JPH0478225A - Sound reproducing device - Google Patents

Abstract

PURPOSE:To reduce the effect of residual noise and distortion at a low level signal and to improve S/N by adopting a cut-off frequency variable type HPF for the HPF connecting to an LPF and changing the cut-off frequency proportional to an input level of a D/A converter. CONSTITUTION:An LPF 7 and an HPF 8 eliminate an undesired harmonic wave component from an output of a D/A converter 6, and a cut-off frequency variable type filter is adopted for the HPF 8. The cut-off frequency of a frequency characteristic (a) of the HPF 8 is least as 20Hz and the characteristic is used when an input to the D/A converter 6 is close to a maximum level. The cut-off frequency is sequentially increased in other characteristics b, c, d. A control microcomputer 11 monitors an output level of a digital signal processor DSP 5 and selects the characteristic from the characteristic (a) toward the characteristic (d) as the level is decreased. The characteristic (d) is used when the output level of the DSP 5 is least and then a prescribed level of the output of the D/A converter 6 is cut off and the effect of distortion and noise is reduced. Similarly, the characteristics (b), (c) are used at an intermediate level with respect to the characteristics a, d.

Description

[Detailed Description of the Invention] [Summary] Reducing the effects of output distortion and residual noise of a D/A converter that restores the digital audio signal to an analog audio signal, regarding a sound reproduction device that receives a digital audio signal as input. In a sound reproduction device that restores a digital audio signal to an analog audio signal using a D/A converter and reproduces the analog audio signal through a low-pass filter, the high-pass filter connected to the low-pass filter is provided. In addition to being of a variable cutoff frequency type, the input level of the D/A converter is monitored by a control computer, and the cutoff frequency of the high pass filter is variably controlled in proportion to the level of human power.

[Industrial application field]

The present invention relates to a sound reproduction device that receives digital audio signals as input.

Sound reproduction devices using digital signal processors (DSP) are capable of converting digital audio signals (hereinafter simply referred to as digital signals) into digital audio signals after processing.
It is necessary to restore it to an analog audio signal (hereinafter simply referred to as an analog signal) using an A converter. At this time, if noise or distortion occurs in the D/A converter, it is not good for the auditory sense, so countermeasures are desired.

[Conventional technology]

Audio source: CD (compact disc), DA
T (digital audio tape), BS (satellite broadcasting)
In the case of a digital system such as, the audio signal is digitized in advance. Further, when the audio source is an analog system such as a cassette tape or radio broadcast, the audio signal is an analog signal, but it can be digitized by using an A/D converter.

The volume, sound quality, surround sound, etc. of the digitized audio signal can be adjusted over a wide range by using a simpler configuration than the analog system, that is, by using a DSP.

[Problem to be solved by the invention]

Since the DSP output described above is a digital signal, a D/A converter is required to restore it to an analog signal. However, when the output level of the D/A converter is high, the distortion is small as shown in Figure 7, but if the input level of the DSP is low or the volume is lowered within the DSP, the input level of the D/A converter may change. Therefore, when the output level decreases, the distortion rate increases, making the sound difficult to hear. Further, even if the volume decreases, the noise generated by the D/A converter remains constant, so the S/N ratio deteriorates.

The present invention attempts to improve such problems by changing the characteristics of a high-pass filter (LPF) downstream of the D/A converter.

[Means to solve the problem]

The present invention restores a digital audio signal to an analog audio signal using a D/A converter, and restores the analog audio signal.
In a sound reproduction device that reproduces an audio signal through a low-pass filter, the high-pass filter connected to the low-pass filter is of a variable cutoff frequency type, and the input level of the D/A converter is monitored by a control combination; The cutoff frequency of the high-pass filter is variably controlled in proportion to the input level.

Furthermore, the low-pass filter 7 may be of a variable cutoff frequency type, and the cutoff frequency of the low-pass filter 7 may be variably controlled in proportion to the input level.

