JP2007110650A - Signal reproducing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a signal reproducing apparatus capable of processing a received multi-bit signal as a digital signal just before a switching amplifier thereby capable of minimizing signal deterioration. <P>SOLUTION: In the signal reproducing apparatus, a delta sigma modulation circuit 1 modulates a received PCM signal into a PDM signal, the switching amplifier 2 amplifies a PDM signal by its switching operation and a low pass filter 3 converts an output signal from the switching amplifier 2 into an analog audio signal, and the output signal from the switching amplifier 2 converted into a digital signal by an A/D conversion circuit 8 is negatively fed back to an input side of a 1-bit digital signal modulation means. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention modulates an input PCM signal, which is a multi-bit signal, into a 1-bit digital signal such as a pulse density modulation (PDM) signal or a pulse width modulation (PWM) signal, and converts the modulated 1-bit digital signal to a switching amplifier. The present invention relates to a signal reproducing apparatus that amplifies by switching operation and reproduces by an output signal from a switching amplifier, and more particularly relates to a signal reproducing apparatus that suppresses signal distortion by negative feedback.

FIG. 4 is a diagram showing a configuration of a conventional signal reproducing apparatus.
2. Description of the Related Art Conventionally, a signal reproducing apparatus that performs reproduction by modulating a PCM signal that is a multi-bit signal into a 1-bit digital signal, as shown in FIG. The sigma modulation circuit 1, the switching amplifier 2, the low-pass filter 3, and the speaker 4, and the input PCM signal is modulated and modulated by the delta sigma modulation circuit 1 into a PDM signal that is a 1-bit digital signal. The PDM signal is level-shifted (power amplified) to a pulse signal having a predetermined constant voltage by the switching amplifier 2, demodulated into an analog audio signal by the low-pass filter 3, and sonicated by the speaker 4.

  In the signal reproduction device shown in FIG. 4A, since the distortion of the signal generated due to various factors in the switching amplifier 2 cannot be suppressed, the applicant of the present invention is able to reduce the signal generated in the switching amplifier 2 by negative feedback. A signal reproducing apparatus capable of suppressing distortion was proposed. As shown in FIG. 4B, the signal reproducing apparatus includes a first delta-sigma modulation circuit 11, a switching amplifier 2, a low-pass filter 3, a speaker 4, an attenuator 5, an adder 6, The adder 6 and the second delta sigma modulation circuit 12 are provided between the first delta sigma modulation circuit 11 and the switching amplifier 2 and are connected via an attenuator 5. The output signal of the switching amplifier 2 is fed back to an adder 6 provided between the first delta sigma modulation circuit 11 and the second delta sigma modulation circuit 12 (for example, Patent Document 1). reference).

  However, in the prior art, it is possible to suppress the distortion of the signal generated in the switching amplifier 2 by negative feedback. However, although the first delta-sigma modulation circuit 11 is a digital circuit, the output of the first delta-sigma modulation circuit 11 However, there is a problem that the signal is distorted due to variations in analog elements and external noise.

That is, the PDM signal output from the first delta-sigma modulation circuit 11 and the power-amplified PDM signal output from the switching amplifier 2 are often referred to as 1-bit digital signals. Both PDM signals are processed as analog signals having signal voltage fluctuation elements. Actually, the adder 6 and thereafter are configured by analog elements such as operational amplifiers. Therefore, the multi-bit signal that was originally a digital signal is essentially converted to an analog signal, resulting in distortion of the signal due to variations in analog elements and external noise.
JP 2004-172735 A

  The present invention has been made in view of such problems, and an object of the present invention is to treat an input multi-bit signal as a digital signal until just before the switching amplifier, thereby minimizing signal degradation. The object is to provide a signal reproduction apparatus that can be suppressed.

In order to solve the above problems, the present invention has the following configuration.
The signal reproducing apparatus of the present invention modulates an input multi-bit signal into a 1-bit digital signal by 1-bit digital signal modulation means, amplifies the 1-bit digital signal by a switching operation of a switching amplifier, and outputs an output signal from the switching amplifier. A signal reproducing device for reproducing by the A / D conversion means for converting the output signal from the switching amplifier into a digital signal, and the digital signal output from the A / D conversion means from the input multi-bit signal. Subtracting means for subtracting, and the subtracting means is located in the preceding stage of the 1-bit digital signal modulating means.

