JPS61242116A - Signal processing circuit - Google Patents

Abstract

PURPOSE:To prevent the disturbance of a signal waveform by correcting the group delay characteristics of a low pass filter LPF before A/D or D/A conversion. CONSTITUTION:The group delays of the digital all pass filters DAPF 10 and 11 provided after an A/D converter 2 and before a D/A converter 4 respectively can be set optionally. For instance, the analog LPF 1 and 5 have the constant frequency and group delay characteristics and these group delay characteristics are not flat at a point near the cut-off frequency. Under such conditions, the levels of the DAPF 10 and 11 are set constant to the frequency together with their group delay times are small at the point near the cut-off frequency unlike the LPF 1 and 5. The DAPF 10 corrects the group delay of the signal passed through the LPF 1; while the DAPF 11 corrects previously the group delay of the signal passing through the LPF 5 respectively. Thus the disturbance of a signal waveform is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a signal processing circuit that processes digital signals.

[Technical background of the invention and its problems]

In the field of digital signal processing, it is sometimes necessary to obtain a digital signal to be processed by A/D converting an analog signal, or to D/A convert a processed digital signal into an analog signal.

A circuit of this kind that performs both of the above operations is shown in FIG. In the figure, 1 is an analog low-pass filter (LPF) that cants high-frequency components higher than half the sampling frequency Is in the analog signal input to the input terminal IN, and 2 is the analog signal that has passed through LPF 1. Analog/digital (A
/D) converter, 3 is a digital signal processing circuit (DSP) that processes the digital signal from the A/D converter 2, and 4 is a D/A that converts the digital signal processed by the DSP 3 into an analog signal. The converter 5 is an analog low-pass filter (LPF) that takes out only the fundamental frequency component in the analog signal from the D/A converter 4, and the signal after signal processing obtained at the output of the LPF 5 is output from the output terminal OUT. Ru.

In the above configuration, the analog signal input to the input terminal IN is converted into a digital signal by the A/D converter 2 after its high frequency components are cut by the LPF 1 to remove aliasing distortion. The digital signal processing circuit 3 receives the digital signal from the A/D converter 2 and performs various digital processing. The digital signal after digital signal processing is D/A
The converter 4 converts it back into an analog signal. And LP
F5 extracts only the fundamental frequency component from the output signal of the D/A converter 4 and outputs it as an analog signal from the output terminal OUT.

By the way, when the cutoff frequency of LPFs 1 and 5 is brought close to the Nyquist frequency, that is, one half of the sampling frequency fs, the high-frequency attenuation characteristic of the LPF needs to be steep, and the phase decreases near the cutoff frequency. turns,
Group delay characteristics are no longer flat. If the group delay characteristic is not flat, only the frequency components in the signal passing through the LPF will be delayed from other components, resulting in a signal having a waveform different from the input signal waveform being output.

[Purpose of the invention]

The present invention has been made to eliminate the drawbacks of the conventional circuits described above, and is a signal that eliminates the problems caused by the non-flat group delay characteristics of the low-pass filter through which analog signals are passed before or after digital signal processing. The purpose is to provide processing circuits.

[Summary of the invention]

In the signal processing circuit according to the present invention, in a circuit that performs A/D or D/A conversion before or after digital signal processing, the group delay characteristic of a low-pass filter through which an analog signal is passed before or after A/D conversion or after D/A conversion Disturbances in the signal waveform caused by the unevenness of the signal waveform can be eliminated by correcting the group delay characteristics of the low-pass filter.

〔Example〕

Embodiments of the present invention will be described below based on the drawings.

FIG. 1 is a block diagram showing an embodiment of a signal processing circuit according to the present invention, and in the figure, the same reference numerals are given to the same parts as those described above with respect to FIG.

10 and 11 are an LPFI provided before the A/D converter 2 and an LP provided after the D/A converter 4.
In order to correct the group delay characteristics of F5, a digital all-pass filter (DAPF) is provided after the A/D converter 2 and before the D/A converter 4, and the group delay of the filter can be adjusted arbitrarily. Can be set.

