal deposition type magnetic tape 26, obtained by forming a 0.15-thick magnetic layer 26A on a base 26B by sputtering, is used, signal components in the a low frequency range pass through the magnetic layer 26A during recording since the magnetic layer 26A is thin. Accordingly, as shown by the curve ME in fig 13, the trailing characteristic appears in the low frequency range, and the signal level near 2.9 (MHz) decrease by about ΔG₁₁ = 1 (dB). On the contrary, in the high frequency range, the characteristic extends toward a higher range than with the coating type magnetic tape 25, and the amplitude is not decreased near f₂ = 5.8 (MHz). As described above, two types of magnetic tapes, 25 and 26 having different frequency characteristics are sometimes used as a magnetic tape 4 of a VTR. In those circumstances, if the conventional equalizing circuit 16 is used, it is very difficult to produce an equalizer output ADX of which the ratio of signal levels of the signals S₁ and S₂ has an optimal value (ie -6 (dB)) for both the coating type and vapour deposition type magnetic tapes 25 and 26. The present invention has been made in consideration of the above situation, and has as its object to provide a data reproducing apparatus in which, when the frequency characteristic of reproduced data picked up from a video head 11 changes in accordance with different types of magnetic tape, the equalization characteristic of an equalizing circuit can be adjusted to an optimal value in accordance therewith. According to the present invention, there is provided a data reproducing apparatus for reproducing data recorded on a magnetic recording medium using a magnetic transducing head by modulating an encoded digital information signal as first and second frequency components in accordance with logic states "0" and "1", comprising an equalizing circuit interposed in a signal path between a reproducing magnetic transducing head and a decoder or demodulator for the encoded digital information signal, for compensating for signal degradation caused by the electromagnetic transducing characteristic between the magnetic recording medium and said recording and reproducing transducing head, characterised in that said equalizing circuit comprises: a) low-pass and high-pass filters for controlling at least the amplitude of the second frequency signal component of the signal in said signal path and at least the amplitude of the first frequency component of the signal in said signal path respectively;and b) switching means for switching at least the cut off frequency of one of said low-pass filter and said high-pass filter in accordance with the presence of a respective one of a plurality of types of the recording medium, and thereby maximizing an aperture opening ratio of an eye-pattern by setting the ratio of the amplitudes of the first and second frequency components to a predetermined range independent of the respective type of recording medium. When the type of magnetic tape is changed, the amplitude characteristics and the phase characteristics of the output of the reproducing transducer head change. Therefore, the eye-opening ration of an eye-pattern of the head output is decreased, and the bit error rate of demodulated data may be increased. When in the present invention the characteristic of the equalizing circuit is changed, an equalization characteristic is switched in accordance with the characteristic change. Therefore, equalization processing is performed such that the eye-opening ration of the eye-pattern of the equalizer output is maximized, thereby decreasing the bit error rate of the reproduced data. Reference will now be made to the accompanying drawings, in which: Fig 1 is a circuit diagram showing an arrangement for explaining the principle of a data reproducing apparatus according to the present invention; Figs 2A to 2C and figs 3A to 3C are views of characteristic curves for explaining improvements in the frequency characteristic and phase characteristic obtained with the apparatus shown in Fig 1; Figs 4A and 4B are views of characteristic curves for explaining an improvement in an amplitude characteristic according to the type of a magnetic tape; Fig 5 is a circuit diagram showing a first embodiment of the present invention; Fig 6 is a view of characteristic curves showing an improvement in a bit error rate of the first embodiment shown in fig 5; Fig 7 is a circuit diagram showing a second embodiment of the present invention; Fig 8 is a circuit diagram showing a third embodiment of the present invention; Figs 9A to 9C are views of signal waveforms for explaining data reproduction by a biphase-mark system; Fig 10 is a circuit diagram showing a conventional data reproducing apparatus; Figs 11A and 11 B are views of signal waveforms for explaining degradation in the eye-opening ratio; Fig 12 is a schematic sectional view showing an arrangement of a coating type magnetic tape; Fig 13 is a view of characteristic curves showing an amplitude characteristic of the tape shown in fig 12; Fig 14 is