[For production]

HPF and LPP are used to remove unnecessary harmonic components from the output of the D/A converter, so the cutoff frequency for HPF is generally 20Hz.

The LPF is set to about 20 KHz. In the present invention, this HPFO cutoff frequency is made variable and is changed in proportion to the input level of the D/A converter. For example, D/
If the cutoff frequency when the input to the A converter is in the full range is 20 Hz, the cutoff frequency is gradually raised toward the audible range as the input level decreases.

An increase in the cutoff frequency of the HPF suppresses low frequency components included in the D/A converter output. This low frequency component is D/
Since it is susceptible to the output distortion of the A converter and noise increase due to S/N reduction, suppressing this produces a reproduction state with less distortion and noise in terms of hearing.

There is also a method of adjusting the volume using a volume installed after the HPF while keeping the cutoff frequency of the HPF constant, but this method suppresses the low range and also suppresses the middle and high ranges, resulting in insufficient volume or unnatural sound. It has the disadvantage of producing a harsh sound.

Furthermore, the increase in the cutoff frequency of the HPF and the decrease in the cutoff frequency of the LPF suppress not only the low frequency components but also the high frequency components contained in the D/A converter output. Through this suppression, a reproduction state with even less distortion and noise can be obtained.

〔Example〕

FIG. 1 is a diagram showing a sound reproduction device according to a first embodiment of the present invention, in which 1 is an analog source such as a cassette, 2 is an A/D converter that digitizes the output (analog signal), and 3 is a CD 4 is a source selection switch, 5 is a DSP, 6 is a D/A converter, 7 is an LPF,
8 is HPF.

9 is an amplifier, 10 is a speaker, 11 is a control microcomputer, and 12 is a display/operation section.

DSP 5 is a high-speed processor dedicated to digital processing that performs volume adjustment, sound quality adjustment, surround processing, etc. on digital signals input manually through switch 4, and the processing contents are displayed and displayed.
It is set (changed) by the output of the control microcomputer 11 that receives instructions from the operation unit 12. D/A converter 6 is this DS
The output of P5 (digital signal after processing) is restored to an analog signal. The LPF 7° HPF 8 removes unnecessary harmonic components from the output of the D/A converter 6, and in the first embodiment of the present invention, the HPF 8 is of a variable cutoff frequency type.

FIG. 3 is a diagram showing the frequency characteristics of HPF 7, where a has the lowest cutoff frequency of 20 Hz. This property a is D/
Used when the input of A converter 6 is close to the maximum level. The cutoff frequencies of the other characteristics (c, d) increase in sequence. The control microcomputer 11 monitors the output level of the DSP 5, and selects characteristics from a to d as the level decreases. Characteristic d is the case where the output level of the DSP 5 is the lowest, and in this case, the output of the D/A converter 6 below a predetermined value is cut to reduce the effects of distortion and noise. Similarly, characteristic c is used at an intermediate level between characteristics a and d.

FIG. 2 is a concrete example of IIPL 8. In this example, first, a 3rd order LP of 18 dB 10 ct is connected to the input side of the operational amplifier OP with resistors R1 to R3 and capacitors 01 to C3.
Indicates F7. Next, capacitor C is connected to the output side of amplifier ○P.
4 and resistors R4 and R in parallel with the capacitor C4.
Taking HPF 8 as an example, in which R6 and R7 are connected, switching transistors T1 to T4 are connected between the respective resistors R9 to R7 and the ground, and their on and off
The off state is controlled by a switching signal from the control microcomputer 11. Note that here R4>R5>Rg>R7.

When transistor T1 is on and T2 to T are off, HP
The characteristics of F8 are shown in a of FIG. When only the transistor T2 is turned on, the resistor R5 is connected in series with the capacitor C1, and the characteristic of the LPF 8 becomes as shown in FIG. 3b. Also,
When only the transistor T3 is turned on, the resistor R6 is connected in series with the capacitor C4, resulting in characteristic C shown in FIG. Similarly, when only the transistor T is turned on, the characteristic d in Fig. 3 is
become.