  Furthermore, the signal reproduction apparatus of the present invention is characterized by comprising a multiplier for adjusting the gain of the output signal of the subtracting means.

  Furthermore, the signal reproducing apparatus of the present invention is characterized by comprising a low-pass filter for limiting the frequency band of the output signal of the subtracting means.

  Further, in the signal reproducing apparatus of the present invention, the 1-bit digital signal modulation means and the A / D conversion means are delta-sigma modulation circuits that modulate an input signal into a PDM signal.

  Furthermore, the signal reproducing apparatus of the present invention is characterized in that the 1-bit digital signal modulating means and the A / D converting means operate with clocks having the same frequency.

  The signal reproducing apparatus of the present invention modulates a multi-bit signal input by a 1-bit digital signal modulation means into a 1-bit digital signal, amplifies the 1-bit digital signal by a switching operation of the switching amplifier, and outputs an output signal from the switching amplifier. Is a signal reproduction device that converts an analog audio signal into a digital signal by the A / D conversion means and negatively feeds back the output signal from the switching amplifier to the input side of the 1-bit digital signal modulation means. Thus, an adder that negatively feeds back the output signal from the switching amplifier converted into a digital signal by the A / D conversion means to the input side of the 1-bit digital signal modulation means, and the 1-bit digital signal modulation means as a digital circuit So that the input multi-bit signal can be The can be handled as a digital signal immediately before the switching amplifier, with an effect that degradation of the signal can be minimized, an effect that the LSI implementation can be easily carried out as a digital circuit.

  Furthermore, the signal reproducing apparatus of the present invention provides a multiplier that adjusts the gain of the output signal of the subtracting means, thereby setting the gain of the multiplier very large, thereby generating a quantum generated by the 1-bit digital signal modulating means. There is an effect that the noise can be reduced.

  Furthermore, the signal reproducing apparatus of the present invention is provided with a low-pass filter that limits the frequency band of the output signal of the subtracting means, so that quantization noise generated by the 1-bit digital signal modulating means and signal distortion generated by the switching amplifier are provided. The effect that it can be reduced is produced.

  Furthermore, the signal reproduction apparatus of the present invention uses the delta-sigma modulation circuit that modulates the input signal into the PDM signal as the 1-bit digital signal modulation means and the A / D conversion means, which may be generated by the A / D conversion means. There is an effect that a certain group delay problem can be avoided.

  Furthermore, the signal reproduction apparatus of the present invention operates an 1-bit digital signal modulation means and an A / D conversion means with a clock having the same frequency, so that an oversampling circuit, a decimation filter, etc. The frequency of the multi-bit signal input to the 1-bit digital signal modulation means and the output signal from the switching amplifier negatively fed back to the input side of the 1-bit digital signal modulation means can be matched without providing an extra circuit. There is an effect.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 is a diagram showing a configuration of a first embodiment of a signal reproduction apparatus according to the present invention.

  In the first embodiment, referring to FIG. 1, a delta-sigma modulation circuit 1, which is a 1-bit digital signal modulation means, a switching amplifier 2, a low-pass filter 3, a speaker 4, a low-pass filter 7, The D converter circuit 8 and the adder 9 are provided. The adder 9 is provided on the input side of the delta sigma modulation circuit 1 and the output signal from the switching amplifier 2 through the low pass filter 7 and the A / D conversion circuit 8. Is negatively fed back to the adder 9

  The delta-sigma modulation circuit 1 is composed of logic circuits such as an adder and a delay device, modulates an input PCM signal that is a multi-bit signal into a PDM signal that is a 1-bit digital signal, and switches the modulated PDM signal to a switching amplifier Output to 2.

  The switching amplifier 2 is supplied with a constant voltage power supply. The PDM signal input from the delta-sigma modulation circuit 1 is level-shifted to a pulse signal of the constant voltage power supply level and output to the low-pass filter 3 and the feedback loop FL. Output as a negative feedback signal.

