JP2005064700A - Digital signal processing system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a digital processing system with which distortion can be reduced much more than a conventional case and a soft clip is realized with less operation. <P>SOLUTION: When an input digital signal is set to be (x), a gain coefficient to be (g) and an output digital signal to be X in the digital signal processing system, a linear processing of X=gx is performed up to a threshold P. When an input signal level exceeds the threshold P, a non-linear processing of X=1-ä1-¾x¾}<SP>n</SP>is performed. When the input signal level is smaller than the threshold P, the linear processing is performed. Thus, deterioration of a distortion rate of an output signal is prevented. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a digital signal processing system, and more particularly to gain adjustment of a digital audio signal or the like.

  With the development of semiconductor technology and digitization technology, digital signals are widely processed in audio devices, navigation devices, home appliances, and the like. In the case of analog signal processing, although there is a problem of variation due to circuit characteristics and temperature characteristics, digital signal processing is excellent in reproducibility. On the other hand, if high precision processing is to be realized, Accordingly, the amount of data to be processed becomes enormous. However, a large amount of data can be processed in real time by a semiconductor processing circuit such as a DSP, and an ideal analog signal waveform can be obtained by digital signal processing.

但し、指数値ｎは圧縮比を決める整数であり、｜ｘ｜は入力信号ｘの絶対値である。 The present applicant has disclosed a technique relating to a digital variable compressor for an input audio signal, as shown in Patent Document 1, for example. When the input audio signal is x, the following equation (4) (hereinafter referred to as the conventional method (4)) is calculated to add a compression characteristic to the output audio signal X and to vary the n value. This changes the compression characteristics.

However, the exponent value n is an integer that determines the compression ratio, and | x | is the absolute value of the input signal x.

  By applying the calculation shown in the conventional method (4) to the input digital signal x, the gain of the output digital signal X can be adjusted, and a soft clip can be realized without overflowing all input signal levels. it can. In addition, an output digital signal can be obtained in real time by executing the processing of the conventional method (4) by the DSP.

Japanese Patent No. 3056815

However, the signal processing method disclosed in Patent Document 1 has the following problems. For example, when the signal processing of the conventional method (4) is applied to a digital amplifier and the gain due to the power supply voltage of the digital amplifier is G [dB], the exponent value n is 1, 2, 3, 4,. When variable, the gain of the output signal is G, G + 6, G + 9, G + 12,... [DB]. When the exponent value n is 2 or more, the conventional method (4) works as nonlinear processing, and the gain includes distortion. For example, when the index value n is 2, the output signal X, distortion component x ² is superimposed, when the index value n is 3, the distortion component of x ³ are superimposed in addition to this. This further deteriorates the distortion rate of the output signal even in a reproduction state where the input signal level is low. Although the degree of deterioration of the distortion rate varies depending on the set gain value, that is, the exponent value n, the distortion value is increased by about one digit.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a digital processing method capable of reducing distortion as compared with the processing according to the conventional method (4).
Another object of the present invention is to provide a digital processing system capable of realizing a soft clip with few operations.
Still another object is to provide a digital processing system capable of performing linear processing when the level of an input digital signal is low and performing nonlinear processing when the level is high.

In the digital signal processing system according to the present invention, when the input digital signal is x, the gain coefficient is g, and the output digital signal X, up to the threshold value P obtained by the conditional expression (1), the calculation according to the expression (2) is performed. When the threshold value P is exceeded, the calculation according to equation (3) is performed.

  According to the present invention, the input digital signal is subjected to the conditional processing according to the conditional expression (1), the linear processing according to the expression (2) is performed up to the threshold P, and the nonlinear processing according to the expression (3) is performed when the input signal level is higher than the threshold P. As a result, good distortion performance can be obtained by complete linear processing in the range where the input signal level is low, and soft clip can be realized without overflow in the range where the input signal level is high. it can. Further, in the case of the processing by the conventional method (4), since the step width is determined by the exponent value n that allows only positive real numbers, the gain values can be obtained only as discrete values as G + 6, G + 9, G + 12. However, in the present invention, an arbitrary gain value can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail. The digital signal processing according to the present invention is preferably performed using a DSP (digital signal processor) equipped with a multiplier or the like and capable of high-speed computation.