The on/off states of the transistors T1 to T4 and the values of the capacitor C1 and the resistors R4 to R1 can be set arbitrarily.

The control microcomputer 11 monitors the output level of the DSP 5, which is averaged by the internal processing of the DSP 5, and changes the cutoff frequency switching signal of the LPF 8. The control microcomputer 11 may monitor any level up to the input of the LPF 8, but since the output of the DSP 5 (input of the D/A converter 6) is digital, it is easiest to process.

Incidentally, the present invention can also be applied to an audio reproduction device that simply restores an input digital signal to an analog signal using a D/A converter without using a DSP.

FIG. 4 is a diagram showing a sound reproduction device according to a second embodiment of the present invention. The difference between this diagram and Figure 2 is that the LPF 7 is also of a variable cutoff frequency type, and the control microcomputer 11 is an LPF.
7 and HPF 8, respectively.

FIG. 5 is a diagram showing specific circuits of the LPF and HPF. What differs from Figure 2 in this diagram is the LPF 7, and a capacitor C5 for variable characteristics is connected in parallel to the capacitor C3.
~C1 can be connected. Capacitor 05~
Switching transistors T5 to T are connected between C7 and ground, respectively, and their on and off states are controlled together with transistors T1 to T by switching signals from the control microcomputer 11. Here, it is set as C7<C6<C5<C3. FIG. 6 is a diagram showing LPF and HPF frequency characteristics. In this diagram, when only transistor T1 is on and all others are off, LPF 7 and HPF
The characteristic of No. 8 is shown in a of FIG. When only transistors T2 and T5 are turned on, capacitor Cs is connected in parallel with capacitor C3 for LPF 7, and LPF 7
And the characteristics of HPF 8 are as shown in Fig. 3b. Further, when only transistors T3 and T6 are turned on, capacitor C6 is connected in parallel to capacitor C3, resulting in characteristic C shown in FIG. Similarly, when only transistors T4 and T7 are turned on, the characteristic d shown in FIG. 3 is obtained. In addition, here LP
For F 7 and HPF 8, a, b, c at the same time
, d and the cutoff frequency were changed, but first the LPF
7 a.

Switch to b, c, d, then change HPF 8 to a, bc,
It is also possible to switch to d, and vice versa.

〔Effect of the invention〕

As described above, according to the present invention, in a sound reproduction device using a D/A converter, there is an advantage that the effects of residual noise and distortion at low level signals can be reduced, and the perceptual S/N can be improved. .

[Brief explanation of drawings]

FIG. 1 is a diagram showing a sound reproduction device according to the first embodiment of the present invention, FIG. 2 is a diagram showing a circuit of a specific example of HPF, and FIG. 3 is a diagram showing a circuit of a specific example of HPF.
FIG. 4 is a diagram showing the sound reproduction device according to the second embodiment of the present invention. FIG. 5 is a diagram showing specific circuits of the LPF and HPF. FIG. 6 is a diagram showing the LPF and HPF. FIG. 7 is a diagram showing the frequency characteristics. FIG. 7 is a diagram showing the output distortion characteristics of the D/A converter. In the figure, 5...digital signal processor (DSP), 6...D/
A converter, 7...low-pass filter, 7...high-pass filter,
11...Control computer.

Claims (1)

[Claims] 1. A digital audio signal is converted into a D/A converter (6
) to restore the analog audio signal to the analog audio signal.
In a sound reproduction device that reproduces an audio signal through a low-pass filter (7), the low-pass filter (7)
) is a variable cutoff frequency type high-pass filter (8) connected to the D/A converter (6).
A sound reproduction device characterized in that an input level of the high-pass filter (8) is monitored by a control computer (11), and the cutoff frequency of the high-pass filter (8) is variably controlled in proportion to the input level. 2. The sound reproduction device according to claim 1, wherein the low-pass filter (7) is of a variable cutoff frequency type, and the cutoff frequency of the low-pass filter (7) is variably controlled in proportion to the input level.