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BACKGROUND OF THE INVENTION
The present invention generally relates to rotary head type digital signal reproducing apparatuses, and more particularly to a rotary head type digital signal reproducing apparatus for playing a magnetic tape which is pre-recorded with a tracking
reference signal in predetermined starting and ending portions of each track which is formed obliquely to the longitudinal direction of the magnetic tape and with a digital audio signal in an intermediate portion between the starting and ending portions
of each track.
In a digital audio tape recorder, an analog audio signal is modulated into PCM audio data by a pulse code modulation (PCM), and the PCM audio data are recorded on and reproduced from a magnetic tape. In a rotary head type digital audio tape
recorder which employs rotary magnetic heads, data are successively recorded on and reproduced from tracks formed obliquely to a longitudinal direction of the magnetic tape without a guard band between two mutually adjacent tracks, alternately by a pair
of rotary heads having gaps of mutually different azimuth angles. A tracking reference signal is recorded on and reproduced from starting and ending portions of each track, while the PCM audio data are recorded and reproduced from an intermediate
portion between the starting and ending portions of each track with a predetermined signal format.
The PCM audio data are recorded and reproduced with at least two kinds of modes. In a linear standard mode, the PCM audio data have a sampling frequency of 48 kHz, two channels and a quantization number of sixteen bits. On the other hand, in a
non-linear long-time mode (hereinafter referred to as a half-speed mode), the PCM audio data have a sampling frequency of 32 kHz, two channels and a quantization number of twelve bits. Actually, there are other non-linear modes such as a mode in which
the PCM audio data have a sampling frequency of 44.1 kHz, four channels and a quantization number of twelve bits, however, these other modes all have the same play time as the standard mode.
In the half-speed mode, the rotational speed of a rotary drum on which the rotary heads are mounted and the tape transport speed are respectively set to speeds which are one-half those in the standard mode. In addition, the frequencies of
digital signals (more accurately, the frequencies of clock pulses for producing the PCM audio data and a the tracking reference signal) are set to one-half those in the standard mode. In other words, the operation speed of the digital audio tape
recorder as a whole in the half-speed mode is set to one-half that in the standard mode, except for a part of the digital audio tape recorder where a conversion is carried out between the analog audio signal and the digital signal.
The data rate in the standard mode is 48 (kHz).times.2.times.16=1536 (kbits/sec), and the data rate in the half-speed mode is 32 (kHz).times.2.times.12=768 (kbits/sec). Accordingly, the sound quality obtained in the half-speed mode is slightly
deteriorated when compared to that obtained in the standard mode, but there is an advantage in that the play time in the half-speed mode is two times that in the standard mode for a given length of the magnetic tape because the operation speed of the
digital audio tape recorder in the half-speed mode is set to one-half that in the standard mode.
It is desirable that the half-speed mode is added to the rotary head type digital audio tape recorder having the standard mode. However, due to the following problems, the realization of such a digital audio tape recorder is difficult costwise
and technically, and would cause deterioration in the quality of the digital audio tape recorder.
Firstly, the tracking reference signal frequency and the carrier frequency of the PCM audio data in the half-speed mode become one-half those in the standard mode. For this reason, particularly in the reproducing mode, the operation frequencies
of an analog filter circuit part and a phase locked loop (PLL) circuit part for reading data within a signal processing circuit for processing the tracking reference signal and the PCM audio data must be switched between the standard and half-speed
modes. Alternatively, it is necessary to provide a circuit part exclusively for use in the standard mode and another circuit part exclusively for use in the half-speed mode.
Secondly, the coupling between the rotary heads mounted on the rotary drum and a recording amplifier and a reproducing amplifier is normally made through a rotary transformer. However, since the signal frequency in the half-speed mode becomes
one-half that in the standard mode, the coupling in the low frequency range becomes deteriorated in the half-speed mode.
Thirdly, when the setting is made so that an optimum carrier-to-noise ratio is obtained in the standard mode, an output voltage of the reproducing rotary head in the half-speed mode becomes one-half that in the standard mode because the relative
linear velocity between the magnetic tape and the rotary head in the half-speed mode is one-half that in the standard mode. As a result, according to this setting, the carrier-to-noise ratio becomes deteriorated in the half-speed mode.
