JPS58117741A - Signal processing circuit - Google Patents

Abstract

PURPOSE:To avoid an adverse effect on the operation of an analog compander inserted in a PCM transmission system, by using dither noise having a spectrum with a large energy in spite of a low human listening sensitivity, as introduced dither noise. CONSTITUTION:An analog input signal is applied to an adder 43, where the signal is summed with dither noise from a dither noise generator 40. The output of the adder 43 is given to a low-pass filter 12 and the component of frequency at >=1/2 of the sampling frequency is eliminated. The output of the filter 12 is applied to a compressor circuit 20 and compressed depending on the signal level. The output of the compressor 20 is converted into a PCM signal and transmitted. The transmitted signal is applied to an expander 30, where the signal is expanded depending on the signal level to be an output signal. As concrete dither noise, a noise signal in which >=0.5kHz frequency component is emphasized in accordance with the minimum listening value of the Fletcher-Munson curve, is used.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a signal processing circuit that can secure a wider dynamic range with a smaller number of bits when transmitting signals such as audio and images using a digital signal transmission system such as a PCM transmission system. Regarding.

First, a PCM transmission circuit system will be described with reference to FIG. 1 as an example of a digital signal transmission system to which the signal processing circuit according to the present invention is applied.

In FIG. 1, an input terminal 1 is supplied with an analog signal such as a video signal or an audio signal, and this analog input signal is processed by a low-pass filter circuit that removes unnecessary frequency components during digital conversion. Sent to filter 2. The band-limited analog signal from the low-pass filter 2 is sent to a sample-and-hold circuit 3, where it is converted into analog values at fixed sampling intervals, and so-called sampling is performed. This sampled analog value signal is passed through an analog-to-digital converter (A-D
Converter) 4 performs quantization and encoding, resulting in so-called PCM (Pulse Code Modulati
on) signal. This PCM signal is transmitted through a signal transmission medium (
Also includes recording and reproducing media. )5, and the receiving side (
or playback output side) digital-to-analog converter (D
-A converter) 6 converts it into a time-divided analog value signal. This signal passes through a low-pass filter 7 to remove the sampling clock component, and is sent to an output terminal 8 as a temporally continuous analog signal.

In such a PCM transmission circuit system, the A-D converter 4
When quantizing within, it is intermittent on the time axis,
Since an analog signal having continuous amplitude values is made to correspond to a finite, discrete amplitude value, an error equal to the quantization width at maximum inevitably occurs, resulting in so-called quantization noise. In order to prevent the adverse effects of this quantization noise on the auditory sense and to reduce the dependence of the spectrum on the input signal, dithering is used.
) is added to the analog input signal at input terminal 1 from a noisy dither generator 9 .

By the way, there is a limit to the number of bits transmitted per unit time of the signal transmission medium (pit-ray speed). A decrease in bit rate is inevitable, and if a constant frequency characteristic is to be obtained, a decrease in dynamic range is unavoidable. This dynamic range is generally determined by the A-D converter 4.
It is known that the apparent dynamic range can be expanded by performing this quantization non-linearly (non-linear quantization). This allows AD conversion of an input signal with a larger amplitude than linear quantization by widening the quantization width when the analog amplitude value is large. Therefore, this non-linear quantization is equivalent to compressing the amplitude value of an analog signal using a non-linear element such as a diode limiter, and the waveform itself in the transmitted signal becomes distorted. ing. Also, low frequencies (
For example, if an input signal of 20H2 or less) is input,
The disadvantage is that the signal tone is hardly perceptible, and only the level fluctuations of the quantization noise are audible. Furthermore, when injecting the dither described above, cross-modulation between noise and signal may occur.

In order to eliminate such conventional drawbacks, the applicant has already proposed a system in which analog companders are provided before and after the PCM transmission circuit system.

This has a great effect on expanding the dynamic range of the PCM transmission line and reducing noise modulation.

The present invention further improves the prior art in which analog companders are installed before and after a digital signal transmission system, and uses a spectrum that has high energy despite the low viewing sensitivity of humans as injected dither noise. It is an object of the present invention to provide a signal processing circuit configured to use dither noise having a structure such that the dither noise does not adversely affect the operation of the analog compander.

