JPH02105368A - Pwm reproducing device - Google Patents

Abstract

PURPOSE:To prevent the generation of the abnormal sounds, etc., by sampling and holding the pulse width reproduction signal undergone the waveform shaping and holding the level of the signal which is sampled and held right before the drop-out in a section corresponding to the drop-out. CONSTITUTION:A PWM (pulse width modulation) reproduction signal undergoes the waveform shaping via a PWM reproduction circuit 3 and is inputted to a programmable PLPF 4. At the same time, a speed data production circuit 10 counts the sampling frequency to convert this counted frequency into the set data of the cut-off frequency of PLPF 4 and 6 respectively via a PWM sampling frequency extracting circuit 8 based on the high frequency clock produced by a high frequency reproducing clock generator 9. Then the set data are sent to both PLPF 4 and 6. An S/H (sampling/holding) signal production circuit 11 detects the drop-out and produces an S/H control signal and an S/H signal to send these signals to an S/H circuit 5. The circuit 5 holds the level of the signal sampled and held right before the drop-out in a section corresponding to the drop-out section. Thus an abnormal signal waveform is never produced.

Description

[Detailed description of the invention] [Industrial application field]

This invention is based on PWM recorded on a recording medium such as a magnetic tape.
The present invention relates to a PWM playback device that performs variable speed playback of signals.

[Conventional technology]

FIG. 3 is a block diagram showing a PWM playback device used in a digital tape recorder that samples an analog signal at a certain sampling frequency fs, pulse width modulates each sample value, converts it into PWM data, and records and plays back the data. In the figure, 1 is a magnetic tape that is a recording medium, 2 is a playback head that plays back information recorded on the magnetic tape 1, and 3 is a PWM signal after removing unnecessary band components of the playback signal from the playback head 2 and shaping the waveform. PWM regeneration circuit that returns to , 4 is PW
7 is a programmable low-pass filter (hereinafter abbreviated as PLPF) that removes unnecessary band components of the PWM reproduction signal reproduced by the M reproduction circuit 3 and demodulates the original analog signal;
Output terminal 8 that outputs the analog signal passed through PF4
9 is a PWM sampling frequency extraction circuit that extracts the sampling frequency from the PWM reproduction signal reproduced by the PWM reproduction circuit 3; 9 is a high frequency clock generator;
The IO counts the period of the sampling frequency extracted by the PWM sampling frequency extraction circuit 8 using the high frequency clock generated by the high frequency clock generator 9, and uses the counted number to determine the PLPF according to the playback speed of the magnetic tape 1.
This is a speed data creation circuit that creates setting data for the cutoff frequency of No. 4. Next, the operation will be explained. Before explaining the operation shown in FIG. 3, when performing variable speed playback of the magnetic tape 1 recorded by the PWM method with variable tape speed, the playback speed (
The reason why the cutoff frequency of the PLPF 4 must be made variable according to the tape speed will be briefly explained with reference to the fifth drawing. Generally, the human audible band is said to be approximately 20 Hz to 20 KHz, and in order to PWM modulate the entire audible band, a sampling frequency fs of at least fs = 40 KHz is required according to the sampling theorem. On the other hand, when recording and reproducing with a PWM digital tape recorder, there are limitations on the electromagnetic conversion system such as the reproducing head 2, and it is not possible to increase the sampling frequency fs unnecessarily. Therefore, in reality, the band is set to about 20 Hz to 10 KHz, and the sampling frequency fs is set to about 24 KHz to 30 KHz. This sampling frequency fs is also called a carrier frequency, and based on the sampling theorem, when an analog signal having a frequency spectrum as shown in FIG. 5(a) is sampled, ), a new frequency spectrum called a sideband centered at the carrier frequency fs (sampling frequency) is added to the frequency spectrum of the original analog signal. Therefore, PWM
In order to demodulate the original analog signal from the modulated signal, it is sufficient to remove the added extra frequency spectrum. To do this, the PWM modulated signal must be passed through a low-pass filter (LPF) with a cutoff frequency fc that is in the passband over a wide range of the frequency spectrum of the original analog signal, and out-band beyond that. good. On the other hand, when a magnetic tape recorded using PWM modulation at a certain sampling frequency fs during recording is subjected to variable speed playback in which the tape speed is varied during playback, the pulse width changes in proportion to the playback speed (tape speed). Therefore, the frequency spectrum changes. For example, as shown in Figure 5(C), the playback speed is 17.
10, the frequency spectrum of the original analog signal also becomes 1/10, and the sampling frequency fs and the sideband spectrum also become 1/10. Therefore, if the cutoff frequency fs of the LPF, which is a demodulation filter for obtaining the original analog signal from the PWM signal, is kept fixed, the sampling frequency fs and the sideband, which are frequency spectrum enters the bass band of the LPF, significantly deteriorating the amount of reproduction. Therefore, it becomes necessary to make the cutoff frequency fc variable according to the playback speed, and for this purpose, a PLPF 4 that makes the cutoff frequency fc variable is used. Next, the operation shown in FIG. 3 will be explained in conjunction with FIG. 4. Magnetic tape 1 recorded by PWM at a certain sampling frequency fs
is reproduced by the reproducing head 2, unnecessary bands are removed by the PWM reproducing circuit 3, and waveform shaping is performed.
Obtain the original PWM reproduction signal shown in a). In the same figure,
The black portion indicates that the pulse width varies within this range. On the other hand, from this PWM reproduction signal and the high frequency clock shown in FIG. 4(C) generated by the high frequency clock generator 9, the PWM sampling frequency extraction circuit 8 generates the PWM reproduction signal at the sampling frequency shown in FIG. 4(b). Extract. In addition, the speed data creation circuit 1o counts the period of the sampling frequency using the high frequency clock, obtains a count number corresponding to the playback speed of the magnetic tape 1, and converts this into a PLPF4 which demodulates the PWM playback signal.
Convert to setting data of cutoff frequency fc of P L
Send to PF4. The PWM reproduction signal that is the output of the PWM reproduction circuit 3 passes through the PLPF 4, which is set to a cutoff frequency according to the reproduction speed.
Unnecessary frequency components are removed, resulting in the original analog signal shown in FIG. 4(d), which is output to the output terminal 7.