a schematic sectional view showing an arrangement of a vapour deposition type magnetic tape; Fig 15 is a view of characteristic curves showing an ideal phase characteristic; Fig 16 is a view of signal waveforms for explaining a problem posed when a phase shift occurs; and Fig 17 is a view of characteristic curves showing phase characteristics of various magnetic tapes. An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. Fig 1 shows an equalizing circuit, in which the same parts as in fig 10 are denoted by the same reference numerals. As for a normal equalizer output ADX obtained at an output terminal of an equalizing circuit 16, conditions such that zero crossing points of two signals S₁ and S₂ coincide with each other and the ratio of their amplitudes is -6 (dB) are established, as described above with reference to figs 9A to 9C. For this purpose, it can be assumed that the equalizing circuit 16 need only have an equalization characteristic by which the eye-opening ratio (a ratio of areas surrounded by waveforms of the signals S₁ and S₂) of the so called eye-pattern formed by the frequency signals S₁ and S₂ is maximized. For this purpose, it can be assumed that the ratio of the amplitude of the signal S₂ with respect to that of the signal S₁ included in the equalizer output ADX need only be -6 (dB), and that it is necessary only to form a signal having a linear phase rotation, (ie the phases of the frequency signals S₁ and S₂ with respect to the frequency f are positioned on a straight line K, as in fig 15). That is, first, assume that the ratio of the amplitudes of the frequency signals S₁ and S₂ is set at -6 (dB). In this case, as described above with reference to figs 11A and 11B, even if a picked-up signal with a poor eye-opening ratio is obtained, it is assumed that the frequency signals S₁ and S₂ having the same eye-opening ratio as that of recording signals having a correct relationship as described above with reference to figs 9A to 9C can be obtained. Second, when the phases of the picked-up frequency signals S₁ and S₂ are not on the straight line K of fig 15 but are in a non linear relationship, the phases of zero crossing points of the frequency signals S₁ and S₂ are shifted from each other as shown in fig. 16. In this case, the phase of a PLL clock PLL₀ which serves as the reference phase for the sampling pulse Ps is shifted, and the error rate is increased in a PCM demodulator 21 (fig 10) upon demodulation of audio data. If the phase relationship between the frequency signals S₁ and S₂ can be equalized to be a linear relationship, the phase difference between the zero crossing points described above with reference to fig 16 can be corrected. Phase rotation generated in circuits before the equalizing circuit 16 can thereby be corrected, which reduces the bit error rate. Assume that a coating type magnetic tape 25 and a vapour deposition type magnetic tape 26 described above with reference to figs 12 to 14 are used. When the coating type magnetic tape 25 is used, the tape itself has an intrinsic phase characteristic. Therefore, the phase shift Δϑ at the zero crossing points of the reproduced signals S₁ and S₂ is generated on a straight line K ME , the phase of which is shifted from an ideal straight line K (crossing a point 0) by about +20°, as shown in fig 17. On the other hand, when the vapour deposition type magnetic tape 26 is used, a phase shift is generated on a straight line K MP shifted by about -5° - -10° from the line K. Generally, in the metal coating type tape as shown in fig 12, it is known that magnetic particles having their orientation axes P at 0° and 90° are both present. In the vapour deposition type tapes as shown in fig 14, it is known that an oblique orientation axis P is obtained in accordance with the sputtering direction. Phase rotations of reproduced signals are generated in accordance with the frequencies of signals recorded in a magnetic layer. According to the present invention, the amplitude and phase characteristics are equalized by the arrangement shown in fig 1 so as to maximize the eye-opening ratio of the frequency signals S₁ and S₂ of the equalizer output ADX including the above phase shifts. In the equalizing circuit shown in fig 1, a capacitor C11, consisting of a variable capacitance circuit, is provided between an input terminal of an operational amplifier 17 and ground in a low-pass filter 18, and a variable resistance circuit is used as a resistor R3 in a high-pass filter 19. In the arrangement shown in fig 1, the audio data AD supplied to the input terminal of the equalizing circuit 16 may have the frequency characteristic shown in fig 2A. That is, low frequency components near a first frequency F₁ = 2.9 (MHz) or less exhibit a drooping characteristic caused by a rotary transformer (not shown) provided between the video head 11 and the reproducing amplifier 12 (fig 10). High frequency components near a second frequency f₂ = 5.8 (MHz) or more tend to decrease due to a characteristic of the video head. With respect to