Now, each of the LPFIs 5 has frequency characteristics and group delay characteristics as shown in Fig. 2, and the group delay characteristics have a cutoff frequency f
If it is not flat near c (= r s/2),
As shown in Fig. 3, DAPFIo, 11 has a level constant with respect to frequency, and a group delay time of LPFI, 5.
On the contrary, if you use one that decreases around the frequency fc (DAPFIO corrects the group delay of the signal after passing through the LPFI, and DAPFII
The group delay of the signal passing through PF5 is corrected in advance. In addition,
If LPFI, 5 have the same characteristics, DAPFlo
, 11 may be the same.

An IIR (cyclic) type digital filter generally has a configuration as shown in FIG. In the figure, A and B are multipliers, Z-1 is a 1-sample delay circuit, and + is an adder.

The transfer function H(Z) of the filter with the illustrated configuration is expressed by the coefficients AO, A + −At , BI, B
By changing t, various filter characteristics such as low-pass and band-pass can be obtained.

In order to make such a filter into an all-pass filter (APF), coefficients may be determined, for example, as in the transfer function of the following formula.

Cr" 2T cos θZ-1+ Z -2H(Z)=
-=□ γ"1-27" cos θZ -1+r 2 Z -, where γ and θ are real values indicated by the positions of the poles on the Z plane of the filter (FIG. 5), and C is a constant.

The amplitude characteristic of the filter in the above formula is IH(ω)l=lH(Z)lz=e''=l l−γ2, and the frequency-amplitude characteristic is flat.In addition, the phase characteristic is expressed as, The group delay characteristic is as follows.

Based on the above theory, in order to design a DAPF that corrects the analog LPFO group delay characteristic, iterative calculations using a computer are performed.

In addition, in the above-mentioned embodiment, the LPF at the front stage of the A/D converter
and the group delay of the LPF after the D/A converter are separated into separate DAPs.
Although correction is made using F, either one of the DAPFs may be used for this purpose.

In addition, in the above embodiment, correction is performed on both the PF in the front stage of the A/D converter and the LPF in the rear stage of the D/A converter, but it is also possible to correct only one of them. It can also be effectively applied to a digital human power-analog output system.

Furthermore, although a DAPF is used to correct the group delay of the LPF, it only needs to have flat frequency characteristics in the passband of the analog LPF, and does not necessarily need to be all-pass.

〔Effect of the invention〕

In devices that perform A/D or D/A conversion before or after digital signal processing, the signal waveform may change due to uneven group delay characteristics of the low-pass filter through which the analog signal is passed before or after A/D conversion or D/A conversion. An effect is obtained in which the disturbance is eliminated by correcting the group delay characteristic of the low-pass filter.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of a signal processing circuit according to the present invention, FIG. 2 is a characteristic graph showing the frequency characteristics and group delay characteristics of the LPF in FIG. 1, and FIG. D.A.
A characteristic graph showing the frequency characteristics and group delay characteristics of the PF, FIG. 4 is a block diagram showing the general configuration of a digital filter, FIG. 5 is an explanatory diagram for explaining the transfer function of the all-pass filter, and FIG. The figure is a block diagram showing an example of a conventional signal processing circuit. 1.5...LPF, 2...A/D converter, 3...
DSP, 4...D/A converter, 10, 11...DA
P.F. Patent applicant Pioneer Co., Ltd. 111 Figure 20 c g'l Rinsing number Figure 3 1'l; Return table

Claims (3)

[Claims] (1) A signal processing circuit that has a low-pass filter in front of an A/D converter that converts an analog signal into a digital signal, and processes a digital signal obtained by A/D conversion into a digital signal, A signal processing circuit comprising means for correcting group delay characteristics of the low-pass filter at a stage subsequent to the D converter. (2) A signal processing circuit that has a low-pass filter at a subsequent stage of a D/A converter that D/A converts a digital signal that has undergone digital signal processing, and outputs an analog signal that has passed through the low-pass filter; A signal processing circuit comprising means for pre-compensating the group delay characteristic of the low-pass filter before the A converter. (3) A low-pass filter that cuts high-frequency components in an analog signal, an A/D converter that converts the output of the low-pass filter into A/D, and a digital signal that converts the output of the A/D converter into a digital signal. A digital signal processing circuit that performs signal processing, a D/A converter that performs D/A conversion of the digital signal processed by the digital signal processing circuit, and a fundamental frequency in the analog signal obtained at the output of the D/A converter. What is claimed is: 1. A signal processing circuit comprising: a low-pass filter that passes only components, the signal processing circuit comprising means for correcting group delay characteristics of both of the low-pass filters.