That is, the feature of the signal processing circuit according to the present invention is that the PCM
A digital signal transmission system such as a transmission circuit system, an analog compander placed before and after this digital signal transmission system, and a dither noise signal with low sensitivity and a large energy spectrum corresponding to the sensitivity characteristics of the viewing sense. The analog compander is equipped with a dither generator for generating dither, and a low-pass filter for antialiasing connected in front of the compressor circuit of the analog compander. This antialiasing low-pass filter is a low-pass filter having a steep frequency characteristic curve that suppresses frequency components of at least 1/2 or more of the sampling frequency of the original analog signal. Furthermore, for dither noise that has a spectrum with low sensitivity in the hearing room, for example, frequency components of 5 kHz or higher are emphasized, in addition to the minimum audible value curve, such as the characteristic generally known as the Flexure-Manson isosensitivity curve. It is preferable to use a noise signal.

Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a block circuit diagram showing one embodiment of the present invention. In FIG. 2, an input terminal 11 is supplied with an analog input signal such as a video signal or an audio signal to be transmitted or recorded. This analog input signal is added by an adder 43 to dither noise having a spectrum corresponding to the above-mentioned viewing sensitivity, and sent to the lohas filter 12 for preventing aliasing. The analog signal from this low-pass filter 12 is sent to an analog compressor circuit 20 where level compression is performed. The compressor circuit 20 has a gain that changes depending on the level of the input signal, and no distortion occurs in the output waveform itself. In other words, in the analog signal transmission path,
A variable gain circuit whose gain changes according to a control signal, such as a VCA (Voltage Controlled A
mp, , voltage controlled amplifier) 21 is inserted and connected, the level of the output signal of the VCA 21 is detected by the level detector 22, and the level is converted into a DC control voltage Vc according to the level, and the control input of the VCA 2 l is detected. Supplied to the terminal.

This level detector 22 mainly performs rectification and smoothing to convert an AC signal into a DC control voltage, and has a predetermined value that determines the attack time when the AC signal level rises and the recovery time when it falls. 1 having a time constant of , and the input signal to the level detector 22 is:
Not only the output signal of the VCA 21 but also the input signal of the VCA 21 or the sum or difference signal between the input and the output can be used.

An example of the input/output characteristics of such a compressor circuit 20 is shown in FIG. In this FIG. 3, the input level is a predetermined value P.
(For example, about −40 dB) or more, level compression is performed such that the compression ratio is a constant value (for example, 2), and the input/output characteristics satisfy a loglinear relationship.

This level-compressed analog signal is sampled by a sample hold circuit 13 of the digital signal transmission system, and converted to a digital signal by an analog-to-digital converter (A-D converter) 14. The information is sent to a transmission medium 15 such as a recording medium. This digital signal from the transmission medium 15 is converted into a peak value signal in a sampled state by a digital-to-analog converter (DA converter) 16, and a sampling clock frequency component is removed by a low-pass filter 17. A continuous digital signal is sent to the expander circuit 30.

This expander circuit 30 has an opposite (complementary) expansion characteristic to the compressor circuit 20 described above, and uses a VCA 31 which is a variable gain circuit and a level detector 32 to adjust the expansion characteristics according to the input level. By controlling the gain, input/output characteristics as shown, for example, by the broken line in FIG. 3 are realized. The expander circuit 30 restores the signal to a signal having the same dynamic range as the original input analog signal, and outputs it from the analog signal output terminal 18.

In this way, by placing analog companders before and after the digital signal transmission system and by injecting dither before the compressor, the effect of expanding the signal dynamic range can be obtained at the analog signal stage, and the conventional nonlinear High-quality dynamic range expansion without waveform distortion such as quantization can be achieved, and noise modulation can be reduced.

Next, it is important that the dither noise signal to be added to the input analog signal in the adder 43 be selected to have a spectrum that is difficult to detect and has a large energy, corresponding to the sensitivity of human viewing sense. For example, in the case of digital transmission of audio signals, the sensitivity characteristics of the listening room, e.g.
Corresponding to the so-called Flexure-Manson's isosensitivity curve shown by the solid line in Figure 4 and the Church King curve shown by the broken line in Figure 4, the lowest level audible to the human ear (for example, O It is preferable to use dither noise having a spectrum equal to or less than the isosensitivity curve of Hong Kong. in this case,
Ignore the low frequency component as it has small energy, and set it to 5kHz.
It is sufficient to use a dither generator 40 that generates dither noise with a curved spectrum that emphasizes the spectrum above z. For example, the white noise from a white noise generator (noise generator) 41 with a uniform spectrum is transferred to a bypass filter 4 having a frequency characteristic as shown in the fourth diagram.
It can be easily obtained by using 2.