the audio data AD as described above, the low-pass filter 18 and the high-pass filter 19 of the equalizing circuit 16 have cut-off frequencies respectively set at f₂ = 5.8 (MHz) and f₁ = 2.9 (MHz) as shown in fig 2B, and are arranged to have a circuit constant which exhibits a cut-off characteristic by which the amplitude ratio of the frequencies f₂ and f₁ of the equalizer output ADX becomes -6 (dB). In addition, the phase characteristic of the audio data AD is shown in fig 3A. When the recording data recorded on the running magnetic tape 4 is picked up by magnetic heads 11 and 11′, substantially the same effect as subjecting the recorded data to differential processing is obtained. Accordingly, phase rotation of 90° is generated in the picked-up audio data AD. In addition, by performing high-frequency range peaking processing to the video signal in an intermediate tuning circuit 13 (fig 16), the audio data AD is affected so that the phase of the high frequency component is further rotated in a positive direction. With respect to the audio data AD having such a phase characteristic, the equalizing circuit 16 gives an equalizing characteristic which sets a phase rotation of about 90° with respect to a low frequency component, and rotates the phase toward the 0° direction, as the frequency is increased, by utilizing the cut-off characteristic of the high-pass filter 19, as shown in fig 3B. As a result, as shown in fig 3C, the equalizer output ADX gives substantially constant phase rotation to the low and high frequency components. As described above with reference to figs 13 and 17, the amplitude and phase characteristics of the audio data AD change according to the type of the magnetic tape 4. In the equalizing circuit 16 of fig 1, changes in the amplitude and phase characteristics are adjusted by changing the capacitance of the capacitor C11 consisting of a variable capacitance circuit and/or the resistance of the resistor R3 consisting of a variable resistance circuit, thereby realizing the equalizing function described above with reference to figs 2 and 3. When the coating type magnetic tape 25 is used as the magnetic tape 4, the cut-off characteristic of the low-pass filter 18 and/or the high-pass filter 19 is adjusted by changing the capacitance and resistance of the capacitor C11 and the resistor R3, thereby adjusting the amplitude characteristic of the equalizer output, as shown by MP in fig 4A. Thus, as described above with reference to figs 2A to 2C the amplitude characteristic of the equalizer output ADX is adjusted by adjusting the cut-off characteristics of the low-pass filter 18 and the high-pass filter 19 so that the maximum eye-opening ratio of the eye-pattern of the equalizer output ADX can be obtained. In addition, as described above with reference to figs 3A to 3C, by correcting the phase characteristic of the equalizer output ADX utilizing the cut-off phase rotation represented by the straight line K ME in fig 17, the phase of the signal being processed is corrected to the phase rotation represented by the straight line K. Therefore, flat phase rotation is applied to frequency components over substantially the entire range of the equalizer output ADX. In this state, assume that the vapour deposition type magnetic tape 26 is used as the magnetic tape 4. In this case, since the amplitude characteristic of the vapour metal deposition type magnetic tape 26 differs from that of the coating type magnetic tape 25, as described above with reference to fig 13, the amplitude characteristic of the equalizer output ADX will change in accordance with the difference between the two tapes, unless the capacitance of the capacitor C11 of the low-pass filter 18 and the resistance of the resistor R3 of the high-pass filter 19 are adjusted again. As a result, the eye-opening ratio of the eye-pattern of the equalizer output ADX is decreased. As for the phase characteristic, since phase rotation is generated in a negative direction in the phase characteristic of the vapour deposition type magnetic tape 26 as represented by the straight line K MP of fig 17, the phase characteristic of the equalizer output ADX is changed in accordance therewith. In order to correct such a change, the capacitance of the capacitor C11 of the low-pass filter 18 is switched to correspond to the vapour deposition type magnetic tape 26. At this time, as shown by the curve Me in fig 4B, the cut-off characteristic of the low-pass filter 18 is switched in a direction to increase the drooping characteristic as compared with the curve MP. This reduces the amplitude of frequency components fo the second frequency f₂ = 5.8 (MHz) by a gain - ΔG₂₂. Contrary to the coating type magnetic tape 25, the vapour deposition type magnetic tape 26 outputs a signal of a level higher than the frequency components of the second frequency f₂ by ΔG₁₂ as the audio data AD (fig 13). The amount of reduction, - ΔG₂₂, cancels ΔG₁₂. Accordingly, the amplitude of the signal components of the frequency f₂ included in the equalizer output