Fourthly, a drum motor for rotating the rotary drum and a capstan motor for rotating a capstan which drives the magnetic tape must have predetermined performances in the two rotational speeds corresponding to the standard and half-speed modes,
where the predetermined performances refer to the tolerable range of the jitter in the rotation of the rotary drum for maintaining phase synchronism between the rotation of the rotary drum and an electrical circuit, the tolerable range of instability of
the rotation of the capstan motor and the like.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful rotary head type digital signal reproducing apparatus in which the first two problems described before are eliminated and the latter two problems described
before are substantially overcome.
Another and more specific object of the present invention is to provide a rotary head type digital signal reproducing apparatus for playing a magnetic tape pre-recorded with a time division multiplexed signal in one of first and second modes,
where the time division multiplexed signal comprises a PCM audio data which is obtained by subjecting an orignal audio signal to a pulse code modulation and a tracking reference signal. The tracking reference signal which amounts to a predetermined time
period is time division multiplexed before and after the PCM audio data which amounts to a certain time period, and the time division multiplexed signal is recorded on successive tracks formed obliquely to a longitudinal direction of the magnetic tape by
two rotary heads. In the second mode, a data quantity per unit time, a frequency of the tracking reference signal, a rotational speed of the rotary heads and a tape transport speed of the magnetic tape are one-half those in the first mode. The rotary
head type digital signal reproducing apparatus comprises a reproducing circuit including two rotary heads for reproducing pre-recorded signals from the magnetic tape, a circuit for setting the tape transport speed to a speed identical to that in the
second mode and for setting the rotational speed of the rotary heads to a speed identical to that in the first mode when playing the magnetic tape pre-recorded in the second mode, a tracking control circuit for controlling tracking of the rotary heads
based on reproduced tracking reference signals reproduced by the reproducing circuit, where the reproduced tracking reference signals include out of reproduced signals obtained from the reproducing circuit during a total of four successive scans made by
the rotary heads with respect to two mutually adjacent tracks on the magnetic tape at least a reproduced tracking reference signal obtained in an ending portion of a first scan and a reproduced tracking reference signal obtained in a beginning portion of
a second scan immediately after the first scan, and the first and second scans are successive scans in which a large reproduced output is obtainable by the rotary heads from the two mutually adjacent tracks, and a decoding circuit for decoding the
reproduced signals obtained from the reproducing circuit into the original audio signal based on a reproduced PCM audio data reproduced from one of the two mutually adjacent tracks during the first scan and a reproduced PCM audio data reproduced from the
other of the two mutually adjacent tracks during the second scan.
According to the apparatus of the present invention, the frequencies of the tracking reference signal and the modulated PCM audio data when playing the magnetic tape pre-recorded in the second mode can be made substantially the same as those in
the first mode, since the magnetic tape recorded in the second mode is played with the rotational speed of the rotary heads set to two times that at the time of the recording. For this reason, circuit parts of the apparatus can be used in common when
playing the magnetic tape pre-recorded in the first mode and when playing the magnetic tape pre-recorded in the second mode.
Still another object of the present invention is to provide a rotary head type digital signal reproducing apparatus in which the decoding circuit comprises a memory for temporarily storing the reproduced PCM audio data including parity codes, and
an error detecting and correcting circuit for detecting and correcting errors in the reproduced signals by carrying out an error detecting and correcting operation on the PCM audio data including parity codes stored in the memory. The error detecting
and correcting operation is carried out at the same speed when playing the magnetic tape pre-recorded in the first mode and the magnetic tape pre-recorded in the second mode, and the error detecting and correcting operation is repeated two times when
playing the magnetic tape pre-recorded in the second mode.
