JPH01298579A - Pulse width modulation signal reproducing circuit - Google Patents

Abstract

PURPOSE:To prevent a reproduced sound from being deteriorated by allowing a signal obtained by deleting the frequency spectrum of a sampling frequency component from the frequency spectrum of a PWM signal to pass a programmable low pass filter (LPF) and then executing the PWM demodulation of the signal. CONSTITUTION:A PWM signal outputted from a waveform shaping circuit 4 and a PWM signal having 180 deg. phase difference generated from a 180 deg. phase shifter 5 are inputted to a sampling frequency extractor 6, a sampling frequency fs component at the time of PWM modulation is extracted and the extracted component is inputted to a subtractor 7 to obtain a PWM signal formed by deleting the sampling frequency component from the frequency spectrum included in the PWM signal. The PWM signal is PWM demodulated through the programmable LPF 8 for programming cut-off frequency corresponding to a reproducing speed generated from a speed data generating circuit 9 and the original analog signal is outputted to an output terminal 10. Consequently, the generation of noise can be prevented and a reproducing speed variable type PWM reproducing circuit with effective reproducing sound quality can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a pulse width modulation signal reproducing circuit that performs variable speed reproduction of a pulse width modulation signal.

[Conventional technology]

FIG. 4 shows a block connection diagram showing a conventional digital tape recorder such as the X-86 model manufactured by Mitsubishi Electric Corporation. In the figure, 1 is a magnetic tape as a recording medium, 2 is a playback head, 13 is a pulse code modulation circuit (hereinafter referred to as PCM demodulation circuit) which is the main reproduction system.
1 is an output terminal that outputs the analog signal demodulated by the PCM demodulation circuit 11, 12 is a pulse width modulation circuit (hereinafter referred to as PWM demodulation circuit) which is a sub-reproduction system, and 10 is a P
An output terminal 9 outputs the analog signal demodulated by the WM demodulation circuit 12, and a speed data generation circuit 9 detects the playback speed as the running speed of the magnetic tape 1 and generates speed data proportional to the playback speed. .

Next, the operation will be explained. This digital tape recorder uses both pulse code modulation (hereinafter referred to as PCM) and pulse width modulation (hereinafter referred to as PWM) to record signals on separate tracks on the magnetic tape, and when playing back, the signals are recorded on separate tracks on the magnetic tape.
Both the CM method and PWM method can be played, and each method can be played according to the purpose. here,
The PCM method is called the main playback system, and the PWM method is called the sub playback system.

The analog signals recorded with both of these methods are magnetic tape 1.
In the above, although the track positions are different, the positions in the longitudinal direction of the magnetic tape 1 are made to match.

Normally, the main purpose of digital tape recorders is to faithfully record and play back input analog signals, so it is difficult to play back by changing the tape speed significantly, but high quality sound can be achieved by playing back at a steady speed. Up to now, the PCM method has been used because it is easy to process error correction codes and the like.

However, on the other hand, there is also a need to vary the playback speed, such as finding the beginning of the playback sound, finding the stamp point for editing the stamp, and queuing to quickly find the gaps between songs, so the sound quality is not so good. is PWM that allows easy variable speed playback.
A method of recording on separate tracks using a method is used.

Here, the concept of PWM modulating an analog signal is simply shown in the signal waveform diagram of FIG. That is, a in FIG. 5 is an analog signal, and b in FIG. 5 is a sawtooth wave having a frequency that is more than twice the frequency of the analog signal, according to the sampling theorem.
C in FIG. 5 is a PWM modulation waveform.

The PWM sub-playback system of this digital tape recorder is constructed as shown in FIGS. 6 and 7.

First, in FIG. 6, 1 is a magnetic tape which is a recording medium, 2 is a reproducing head, and 3 is a read amplifier equipped with an automatic gain control circuit that can always output a constant output regardless of the magnitude of the output of the reproducing head 2. 4 is a waveform shaping circuit that shapes the signal reproduced by this read amplifier 3 into a PWM waveform; 1
4 to 16 are low-pass filters each having a different cutoff frequency; 9 is a speed data generation circuit that detects the running speed of the magnetic tape 1, that is, the playback speed, and generates speed data proportional to the playback speed; 17 is a speed data generation circuit 9 is a selector circuit that selects one of the low-pass filters 14 to 16 having a necessary cutoff frequency according to the speed data; 10 is a low-pass filter 14 to 16;
This is an output terminal that outputs an analog signal that has been PWM demodulated.

