JPH09191232A - Da converting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a noise sensation as small as possible at the time of fine level reproduction. SOLUTION: A difference between samples of input data is taken out and a level detection part 2 detects the difference being lower than a specific level. Consequently, a variable cutoff frequency is controlled to narrow down the band of a variable low-pass filter 5. Consequently, high-frequency noise and high-frequency distortion are reduced and a filter output is outputted to less than its input bit length to make the band switching speed of a filter at the time of the removal of quantization distortion and high-frequency noise of noise shaping slow when the level of input data is fine or varies finely, thereby preventing noise due to filter switching from being generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to control of a digital filter of a DA converter.

[0002]

2. Description of the Related Art When performing D / A conversion, a dither signal is added in order to bring it closer to the original analog signal before A / D conversion,
DA conversion is performed by noise shaping or the like to express the digital signal to a level equal to or lower than the minimum bit.

[0003]

When DA conversion is performed,
High frequency signals and harmonics cause aliasing noise. In order to prevent this, when the input signal changes at a minute level close to the dither signal added as a signal in the no-signal section, the filter means for removing the high frequency signal is turned on to remove noise, and the input signal level filter means is provided. When it exceeds the level to turn on, the filter means is turned off, or a signal is input to the DA converter through a filter having a wide pass band and converted into an analog signal. Therefore, the filter is turned on at high speed,
Since the signal is turned off or switched, the linearity is deteriorated in a high frequency signal, and a bursty high frequency signal output or the like is generated, and there is a feeling of noise and noise in the sense of hearing. It is an object of the present invention to provide a DA converter that has less noise and less noise.

[0004]

The present invention provides an apparatus for converting input digital data into an analog signal,
Level detection means for detecting the level of the input digital data, a variable low-pass filter for removing the high frequency band of the input digital data, and the level of the input digital data by the level detection means is below a predetermined level. When it is detected that the cutoff frequency of the variable low-pass filter is gradually lowered, and when it is detected that the predetermined level or a level slightly larger than the predetermined level is exceeded, the cutoff frequency is gradually returned to the dimension. It is a DA converter including a filter control means for controlling.

Further, the present invention is a DA converter in which the level detecting means detects a change in bit of the input digital data below a predetermined level.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a DA converter according to the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of the DA converter of the present invention. The level detection unit 2 detects a small level of a predetermined 1 to 2 bits in the digital input data 1 such as a signal recorded and reproduced by adding a minute signal of noise by a dither signal or a noise shaper to the digital data, and the level is detected. The section 2 outputs a detection signal b to the filter control section 3 for filter control.

Output f and input data 1 of the filter controller 3
The data g is input to the variable low-pass filter 5 via the buffer 4 which delays the input data 1 in order to form the same phase as the data a of the variable low-pass filter 5 and the variable low-pass filter 5 is synchronized with the output f of the filter control unit 3. In this way, a high cut filter process is performed to extend the bit length and remove noise.

The output h of the variable low-pass filter 5 has n bits of input data 1 and is a general CD (compact disk).
In addition to this n bits, m bits, generally 2 to 4 bits, are newly added as lower bits in addition to the n bits.
A-converted and an analog signal output is obtained.

The variable low-pass filter 5 has an n-bit data g as shown in FIG.
-1 is input, and the cutoff frequency of the low-pass filter is 20 KH by switching the filter coefficients k0 to kn.
It is controlled by the input f so as to be variable from z or filter through to several KHz, and is added by the adder 5-3 to n + m.
Get the output h of the bits.

Conventionally, when the change level of input data is detected and the difference between samples is detected and the coefficient 5-2 is controlled by the change of LSB or 2LSB, for example, a gentle level change is made especially at a low frequency. Although it is possible to obtain a gentle reproduced signal without high frequency noise, when the cutoff characteristic of the filter is suddenly switched from 20 KHz to several KHz by high-speed arithmetic processing at a small signal level, both distorted noise and noise are generated. Noise that is unpleasant to hear is generated.

The present invention uses the output f of the filter controlless 3 for controlling the variable low-pass filter 5 so that the predetermined level V
The time t1 when the level becomes equal to or less than (1 to 2 LSB) and the time t2 when the level exceeds a predetermined level or a level slightly larger than the predetermined level are detected, and the coefficient of the variable low-pass filter 5 when the level is detected. Of the filter is controlled so that the cutoff frequency of the filter is switched gently.