  FIG. 1 is a block diagram showing the configuration of a DSP that performs digital signal processing according to the present invention. The DSP 10 includes a data space and a program space, and the data space includes an I / O port unit 22, a multiplier / ALU unit 24, and a data memory 26 connected to the data bus 20. The program space includes a control unit 32 and a program memory 34 that control the operation of each unit connected to the program bus 30.

  The program memory 34 stores a program for executing the processing of the following equations (1), (2), and (3), and the control unit 32 follows the program according to the multiplier / ALU unit 24, data The memory 26, the I / O port unit 22 and the like are controlled. The set gain coefficient g is stored in the data memory 26. The input digital signal x is input to the I / O port 22, and then subjected to digital processing in the multiplier / ALU unit 24 and the like under the control of the program operation, and is output again from the I / O port 22 as the output digital signal X. The

  FIG. 2 is a functional block for explaining the operations of the above formulas (1), (2) and (3), and FIG. 3 is a flowchart showing the operation. When the input digital signal x is input to the DSP 10 (step S101), the process determination unit 42 reads a preset gain coefficient g from the gain coefficient holding unit 44 (step S102), and the gain coefficient g is 2 or more. It is determined whether or not there is (step S103).

Further, when the gain coefficient g is smaller than 2 (1 <g <2), the process determination unit 42 calculates a threshold value P according to the following equation (step S104).

On the other hand, when the gain coefficient g is 2 or more (n <g <n + 1, where n is a positive integer of 2 or more), the threshold value P is calculated according to the following equation (step S105).

  The threshold value P is a boundary point between linear processing and non-linear processing. For an input digital signal having a level smaller than the threshold value P, a linear processing algorithm is executed. A nonlinear processing algorithm is executed.

  Next, the process determination unit 42 compares the input digital signal x with the threshold value P (step S106), and when it is determined that the input signal is less than the threshold value P, an output that instructs the linear processing unit 44 to be operable. 42a is given. Accordingly, the linear processing unit 44 performs linear processing for multiplying the input digital signal x by the gain coefficient g, that is, processing of the above formula (2) (step S107). On the other hand, when the input digital signal x is equal to or greater than the threshold value P, the process determination unit 42 gives an output 42b that instructs the non-linear processing unit 46 to be operable. Thereby, the non-linear processing unit 46 performs non-linear processing of the input digital signal x, that is, the processing of the above equation (3) (step S108). The calculation for the non-linear processing is described in detail in Patent Document 1 described above. The above linear and nonlinear operations are executed in real time by the DSP 10 and the digital signal X is output (step S109).

  FIG. 4 is a diagram showing input / output characteristics when digital processing according to this embodiment is performed. The horizontal axis and the vertical axis are logarithmic (dB) scales, the horizontal axis indicates the input signal, and the vertical axis indicates the level of the output signal. The solid line S1 (gain coefficient g = 1), the solid line S2 (1 <g <2), the solid line S3 (2 <g <3), and the solid line S4 (3 <g <4) indicate linear processing according to the equation (2). The wavy line (exponential value n = 2), the dotted line (exponential value n = 3), and the alternate long and short dash line (exponential value n = 4) indicate nonlinear processing according to the equation (3), and P indicates a threshold value. For example, when the gain coefficient is 1 <g <2, as shown by the solid line S2, linear processing is performed until the input digital signal x reaches a level of about −13 dB, that is, the threshold P, and at a level higher than that, Then, nonlinear processing with an exponent value n = 2 is performed. The threshold P is the intersection of the solid line S2 and the curve with the exponent value n = 2. When the gain coefficient is 2 <g <3, the linear processing as indicated by the solid line S3 is performed up to the threshold P, and the nonlinear processing as indicated by the exponent value n = 3 is performed in the portion exceeding the threshold P. Similarly, for a gain coefficient of 3 <g <4, linear processing as shown by the solid line S4 is performed up to the threshold value P, and nonlinear processing as shown by the exponent value n = 4 is performed in a portion exceeding the threshold value P.

