JP5409101B2 - Digital waveform compensation circuit - Google Patents

Description

  The present invention relates to a digital waveform compensation circuit suitable for use in an amplifier to which a vehicle-mounted speaker is connected.

  In-vehicle speakers used in the vehicle interior acoustic space are distorted waveforms with poor responsiveness compared to ideal sound waveforms due to insufficient strength of vehicle structures such as iron plates and interior materials used as enclosures around the speakers. It is known that many components (undershoot and overshoot) occur. This distortion component waveform is an unnecessary component that is not included in the original waveform, and is one of the causes that adversely affect the sound quality.

For this reason, although the improvement tendency is seen by ensuring the intensity | strength etc. of a vehicle structure, a complete countermeasure is difficult and the cost for it was required. In addition, in order to ensure strength, an increase in weight is caused, and there is an adverse effect on fuel consumption. Therefore, sufficient measures cannot be taken.
Therefore, conventionally, measures have been taken on the amplifier side to which the vehicle-mounted speaker is connected. For example, distortion component waveform reduction processing is performed by convolution calculation of an impulse response using an adaptive filter (FIR: Finite Impulse Response). It had been.

  On the other hand, the frequency of the reflected sound that is conventionally inserted between the delay means and the level adjusting means for the non-uniformity of the frequency characteristics at the viewing point due to the interference between the direct sound from the speaker and the reflected sound from the wall or the like. There has been proposed a sound field correction apparatus that compensates with a low-pass filter (LPF) whose filter characteristics are determined based on the characteristics (see, for example, Patent Document 1).

JP 60-1937776 A

However, according to the distortion component waveform reduction processing by the convolution calculation using the adaptive filter described above, a huge amount of calculation is required, resulting in an increase in cost.
The technique disclosed in Patent Document 1 is a technique related to sound field correction for interference between direct sound and reflected sound, and is not sound field correction for direct sound on the front surface of the speaker.

  The present invention has been made to solve the above-described problems, and improves transient characteristics immediately before a vehicle-mounted speaker with a small amount of calculation to improve sound quality at a low cost. As a result, the sound field feeling is improved. It is an object of the present invention to provide a digital waveform compensation circuit that can be used.

In order to solve the above-described problems, the digital waveform compensation circuit according to the present invention provides an input signal until an overshoot and undershoot of the first wave at the time of impulse input obtained from an impulse response measurement result of a vehicle-mounted speaker occurs. A delay unit that delays by a delay time and a low frequency band that generates a waveform having a width corresponding to the overshoot and undershoot of the first wave of the delayer output based on the filter characteristics obtained from the impulse response measurement result A pass filter, a phase inverting circuit for generating an antiphase signal component that reduces overshoot and undershoot of the first wave from the output of the low-pass filter, and the input signal and the output of the phase inverting circuit. And an adder for adding.

  According to the digital waveform compensation circuit of the present invention, it is possible to improve transient characteristics immediately before an in-vehicle speaker with a small amount of calculation and improve sound quality at a low cost. As a result, it is possible to improve the sound field feeling.

It is a figure which shows the structural example of the vehicle-mounted audio equipment in which the digital waveform compensation circuit which concerns on Embodiment 1 of this invention is used. It is the figure which showed the internal structure of the digital waveform compensation circuit which concerns on Embodiment 1 of this invention with the equivalent circuit. It is the figure which showed typically the operation | movement of the digital waveform compensation circuit which concerns on Embodiment 1 of this invention. It is the figure which showed the internal structure of the digital waveform compensation circuit which concerns on Embodiment 2 of this invention with the equivalent circuit. It is the figure which showed the internal structure of the digital waveform compensation circuit which concerns on Embodiment 3 of this invention with the equivalent circuit. It is the figure which showed the frequency characteristic by the digital waveform compensation circuit which concerns on Embodiment 1 of this invention in contrast with the prior art example.

