WO2017193877A1

WO2017193877A1 - Pulse density modulation switching circuit and method and storage medium

Info

Publication number: WO2017193877A1
Application number: PCT/CN2017/083273
Authority: WO
Inventors: 赵恒正; 李安
Original assignee: 深圳市中兴微电子技术有限公司
Priority date: 2016-05-10
Filing date: 2017-05-05
Publication date: 2017-11-16
Also published as: CN107359868A

Abstract

A pulse density modulation switching circuit, pulse density modulation switching method and storage medium, the circuit comprising: a cascade integrator (11), a downsampling decimator (12) and a cascade comb filter (13); the cascade integrator (11) is configured to receive a pulse density modulation (PDM) audio signal, accumulate the pulse density modulation (PDM) audio signal, and output to the downsampling decimator (12) the pulse density modulation (PDM) audio signals accumulated by an integrator at each level; the downsampling decimator (12) is configured to decimate the accumulated pulse density modulation (PDM) audio signals at a predetermined interval; and the cascade comb filter (13) is configured to receive the pulse density modulated (PDM) audio signals as decimated by the downsampling decimator (12) and subtract the extracted pulse density modulated (PDM) audio signals to obtain a pulse code modulation (PCM) audio signal. The present circuit simplifies the complex structure of original cascade comb filters, thereby facilitating the implementation of hardware circuits and reducing the amount of occupied resources.

Description

Pulse density modulation conversion circuit and method, storage medium

Cross-reference to related applications

The present application is based on a Chinese patent application filed on Jan. 10, 2016, the filing date of which is hereby incorporated by reference.

Technical field

The present invention relates to signal processing technologies, and in particular, to a pulse density modulation conversion circuit and method, and a storage medium.

Background technique

Digital microphone is a new type of processing device that converts analog audio signals into digital signals. Compared with analog microphones, it has the advantages of strong anti-interference ability, high signal-to-noise ratio and low distortion. The output signal of a digital microphone is typically in the form of pulse density modulation (PDM). However, since the promotion time of the PDM format is later than the pulse code modulation (PCM), most audio processing and playback devices currently only support PCM, so the PDM audio signal obtained from the digital microphone needs to be converted into a PCM audio signal. For routine processing or application.

The existing PDM to PCM method has a cascade integrator comb (CIC) filter, and FIG. 1 is a schematic diagram of a conventional cascaded comb (CIC) filter structure, as shown in FIG. Integral comb (CIC) filtering consists of a cascade integrator n-1, a cascade comb filter nD, and a downsampled decimation R. 2 is an equivalent structure diagram of a conventional comb filter portion. As shown in FIG. 2, the cascade comb filter nD can be equivalent to a plurality of delay units Z (in FIG. 2, 32 delay units are taken as an example for illustration. ). Cascaded Integral Comb (CIC) filtering with a downsampling rate R equal to that of a cascaded comb filter Differential delay D. The PDM audio signal is input from the cascade integrator, and after cascading comb filter and downsampling extraction process, the output is extracted from the downsampled, and then converted into a PCM audio signal.

As can be seen from FIG. 2, the cascaded comb filter in the existing cascaded comb-like (CIC) filter has a large order and a large resource occupation.

Summary of the invention

In order to solve the above technical problem, embodiments of the present invention are expected to provide a pulse density modulation conversion circuit and method, and a storage medium, which are used to simplify the filter structure, facilitate hardware circuit implementation, and reduce resource occupation.

The technical solution of the present invention is implemented as follows:

Embodiments of the present invention provide a pulse density modulation conversion circuit, the circuit including: a cascade integrator, a downsampling extractor, and a cascade comb filter;

The cascade integrator is configured to receive a pulse density modulated PDM audio signal, and perform an accumulating process on the PDM audio signal, and output the PDM audio signal accumulated by the integrators of each stage to the downsampler extractor;

The downsampling decimator is configured to extract the accumulated PDM audio signal every preset interval;

The cascaded comb filter is configured to receive the PDM audio signal extracted by the downsampling decimator, and perform subtraction on the extracted PDM audio signal to obtain a pulse code modulated PCM audio signal.

In the above solution, the delay of the cascaded comb filter is the ratio of the original delay of the cascaded comb filter to the downsampled rate of the downsampler decimator.