  The low-pass filter 3 includes a coil, a capacitor, and the like. The low-pass filter 3 integrates the PDM signal amplified by the switching amplifier 2 and converts it into an analog audio signal. The converted analog audio signal is output to the speaker 4. Let

  The low-pass filter 7 is a filter that is provided on the feedback loop FL and cuts off the aliasing noise component in the subsequent A / D conversion circuit 8 from the negative feedback signal, and has a sufficiently high cutoff frequency. If the negative feedback signal does not include a component that causes aliasing noise in the A / D conversion circuit 8, the low-pass filter 7 is omitted and the negative feedback signal is directly input to the A / D conversion circuit 8. May be.

  The A / D conversion circuit 8 converts the negative feedback signal from which the aliasing noise component is cut off by the low-pass filter 7 into a digital signal, negatively feeds back to the adder 9, and the PCM signal input to the delta-sigma modulation circuit 1. The negative feedback signal converted into a digital signal is subtracted. Note that the sampling frequency of the A / D conversion circuit 8 is set to be the same as the sampling frequency of the delta-sigma modulation circuit 1, and the adder 9 does not provide an extra circuit after the A / D conversion circuit 8. The frequencies of the PCM signal and the negative feedback signal can be matched. When a subtle deviation occurs in the sampling frequency between the A / D conversion circuit 8 and the delta sigma modulation circuit 1, there is a possibility that a beat may occur and the sampling between the A / D conversion circuit 8 and the delta sigma modulation circuit 1 In the case where the frequency is set to equal magnification / division, an extra circuit such as an oversampling circuit or a decimation filter is required after the A / D conversion circuit 8.

Next, the principle of correcting the signal distortion in the switching amplifier 2 according to the first embodiment will be described in detail with reference to FIG.
FIG. 2 is an explanatory diagram for explaining the effect of correcting the signal distortion in the signal reproducing apparatus shown in FIG. It is.

As a general servo story, in reality, gain is given somewhere in the servo loop, or frequency characteristics are given. Therefore, considering the model shown in FIG. 3 as a relatively simple example, a multiplier 20 having a very large gain and a low-pass filter 21 having a cutoff frequency around the audible band are arranged on the input side of the delta-sigma modulation circuit 1. did. Here, the transfer function of each element is expressed as follows: the multiplier 20 is G, the low-pass filter 21 is L ₁ , the delta-sigma modulation circuit 1 is flat gain characteristics (1 time), the switching amplifier 2 is A, low-pass The filter 7 is L ₂ , the A / D conversion circuit 8 is a flat gain characteristic (1 time), the input signal is I, the output signal is O, and the quantization noise generated by the delta-sigma modulation circuit 1 is N ₁ , Assuming that the distortion of the signal generated in the switching amplifier 2 is N ₀ , the characteristic of the output O is expressed by the following formula A

Here the low-pass filter 7, since a sufficiently high cut-off frequency, the L ₂ with the a flat characteristic (1x), when the transfer function A = 1 of the switching amplifier 2, characteristics of the output O is It can be represented by the following formula B.

Next, the attenuation characteristic of the L ₁ low-pass filter 21 is divided into three bands, and the expression B is evaluated.
(1) Since L ₁ ≈1 times the frequency band G >> ₁ and L ₁ ≈1 times, GL ₁ >> ₁ and thus the denominator “1+” can be ignored. Therefore, as shown in the following equation, the quantization noise N ₁ generated in the delta-sigma modulation circuit 1 and the distortion N _{0 of} the signal generated in the switching amplifier 2 are eliminated in the equation B, and the input signal I is directly output as the output signal O. I can see it coming.

(2) The gain of the frequency band G that is L ₁ ≈ (1 / G) times and the attenuation characteristic of L ₁ cancel each other, and GL ₁ ≈1 times, and the denominator “1+” cannot be ignored. In addition, the quantization noise N ₁ generated in the delta-sigma modulation circuit 1 and the distortion N _{0 of} the signal generated in the switching amplifier 2 are halved, but the input signal I is also halved. If the band is audible, the sound signal is attenuated and heard. Therefore, at the time of design, it is necessary to bring a frequency band L ₁ ≈ (1 / G) times in a frequency band sufficiently higher than the audible band.