[Problem to be solved by the invention]

Conventional PWM playback devices were configured as described above, and due to dropouts peculiar to magnetic tapes (intermittent playback output from the head due to missing magnetic powder or addition of dust, etc.), the PWM playback signal may be distorted as shown in Figure 4. If there is a dropout as shown by the dotted line in (a), the demodulation during that period will be abnormal, and for example, the fourth
An abnormal signal waveform as shown by the dotted line in figure (d) is output,
There were problems such as abnormal noises. This invention was made to solve the above-mentioned problems, and even if the playback speed changes significantly, the cutoff frequency of the PLPF required for demodulating the PWM signal can be set accordingly. , When a dropout occurs, pre-holding is performed, and the analog signal waveform after demodulation is output relatively smoothly, and abnormal noise does not occur using PWM.
The purpose is to obtain a playback device.

[Means to solve the problem]

The PWM reproducing device according to the present invention provides PWM after waveform shaping.
Setting data for setting the cutoff frequency of the PLPF and a sample hold signal are created from the playback signal, and after demodulating the PWM playback signal including dropout using the first PLPF, the sample hold is performed based on the sample hold signal. After that, the analog signal is reproduced through a second PLPF in order to remove the sample-and-hold crotter component from the sample-and-hold signal.

[Effect]

The sample-and-hold signal in this invention is created from the PWM playback signal after waveform shaping, so when a dropout occurs, no sample-and-hold command occurs in that dropout section. The sample data before dropout is held in advance, and the abnormal demodulated analog waveform at the time of dropout can be deleted, so an analog signal with less abnormal sounds etc. can be reproduced.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. 1st
In the figure, 1 to 4 and 7 to 10 correspond to the same reference numerals in FIG. 3, so their explanation will be omitted. Note that P L P F 4 is the first PLPF 4 here.
It's called. 5 is a sample and hold circuit (hereinafter abbreviated as S/H circuit) that samples and holds the analog signal output from the first PLPF 4; 6 is a sample and hold circuit that extracts a sample and hold crotter component from the analog signal sampled and held in the S/H circuit 5; A second PLPF removes the original analog signal, 7 is an output terminal that outputs the analog signal demodulated by the second PLPF 6, and 11 is an S/H circuit from which the sampling frequency extracted by the PWM sampling frequency extraction circuit 8 is extracted. This is a sample and hold signal generation circuit (hereinafter abbreviated as S/H signal generation circuit) that generates the sample and hold signal of No. 5. In addition, the first and second PLPF4.6
The cutoff frequency is set by the setting data created by the speed data creation circuit 10. Next, the operation will be explained. To explain with reference to the time chart in FIG. 2, the information recorded on the magnetic tape 1 that has been PWM recorded at a certain sampling frequency fs is
After removing unnecessary bands in the PWM reproducing circuit 3, waveform shaping is performed to obtain the original PWM signal, not shown in FIG. 2(a). On the other hand, from the PWM reproduction signal and the high frequency clock shown in FIG. 2(C) generated by the high frequency clock generator 9, the PWM sampling frequency extraction circuit 8 extracts the sampling frequency shown in FIG. 2(b) of the PWM reproduction signal. Extract. In the speed data creation circuit 10,