Hereinafter, a digital waveform compensation circuit according to an embodiment of the present invention will be described in detail with reference to the drawings.
Embodiment 1 FIG.
FIG. 1 is a diagram illustrating a configuration example of an in-vehicle acoustic device in which the digital waveform compensation circuit according to the first embodiment of the present invention is used.

  As shown in FIG. 1, an in-vehicle audio device 1 includes an ADC (Analog Digital Converter) 11, a digital waveform compensation circuit 12 according to Embodiment 1 of the present invention, a DAC (Digital Analog Converter) 13, and a power amplifier. 14 and a vehicle-mounted speaker 15.

The ADC 11 converts the analog input audio signal into a digital audio signal and outputs the digital audio signal to the digital waveform compensation circuit 12.
As will be described later, the digital waveform compensation circuit 12 uses a digital audio signal output from the ADC 11 as a first wave (overshoot / undershoot) at the time of impulse input obtained from an impulse response measurement result of the in-vehicle speaker 15 in advance. The delay output is delayed by the delay time until the distortion component is generated. Based on the filter characteristics obtained from the impulse response measurement result, a waveform having a width corresponding to the distortion waveform of the first wave of the delay output is obtained. Then, a signal component having a reverse phase that reduces the distortion component of the first wave is generated, added to the input digital audio signal, and output to the DAC 13. Details will be described later.

The DAC 13 converts the digital audio signal output from the digital waveform compensation circuit 12 into an analog audio signal and outputs the analog audio signal to the power amplifier 14.
The power amplifier 14 amplifies the analog audio signal output from the DAC 13 and outputs the amplified signal to the vehicle-mounted speaker 15 that is an electromagnetic acoustic converter.

  Note that each of the ADC 11, the digital waveform compensation circuit 12, and the DAC 13 is implemented in the digital signal processing apparatus 10 (DSP) and processed by the DSP 10. Details will be described later.

FIG. 2 is an equivalent circuit diagram showing the internal configuration of the digital waveform compensation circuit 12 according to the first embodiment of the present invention.
The digital waveform compensation circuit 12 includes an IIR (Infinite Impulse Response) filter having an impulse response function that returns a non-zero value in an infinitely long time. The delay circuit (Delay) 121 and a low-pass filter 122 (hereinafter, LPF 122). A phase inversion circuit 123, a level adjustment circuit 124, and an adder 125.

  Delay time until the first waveform distortion waveform component related to overshoot and undershoot at the time of impulse input obtained from the impulse response measurement result of the in-vehicle speaker 15 from the delay device 121 and the input signal (INPUT) in advance. The output is delayed by (td) and output to the LPF 122.

The LPF 122 generates a waveform having a width corresponding to the distortion waveform of the first wave output from the delay device 121 based on the filter characteristics obtained from the previous impulse response measurement result, and outputs the waveform to the phase inverting circuit 123.
The phase inversion circuit 123 generates an antiphase signal component that reduces the distortion component of the first wave from the output of the LPF 122 and outputs the signal component to the adder 125.

  The adder 125 adds the signal obtained by adjusting the level of the input signal (INPUT) by the level adjustment circuit 124 and the output of the phase inverting circuit 123 described above, and outputs the result as an OUTPUT signal to the power amplifier 14.

FIG. 3 is a diagram schematically showing the operation of the digital waveform compensation circuit 12 according to the first embodiment of the present invention.
The operation of the digital waveform compensation circuit 12 according to the first embodiment of the present invention shown in FIGS. 1 and 2 will be described below with reference to FIG.

As described above, the vehicle-mounted speaker 15 generates an overshoot or undershoot distortion waveform having a poor transient response with respect to an input audio signal. Here, waveform compensation in the digital domain is performed in order to cancel out this distortion waveform component.
For this reason, the vehicle-mounted speaker 15 is installed in advance in the vehicle interior, the impulse response waveform on the front surface of the vehicle-mounted speaker 15 is measured (a), and attention is paid to the distortion waveform component generated after the fundamental wave drive. Parameters including delay time and filter characteristics for simulating the distortion component of the first wave for the shoot need to be set in advance in the DSP 10 (digital waveform compensation circuit 12).

