JPH0576105B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION For example, a digital audio device that converts an audio signal into PCM and records and reproduces it on a magnetic tape using a rotating head has been put into practical use.

In such devices, various servos are applied to drive the head motor, the capstan motor for transporting the tape, the reel motor, etc. in accordance with recording signals, reproduction signals, etc. In this case, the most effective method in terms of ease of control and low power consumption is to PWM the motor power supply in accordance with the servo signal and thereby control the motor drive.

However, as mentioned above, when the audio signal is converted to PCM and recorded and played back, the pulse of the PWM signal for the servo leaks into the audio signal system, causing a beat with the PCM sampling frequency and generating audible frequency noise. was found to interfere with acoustic signals.

That is, for example, if the sampling frequency is 44.1kHz,
When the carrier frequency of the WPM signal is 20kHz,
4.1kHz between 44.1kHz and 40kHz twice the carrier frequency
, and various beats are generated to generate audible frequency noise.

In view of these points, the present invention is intended to prevent the above-mentioned noise from occurring with a simple configuration. That is, in the present invention, first, the carrier frequency of the PWM signal is selected to be an integral multiple of 1/2 of the sampling frequency, and is synchronized with sampling. However, if this is done alone, the sideband components may cause a beat with the sampling frequency because the PWM signal is modulated. Therefore, the second method is to fix one edge of the PWM signal pulse, modulate it at the other edge, and limit the slew rate of this modulated edge to be smaller than the slew rate of the fixed edge. It is.

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, an acoustic signal supplied to input terminal 1 is supplied to sample-and-hold circuit 2, and a 44.1kHz sampling signal from oscillator 3 is supplied to circuit 2, and at the timing of this sampling signal. be sampled. This sampled signal is supplied to the PCM circuit 5 through the AD conversion circuit 4, and is
It is assumed to be a PCM signal. This PCM signal is supplied to the rotary head 8 through the recording amplifier 6 and the recording side contact R of the recording/reproducing switch 7, and is recorded on the tape 9.

In addition, the signal reproduced by the rotary head 8 passes through the reproduction side contact P of the switch 7 and the reproduction amplifier 10.
The signal is supplied to the PCM demodulation circuit 11. The PCM demodulated signal is then taken out to the output terminal 13 through the DA conversion circuit 12.

Further, the sampling signal from the oscillator 3 is supplied to the 1/2 frequency divider circuit 14 to form a carrier wave of the 22.05kHz PWM signal. This signal is supplied to a ²⁶ multiplier circuit 15 to form a clock signal. This clock signal is applied to the clock terminal of a 6-bit counter 16. Further, the carrier wave from the frequency dividing circuit 14 is supplied to the load terminal of the counter 16. The carry output of this counter 16 is supplied to the count enable terminal of the counter 16 through an inverter 17.

Further, the output of the inverter 17 is grounded through a resistor 18 and a capacitor 19. This resistor 1
8 and the capacitor 19 are connected to the base of the transistor 20, and the capacitor 2
1. Connected to a 5V power supply terminal 23 through a Schottky diode 22. Further, the collector of the transistor 20 is grounded, the emitter is connected to the midpoint between the capacitor 21 and the diode 22, and the motor 24 is connected between the emitter and the power supply terminal 23.

The rotary head 8 is driven by this motor 24. Further, a pulse generator and a frequency generator 25 are provided on the rotating shaft of the motor 24, and this output signal and a synchronization signal from the PCM circuit 5 are supplied to a servo circuit 26, which outputs predetermined servo data. This data signal is applied to the load data terminal of the counter 16.

In this device, from the multiplier circuit 15 to FIG.
from the clock signal and frequency divider circuit 14 such as
When a carrier wave as shown in Figure B is supplied to the counter 16, the data L from the servo circuit 26 is loaded into the counter 16 at the time when the carrier wave is supplied, and from then on, data L is loaded into the counter 16 at the timing of the clock signal as shown in Figure 2C. The value of L is counted up by 1. When the count value reaches 63, the carry signal falls as shown in FIG. 2D, and counting is stopped thereafter and the count value remains at 63 until the next data is loaded.

Furthermore, by supplying a signal from the inverter 17, the potential at the midpoint of the connection between the resistor 18 and the capacitor 19 becomes as shown in FIG. 2E. By supplying this potential to the base of the transistor 20, the collector potential of the transistor 20 becomes as shown in FIG. 2F, and the motor 24 is driven while the collector potential is at a low potential.

Therefore, in this circuit, depending on the value of data L from the servo circuit 26, when L is large, the timing at which the transistor 20 is turned off becomes earlier, and when L is small, the timing at which the transistor 20 is turned off is delayed. , L can be adjusted to control the rotation of the motor 24.

That is, the edge at which the transistor 20 is turned on is fixed in synchronization with the sampling signal, and the edge at which the transistor 20 is turned off is modulated in accordance with the servo data L.

Furthermore, the slew rate when the transistor 20 is turned on is determined by 5-0.7/C・R, where the resistance value of the resistor 18 is R and the capacitance value of the capacitor 21 is C. The rate is 0.7/C·R, and the slew rate of the modulated edge is small.

In this way, it is possible to drive a motor in a digital audio device, and according to the present invention, the carrier frequency of the PWM signal for driving the motor is
Since it is an integral multiple of 1/2 of the PCM sampling frequency and these are synchronized, these signals cause the beat to be a DC component or 22.05kHz, which is outside the audible frequency range. In addition, one edge of the PWM signal pulse is fixed and the other edge is used for modulation, and the slew rate of the modulated edge is small, so sideband harmonics due to this modulation are reduced. , interference due to beats with the sampling frequency is reduced.

Note that the above example is a case where the integer is 1 and the PWM carrier frequency is 1/2 of the sampling frequency, but the same effect can be obtained by setting the integer to other values, such as setting the integer to 3 and multiplying it by 3/2. be.

The present invention can also be applied to carbstan motors, reel motors, and the like. Furthermore, if these are to be controlled simultaneously, it is only necessary to provide each circuit after the counter 16.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an example of the present invention, and FIG. 2 is a waveform diagram for explaining the same. 2 is a sample hold circuit, 3 is an oscillator, 16
is a counter, 24 is a motor, and 26 is a servo circuit.

Claims (1)

[Claims] 1. Sampling an acoustic signal at a predetermined sampling frequency,
In a motor drive circuit of a digital audio device that records or reproduces the converted digital signal and drives a motor used for this recording or reproduction with a PWM signal, a circuit that generates the sampling frequency; A circuit that generates a carrier frequency of the PWM signal selected to be an integral multiple of 1/2 of the sampling frequency and synchronized with the sampling; In addition to performing modulation, the slew rate of this modulated edge is limited to be smaller than the slew rate of the fixed edge.
A motor drive circuit for a digital audio device, characterized in that it is equipped with a PWM modulation circuit.