The period of the sampling frequency is counted by the high frequency clock, a count number corresponding to the reproduction speed of the magnetic tape l is obtained, and this is used to demodulate the PWM reproduction signal.
The data is converted into cutoff frequency setting data of the PLPF 4.6 and sent to the first and second PLPFs 4 and 6. Further, based on the sampling frequency and the high frequency clock, the S/H signal generation circuit 11 performs dropout detection as shown in FIG. 2(d), and based on this, the second
The S/H control signal shown in figure (e) is created, and this S/H control signal is
/Hf! The S/H signal shown in FIG. 2(f) is created using the I control signal and the sampling frequency and sent to the S/H circuit 5. The PWM playback signal that is the output of the PWM playback circuit 3 is first demodulated into an analog signal by passing through the first PLPF 4 whose cutoff frequency is set according to the playback speed, and then demodulated into an analog signal. The sample and hold shown in FIG. 2 ((2)) is performed based on the sample and hold signal.Next, this sample and hold signal is subjected to a cut similar to the first PLPF4 in order to remove the sample and hold crotter component. 2nd set to off frequency
By passing through the PLPF 6, the analog signal shown in FIG. 2(h) is outputted to the output terminal 7. In the signal shown in FIG. 2(h), the section corresponding to the dropout section is a pre-hold section in which the magnitude of the signal sampled and held immediately before dropout by the S/H signal continues to be held. No abnormal signal waveform occurs.

【Effect of the invention】

As described above, according to the present invention, the cutoff frequency setting data and the S/H signal of the first and second PLPF are created from the PWM reproduction signal, and the first and second PLPFs controlled by the cutoff frequency setting data and the S/H signal are The second PLPF, 378 circuit is connected to the PWM reproduction signal from the first PLPF-5/H circuit to the second PLP
Since the signal is configured to pass through F, pre-holding is performed in the dropout section, which has the effect of making it possible to obtain a smooth analog signal with less abnormal sounds.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a PWM reproducing device according to an embodiment of the present invention, FIG. 2 is a timing chart showing the operation of the same device, FIG. 3 is a block diagram showing a conventional PWM reproducing device, and FIG. 4 is a block diagram showing a PWM reproducing device according to an embodiment of the present invention. FIG. 5 is a timing chart showing the operation of the apparatus, and a spectrum diagram illustrating the necessity of making the cutoff frequency of the low-pass filter variable in accordance with variable speed reproduction. 4 is a first PLPF, 5 is a sample hold circuit, 6 is a second PLPF, 10 is a speed data creation circuit, 11
is a sample and hold signal generation circuit. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] A speed data creation circuit that creates setting data according to the playback speed from the PWM playback signal, a sample hold signal creation circuit that creates a sample hold signal according to the playback speed from the PWM playback signal, and a cutoff based on the setting data. a first programmable low-pass filter to which the frequency is set and the PWM reproduction signal is supplied;
a sample and hold circuit that samples and holds the output signal of the first programmable low-pass filter using the sample and hold signal; and a second programmable low-pass whose cutoff frequency is set according to the setting data and to which the output signal of the sample and hold circuit is supplied. A PWM regenerator equipped with a filter.