  In this way, since the process is completely linear when the input digital signal is in a low level below a predetermined level, the distortion of the output signal can be extremely reduced. A soft clip with a waveform can be realized. Further, in the case of processing according to the conventional method (4), only a positive integer is allowed for the exponent value n, so that the gain of the output signal is limited to a step width of G + 6, G + 9, G + 12. However, in this embodiment, it is also possible to obtain an arbitrary gain during that time.

  FIG. 5 is a block diagram showing an example in which the digital signal processing according to this embodiment is applied to a digital amplifier. The digital amplifier 100 includes a volume adjusting unit 110, a gain adjusting unit 120, a gain coefficient setting unit 130, a PWM unit 140, and a driving unit 150. An m-bit digital audio signal optically read from a recording medium such as a CD or DVD and subjected to A / D conversion is input to the volume adjustment unit 110. Here, the digital audio signal corresponding to the volume instruction from the user is converted into a digital audio signal. Variable. The gain adjusting unit 120 is a part that executes the digital signal processing of the equations (1) to (3) according to the above-described embodiment. The gain coefficient setting unit 130 holds the set gain coefficient g. However, the gain coefficient g can be changed according to an instruction from the user, or automatically according to an external noise level. It is also possible to vary the gain coefficient g. The PWM unit 140 generates a PWM signal having a duty ratio corresponding to the input digital data. The driving unit 150 performs a switching operation for selectively driving the speaker in response to the PWM signal.

  The gain adjusting unit 120 can adjust the gain according to the level of the audio digital signal input from the volume adjusting unit 110. In particular, when the signal has a low volume level, the gain adjustment unit 120 can perform linear processing of the input signal, and thus distortion can be suppressed to a very low level. Thus, by applying the digital signal processing system according to the present embodiment to the gain adjustment unit 120 of the digital amplifier, an excellent audio output signal close to the analog system can be obtained.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific embodiments, and various modifications can be made within the scope of the gist of the present invention described in the claims. Deformation / change is possible.

  The digital signal processing system according to the present invention can be applied to processing of any digital signal. It can be used in digital devices such as audio devices, home appliances, and navigation devices that perform digital signal processing.

1 is a block diagram of a DSP according to an embodiment of the present invention. It is a functional block of digital signal processing concerning this example. It is a figure which shows the operation | movement flow of the digital signal processing which concerns on a present Example. It is a figure which shows the input-output characteristic by the digital signal processing based on a present Example. It is a block diagram of a digital amplifier to which an embodiment of the present invention is applied.

Explanation of symbols

10: DSP
42: Processing determination unit 44: Gain coefficient holding unit 46: Linear processing unit 48: Non-linear processing unit

Claims (4)

When the input digital signal is x, the gain coefficient is g, and the output digital signal X, the calculation according to the expression (2) is performed up to the threshold P obtained by the conditional expression (1), and when the threshold P is exceeded, the expression (3 ) Digital signal processing method.

The digital signal processing system according to claim 1, wherein the input digital signal is a digital audio signal, and the gain coefficient g is a gain coefficient of a digital amplifier. A digital signal processing method for processing a digital signal,
Receiving an input digital signal x;
It is determined whether or not the signal level of the input digital signal x is smaller than the threshold value P obtained by the conditional expression (1). When the signal level is smaller than the threshold value P, the calculation according to the expression (2) is performed. ) To perform the calculation by
And a step of outputting an output digital signal X obtained by the above calculation.

4. The digital signal processing system according to claim 3, wherein each step of the digital signal processing system is performed when the gain of the input audio signal is adjusted in the digital amplifier.

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