According to the apparatus of the present invention, the error detecting and correcting capability is improved compared to the conventional apparatus, and it is possible to obtain a reproduced audio signal of a high sound quality. A further
object of the present invention is to provide a rotary head type digital signal reproducing apparatus in which the error correcting operation is carried out within first correcting time periods during a first of two successive revolutions of the rotary
heads and also within second correcting time periods during a latter of the two successive revolutions when playing the magnetic tape pre-recorded in the second mode. The first correcting time periods respectively start a predetermined time after a
beginning of a scan to an end of the scan made by a corresponding one of the two rotary heads during the first revolution, and the second correcting time periods respectively start a predetermined time after a beginning of a scan to an end of the scan
made by a corresponding one of the two rotary heads during the second revolution. The error correcting operation is carried out also within at least one of third correcting time periods. One of the third correcting time periods is defined by an end of
one of the first correcting time periods corresponding to an end of the first revolution and a start of one of the second correcting time periods corresponding to the predetermined time after a beginning of the second revolution. The other of the third
correcting time periods is defined by an end of the one of the second correcting time periods and a start of the other of the second correcting time periods.
According to the apparatus of the present invention, the error correcting capability is further improved because the third correcting time period is effectively utilized for the correction.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a system block diagram showing a first embodiment of the rotary head type digital signal reproducing apparatus according to the present invention;
FIG. 2 shows a track pattern on a magnetic tape played on the apparatus shown in FIG. 1;
FIG. 3 is a diagram for explaining scanning loci of rotary heads of the apparatus shown in FIG. 1 in a half-speed mode;
FIG. 4 is a diagram for explaining contact areas of the rotary heads with respect to recorded tracks on the magnetic tape;
FIG. 5 is a diagram for explaining output signal levels of the rotary heads for the case shown in FIG. 5;
FIG. 6 shows a recording pattern of a tracking reference signal on the magnetic tape;
FIG. 7 shows scanning areas of the rotary heads with respect to a home track and tracks adjacent thereto;
FIG. 8 schematically shows a reproduced signal comprising the tracking reference signal and a PCM audio signal;
FIG. 9 is a diagram for explaining tracking offsets between tracking reference signal recording sections and the scanning loci of the rotary head;
FIG. 10 shows a tracking reference signal detection output versus tracking error characteristic;
FIG. 11 is a system block diagram showing an embodiment of a tracking reference signal detecting circuit in the block system shown in FIG. 1;
FIG. 12(A) through 12(D) and FIGS. 13(A) through 13(I) show signal waveforms for explaining the operation of the block system shown in FIG. 11;
FIGS. 14(A) through 14(E) and FIGS. 15(A) through 15(E) are timing charts for explaining the operation of a memory and the like in the half-speed mode and the standard mode, respectively;
FIG. 16 shows a memory map of the memory shown in FIG. 1;
FIG. 17 is a diagram for explaining an error correcting operation;
FIG. 18 is a system block diagram showing an embodiment of a timing generating circuit in the block system shown in FIG. 11;
FIGS. 19(A) through 19(I) and FIGS. 20(A) through 20(J) show signal waveforms for explaining the operation of the block system shown in FIG. 18;
FIG. 21 is a system circuit diagram showing an embodiment of a timing control circuit in the block system shown in FIG. 1;
FIG. 22 is a system circuit diagram showing an embodiment of a demodulating circuit in the block system shown in FIG. 1;
FIGS. 23(A) through 23(D) are timing charts for explaining unused time periods introduced during the error correcting operation;
FIG. 24 is a system block diagram showing an essential part of a second embodiment of the rotary head type digital signal reproducing apparatus according to the present invention;
FIG. 25 is a system circuit diagram showing an embodiment of a correction start signal producing circuit in the block system shown in FIG. 24;
FIGS. 26(A) through 26(I) are timing charts for explaining the operation of the circuit system shown in FIG. 25; and
FIG. 27 is a system block diagram showing an essential part of a modification of the second embodiment of the rotary head type digital signal reproducing apparatus according to the present invention.
DETAILED DESCRIPTION
FIG. 1 shows a first embodiment of the rotary head type digital signal reproducing apparatus according to the present invention. In order to facilitate the understanding of the present invention, FIG. 1 also shows an essential part of a
recording system which is known, and in this respect, the apparatus shown in FIG. 1 is actually a recording and reproducing apparatus.
First, although not directly related to the subject matter of the present invention, a description will be given on the recording operation. An analog audio signal applied to an input terminal 33 is converted into PCM audio data in an
analog-to-digital (A/D) converter 34, and the PCM audio data are thereafter written into a memory 35. The PCM audio data are added with a parity code generated in an encoder 36, and are supplied to a modulating circuit 37 after being interleaved and
being subjected to a time base compression. The modulating circuit 37 produces a modulated digital signal by modulating the PCM audio data and the added parity code based on a known modulation system (for example, the 8-10 conversion).