Next, the configuration of FIG. 7, which is another conventional example, will be explained. In the figure, 1 is a magnetic tape which is a recording medium, 2 is a playback head, 3 is a read amplifier with an automatic gain control circuit that can always output a constant output regardless of the magnitude of the output of the playback head 2, and 4 is this read amplifier. lead amp 3
9 is a speed data generating circuit that detects the running speed of the magnetic tape 10, that is, the playback speed, and generates speed data proportional to this playback speed; 8 is a switched drive circuit; A programmable low-pass filter that uses capacitor technology and whose cutoff frequency can be varied in steps according to the playback speed according to the input speed data, 10 outputs an analog signal that has been PWM demodulated by the programmable low-pass filter 8. It is an output terminal.

Next, the operation of each of the circuit diagrams shown in FIGS. 6 and 7 will be explained. First, prior to this explanation, we will briefly explain why the cutoff frequency must be varied according to the playback speed when variable-speed playback is performed by varying the running speed of a magnetic tape 1 recorded using the PWM method. Explain.

Generally, the human audible range is said to be about 20 Hz to 20 KE (about 20 Hz), and in order to perform PWM modulation over the entire audible range,
According to the sampling theorem, a sampling frequency of at least f8=40 KHz is required. On the other hand, when attempting to record and reproduce data on a digital tape recorder using the PWM method, there are limitations in the electromagnetic conversion system, making it impossible to increase the sampling frequency unnecessarily. Therefore, in reality, the band is set to about 20 Hz to 10 -, and the sampling frequency is set to about 24 KHz to 30 KHz.

This sampling frequency is also called the carrier frequency,
When an analog signal is sampled based on the sampling theorem in this way, in addition to the frequency spectrum of the original analog signal as shown in Figure 8(a), the carrier frequency f, C sampling frequency), and fl A new separate frequency spectrum is added, as shown in Figure 8, called the centered sideband. Therefore, in order to demodulate the original analog signal from the PWM modulated signal, it is sufficient to remove the added extra frequency spectrum. If you pass it through a low-pass filter with a cutoff frequency f0 that is in the passband within the spectrum range and outband beyond that, it will be fine.

On the other hand, when a magnetic tape recorded with PWM modulation at a certain sampling frequency f8 during recording is reproduced at variable speed by varying the tape running speed, the frequency spectrum changes in proportion to the reproduction speed. K as shown in Figure 8(C)
For example, when the playback speed becomes 1/10, the frequency spectrum of the original analog signal also becomes 1/10, and both the sampling frequency fB and the sideband spectrum become 1/10.

Therefore, if the cutoff frequency of the low-pass filter, which is a demodulation filter for reproducing the original analog signal from the PWM signal, is kept fixed, the sampling frequency,
The extra frequency spectrum added by sampling, such as sidebands, enters the passband of the low-pass filter and significantly degrades the reproduced sound.

Therefore, it is necessary to vary the cutoff frequency depending on the playback speed.

Next, the operation will be explained with reference to FIG.

Magnetic tape 1 recorded by PWM at a certain sampling frequency fs
When reproduced by the playback head 2, the output level of the playback head 2 changes depending on the playback speed (tape running speed), but the output level of the read amplifier 3 changes regardless of the magnitude of the input.
The signal is amplified to a constant output level, and then waveform-shaped by the waveform shaping circuit 4 to output the original PWM signal.

On the other hand, the speed data generation circuit 9 generates speed data in proportion to the tape running speed, and based on this speed data, the selector circuit 17 selects one of the low-pass filters 14 to 16 having a cutoff frequency corresponding to the playback speed. Select the PWM signal to pass through its low-pass filter 14~
b PWM demodulated by passing through one of 16,
It is output to the output terminal 10 as the original analog signal.

On the other hand, the operation in FIG. 7 is as follows.

When a magnetic tape 1 recorded by PWM at a certain sampling frequency fll is played back by the playback head 2, the output level of the playback head 2 changes depending on the playback speed (tape running speed). Regardless of the magnitude of the input level, it is amplified to a constant output level, and then waveform-shaped by the waveform shaping circuit 4 to output the original PWM signal.

On the other hand, the speed data generation circuit 9 generates speed data in proportion to the tape running speed, and this speed data is input to the programmable low-noise filter 8, so that it becomes a low-pass filter with a cutoff frequency according to the playback speed. By passing the programmed PWM signal through this programmer filter 8,
PwM demodulated and output terminal 1 as the original analog signal
Output to 0.