  After the above-described preparation processing, the digital waveform compensation circuit 12 delays the signal branched from the input signal (INPUT) by the time td according to the parameter set by the delay device 121 and passes it through the LPF 122. Accordingly, a waveform corresponding to the width of the distortion waveform component is generated and output to the phase inversion circuit 123.

  The phase inversion circuit 123 generates a signal (distortion compensation waveform) having an opposite phase to the distortion waveform of the first wave from the output of the LPF 122 (b), and the adder 125 and the level adjustment circuit 124 input the input signal (INPUT). ) Is added to the level-converted signal to cancel the distortion waveform (c) and output to the power amplifier 14 to reduce the distortion component described above.

  Here, td is the delay time until the first wave distortion component at the time of impulse input obtained from the impulse response measurement result of the vehicle-mounted speaker 15 performed in advance, and the filter characteristics A parameter for generating a waveform having a width corresponding to the distortion waveform of the first wave obtained from the impulse response measurement result of the on-vehicle speaker 15 to be performed.

According to the digital waveform compensation circuit according to the first embodiment of the present invention described above, only the delay time until the distortion component of the first wave at the time of impulse input obtained from the impulse response measurement result of the vehicle-mounted speaker 15 is generated. The delayed output is delayed, and a distortion waveform component having a width corresponding to the distortion waveform is generated based on a filter characteristic similarly obtained from the impulse response measurement result, and a reverse-phase signal component that reduces the distortion component of the first wave is generated. By adding the digital audio signal inputted in this way and outputting it, digital waveform compensation is realized and the sound quality is improved.
In addition, the digital waveform compensation circuit 12 described above is constituted by an IIR filter, and signal processing is performed by the DSP 10, so that the amount of calculation is reduced and the sound quality is improved at a low cost. As a result, the sound field feeling of the in-vehicle speaker 15 is improved. Improvements can be made.

Embodiment 2. FIG.
FIG. 4 is an equivalent circuit diagram showing the internal configuration of the digital waveform compensation circuit according to Embodiment 2 of the present invention.

As shown in FIG. 4, the digital waveform compensation circuit according to the second embodiment of the present invention connects the digital waveform compensation circuit 12 according to the first embodiment of the present invention shown in FIG. On the other hand, more effective waveform compensation can be performed by performing a plurality of digital waveform compensation including the second wave. Here, a second-order IIR filter is shown.
Therefore, the sound quality is improved, and as a result, the sound field feeling of the vehicle-mounted speaker 15 can be improved. Further, by performing signal processing on each digital waveform compensation circuit 12 connected in parallel by the DSP 10, the amount of calculation is small, and sound quality can be improved at low cost.

  In the second embodiment described above, as in the first embodiment, the impulse response waveform on the front surface of the vehicle-mounted speaker 15 installed in the vehicle interior is measured in advance, and the distortion component is transmitted to the DSP 10 (digital waveform compensation circuit 12). It is indispensable to set parameters related to delay time and filter characteristics.

Embodiment 3 FIG.
FIG. 5 is an equivalent circuit diagram showing the internal configuration of the digital waveform compensation circuit according to Embodiment 3 of the present invention.

As shown in FIG. 5, the digital waveform compensation circuit according to the third embodiment of the present invention is cascade-connected to the digital waveform compensation circuit 12 according to the first embodiment of the present invention shown in FIG.
In the second embodiment described above, impulse responses are also added and subtracted by connecting the digital waveform compensation circuit 12 in parallel. However, as shown in the third embodiment, when the digital waveform compensation circuit 12 is cascade-connected. Further, more effective waveform compensation can be performed by convolution of the digital waveform compensation circuits 12 connected in cascade.