On the other hand, a timing control circuit 19 generates a gate signal, and a tracking reference signal such as a synchronizing signal f.sub.S and a pilot signal f.sub.P during an interval corresponding to a tracking reference signal recording
section of each track on a magnetic tape 16. The gate signal and the tracking reference signal are supplied to the modulating circuit 37, and the modulating circuit selectively outputs the modulated digital signal or the tracking reference signal
responsive to the gate signal.
Accordingly, the modulating circuit 37 produces a time division multiplexed signal in which the modulated digital signal is transmitted in a certain interval and the tracking reference signal is transmitted in predetermined intervals before and
after the certain interval. This time division multiplexed signal is passed through a recording amplifier 38 and a rotary transformer (not shown) and is supplied to two rotary magnetic heads mounted at diametrical positions on a rotary drum 14, and the
time division multiplexed signal is alternately recorded on a magnetic tape 16 by the two rotary magnetic heads in either the standard more or the half-speed mode. As a result, a track pattern such as that shown in FIG. 2 is formed on the tape 16.
In FIG. 1, a rotary shaft 12 of a drum motor 11 penetrates a central portion of a stationary drum 13 and is fixed to a central portion of the rotary drum 14. A rotary head 15 and another rotary head (not shown) are mounted at the diametrical
positions on a rotational plane of the rotary drum 14. The tape 16 is wrapped obliquely around a peripheral surface of the rotary drum 14 for an angular range of approximately 90.degree.. The tape 16 is transported in a direction A in a state pinched
between a capstan 17 and a pinch roller 39.
The rotary head 15 and the other rotary head have gaps of mutually different azimuth angles, and the two rotary heads have a track width greater than a track width of the recorded tracks on the tape 16. For example, the track width of the two
rotary heads are 1.5 times the track width of the recorded tracks. The rotary shaft 12 rotates unitarily with the rotary drum 14. In the standard mode, the rotary heads rotate at a rotational speed of 2000 rpm, for example, and the rotary heads also
rotate at this rotational speed of 2000 rpm in the half-speed mode of the present invention. In the half-speed mode of the present invention, the tape transport speed of the tape 16 which is driven by the capstan 17 is set to one-half the tape transport
speed in the standard mode.
The tape 16 has the track pattern shown in FIG. 2. In each track shown in FIG. 2, the PCM audio data amounting to a predetermined time is recorded in a track portion 40b indicated by a hatching, and the tracking reference signal (ATF1) is
recorded in a track portion 40 a between the beginning of the track and the beginning of the track portion 40 b and the tracking reference signal (ATF2) is also recorded in a track portion 40 c between the end of the track portion 40 b and the end of the
track.
The tape 16 is pre-recorded in the conventional half-speed mode, and at the time of the recording in the half-speed mode, two rotary heads rotate at 1000 rpm. However, in the half-speed mode of the present invention, this tape 16 is played by
rotating the two rotary heads at 2000 rpm which is two times the speed at the time of the recording. A drum servo circuit 20 controls the rotation of the drum motor 11 and the rotary drum 14, based on a drum servo signal supplied from a timing control
circuit 19. As is well known, the drum servo signal is compared in the servo circuit 20 with a drum pulse signal which is obtained by detecting the rotation of the rotary drum 14 by a stationary detection head 21, and the rotational speed and phase of
the rotary drum 14 are controlled by the drum servo circuit 20.
On the other hand, the tape transport speed in the half-speed mode of the present invention is identical to that at the time of the recording. As a result, the scanning loci of the two rotary heads which rotate at two times the rotational speed
at the time of the recording do not coincide with the tracks on the tape 16. As shown in FIG. 3, center lines of the successive scans become as indicated by phantom lines 1 through 4 with respect to two mutually adjacent tracks T.sub.2n and T.sub.2n+1.
In other words, four scans are made by the two rotary heads with respect to the two mutually adjacent tracks T.sub.2n and T.sub.2n+1.