In this case, a programmable ronosus filter 8 using switched canopy technology is inexpensive, but the programmable ronosus filter 8
Since the double signal is further sampled, in addition to the frequency spectrum components in the above example, a frequency spectrum component based on the sampling frequency FS of the programmable low-pass filter is added as shown in FIG.

Note that Fs-fB has the highest level as the aliasing noise component at this time, and when the aliasing noise of Fs-fB enters the audible band depending on the reproduction speed, it significantly deteriorates the reproduced sound.

[Problem to be solved by the invention]

Since the conventional pulse width modulation signal reproducing circuit is configured as described above, as shown in FIG.
For example, in the case where passive low-pass filters or active low-pass filters are used as the filters 4 to 16, if it is desired to vary the playback speed significantly, a large number of low-pass filters 14 to 16 having cutoff frequencies corresponding to the playback speed are required. There were problems such as being uneconomical. Furthermore, as shown in Fig. 7, a programmable low-pass filter 8 using switched capacitor technology has the advantage that even if the playback speed is varied considerably, the cutoff frequency can also be varied accordingly. , Since the input signal of the programmer filter 8 is further sampled, the sampling frequency during PwM modulation! In addition to 8, the sampling frequency F of the programmable low-pass filter 8
B - A sideband is added to the frequency spectrum, and the difference between the sampling frequency F of the programmable low-pass filter 8 and the sampling frequency f during PWM modulation, that is, the frequencies F and −f, is There has been a problem in that when aliasing noise enters the audible range depending on the playback speed, it significantly degrades the playback sound.

This invention was made to solve the above-mentioned problems, and even if the playback speed is changed significantly, the cutoff frequency can be varied accordingly, and the generation of the above-mentioned aliasing noise can be prevented. In particular, it is an object of the present invention to obtain a pulse width modulation signal reproducing circuit that can prevent deterioration of reproduced sound.

[Means to solve the problem]

The PWM signal reproducing circuit according to the present invention subtracts the sampling frequency component during PWM modulation from the PWM signal of the waveform shaping circuit using a subtracter, and deletes the frequency spectrum of the sampling frequency component from the frequency spectrum of the PWM signal. The resulting signal is passed through a programmable low-pass filter and demodulated at 2wM.

[Effect]

The PWM signal reproducing circuit of the present invention passes a signal obtained by removing the sampling frequency component during PWM modulation from the PWM signal after waveform shaping to a programmable ronos filter. The aliasing noise FS-f8 between the sampling frequency fIS during PWM modulation and the sampling frequency fIS during PWM modulation can be removed, and even when the playback speed becomes such that FB-fs falls into the audible band, the playback sound will not deteriorate. Try not to let it happen.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
In the figure, 1 is a magnetic tape which is a recording medium, 2 is a playback head, and 3 is a playback head 2, regardless of its output size.
A read amplifier having an automatic gain control circuit that can always output a constant output, 4 a waveform shaping circuit that shapes the signal reproduced by the read amplifier 3 into a PWM waveform, and 5 a waveform shaping circuit 4 that converts the signal regenerated into a PWM waveform into a PWM waveform. 1800 phase shifter 6 generates a PWM signal having a phase difference of 180';
In addition to the PWM signal having a phase difference of 1800 generated by the phase shifter 5, a sampling frequency extractor 7 generates a sampling frequency during PWM modulation; A subtractor that performs subtraction with the sampling frequency extracted by the sampling frequency extractor 6, 8 a programmable low-pass filter whose cutoff frequency can be varied in steps according to the input speed data using a switched capacitor, 9 10 is a speed data generation circuit that detects the running speed of the magnetic tape 1, that is, the playback speed, and generates speed data proportional to the playback speed; 10 is a PWM
When the signal passes through the programmable low-pass filter 8, it is demodulated back to the original analog signal and is an output terminal for outputting the analog signal.

Next, the operation will be explained.

A magnetic tape 1 recorded by PWM at a certain sampling frequency f.
When the playback head 2 plays back the tape, the playback head 2 changes as shown in Fig. 2(a) due to changes in playback speed (tape running speed).
The output level of the read amplifier 3 changes, but regardless of the magnitude of the input, it is amplified to a constant output level as shown in Fig. 2 (Fig. 2), and then the waveform shaping circuit 4 The waveform is shaped as shown in c), and the original PWM signal is output.