FIG. 2 is a block diagram showing an example of the filter control section 3. FIG. 4 shows signal waveforms of respective sections and FIG. 5 shows an example of filter characteristics. Digital input data 1 has a predetermined level V
Output b from the level detection unit 2 which detects that
Is input to the filter control unit 3, it is delayed by the shift register 3-1 of several sample clocks or more, and the output c
The detection output d for detecting the filter start time t1 ′ at the rising edge of the waveform is obtained through the filter start detection 3-3 for extracting the time t1 at which the high frequency band removing filter starts.

The detection output d is the up / down counter 3-4.
Is counted up and counted up by the sample clock fs to obtain a predetermined value A of several samples or more.
The counter of the up-down counter 3-4 is set so as to be saturated at.

The output f of the up / down counter 3-4 is changed so as to change the value of the cutoff frequency of the variable low-pass filter 5 corresponding to the output value A of the up / down counter 3-4.
And the coefficient 5-2 are associated with each other, and the cutoff frequency is lowered along with the count number in the arrow max direction. Further, the buffer 4 is provided to make the shift time of the shift register 5-1 coincide with the start time of the filter. That is, the delay by the register 3-1 and the input data 1 are made to match the delays in the buffer 4 and the shift register 5-1 of the variable low-pass filter 5.

When the input data 1 exceeds a predetermined level, the time t2 is detected by the level detector 2 and the trailing edge of the output b is detected by the filter end detection 3-2. 4 is set to the countdown mode, the output value A is counted down by the sample clock fs, gradually decreased to 0, and saturated at 0.
As a result, the cut-off characteristics of the filter of the low-pass filter 5 similarly gradually increase the cut-off frequency and change the band of the low-pass filter 5 to a wide band.

As shown in FIG. 4, the digital audio data generally includes a component such as a noise shaper or a dither, as shown by a, so that aliasing noise after DA conversion does not occur. When the amplification degree of is large, a signal level exceeding a predetermined level, for example, ± 2 LSB level range V, is a wide band (cut-off 20 kHz) in order to reduce the audible noise at a small signal level, and at a small signal level. Gradually narrow band (cutoff 5k
HZ), the variable low-pass filter is operated in a wide band as in the past at a predetermined minute signal level or more, and is formed in a gradually narrow band or a wide band of gradual dimension according to the present invention at a predetermined minute signal level or less. It is possible to prevent a feeling of noise appearing as sidebands that occurs when the filter is switched at high speed, and it is possible to obtain a DA converter with significantly reduced noise and improved sound quality.

In the above embodiment, the hardware circuit has been described, but a digital filter which gradually changes the cut-off frequency can be realized by a program processing using a digital signal processor (DSP). Although the detection level at the start of the filter and the detection level at the end of the filter are shown to be the same, the detection level is slightly increased at the end to provide a hysteresis characteristic to reduce the frequency of filter switching. The switching noise can be reduced.

[0018]

According to the present invention, since the digital signal can be converted into an analog signal by gently controlling the filter switching at the small signal level to reduce the generation of noise, DA conversion with less noise feeling is possible. You can get a vessel.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a DA converter according to the present invention.

FIG. 2 is a block diagram showing an example of a filter control unit.

FIG. 3 is a block diagram showing an example of a variable low-pass filter.

FIG. 4 is a diagram showing a signal waveform of each part.

FIG. 5 is a diagram showing changes in cutoff characteristics of a filter.

[Explanation of symbols]

 1 Input data 2 Level detection unit 3 Filter control unit 4 Buffer 5 Variable low-pass filter 6 DA conversion unit

Claims (2)

[Claims] 1. A device for converting input digital data into an analog signal, a level detecting means for detecting a level of the input digital data, and a variable low-pass filter for removing a high frequency band of the input digital data. When the level detecting means detects that the level of the input digital data is below a predetermined level, the cut-off frequency of the variable low-pass filter is controlled to be gradually lower, and the predetermined level or slightly lower than the predetermined level. And a filter control means for controlling the cutoff frequency to gradually return to a dimensional level when it is detected that the level exceeds a large level. 2. The DA converter according to claim 1, wherein the level detecting means detects a change in bit of the input digital data below a predetermined level.