In FIG. 3, the tracks T.sub.2n-1, T.sub.2n+1 and T.sub.2n+3 are tracks recorded by the rotary head having the gap of a negative azimuth angle, while the tracks T.sub.2n, T.sub.2n+2 and T.sub.2n+4 are tracks recorded by the rotary head having the
gap of a positive azimuth angle. No guard band is formed between two mutually adjacent tracks. The phantom lines 1 and 3 indicate the center lines of the scans made by the rotary head (for example, the rotary head provided diametrically to the rotary
head 15) having the gap of the negative azimuth angle, and the phantom lines 2 and 4 indicate the center lines of the scans made by the rotary head (for example, the rotary head 15) having the gap of the positive azimuth angle.
FIG. 4 shows the contact areas of the two rotary heads with respect to the tracks during the successive scans the center lines of which are indicated by 1 through 5 . Out of the signals alternately reproduced by the two rotary heads, a
reproduced signal having a frequency over a predetermined value is obtained from the rotary head only when the rotary head scans the track which has been recorded by the rotary head having the gap of the same azimuth angle, due to the well known azimuth
loss effect. In addition, the level of the reproduced signal is approximately proportional to the contact area of the rotary head which scans the track. Accordingly, when the two rotary heads scan the tape 16 shown in FIG. 3, the relationship of the
tracks from which the reproduced signals are obtained by the two rotary heads and the signal levels thereof becomes as shown in FIG. 5.
As may be seen from FIG. 5, out of the four scans 1 through 4 made by the two rotary heads with respect to the two mutually adjacent tracks T.sub.2n and T.sub.2n+1, the reproduced signal levels obtained from the tracks T.sub.2n and T.sub.2n+1
during the second and third scans 2 and 3 are large compared to the reproduced signals levels obtained from the tracks T.sub.2n and T.sub.2n+1 during the first and fourth scans 1 and 4 . Accordingly, in the present embodiment, the demodulation is
carried out on the modulated PCM audio data with in the reproduced signals obtained from the two mutually adjacent tracks T.sub.2n and T.sub.2n+1 during the second and third scans, so that a satisfactory signal-to-noise (S/N) ratio is obtainable.
Next, a description will be given on the reproduction of the tracking reference signal. As described before, the tracking reference signal is recorded in the beginning portion and the ending portion of each track. The tracking reference signal
is recorded with a signal pattern shown in FIG. 6. In FIG. 6, those parts which are the same as those corresponding parts in FIGS. 2 and 3 are designated by the same reference numerals. As shown in FIG. 6, the tracking reference signal comprises the
synchronizing signal f.sub.S for controlling the sample and hold timing of the detected output of the tracking reference signal, and the pilot signal f.sub.P. The frequency of the synchronizing signal f.sub.S is set to a relatively high frequency so
that a sufficient azimuth loss effect would occur. For example, the synchronizing signal f.sub.S recorded on one of the two mutually adjacent tracks has a frequency of approximately 500 kHz while the synchronizing signal f.sub.S recorded on the other of
the two mutually adjacent tracks has a frequency of approximately 780 kHz. On the other hand, the frequency of the pilot signal f.sub.P is set to a constant low frequency so that only a small azimuth loss effect would occur and the pilot signal f.sub.P
would be reproduced as crosstalk from an adjacent track. For example, the pilot signal f.sub.P has a frequency of approximately 130 kHz.
As may be seen from FIG. 6, in each track portion where the tracking reference signal is recorded, the synchronizing signal f.sub.S is recorded in a synchronizing signal recording section, a signal recording section of approximately 1.56 MHz, for
example, is provided after the synchronizing signal recording section for erasing the previous signal as indicated by a hatching, and the pilot signal f.sub.P is recorded in a pilot signal recording section. The signal recording sections are arranged so
that the pilot signal recording section in one track is adjacent to the signal recording section indicated by the hatching in a track immediately preceding the one track, and the synchronizing signal recording section and the pilot signal recording
section are not adjacent to each other between two mutually adjacent tracks. This signal pattern of the tracking reference signal is known.