The PWM signal output from the waveform shaping circuit 4 passes through the 180° phase shifter 5, and is converted to the original PWM signal as shown in FIG. 2(d).
The PWM signal has a 180° phase difference with respect to the WM signal. The PWM signal output from the waveform shaping circuit 4 and 1
PW with 180° phase difference generated by 80° phase shifter 5
By inputting the M signal to the sampling frequency extractor 6, the second
As shown in figure (e), the sampling frequency component during PWM modulation is extracted, and then the PWM signal output from the waveform shaping circuit 4 and the sampling frequency component output from the sampling frequency extractor 6 are subtracted. By inputting it to the frequency spectrum of the PWM signal, the sampling frequency component is removed from the frequency spectrum of the PWM signal, and the second
A PWM signal from which the sampling frequency component is removed is obtained as shown in Figure (f). The PWM signal from which the sampling frequency component has been removed, which is output from the subtracter 7, is cut according to the playback speed according to the speed data from the speed data generation circuit 9, which generates speed data in proportion to the playback speed. The signal is PWM demodulated through a programmable low-pass filter 8 whose off frequency is programmed, and the original analog signal is outputted to an output terminal 10 as shown in FIG. 2 (b).

Figure 3 shows the essential part of the above operation explanation from the angle of the frequency spectrum. Figure 3(a) shows the main frequency spectrum of the PWM signal output from the waveform shaping circuit 4, Figure 3) is the main frequency spectrum of the sampling frequency component output from the sampling frequency extractor 6,
Figure 3 (C) shows that the subtracter T removes the sampled frequency component from the frequency spectrum of the PWM signal.
Main frequency spectrum, Fig. 3(d) is the main frequency spectrum after passing through the programmable low-pass filter (however, frequency components above fc are attenuated by the cutoff frequency f0 of the programmable low-pass filter 8). .

Therefore, among the aliasing noises with respect to the sampling frequency of the programmable low-pass filter 8, Fs(
Sampling frequency of programmable low-pass filter 8)
Since the fs (sampling frequency during PWM modulation) component is deleted, the quality of the reproduced sound at the output terminal 10 is improved.

Note that although the PWM method was used as the recording and reproducing method in the above embodiment, the same effects as in the above embodiment can be obtained even if the PAM method is used.

In addition, the PWM signal of one channel is converted to a high frequency clock to create a square wave whose count number is equal to the sampling frequency, this square wave signal is subtracted from the above PWM signal, and demodulated into an analog signal by a demodulation filter. You can also.

Furthermore, when recording 2-channel PWM signals, 18
It is also possible to record with a phase difference of 0.00, create a square wave equal to the sampling frequency of the two channels of PWM signals during playback, subtract that signal from the PWM signal, and demodulate it to an analog signal using a demodulation filter. can.

〔Effect of the invention〕

As described above, according to the present invention, PWM modulated PW
After removing the sampling frequency component during PWM modulation from the M signal, the input data is filtered using a programmable low-pass filter whose frequency can be varied in steps.
Since the configuration is configured to perform wM demodulation, the PWM for the sampling frequency F, which is unique to the programmable low-pass filter, is
It is possible to prevent the difference in sampling frequency f8 during modulation, that is, the generation of aliasing noise of Fs-fB, and there is an effect that a variable reproduction speed type PWM reproduction circuit with good reproduction sound quality can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block connection diagram showing a pulse width modulation signal reproducing circuit according to an embodiment of the present invention, FIG. 2 is a signal waveform diagram for explaining the operation of each part of the block in FIG. 1, and FIG. Frequency spectrum diagram to explain the operation of Figure 1,
Figure 4 is a block connection diagram showing the reproduction system circuit in a conventional digital tape recorder, Figure 5 is a signal waveform diagram showing the concept of PWM modulation, and Figures 6 and 7 are diagrams showing the conventional pulse width modulation signal reproduction circuit. 8 is a frequency spectrum diagram explaining the operation of the pulse width modulation signal reproducing circuit shown in FIG. 6, and FIG. 9 is a frequency spectrum diagram explaining the operation of the pulse width modulation signal reproducing circuit shown in FIG. 7. It is a spectrum diagram. 4 is a waveform shaping circuit, 7 is a subtracter, and 8 is a programmable low-pass filter. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] A waveform shaping circuit that shapes the waveform of the pulse width modulation signal to be reproduced, and subtraction between the pulse width modulation signal obtained from this waveform shaping circuit and the sampling frequency component extracted from this pulse width modulation signal, and the above pulse width A subtracter that deletes the frequency spectrum of the sampled frequency component from the frequency spectrum of the modulated signal, and a programmable programmable device that demodulates the pulse width modulation signal output by the subtracter at a cutoff frequency that corresponds to the reproduction speed. A pulse width modulation signal regeneration circuit equipped with a low pass filter.