In the above solution, the output of the PDM audio signal accumulated by the level integrators after being extracted by the downsampling extractor is:

When n is equal to N*R, the output is the PDM audio signal accumulated by the nth stage integrator, otherwise, when n is not equal to N*R, the output is 0; wherein N is the clock of the cascaded comb filter Frequency, R represents R times downsampling, and n is the number of stages in which the integrator is located.

In the above solution, the clock frequency of the cascaded integrator is the product of the clock frequency of the cascade comb filter and the downsampling rate.

In the above solution, the cascade comb filter includes an M-stage delayer and an M-stage subtractor; wherein the M is a positive integer;

When the M is greater than or equal to 2, the Mth stage delayer is configured to save an operation result of the M-1th stage subtractor; when the M is equal to 1, the 1st stage delayer is configured to save the Extracted PDM audio signal;

The Mth stage subtractor is configured to subtract the extracted Mth PDM audio signal and the M-1th audio signal.

The embodiment of the invention further provides a pulse density modulation conversion method, the method comprising:

The cascade integrator receives the pulse density modulated PDM audio signal, performs an accumulating process on the PDM audio signal, and outputs the PDM audio signal accumulated by the integrators of each stage to the downsampling extractor;

The downsampling decimator receives the accumulated PDM audio signal, and extracts the accumulated PDM audio signal every preset interval;

The cascaded comb filter receives the PDM audio signal extracted by the downsampled decimator, and performs subtraction on the extracted PDM audio signal to obtain a pulse code modulated PCM audio signal.

In the above solution, the downsampling decimator receives the accumulated PDM audio signal, and extracts the accumulated PDM audio signal at a preset interval, specifically:

When n is equal to N*R, the downsampling decimator extracts the PDM audio signal accumulated by the nth stage integrator as the output of the downsampled decimator, otherwise, when n is not equal to N*R, the downsampling The output of the decimator is 0; where N is the clock frequency of the cascaded comb filter, R table Show R times downsampling, where n is the number of stages in which the integrator is located.

Correspondingly, the cascaded comb filter receives the PDM audio signal extracted by the downsampled decimator, and performs subtraction on the extracted PDM audio signal to obtain a pulse code modulated PCM audio signal:

When the M is greater than or equal to 2, the Mth stage subtractor subtracts the extracted Mth PDM audio signal and the extracted M-1th audio signal to obtain a pulse code modulated PCM audio signal, and the The operation result of the M-1 level subtractor is stored in the Mth stage delayer;

When the M is equal to 1, the PDM audio signal extracted by the downsampling decimator is saved in the first stage delayer.

Embodiments of the present invention provide a pulse density modulation conversion circuit and method, and a storage medium, including a cascade integrator, a downsampling extractor, and a cascade comb filter; the cascade integrator is configured to receive pulse density modulated PDM audio Signal, and accumulate the PDM audio signal, output the PDM audio signal accumulated by the integrators of each stage to the downsampler decimator; the downsampler decimator is configured to extract the accumulated PDM audio signal at every preset interval, cascading comb filter The device is configured as a PDM audio signal extracted by the downsampling decimator, and performs subtraction on the extracted PDM audio signal to obtain a pulse code modulated PCM audio signal. The conversion circuit advances the downsampled extractor R to the middle of the cascaded integrator and the cascaded comb filter, so that the differential delay D of the cascaded comb filter is only 1/R of the original structure, which is greatly simplified. The structural complexity of the original cascade comb filter facilitates hardware circuit implementation and reduces resource occupation.

DRAWINGS

1 is a schematic diagram of a conventional cascaded comb (CIC) filter structure;

2 is an equivalent structural diagram of a conventional comb filter portion;

3 is a schematic diagram of a PDM audio signal conversion circuit according to an embodiment of the present invention;

4 is a structural block diagram of components included in a PDM audio signal conversion circuit according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of an equivalent structure of a cascade comb filter according to an embodiment of the present invention; FIG.

6 is a schematic diagram of a two-stage cascade process of converting a PDM audio signal into a PCM audio signal;

7 is a two-level cascade circuit diagram of converting a PDM audio signal into a PCM audio signal;

FIG. 8 is a schematic flowchart diagram of a PDM audio signal conversion method according to an embodiment of the present invention.

detailed description

The technical solutions in the embodiments of the present invention will be clearly and completely described in the following with reference to the accompanying drawings.

The pulse density modulation conversion circuit of the embodiment of the present invention improves the downsampling extraction part in the existing cascaded integral comb filter from the last stage of the processing structure to the middle of the integral filtering and the comb filtering, so that After the improvement, the differential delay D of the comb filter is only 1/R of the original structure, which greatly simplifies the structural complexity of the comb filter.