_{(3) L 1 «(1} / G) for damping characteristic of the gain from the L ₁ of the multiple of the frequency band G is strong, approaches GL ₁ ≒ 0 times, as shown in the following equation, generated in the delta sigma modulation circuit 1 It remains distortion N ₀ of the signal generated by the quantization noise N ₁ and switching amplifier 2, the input signal I disappears, both no problem since sufficient higher frequency than the audio band.

As described above, the quantization noise N ₁ generated in the delta-sigma modulation circuit 1 and the distortion N ₀ of the signal generated in the switching amplifier 2 in the frequency band of L ₁ ≈1 can be corrected almost completely. _In the frequency band of 1≈ (1 / G) times, the quantization noise N ₁ generated in the delta sigma modulation circuit 1 and the distortion N ₀ of the signal generated in the switching amplifier 2 can be corrected to ½ times. Actually, attention must be paid to the attenuation characteristic of L ₁ and the servo stability (not oscillating) conditions not shown here, but if designed correctly, the distortion N _{0 of} the signal generated in the switching amplifier 2 As well as the quantization noise N ₁ can be attenuated in the audible band.

When the A / D conversion circuit 8 in the feedback loop FL is an audio multi-bit A / D conversion circuit, the processing time delay is large, the group delay occurs, and it is difficult to satisfy the servo stability condition. In some cases, the problem of group delay can be avoided by using a delta-sigma modulation circuit as the A / D conversion circuit 8. If the input to the adder 9 is a digital signal, it may be a 1-bit signal. At the entrance of the adder 9, for example, “0” becomes “7FFF (hex)” and “1” becomes “8000 (hex)”. Replace with. However, when a delta sigma modulation circuit is used as the A / D conversion circuit 8, the quantization noise N ₂ generated in the delta sigma modulation circuit used as the A / D conversion circuit 8 is generated in the delta sigma modulation circuit 1. Unlike the quantization noise N ₁ , it does not attenuate in the audible band, so it is necessary to keep the quantization noise N ₂ generated in the delta-sigma modulation circuit used as the A / D conversion circuit 8 low.

  In the first embodiment, the example in which the delta-sigma modulation circuit 1 is used as a 1-bit digital signal modulation unit that modulates a PCM signal that is an input multi-bit signal into a 1-bit digital signal has been described. A PWM signal modulation circuit that modulates a PCM signal that is a multi-bit signal into a PWM signal that is a 1-bit digital signal may be used as the 1-bit digital signal modulation means.

(Second Embodiment)
FIG. 3 is a diagram showing the configuration of the second embodiment of the signal reproduction apparatus according to the present invention.

  In the second embodiment, referring to FIG. 3, the playback volume can be controlled by making the voltage value of the constant voltage supplied to the switching amplifier 2 variable. In addition to the configuration of the embodiment, the variable voltage power supply 22 that can change the constant voltage supplied to the switching amplifier 2 and the gain of the output signal from the A / D conversion circuit 8, that is, the signal that is negatively fed back to the adder 9 And a microcomputer 24 for controlling the voltage value of the constant voltage supplied from the variable voltage power source 22 to the switching amplifier 2 and the gain of the gain adjusting means 23.

  The microcomputer 24 sets the voltage value of the constant voltage supplied from the variable voltage power supply 22 to the switching amplifier 2 according to the volume level set by a volume adjusting means (not shown), and also supplies the constant voltage supplied to the switching amplifier 2. When the voltage value of N is multiplied by N, the gain of the gain adjusting unit 23 is configured to be set to 1 / N times. According to the increase or decrease of the voltage value of the constant voltage supplied to the switching amplifier 2, The servo loop can be stabilized by providing a reciprocal gain in the feedback loop FL.

