JPS61166228A - Overload compensation type digital-analog converting circuit - Google Patents

Abstract

PURPOSE:To avoid generation of overload noise by providing a synthesis circuit synthesizing an output of an analog converting circuit and an output of a narrow band low pass filter and making the delay time of both the synthesized outputs identical so as to reproduce an analog waveform. CONSTITUTION:An output of an analog converting circuit 2 has a waveform whose positive/negative peaks are clipped. A time delay of TAB is caused to an input signal at a point A. Then a time delay of TAC is caused to a signal at a point C to the input signal at the point A. The synthesis circuit 9 synthesizes signals B, C and the delay times of TAB, TAC are made identical so that both the phases are made coincident in this case. In matching analogical ly the phases, the phases, the phase of filters 5, 8 is adjusted or an attenuator ATT of the synthesis circuit 9 is adjusted. In matching the phases digitally, a delay adjusting circuit 11 is inserted to any one of digital transmission lines branched into two from a PCM signal processing circuit 1. When the TAB and TAC are identical, the waveform whose clip state is compensated is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a PCM modulation type audio signal reproducing device for digital audio equipment and the like.

[Conventional technology]

In digital audio equipment, audio signals are recorded and played back using the PCM modulation method.

When playing PCM data, normally the D/A
The conversion is performed to obtain an analog signal.

However, in a device that allows the user to record, such as a digital audio tape, the user sets the recording level and adjusts the signal so that it falls within the operating range of the A/D converter. This is difficult for ordinary users, as they sometimes record analog signals that exceed the quantization level range, resulting in overflow noise (overload noise) during playback.

[Problem that the invention seeks to solve]

An object of the present invention is to eliminate the above-mentioned drawbacks, and when a part of the waveform is overloaded and a signal that has been A/D converted is converted into analog, the waveform becomes trapezoidal at the positive and negative peak values. The purpose is to modify the waveform to make it closer to the original waveform and remove overload noise.

[Means for solving problems]

The D/A conversion circuit of the present invention includes an analog conversion circuit that converts into an analog waveform using a D/A converter and a low-pass filter;
an overload detection circuit that detects an overload of the digital signal and generates a constant output pulse during that time; a narrowband low-pass filter that inputs the constant output pulse and provides a constant delay; A synthesizing circuit for synthesizing the output of the low-pass filter is provided, and the analog waveform is reproduced by making the delay time of the synthesized re-output the same.

[Effect]

When A/D converting an analog wave, if some of the positive and negative quantization level ranges are exceeded, all bits of the converted digital signal will be at "H" level or "L" level within that range. The state continues for m times. Therefore, the reproduced analog signal has a clipped waveform, the odd-order distortion components increase, and the audio quality is low. In the present invention, the response waveform of a narrow-band low-pass filter caused by a pulse input with a pulse width corresponding to the overload time interval is superimposed on the clipped waveform to create a gentle waveform and correct the clipped waveform. This approximates the waveform and prevents overload noise from occurring.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit block diagram of the embodiment, and FIG. 2 illustrates signals at each point of the circuit.

The PCM signal from the PCM signal circuit 1 is an input to the overload compensation type D/A conversion circuit of the present invention, and is A/D converted in an overload state at a part of its positive or negative input level. shall be. This PCM signal passes through the analog converter circuit 2, becomes an analog output, and becomes a signal at point B of the adder circuit 9. The analog conversion circuit 2 includes a D/A converter 3. It consists of an aperture circuit 4° and a low-pass filter 5, and is a normal D/A conversion circuit. The output of the analog conversion circuit 2 has a waveform with its positive and negative peaks clipped, as shown in FIG. 2(B). Also, there is a time delay of TAN with respect to the input signal at point A.

On the other hand, the PCM signal is input to the overload detection circuit 6.

If overloaded with a positive input level, all bits are 1.
'' is detected by circuit 61, and as long as there is an overload condition, a positive pulse is generated during that period.If the overload is at a negative input level, circuit B62 detects that all bits are ``θ''. It detects and generates intercalating pulses.

The above pulse is a constant amplitude pulse with a pulse width proportional to the overload duration, and the waveforms at points P and N are shown in Figure 2 (P).
It becomes as shown in (N). The pulses are input to a narrow band low pass filter 8 via an analog adder circuit 7. The narrowband low-pass filter 8 may be of any type as long as it has linear phase characteristics, provides a constant delay, has narrowband characteristics, and provides pulse response characteristics as shown in FIG. For example, a characteristic filter such as a cos'' roll-off or a Gaussian waveform as shown in FIG. It can be set appropriately with the attenuator ATT in the circuit 9.Then, the signal at point 0 with respect to the input signal at point A is TAc.
There will be a time delay.

The synthesis circuit 9 synthesizes the signal B and the doubled signal, but at this time,
TAR so that both phases match.

Make the delay time of T'Ac the same. If the phase is to be matched in an analog manner, the phase of each filter 5.8 is adjusted, or the attenuator ATT of the synthesis circuit 9 is used. Digitally matching the phase is achieved by inserting a delay adjustment circuit 11 into one of the two branched digital transmission lines from the PCM signal processing circuit 1.

If TAll and TAc are the same, the waveform in FIG. 2(C) is superimposed on the clipped part of the output of the analog conversion circuit 2 in FIG. 2(B), resulting in the waveform in FIG. 2(D), A waveform whose clipping state is compensated for can be obtained. The low-pass filter 10 is used to smooth the waveform even if there is a slight phase shift during the synthesis process.

〔Effect of the invention〕

As explained in detail above, during PCM recording, even signals that exceed the quantization level can be directly D/A
If the conversion results in a clipped waveform, the odd-order distortion components due to clipping can be reduced by superimposing a low-pass filter response wave of a pulse having the time width of the clipped period on that portion to compensate. Therefore, even in the case of overload, the auditory characteristics can be significantly improved. Although the present invention does not completely reproduce the original waveform, since overload does not occur often, a compensation circuit such as the one described above can sufficiently cope with the problem.

[Brief explanation of drawings]

FIG. 1 is a circuit block diagram of an embodiment of the present invention, FIG. 2 is a waveform at each point in FIG. 1, and FIGS. 3 and 4 are diagrams showing characteristics of a narrow band low-pass filter. 1-P CM signal processing circuit, 2-analog conversion circuit, 3
-D/A converter, 4-aperture circuit, 5-low-pass filter, 6-overload detection circuit, 8-narrowband low-pass filter, 9-synthesizing circuit, 10-low-pass filter. Agent for utility model registration applicant NEC Home Electronics Co., Ltd.
Patent Attorney Akihiko Sato Figure 2 L3

Claims (1)

[Claims] Regarding a digital signal in which a part of the analog waveform is A/D converted in an overload state, inputting the digital signal,
an analog conversion circuit that converts the digital signal into an analog waveform using a D/A converter/low-pass filter; an overload detection circuit that detects an overload of the digital signal and generates a constant output pulse during that time; and an overload detection circuit that inputs the constant output pulse. A narrowband low-pass filter that provides a constant delay and a synthesis circuit that synthesizes the output of the analog conversion circuit and the output of the narrowband low-pass filter are provided, and the delay time of both outputs to be synthesized is made the same. An overload compensation type D/A conversion circuit characterized by reproducing an analog waveform.