When the track recorded with the tracking reference signal is scanned at the time of the reproduction in the same mode as that at the time of the recording by use of the rotary heads having the gaps of the same azimuth angles as those of the
rotary heads which were used at the time of the recording, the rotary head which scans the track T.sub.2n, for example, scans its home track T.sub.2n and portions of the adjacent tracks T.sub.2n-1 and T.sub.2n-1 on both sides of the home track T.sub.2n
as shown in FIG. 7. (A "home track" of a reproducing rotary head means the track which was recorded by a head having the same azimuth angle as the reproducing head.) As described before, the synchronizing signal f.sub.S is only reproduced from the home
track T.sub.2n, while the pilot signal f.sub.P is not only reproduced from the home track T.sub.2n but is also reproduced as crosstalk from the adjacent tracks T.sub.2n-1 and T.sub.2n+1. As a result, the content of the reproduced signal obtained from
the rotary head, the signal level and the reproduced track become as shown in FIG. 8.
However, FIG. 8 shows the case where the rotary head scans along a scanning locus identical to the track. When the rotary heads scan in the half-speed mode of the present invention the tracks which were recorded in the conventional half-speed
mode, the rotary heads scan along scanning loci different from the recorded tracks as shown in FIG. 9. In FIG. 9, those parts which are the same as those corresponding parts in FIG. 3 are designated by the same reference numerals.
In FIG. 9, with respect to a center line I of the track T.sub.2n, there is a tracking offset +b (average value) in the tracking reference signal recording section at the beginning of the track T.sub.2n and a tracking offset +a (average value) in
the tracking reference signal recording section at the end of the track T.sub.2n during the second scan 2 . In addition, with respect to a center line II of the next track T.sub.2n+1, there is a tracking offset -a (average value) in the tracking
reference signal recording section at the beginning of the track T.sub.2n and a tracking offset -b (average value) in the tracking reference signal recording section at the end of the track T.sub.2n during the third scan 3 .
Accordingly, when the detected outputs of the two tracking reference signals including errors caused by the tracking offsets +a and -a are averaged, that is, when tracking error signals caused by the tracking offsets +a and -a are averaged, the
effects of the tracking offsets become zero, and there is no need to carry out an additional process such as adding an offset voltage to the detected output of the tracking reference signal. Similarly, when the tracking error signals caused by the
tracking offsets +b and -b are averaged, the effects of the tracking offsets also become zero. However, b is greater than a and it is desirable that the tracking error signal is small from the point of view of minimizing the jitter in the tracking
reference signal. In addition, it may be seen from FIG. 10 which shows a tracking reference signal detection output versus tracking error characteristic III (S-curve) that the tolerable range of the jitter in the tracking reference signal, that is, the
jitter margin, is larger for the case where the tracking error signals caused by the tracking offsets +a and -a are averaged.
Accordingly, in the present embodiment, the tracking control is carried out based on the two tracking reference signals respectively reproduced from the tracking reference signal recording section (track portion 40c) at the end portion of the
track scanned during the second scan 2 and from the tracking reference signal recording section (track portion (40a) at the beginning portion of the track scanned during the third scan 3 .
It is of course possible to carry out the tracking control based on the tracking reference signals reproduced from all of the four tracking reference signal recording sections at the beginning and end portions of the two tracks scanned during the
second and third scans 2 and 3 . Furthermore, it is possible to carry out the tracking control based on arbitrarily selected ones of the tracking reference signals reproduced from the four tracking reference signal recording sections of the two tracks
scanned during the second and third scans 2 and 3 , and subtract an appropriate offset voltage when the averaged tracking offset does not become zero.
Returning now to the description of FIG. 1, the signals alternately reproduced from the tape 16 by the two rotary heads are passed through a rotary transformer (not shown) and a reproducing amplifier 22, and are supplied to a wave equalizing
circuit 23 and a filtering and wave equalizing circuit 24. The synchronizing signal f.sub.S and the pilot signal f.sub.P are filtered and then subjected to a wave equalization in the filtering and wave equalizing circuit 24, and are supplied to a
tracking reference signal detecting circuit 25.