3 is a schematic diagram of a PDM audio signal conversion circuit according to an embodiment of the present invention, and FIG. 4 is a structural block diagram of components included in a PDM audio signal conversion circuit according to an embodiment of the present invention, as shown in FIG. 3 and FIG. include:

a cascade integrator 11, a downsampling extractor 12 and a cascade comb filter 13;

The cascade integrator 11 is configured to receive a pulse density modulated PDM audio signal, and perform an accumulation process on the PDM audio signal, and output the PDM audio signal accumulated by the integrators of each stage to the downsampler extractor;

The downsampling decimator 12 is configured to extract the accumulated PDM audio signal every preset interval;

The cascaded comb filter 13 is configured to receive PDM sounds extracted by the downsampling extractor The frequency signal is subtracted from the extracted PDM audio signal to obtain a pulse code modulated PCM audio signal.

It should be noted that, as can be seen in conjunction with FIG. 3 and FIG. 4, 11 in FIG. 3 corresponds to the cascade integrator 11 in FIG. 4, and 12 in FIG. 3 corresponds to the downsampling extractor 12 in FIG. 13 corresponds to the cascaded comb filter 13 in FIG.

Specifically, the PDM audio signal is input from the cascade integrator, and is output from the cascade comb filter after the downsampler decimator. Specific steps are as follows:

(1) The PDM audio signal is input from the cascade integrator, and the delay of the cascade integrator is 1, that is, the current output of each stage integrator is equal to the sum of the current input and the previous output. which is:

y(n)=x(n)+y(n-1)

Where x(n) is the nth input of the cascaded integrator, y(n-1) is the (n-1)th output of the cascaded integrator, and y(n) is the nth of the cascaded integrator Level output, 1 means that the cascaded integrator has a delay of 1, and n represents the number of stages in which each integrator is located, n=1, 2, 3, .

(2) After the PDM audio signal processed by the step (1) is output from the cascade integrator, after the R times down sampling extractor, the number of data becomes 1/R of the original number, that is, the downsampled extractor is every The accumulated value is extracted at a preset interval, where the preset interval is N*R. The downsampling decimator can be realized by the change of the clock frequency: the operating clock frequency used by the cascaded integrator is T, the working clock frequency used by the cascaded comb filter is N, and satisfies n=T=N*R, Then the number of data entering the cascade comb filter is 1/R of the number of data output from the cascade integrator. Formulated as follows:

Where N is the clock frequency of the cascaded comb filter, R represents R times downsampling, n is the number of stages in which the integrator is located, n=N*R, n' is the output result number of the downsampled decimator, n' =1, 2..., y(n) is the output of the nth stage integrator, and y(n') is the nthth output of the downsampled decimator.

The formula indicates that the PDM audio signals accumulated by the integrators of each stage are pumped by the downsampling extractor. The output after fetching is:

When n is equal to N*R, the output is the PDM audio signal accumulated by the nth stage integrator; otherwise, when n is not equal to N*R, the output is 0; where N is the clock frequency of the cascaded comb filter, R represents R times downsampling, and n is the number of stages in which the integrator is located.

(3) After the PDM audio signal processed by step (2) is output from the downsampling decimator, it enters the cascade comb filter, and the delay of the cascade comb filter is greatly reduced due to the forward shift of the downsampler decimator. Small, for example, assuming that the delay of the improved pre-cascade comb filter is 32 and the downsampling rate is 32, then the input-output relationship is expressed as:

z(n")=y(n')-y(n'-32)

Where y(n') is the nth output of the downsampled decimator, y(n'-32) is the (n'-32)th output of the downsampled decimator, and z(n") is cascading The nthth output of the comb filter, n" = 1, 2, ....

The delay of the improved cascaded comb filter is the ratio of the original delay to the downsampling rate of the downsampled decimator, that is, the input-output relationship is expressed as:

z(n")=y(n')-y(n'-1)

It can be seen that, after the improvement, the delay of the cascade comb filter is reduced from 32 to 1, that is, only at least one data needs to be saved, and the improved cascade comb filter is compared with the need to save at least 32 data before the improvement. The delay of the device is reduced a lot. In the actual circuit, it means that the number of data to be saved is greatly reduced, which simplifies the circuit structure and occupied resources.