By the way, when the injection position of such dither noise is placed in front of the analog compressor circuit 20, when the analog compander has a large noise reduction effect and the dither spectrum includes a spectrum exceeding the transmission band of the digital transmission system. Furthermore, the dither causes a phenomenon of mistracking in which the compressor circuit is controlled but the expander circuit is not.

In order to prevent this phenomenon, it is possible to configure the compressor circuit to stop the compression operation at low level input so that the compressor circuit is not controlled by dither, but this is undesirable as it would limit the noise reduction effect. . Inserting a low-pass filter with steep frequency characteristics in front of the compressor circuit causes a large increase in cost, but in the present invention, a low-pass filter for unarchealing in the digital signal transmission system is used. It is placed in front of the analog compressor to prevent compression malfunctions due to dithering.

That is, the input analog signal from the input terminal 11 is added to the adder 43 (by the dither generator 4o) with the dither noise having the spectrum of the audiovisual low sensitivity part, and then the input analog signal from the input terminal 11 is added to the dither noise having the spectrum of the audiovisual low sensitivity part from the dither generator 4o. By passing through the filter 12, it is possible to prevent the compressor from malfunctioning due to the dither, and to prevent aliasing during digital conversion. It has steep frequency characteristics that suppress at least 1/2 or more of the signal component.

The problem in this case is that the analog compressor circuit 2
0, it is necessary that no spectral folding (so-called aliasing) occurs during the conversion operation by the AD converter 14.

Now, the operation of the analog compressor circuit 20 will be considered separately into steady and transient operations.

First, in a steady state, the compressor circuit 2o can be considered as a linear low-pass or bypass filter, so the worst mode is when it has bypass characteristics. Since the above aliasing is not too much, the compressor circuit 20
This can be sufficiently prevented by the low-pass filter 12 in the front position. Next, although it is a transient operation, a compander normally uses an attack time and a recovery time that do not cause distortion within one cycle, so the new spectrum caused by this is very small. Therefore, the generation of a spectrum that may cause new aliasing noise in the analog compressor 20 located after the anti-elite, 7-single low-pass filter 12 can be ignored.

As is clear from the above description, according to the signal processing circuit according to the present invention, dither noise with high energy can be added even though it is difficult to detect visually, and the analog compander malfunctions due to this dither. The prevention and antialiasing effect during AD conversion can be realized by the same low-pass filter 12, and it can be supplied at low cost.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram showing an example of a digital signal transmission system used in the present invention, FIG. 2 is a block circuit diagram showing a signal processing circuit as an embodiment of the present invention, and FIG.
A graph showing an example of the input/output characteristics of the compander circuit in the circuit shown in the figure, and FIG. 4 is a graph showing an auditory sensitivity curve for explaining the spectrum of dither noise. 11... Input terminal 12... Low pass filter 13... Sample hold circuit 14... A-D converter 15... Transmission medium 16... D-A converter 17...・Low pass filter 18... Output terminal 20... Compressor circuit 30... Expander circuit 40... Dither generator 41... Noise generator 42... Bypass filter 43... Addition Equipment Patent Applicant Sony Corporation Representative Patent Attorney Mikai Koike Sakae Tamura

Claims (1)

[Claims] A digital signal transmission system that converts the input analog signal into a digital signal, transmits it, and then converts it back to an analog signal and outputs it, and compresses the level of the input signal by controlling the gain according to the level of the analog input signal. a compressor circuit that outputs the output and supplies it to the input terminal of the digital signal transmission system, and an output from the digital signal transmission system,
It is equipped with an expander circuit that expands the level by controlling the gain according to the level, and a dither generator that generates dither noise having low sensitivity frequency components corresponding to the frequency characteristics of human viewing sensitivity. A signal processing circuit characterized in that a dither noise from the dither generator is added and a filter for preventing aliasing during analog-to-digital conversion is inserted and connected before the compressor circuit.