  As described above, according to the present embodiment, the PCM signal input by the delta-sigma modulation circuit 1 is modulated into the PDM signal, the PDM signal is amplified by the switching operation of the switching amplifier 2, and the A signal reproducing apparatus for reproducing an output signal into an acoustic signal by a low-pass filter 3, and the output signal from the switching amplifier 2 converted into a digital signal by the A / D conversion circuit 8 is input to the input side of the 1-bit digital signal modulating means. An adder 9 that negatively feeds back the output signal from the switching amplifier converted into a digital signal by the A / D conversion means to the input side of the delta sigma modulation circuit 1 by being configured to be negatively fed back, and the delta sigma modulation circuit 1 can be used as a digital circuit. It can be handled as a digital signal until shortly before quenching amplifier 2, with an effect that degradation of the signal can be minimized, an effect that the LSI implementation can be easily carried out as a digital circuit.

  Furthermore, according to the present embodiment, the multiplier 20 that adjusts the gain of the output signal of the adder 9 is provided, so that the gain of the multiplier 20 is set to be very large, so that the delta sigma modulation circuit 1 generates the gain. There is an effect that quantization noise can be reduced.

  Furthermore, according to the present embodiment, by providing the low-pass filter 21 that limits the frequency band of the output signal of the adder 9, the quantization noise generated by the delta-sigma modulation circuit 1 and the signal generated by the switching amplifier 2 are provided. There is an effect that it is possible to reduce the distortion.

  Furthermore, according to the present embodiment, a delta-sigma modulation circuit that modulates an input signal into a PDM signal is used as the A / D conversion means, thereby eliminating the group delay problem that may occur in the A / D conversion circuit 8. There is an effect that it can be avoided.

  Furthermore, according to the present embodiment, by operating the delta sigma modulation circuit 1 and the A / D conversion circuit 8 with a clock having the same frequency, an oversampling circuit, a decimation filter, etc. The frequency of the multi-bit signal input to the delta sigma modulation circuit 1 and the output signal from the switching amplifier 2 that is negatively fed back to the input side of the delta sigma modulation circuit 1 can be matched without providing an extra circuit. There is an effect.

  It should be noted that the present invention is not limited to the above-described embodiments, and it is obvious that each embodiment can be appropriately changed within the scope of the technical idea of the present invention. In addition, the number, position, shape, and the like of the constituent members are not limited to the above-described embodiment, and can be set to a suitable number, position, shape, and the like in practicing the present invention. In each figure, the same numerals are given to the same component.

It is a figure which shows the structure of 1st Embodiment of the signal reproducing | regenerating apparatus based on this invention. It is explanatory drawing for demonstrating the correction effect for signal distortion in the signal reproducing | regenerating apparatus shown in FIG. It is a figure which shows the structure of 2nd Embodiment of the signal reproducing | regenerating apparatus based on this invention. It is a figure which shows the structure of the conventional signal reproducing | regenerating apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Delta-sigma modulation circuit 2 Switching amplifier 3 Low pass filter 4 Speaker 7 Low pass filter 8 A / D conversion circuit 9 Adder 20 Multiplier 21 Low pass filter 22 Variable voltage power supply 23 Gain adjustment means 24 Microcomputer

Claims (5)

A signal reproduction device that modulates an input multi-bit signal into a 1-bit digital signal by a 1-bit digital signal modulation means, amplifies the 1-bit digital signal by a switching operation of a switching amplifier, and reproduces the output signal from the switching amplifier. There,
A / D conversion means for converting an output signal from the switching amplifier into a digital signal;
Subtracting means for subtracting a digital signal that is an output of the A / D conversion means from the input multi-bit signal,
The signal reproducing apparatus according to claim 1, wherein the subtracting means is located upstream of the 1-bit digital signal modulating means.   2. The signal reproducing apparatus according to claim 1, further comprising a multiplier for adjusting a gain of an output signal of the subtracting means.   3. The signal reproducing apparatus according to claim 1, further comprising a low-pass filter that limits a frequency band of an output signal of the subtracting unit.   4. The signal reproducing apparatus according to claim 1, wherein the 1-bit digital signal modulating unit and the A / D converting unit are delta-sigma modulation circuits that modulate an input signal into a PDM signal.   5. The signal reproducing apparatus according to claim 1, wherein the 1-bit digital signal modulating unit and the A / D converting unit operate with clocks having the same frequency.

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