4 is a structural block diagram of components included in a PDM audio signal conversion circuit according to an embodiment of the present invention. As shown in FIG. 4, the conversion circuit includes: a cascade integrator 11, a downsampling extractor 12, and cascade comb filtering. 13. The cascade integrator 11 is configured to receive the PDM audio signal and perform an accumulation process on the PDM audio signal, and output the PDM audio signal accumulated by the integrators of each stage to the downsampler decimator 12; the downsampler decimator 12 is configured to pre-predict The accumulated PDM audio signal is extracted at intervals, and the extracted PDM audio signal is output to the cascade comb filter 13 for processing, and the cascaded comb filter 13 performs subtraction on the extracted PDM audio signal to obtain PCM audio. signal. The preset interval is set according to the system and user requirements, and is not described here.

Wherein, the input end of the cascade integrator 11 receives the PDM audio signal, the output of the cascaded integrator 11 is connected to the input of the downsampler decimator 12, the output of the downsampler decimator 12 and the cascaded comb filter 13 The input terminal is connected, and the output of the cascade comb filter 13 outputs a PCM audio signal.

The pulse density modulation conversion circuit of the embodiment comprises a cascade integrator, a downsampling extractor and a cascade comb filter; the PDM audio signal is input before the cascade integrator, and sequentially passes through the cascade integrator and the downsampler extractor. After processing by the cascade comb filter, it is output from the cascade comb filter. In the circuit, the cascaded comb filter is filtered by the downsampled decimator in the original cascaded comb filter from the last stage of the processing structure to the middle of the cascade integrator and the cascade comb filter. The differential delay D of the device is only 1/R of the original cascaded comb filter structure, which reduces the number of cascaded comb filters, facilitates hardware implementation, simplifies the structure, and saves resources.

FIG. 5 is a schematic diagram of an equivalent structure of a cascaded comb filter according to an embodiment of the present invention. As shown in FIG. 5, comparing FIG. 2 and FIG. 4, it is assumed that the differential delay D and the downsampling multiple R are both 32. The improved PDM audio signal conversion circuit requires 32 stages of delay units, and the improved PDM audio signal conversion circuit requires only one level of delay units.

By verification, the downsampler decimator 12, the system function corresponding to the last stage of the processing structure, and the system function corresponding to the intermediate between the cascaded integrator 11 and the cascaded comb filter 13 are equal to the same moving average The system function of the filter, so by moving the downsampling decimator 12 forward, it simplifies the circuit structure and ensures that the processing effect is the same as that without advance.

Further, the delay of the cascade comb filter is a ratio of an original delay of the cascade comb filter to a down sample rate of the downsampler decimator.

Further, the output of the PDM audio signal accumulated by the level integrators after being extracted by the downsampling extractor is:

When n is equal to N*R, the output is the PDM audio signal accumulated by the nth stage integrator; otherwise, When n is not equal to N*R, the output is 0; where N is the clock frequency of the cascaded comb filter, R is R times downsampling, and n is the number of the integrator.

Further, the clock frequency of the cascaded integrator is the product of the clock frequency of the cascaded comb filter and the downsampling rate.

Further, the cascade comb filter includes an M-stage delayer and an M-stage subtractor; wherein the M is a positive integer;

6 is a schematic diagram of a two-stage cascade process of converting a PDM audio signal into a PCM audio signal; as shown in FIG. 6, the PDM audio signal sequentially passes through a two-stage integrator I, a downsampling extractor R, and a two-stage comb filter C. , converted to PCM audio signal.

7 is a two-stage cascade circuit diagram of converting a PDM audio signal into a PCM audio signal. As shown in FIG. 7, the integrator I is composed of an integrator and a delay, and the comb filter C is composed of a delayer and a subtractor.

FIG. 8 is a schematic flowchart of a PDM audio signal conversion method according to an embodiment of the present invention. As shown in FIG. 8, the method includes:

Step 101: The cascade integrator receives the pulse density modulated PDM audio signal, accumulates the PDM audio signal, and outputs the PDM audio signal accumulated by the integrators of each stage to the downsampler extractor.

In this step, the PDM audio signal is input from the cascade integrator, and after receiving the PDM audio signal, the integrators of each stage sequentially perform the accumulation processing on the PDM audio signal. Specifically, after the PDM audio signal is input from the cascade integrator, the current output of each stage integrator is equal to the current input and The sum of the last output. Expressed by the formula y(n)=x(n)+y(n-1), where x(n) is the nth input of the cascaded integrator and y(n-1) is the cascaded integrator The (n-1)th output, y(n) is the nth output of the cascaded integrator, 1 means the delay of the cascaded integrator is 1, and n is the number of stages in which each integrator is located, n=1, 2,3,...