ADX is equalized to be substantially the same as the amplitude obtained when the coating type magnetic tape 25 is used. In addition, by adjusting the resistor R3 of the high-pass filter 19, its cut-off characteristic is increased to decrease the drooping characteristic as shown by ME in fig 4B. As a result, the amplitude of the signal components of the frequency f₁ is corrected by increasing it by + ΔG₂₁. In this case, the amplitude of the signal components of the frequency f₁ of the audio data AD is reduced as the coating type magnetic tape 25 is switched to the vapour deposition type magnetic tape 26 (fig 13). Therefore, this is equalized by an increased amount +ΔG₂₁. As the coating type magnetic tape 25 is switched to the vapour deposition type magnetic tape 26, the phase of the audio data AD rotates in the direction toward 0° (fig 17). The resistance of the resistor R3 of the high-pass filter 19 is adjusted to provide a phase characteristic compensating for this phase rotation. As described above, the cut-off characteristics of the low-pass filter 18 and the high-pass filter 19 can be adjusted as needed. For this reason, the eye-opening ratio of the eye-pattern of the equalizer output ADX can always be adjusted to be the maximum value, thereby decreasing the error rate of the reproduced digital data. A data reproducing apparatus which can effectively prevent degradation in the eye-opening ratio of the eye-pattern of the equalizer output ADX from occurring when the type of the magnetic tape 4 is changed can be realized by the following arrangements based on the above principal. Fig 5 shows a first embodiment, in which the same parts as in fig 1 are denoted by the same reference numerals. In fig 5, the capacitor C11 is constituted by a capacitor C MP for a coating type magnetic tape and a capacitor C ME for a vapour deposition type magnetic tape are connected in parallel. The capacitor C ME for vapour deposition type is grounded through an NPN switching transistor TR1. The switching transistor TR1 is turned on/off by a detection output DET from a tape type detector 40 via the resistor R5. In the first embodiment, a fixed resistor constitutes the resistor R3 of the high-pass filter 19. The tape type detection may be performed by utilizing a type identification hole formed in a tape cassette housing, or a signal may be manually supplied (eg by the user operating a switch) instead of this detection. In fig 5, when the coating type magnetic tape 25 is mounted in a VTR, the detection output DET from the tape type detector 40 falls to logic state "0" to turn off the switching transistor TR1. As a result, the capacitance of the capacitor C11 is defined by the value of the coating type magnetic tape capacitor C MP . Thus, as described above with reference to figs 4A and 4B, the low-pass filter 18 exhibits the cut-off characteristic for maximizing the eye-opening ratio of the eye-pattern of the equalizer output ADX. In the first embodiment, the resistance of the resistor R3 of the high-pass filter 19 is pre-selected such that the eye-opening ratio of the eye-pattern of the equalizer output ADX is maximised when the coating type magnetic tape 25 is mounted, as described above with reference to figs 4A and 4B. On the other hand, when the vapour deposition type magnetic tape 26 is mounted, the tape type detector 40 detects this and raises the detection output DET to logic state "1", thereby turning on the switching transistor TR1. At this time, the capacitance of the capacitor C11 is the sum of the capacitance of the capacitor C MP for coating type magnetic tape and that of the capacitor C ME for vapour deposition type magnetic tape. The cut-off characteristic of the low-pass filter 18 is switched to exhibit the large drooping characteristic as described above with reference to the curve ME of fig 4B. Therefore, the amplitude characteristic of the frequency f₂ is corrected to the same extent as that obtained when the coating type magnetic tape 25 is used. As a result, the eye-opening ratio of the eye-pattern of the equalizer output ADX can be increased by an amount corresponding to the required amount of correction. Note that as shown in fig 6, the bit error rate with respect to the change in the capacitor C11 is minimized by the relatively low capacitance C1 as shown by the curve MP when the coating type magnetic tape 25 is used. On the other hand, when the vapour deposition type magnetic tape 26 is used, the bit error rate is minimized by the relatively high capacitance C2 as shown by the curve ME. Therefore, when the coating type magnetic tape 25 is mounted in the VTR, the bit error rate is adjusted to be minimized by selecting the capacitance of the capacitor C11 as C MP = C1. On the contrary, when the vapour deposition type magnetic tape 26 is mounted in the VTR, the bit error rate is adjusted to be minimized by selecting the capacitance of the capacitor C11 as C MP + C ME = C2. Fig 7 shows a second embodiment, in which the same parts as in fig 1 are denoted by the same reference