As described before, the tracking reference signal is recorded in the conventional half-speed mode with a frequency which is one-half that in the standard mode. However, during the reproducing in the half-speed mode of the present invention, the
rotary drum 14 rotates at the rotational speed which is two times that at the time of the recording which is carried out in the conventional half-speed mode, while the tape transport speed is set identical to that at the time of the recording. As a
result, the tape transport speed is considerably small compared to the rotational speed of the rotary drum 14, and the tracking reference signal is reproduced with a frequency which is substantially the same as that in the standard mode. Hence, the
filtering and wave equalizing circuit 24 can be used in common for the standard and half-speed modes.
Similarly, in the half-speed mode of the present invention, the carrier frequency of the PCM audio data becomes substantially the same as that in the standard mode. For this reason, the wave equalizing circuit 23, a phase locked loop (PLL)
circuit 28 and the like can be used in common for the standard and half-speed modes.
FIG. 11 shows an embodiment of the tracking reference signal detecting circuit 25. In FIG. 11, the reproduced tracking reference signal is applied to an input terminal 41 and is supplied to a bandpass filter 42 and a lowpass filter 43. The
bandpass filter 42 separates the reproduced synchronizing signal f.sub.S shown in FIG. 12(A) and FIG. 19(A) which will be described later, and supplies the reproduced synchronizing signal f.sub.S to a detecting circuit 44. The detecting circuit 44
detects the envelope of the reproduced synchronizing signal f.sub.S and supplies an output detection signal to a timing generating circuit 46 through a comparator 45. On the other hand, the lowpass filter 43 separates the reproduced pilot signal f.sub.P
shown in FIG. 12(B) and FIG. 19(B) which will be described later.
First and second timing signals respectively shown in FIGS. 13(H) and 13(I) are generated from the timing control circuit 19 shown in FIG. 1 and are supplied to the timing generating circuit 46 through input terminals 47 and 48. As shown in
FIGS. 13(I) and 20(B), the second timing signal is a square wave having a polarity which is inverted for every two track scans in synchronism with the scanning of the tracks by the rotary heads shown schematically in FIGS. 14(A) and 20(A). On the other
hand, as shown in FIG. 13(H), the first timing signal is a square wave having a constant repetition frequency in the order of ten odd times that of the second timing signal, for example. In FIGS. 13 and 20, the reference numerals 1 through 4 correspond
to the first through fourth scans 1 through 4 of the rotary heads shown in FIGS. 2 and 9.
Based on the incoming signals, the timing generating circuit 46 generates a first sampling pulse signal PLS1 shown in FIG. 12(C), a second sampling pulse signal PLS2 shown in FIG. 12(D), a third sampling pulse signal PLS3 shown in FIG. 13(E), and
a fourth sampling pulse signal PLS4 shown in FIG. 13(G).
FIG. 18 shows an embodiment of the timing generating circuit 46. In FIG. 18, those parts which are the same as those corresponding parts in FIG. 11 are designated by the same reference numerals. In FIG. 18, the output signal of the comparator
45 shown in FIG. 19(C)is applied to a terminal 61 and is successively passed through monostable multivibrators 62 and 63, and an output signal of the monostable multivibrator 63 is supplied to one input terminal of a 2-input AND circuit 64. FIG. 19(D)
shows a Q-output signal of the monostable multivibrator 62, and FIG. 19(E) shows a Q-output signal of the monostable multivibrator 63.
The output signal of the comparator 45 is also successively passed through monostable multivibrators 65 and 66, and an output signal of the monostable multivibrator 66 is supplied to one input terminal of a 2-input AND circuit 67. Furthermore,
the output signal of the comparator 45 is successively passed through monostable multivibrators 68 and 69, and an output signal of the monostable multivibrator 69 is supplied to one input terminal of a 2-input AND circuit 70 and to one input terminal of
a 2-input AND circuit 71. FIGS. 19(F) and 19(G) respectively show a Q-output signal of the monostable multivibrator 65 and a Q-output signal of the monostable multivibrator 66. FIG. 19(H) shows a Q-output signal of the monostable multivibrator 68, and
FIG. 19(I) and FIG. 20(F) which will be described later show a Q-output signal PLS34 of the monostable multivibrator 69.
On the other hand, the first timing signal shown in FIG. 13(H) and applied to the input terminal 47 shown in FIG. 18 is applied as a clock pulse signal to clock terminals of delay (D-type) flip-flops 72 and 73 and a counter 75. At the same time,
the second timing signal shown in FIGS. 13(I) and 20(B) and applied to the input terminal 48 is applied to a data input terminal D of the flip-flop 72.