Step 102: The downsampler decimator receives the accumulated PDM audio signal, and extracts the accumulated PDM audio signal at preset intervals.

In this step, the downsampling decimator receives the accumulated PDM audio signal output by the cascaded integrator, and performs an R-time downsampling extraction process on the accumulated PDM audio signal, and extracts data by R times down sampling. The number becomes 1/R of the original number and the extracted PDM audio signal is obtained. The specific downsampling process and the output result are described in detail in the foregoing embodiments, and are not described herein again.

Step 103: The cascading comb filter receives the PDM audio signal extracted by the downsampled decimator, and performs subtraction on the extracted PDM audio signal, and obtains a pulse code modulated PCM audio signal after conversion.

In this step, the cascading comb filter receives the PDM audio signal extracted by the down sampling decimator in step 102, and processes the extracted PDM audio signal through a comb filter, and specifically extracts the PDM audio signal according to the formula z. (n") = y(n') - y(n' - 1) is subtracted to obtain a PCM audio signal, where N is the clock frequency of the cascaded comb filter, R is R times downsampling, and n is The number of stages in which the integrator is located, n=N*R, n' is the output result number of the downsampled decimator, n'=1, 2..., y(n) is the output of the nth stage integrator, y(n' ) is the n'th output of the downsampling decimator. The delay of the cascaded comb filter is greatly reduced due to the forward shift of the downsampled decimator, and the delay of the cascaded comb filter is the original delay and drop. The ratio of the downsampling rate of the sample decimator.

In the pulse density modulation conversion method of the embodiment, the PDM audio signal is input from the cascade integrator, and sequentially processed by the cascade integrator, the downsampling extractor, and the cascade comb filter, after the cascade comb filter Output. In this method, by dropping the original cascaded comb filter The processing of the sample decimator, from the last stage of the processing structure, is advanced to the middle of the cascade integrator and the cascade comb filter, so that the differential delay D of the cascade comb filter is only the original cascaded comb The 1/R of the filtering structure reduces the number of cascaded stages of the comb filter, facilitating hardware implementation, simplifying the structure, and saving resources.

Further, the downsampling decimator receives the accumulated PDM audio signal, and extracts the accumulated PDM audio signal at a preset interval, specifically:

When n is equal to N*R, the downsampling decimator extracts the PDM audio signal accumulated by the nth stage integrator as the output of the downsampled decimator, otherwise, when n is not equal to N*R, the downsampling The output of the decimator is 0; where N is the clock frequency of the cascaded comb filter, R is R times downsampling, and n is the number of stages in which the integrator is located.

When the M is greater than or equal to 2, the Mth stage subtractor subtracts the extracted Mth PDM audio signal and the extracted M-1th audio signal, and converts to obtain a pulse code modulated PCM audio signal, and The operation result of the M-1th class subtractor is stored in the Mth stage delayer;

The embodiment of the invention further describes a computer storage medium, wherein the computer storage medium stores a computer program for performing the pulse density modulation conversion method shown in FIG. 8 in the embodiment of the invention.

Those skilled in the art will appreciate that embodiments of the present invention can be provided as a method, system, or computer program product. Accordingly, the present invention can take the form of a hardware embodiment, a software embodiment, or a combination of software and hardware. Moreover, the invention can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage and optical storage, etc.) including computer usable program code.

The present invention has been described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (system), and computer program products according to embodiments of the invention. It will be understood that each flow and/or block of the flowchart illustrations and/or FIG. These computer program instructions can be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing device to produce a machine for the execution of instructions for execution by a processor of a computer or other programmable data processing device. Means for implementing the functions specified in one or more of the flow or in a block or blocks of the flow chart.

The computer program instructions can also be stored in a computer readable memory that can direct a computer or other programmable data processing device to operate in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture comprising the instruction device. The apparatus implements the functions specified in one or more blocks of a flow or a flow and/or block diagram of the flowchart.

These computer program instructions can also be loaded onto a computer or other programmable data processing device such that a series of operational steps are performed on a computer or other programmable device to produce computer-implemented processing for execution on a computer or other programmable device. The instructions provide steps for implementing the functions specified in one or more of the flow or in a block or blocks of a flow diagram.