numerals. In fig 7, as the resistor R3 of the high-pass filter 19 is constituted by series resistors R31 and R32, connected between input terminals of the output amplifier 20. An NPN switching transistor T2 is connected to the resistor R32. The base of the switching transistor TR2 receives the detection output DET from the tape type detector 40 for detecting the type of the tape mounted in the VTR, via an inverter IN and a resistor R6 (no inverter in fig 5). In the second embodiment, the capacitor C11 of the low-pass filter 18 consists of a fixed capacitor having a capacitance which provides the curve MP of the cut-off drooping characteristic (figs 4A and 4B) optimal for the coating type magnetic tape 25. In the arrangement of fig 7, when the coating type magnetic tape 25 is mounted in the VTR, the detection output from the tape type detector 40 is in logic state "0". The detection output DET is inverted by the inverter IN to turn on the switching transistor TR2. Therefore, only the resistor R31 effectively operates as the resistor R3 of the high-pass filter 19. Thus, as described above with reference to fig 4A, the cut-off drooping characteristic of the low-pass filter 19 has a value suitable for the amplitude and phase characteristics of the coating type magnetic tape 25. As a result, the eye-opening ratio of the eye-pattern of the equalizer output ADX is maximized. On the other hand, when the vapour deposition type magnetic tape 26 is mounted in the VTR, the detection output DET from the tape type detector 40 rises to logic state "1", turning off the switching transistor TR2. At this time, the resistance of resistor 3 is the sum of the series resistors R31 and R32 effectively serves as a resistance of the resistor R3 of the high-pass filter 19. Thus, the high-pass filter 19 raises the cut-off characteristic curve to decrease a drooping amount as described above with reference to the curve of fig 4B and corrects the phase characteristic of the equalizer output ADX as described above with reference to figs 3A to 3C. As described above by changing the amplitude and phase characteristics of the high-pass filter 19 to correspond to the vapour deposition type magnetic tape 26, the eye-opening ratio of the eye-pattern of the equalizer output ADX is improved in accordance with the changed amount. Fig 8 shows a third embodiment, in which the same parts as in figs 1, 5 and 7 are denoted by the same reference numerals. The capacitor C11 of the low-pass filter 18 is constituted by a variable capacitance circuit as in the first embodiment shown in fig 5. In addition, the resistor R3 of the high-pass filter 19 is constituted by a variable resistance circuit as in the second embodiment shown in fig 7. In the arrangement of fig 8, when the coating type magnetic tape 25 is mounted in the VTR, the low-pass filter 18 exhibits the amplitude characteristic according to the cut-off characteristic which corresponds to the coating type magnetic tape capacitor C MP . On the other hand, the high-pass filter 19 has the cut-off characteristic which exhibits the amplitude and phase characteristics corresponding to the coating type magnetic tape 25. On the contrary, when the vapour deposition type magnetic tape 26 is mounted in the VTR, the capacitance of the capacitor C11 of the low-pass filter 18 is the total capacitance value of the capacitor C MP for coating type magnetic tape and the capacitor C ME for vapour deposition type magnetic tape. The cut-off characteristic of the low-pass filter 18 is changed to exhibit the amplitude characteristic corresponding to the vapour deposition type magnetic tape 26, as described above with reference to fig 5. On the other hand, since the series resistors R31 and R32 are used as the resistor R3 in the high-pass filter 19, the amplitude characteristic corresponding to the vapour deposition type magnetic tape 26 is applied to the equalizer output ADX on the basis of the cut-off characteristic of the high-pass filter 19. At the same time, the phase characteristic can be switched to the characteristic optimal for the vapour deposition type magnetic tape 26 on the basis of the cut-off characteristic (figs 3A to 3C). Thus, according to the arrangement of fig 8, regardless of whether the coating type magnetic tape 25 or the vapour deposition type magnetic tape 26 is mounted in the VTR, equalization can be performed such that the eye-opening ratio of the eye-pattern of the equalizer output ADX is maximized. Other Embodiments 1. In the embodiments shown in figs 5, 7 and 8 the circuit characteristics are set such that the cut-off characteristic of the low-pass filter 18 and/or the high-pass filter 19 corresponds to the coating type magnetic tape 25. In addition, when the magnetic tape 4 is replaced with the vapour deposition type magnetic tape 26, degradation in the eye-opening ratio of the equalizer output ADX is improved in a direction toward the eye-opening ratio set for the coating type magnetic tape 25. On the contrary, if the eye-opening ratio is preset for the vapour deposition type magnetic tape and degradation in the eye-opening ratio generated when using the coating type magnetic tape is improved in a direction toward that of the vapour deposition type magnetic tape 26, the same effect as that described above can be obtained. 