An AND circuit 74 is supplied with a Q-output signal of the flip-flop 72 and a Q-output signal of the flip-flop 73 and obtains a logical product of the two signals. An output signal of the AND circuit 74 is applied to a clear terminal CLR of the
counter 75. A counted output signal of the counter 75 is supplied to a decoder 76. The decoder 76 generates three kinds of gate signals G1, G2 and G3 respectively shown in FIGS. 20(C), 20(D) and 20(E) every time the counted value in the counter 75
reaches predetermined values. The gate signal G1 is supplied to the other input terminals of the AND circuits 64 and 67. The gate signal G2 is supplied to the other input terminal of the AND circuit 70, and the gate signal G3 is supplied to the other
input terminal of the AND circuit 71. Accordingly, the sampling pulse signal PLS1 shown in FIGS. 12(C) and 20(G) is obtained from the AND circuit 64, and the sampling pulse signal PLS2 shown in FIGS. 12(D) and 20(H) is obtained from the AND circuit 67.
In addition, the sampling pulse signal PLS3 shown in FIGS. 13(E) and 20(I) is obtained from the AND circuit 70, and the sampling pulse signal PLS4 shown in FIGS. 13(G) and 20(J) is obtained from the AND circuit 71.
Returning now the description of FIG. 11, the reproduced pilot signal f.sub.P shown in FIGS. 12(B) and 19(B) from the lowpass filter 43 is supplied to a detecting circuit 49 wherein the envelope of the reproduced pilot signal f.sub.P is detected. An output detection signal of the detecting circuit 49 is supplied to a sample and hold circuit 50 and a differential amplifier 51. The sample and hold circuit 50 samples and holds the signal reproduced from one of the two adjacent tracks on both sides
of the home track based on the sampling pulse signal PLS1, and supplies a sampled and held voltage to the differential amplifier 51.
An output signal of the differential amplifier 51 shown in FIG. 13(B) is supplied to a sample and hold circuit 52 and is sampled and held based on the sampling pulse signal PLS2 shown in FIG. 12(D) which is generated during a time when the pilot
signal f.sub.P is reproduced from the other of the two adjacent tracks on both sides of the home track. As a result, a sampled and held voltage shown in FIG. 13(C) is obtained from the sample and hold circuit 52. As may be seen from FIG. 13(C), the
sampled and held voltage obtained from the sample and hold circuit 52 is a detection voltage of the tracking reference signal comprising the held voltage of the envelope detection level of the pilot signal f.sub.P reproduced from the tracking reference
signal recording section at the end portion of the track during the second scan 2 and the held voltage of the envelope detection level of the pilot signal f.sub.P reproduced from the tracking reference signal recording section at the beginning portion of
the track during the third scan 3 .
The detection voltage of the tracking reference signal is supplied to sample and hold circuits 53 and 54 shown in FIG. 11 wherein the detection voltage is again sampled and held based on the respective sampling pulse signals PLS3 and PLS4 shown
in FIGS. 13(E) and 13(G). Thus, sampled voltages shown in FIGS. 13(D) and 13(F) are respectively obtained from the sample and hold circuits 53 and 54, and are passed through respective mixing resistors 55 and 56 to be averaged. As a result, an averaged
signal is obtained from a node between the resistors 55 and 56 and is outputted through an output terminal 57 as the tracking error signal.
The detection voltage of the tracking reference signal shown in FIG. 13(C) is sampled again in the sample and hold circuits 53 and 54 based on the sampling pulse signals PLS3 and PLS4 for the following reasons. That is, as may be seen from FIG.
13(C), the hold time of +a and the hold time of -a greatly differ. Since the tracking control is carried out based on the sampled and held voltages and the hold times, the resamplings in the sample and hold circuits 53 and 54 are carried out for the
purpose of preventing undesirable effects caused by the different hold times.
At the time of the reproduction in the standard mode, the sampling pulse signal PLS3 is generated for every time period in which the tracking reference signal is reproduced from the tracking reference signal recording section at the end portion
of each track. On the other hand, the sampling pulse signal | | |