  Therefore, according to the digital waveform compensation circuit according to the third embodiment, the sound quality is improved, and as a result, the sound field feeling of the vehicle-mounted speaker 15 can be improved. Further, the DSP 10 performs signal processing on the digitally connected waveform compensation circuits 12 that are connected in cascade, so that more effective waveform compensation is performed, and the amount of calculation is small, and sound quality can be improved at low cost.

  In the third embodiment described above, as in the first and second embodiments, the impulse response waveform on the front surface of the vehicle-mounted speaker 15 installed in the vehicle interior is measured in advance, and the DSP 10 (digital waveform compensation circuit 12) It is essential to set parameters related to delay time and filter characteristics that simulate distortion components.

FIG. 6 is a diagram showing the frequency characteristics of the digital waveform compensation circuit 12 according to the first embodiment of the present invention described above in comparison with the conventional example.
6A is a conventional example, and FIG. 6B is a response waveform immediately before the vehicle-mounted speaker 15 of the digital waveform compensation circuit 12 according to Embodiment 1 of the present invention, and is shown in FIG. 6B. As can be seen, the first wave distortion components of overshoot and undershoot are reduced. Here, the values set as parameters in the DSP 10 (digital waveform compensation circuit 12) are exemplified by the case where the delay time (td) of the delay device 121 is 0.15 ms and the cutoff frequency (fc) of the LPF 122 is 800 Hz. Yes.

  As described above, according to the digital waveform compensation circuit according to the first to third embodiments of the present invention, the transient characteristic immediately before the vehicle-mounted speaker is improved with a small amount of calculation, and the sound quality is improved at a low cost. As a result, the sound field feeling can be improved.

The functions of the digital waveform compensation circuit 12 shown in FIG. 2, FIG. 4, and FIG. 5 may be all realized by software by the DSP 10, or at least a part thereof may be realized by hardware.
For example, the digital waveform compensation circuit 12 delays the input signal by a delay time until the distortion component of the first wave at the time of impulse input obtained from the impulse response measurement result of the vehicle-mounted speaker 15 is generated, and measures the impulse response. Based on the filter characteristics obtained as a result, a first-wave distortion waveform is generated, a negative-phase signal component for reducing the distortion component of the first wave is generated, and data processing for adding to the input signal is performed by one or more It may be realized on a computer by a program, or at least a part thereof may be realized by hardware.

  DESCRIPTION OF SYMBOLS 1 Car audio equipment, 10 DSP, 11 ADC, 12 Digital waveform compensation circuit, 13 DAC, 14 Power amplifier, 15 Car speaker, 121 Delay device, 122 Low-pass filter (LPF), 123 Phase inversion circuit, 124 Level adjustment Circuit, 125 adder.

Claims (4)

A delay device for delaying an input signal by a delay time until occurrence of overshoot and undershoot of the first wave at the time of impulse input obtained from an impulse response measurement result of a vehicle-mounted speaker;
A low-pass filter that generates a waveform having a width corresponding to the overshoot and undershoot of the first wave of the delayer output based on the filter characteristic obtained from the impulse response measurement result;
A phase inverting circuit for generating an anti-phase signal component that reduces overshoot and undershoot of the first wave from the output of the low-pass filter;
An adder for adding the input signal and the output of the phase inverting circuit;
A digital waveform compensation circuit comprising:   Each of the delay unit, the low-pass filter, the phase inverting circuit, and the adder is a digital signal processing device configured by an IIR filter having an impulse response function that returns a non-zero value in an infinitely long time. The digital waveform compensation circuit according to claim 1, wherein:   The digital waveform compensation circuit according to claim 2, wherein the IIR filters are connected in parallel to reduce overshoot and undershoot of the second wave. The digital waveform compensation circuit according to claim 2, wherein the IIR filters are cascaded to reduce overshoot and undershoot of the second wave.

Images