2. In the embodiments shown in figs 1, 5, 7 and 8, a single equalizing circuit 16 is provided, and by adjusting the capacitance of the capacitor C11 of the low-pass filter 18 and/or the resistance of the resistor R3 of the high-pass filter 19, the cut-off characteristic of the low-pass filter 18 and/or the high-pass filter 19 is switched in accordance with switching between the coating type magnetic tape 25 and the vapour deposition type magnetic tape 26. However, if two equalizing circuits, in which the capacitance of the capacitor C11 and the resistance of the resistor R3 respectively correspond to the coating type magnetic tape 25 and the vapour deposition type magnetic tape 26, are provided and switched to operate in accordance with the detection output DET from the tape type detector 40, the same effect as that described above can be obtained. 3. In the above embodiments, the resistor R3 of the high-pass filter 19 is varied to adjust both the amplitude and phase characteristics of the audio data AD. However, when either amplitude or phase characteristics is principally corrected, the bit error rate of the corrected factor can be effectively improved. 4. In the above embodiments, the description has been made with reference to the case wherein the present invention is applied to a VTR of the 8 mm video format in which a PCM audio signal recorded on an audio track is reproduced. However, the present invention is not limited to the above embodiments but can be widely applied to any case wherein data modulated by a biphase-mark system is demodulated.

Claims: A data reproducing apparatus for reproducing data recorded on a magnetic recording medium using a magnetic transducing head by modulating an encoded digital information signal as first and second frequency components in accordance with logic states "0" and "1", comprising an equalizing circuit (16) interposed in a signal path between a reproducing magnetic transducing head and a decoder or demodulator for the encoded digital information signal, for compensating for signal degradation caused by the electromagnetic transducing characteristic between the magnetic recording medium and said recording and reproducing transducing head, characterised in that said equalizing circuit comprises: a) low-pass and high-pass filters (18,19) for controlling at least the amplitude of the second frequency signal component (S2) of the signal in said signal path and at least the amplitude of the first frequency component (S1) of the signal in said signal path respectively; and b) switching means (40) for switching at least the cut-off frequency of one of said low-pass filter and said high-pass filter in accordance with the presence of a respective one of a plurality of types of the recording medium, and thereby maximizing an aperture opening ratio of an eye-pattern by setting the ratio of the amplitudes of the first and second frequency components to a predetermined range independent of the respective type of recording medium. An apparatus according to claim 1, wherein the characteristics of the equalizing circuit (16) are adapted for a metal particle coating type magnetic tape (25) and a vapour metal deposition type magnetic tape (26). An apparatus according to claim 1 or 2, wherein, in use, said switching means (40, R5, TR1) switches a value of a capacitance in said low-pass filter (18) in accordance with a type of said magnetic recording medium. An apparatus according to claim 1 or 2, wherein, in use, said switching means (40, IN, R6, TR2) switches a value of a resistance in said high-pads filter (19) in accordance with a type of said magnetic recording medium. An apparatus according to claim 1 or 2, wherein, in use, said switching means (40, R5, TR1, IN, R6, TR2) switches the values of both a capacitance (11) and a resistance (R3) respectively of said low-pass filter (18) and said high-pass filter (19) in accordance with the type of said magnetic recording medium. An apparatus according to claim 2 or any one of claims 3 to 5 when appended thereto, wherein the cut-off frequency of said low-pass filter (18) is lower for a vapour deposition type tape than for a metal particle coating type tape. An apparatus according to claim 2 or any one of claims 4 to 6 when appendant thereto, wherein the cut-off frequency of said high-pass filter (18) is higher for a metal particle coating type tape than for a vapour deposition type tape. An apparatus according to any one of the preceding claims, wherein the type of said magnetic recording medium is detected by detecting means provided in a tape cassette housing which, in use, houses the magnetic tape.

Inventors: NAKANO, KENJI
Applicants: SONY CORPORATION
Application year: 1987
Main classification: G11B20
IPC codes: H04N5, G11B20, G11B5