The above is only the preferred embodiment of the present invention and is not intended to limit the scope of the present invention.

Industrial applicability

In the embodiment of the present invention, the differential sampling delay D of the cascaded comb filter is only 1/R of the original structure by moving the downsampling extractor R forward to the middle of the cascade integrator and the cascade comb filter. It simplifies the structural complexity of the original cascade comb filter, facilitates hardware circuit implementation, and reduces resource occupation.

Claims

A pulse density modulation conversion circuit, the circuit comprising: a cascade integrator, a downsampling extractor and a cascade comb filter;

The cascade integrator is configured to receive a pulse density modulated PDM audio signal, and perform an accumulating process on the PDM audio signal, and output the PDM audio signal accumulated by the integrators of each stage to the downsampler extractor;

The downsampling decimator is configured to extract the accumulated PDM audio signal every preset interval;

The cascaded comb filter is configured to receive the PDM audio signal extracted by the downsampling decimator, and perform subtraction on the extracted PDM audio signal to obtain a pulse code modulated PCM audio signal.
The circuit of claim 1 wherein the delay of the cascaded comb filter is the ratio of the original delay of the cascaded comb filter to the downsampled rate of the downsampled decimator.
The circuit of claim 2, wherein the output of the PDM audio signal accumulated by the stages of the integrators after being extracted by the downsampling extractor is:

When n is equal to N*R, the output is the PDM audio signal accumulated by the nth stage integrator, otherwise, when n is not equal to N*R, the output is 0; wherein N is the clock of the cascaded comb filter Frequency, R represents R times downsampling, and n is the number of stages in which the integrator is located.
The circuit of claim 1 wherein the clock frequency of the cascaded integrator is the product of the clock frequency of the cascaded comb filter and the downsampled rate of the downsampled decimator.
The circuit according to any one of claims 1 to 4, wherein said cascade comb filter comprises an M-stage delayer and an M-stage subtractor; wherein said M is a positive integer;

When the M is greater than or equal to 2, the Mth stage delayer is configured to save an operation result of the M-1th stage subtractor; when the M is equal to 1, the 1st stage delayer is configured to save the Extract PDM audio signal;

The Mth stage subtractor is configured to subtract the extracted Mth PDM audio signal and the M-1th audio signal.
A pulse density modulation conversion method, the method comprising:

The cascade integrator receives the pulse density modulated PDM audio signal, performs an accumulating process on the PDM audio signal, and outputs the PDM audio signal accumulated by the integrators of each stage to the downsampling extractor;

The downsampling decimator receives the accumulated PDM audio signal, and extracts the accumulated PDM audio signal every preset interval;

The cascaded comb filter receives the PDM audio signal extracted by the downsampled decimator, and performs subtraction on the extracted PDM audio signal to obtain a pulse code modulated PCM audio signal.
The method of claim 6 wherein the delay of the cascaded comb filter is the ratio of the original delay of the cascaded comb filter to the downsampled rate of the downsampled decimator.
The method of claim 7, wherein the downsampling decimator receives the accumulated PDM audio signal and extracts the accumulated PDM audio signal at predetermined intervals, specifically:

When n is equal to N*R, the downsampling decimator extracts the PDM audio signal accumulated by the nth stage integrator as the output of the downsampled decimator, otherwise, when n is not equal to N*R, the downsampling The output of the decimator is 0; where N is the clock frequency of the cascaded comb filter, R is R times downsampling, and n is the number of stages in which the integrator is located.
The method of claim 6 wherein the clock frequency of the cascaded integrator is the product of the clock frequency of the cascaded comb filter and the downsampled rate of the downsampled decimator.
The method according to any one of claims 6 to 9, wherein the cascade comb filter comprises an M-stage delayer and an M-stage subtractor; wherein the M is a positive integer;

Correspondingly, the cascaded comb filter receives the PDM audio signal extracted by the downsampling extractor And subtracting the extracted PDM audio signal to obtain a pulse code modulated PCM audio signal as:

When the M is greater than or equal to 2, the Mth stage subtractor subtracts the extracted Mth PDM audio signal and the extracted M-1th audio signal to obtain a pulse code modulated PCM audio signal, and the The operation result of the M-1 level subtractor is stored in the Mth stage delayer;

When the M is equal to 1, the PDM audio signal extracted by the downsampling decimator is saved in the first stage delayer.
A computer storage medium storing a computer program for performing the pulse density modulation conversion method according to any one of the preceding claims 6 to 10.