CN102055480B - Sampling frequency conversion device - Google Patents

Abstract

The invention provides a sampling frequency conversion device which can convert an analog audio signal into a digital audio signal with a target frequency. The sampling frequency conversion device comprises a delta-sigma modulator, a first down-conversion filtering module, a down-conversion module and a third down-conversion filtering module. The sigma-delta modulator samples the analog audio signal to generate a digital audio signal at a first frequency. The first frequency-reducing filtering module filters and reduces the frequency of the digital audio signal with the first frequency to generate a digital audio signal with a second frequency. The frequency raising and reducing module raises and reduces the frequency of the digital audio signal with the second frequency to generate a digital audio signal with a third frequency. The third frequency-reducing filter module receives the digital audio signal output by the frequency-increasing and frequency-reducing module to generate a digital audio signal with a target frequency. The ADC of the invention does not need to use an additional quartz oscillator or a phase-locked loop to generate sampling frequency, thereby effectively reducing the circuit area and the manufacturing cost.

Description

Sampling Frequency Converter

technical field

The present invention relates to a sampling frequency conversion device, especially a sampling frequency conversion device applied to an audio analog-digital converter, so that the audio analog-digital converter does not need to use a crystal oscillator or a phase-locked loop to generate sampling frequency.

Background technique

Most of the current mainstream audio formats of multimedia use three reference frequencies of 48kHz, 44.1kHz or 32kHz to multiply or divide by a positive integer N as the sampling frequency of the audio output signal. For example, the sampling frequency of 192kHz of the High Definition Audio specification is four times (48kHz*4) of 48kHz; the sampling frequency of 8kHz of the MPEG-4CELP specification is a quarter of 32kHz (32kHz/4). In order to support different sampling frequencies, the operating frequency used by the audio analog-to-digital converter (ADC) must be an integer multiple of the sampling frequency. Assuming that the above sampling frequency is Fs, common working frequencies include (256*Fs), (384*Fs), (512*Fs) and (768*Fs). However, the above operating frequency does not exist in the original system. In order to cope with the audio playback function of different sampling frequencies, a crystal oscillator (crystal oscillator) or a phase-locked loop (Phase Lock Loop, PLL) circuit that can generate the corresponding operating frequency must be added to the design of the audio ADC.

Please refer to FIG. 1 . FIG. 1 is a schematic diagram of the architecture of an audio ADC 100 in the prior art. The audio ADC 100 includes a delta-sigma modulator (Sigma-Delta Modulator, SDM) 110, a comb filter (comb filter) 120, an N-fold down-converter 130, a down-frequency filter module 140, and an analog image-locking loop (Analog Phase Lock Loop, APLL) 150. The audio ADC100 converts an analog audio signal into a digital audio signal with a target sampling frequency Fs according to the operating frequency generated by the APLL150. The SDM 110 is used for over-sampling an analog audio signal to convert the analog audio signal into a digital audio signal. When sampling a signal, it often causes aliasing, which leads to signal distortion. In order to avoid this situation, the sampling frequency when sampling the signal must be greater than the Nyquist rate of the signal. , which is twice the signal frequency of the signal, and the oversampling rate (Oversampling Rate, OSR) of the SDM110 for oversampling the analog audio signal must be much greater than twice the signal frequency of the analog audio signal. In this way, the quantization noise generated when the analog audio signal is sampled will be evenly distributed among higher bandwidths, so the quantization noise within the signal bandwidth can be effectively reduced.

The comb filter 120 is a cascaded integrated comb (CIC) filter whose main function is to eliminate noise outside the signal frequency band to prevent the noise from entering the signal frequency band (in-band) after sampling. Generally, the CIC filter is used for down-sampling and up-sampling applications. In the application of down-sampling, the CIC filter has the function of anti-aliasing, and in the application of up-sampling, the CIC filter can be used to prevent anti-image. The biggest advantage of using the CIC filter is that it does not need to use a multiplier when implementing the algorithm to hardware, but only uses simple adders and subtractors, thus simplifying hardware complexity and reducing system power. In the architecture of the audio ADC 100 in the prior art, the N-fold down-converter 130 is electrically connected to the comb filter 120 to form a down-sampling filter for down-converting the digital audio signal output by the SDM 110 by a factor of N ( means to divide the frequency of the digital audio signal by N), where N is a positive integer.

The down-frequency filtering module 140 is used for further down-converting the digital audio signal output by the N-fold down-converter 130 by a positive integer (for example, a factor of 4), so as to generate a digital audio signal with a target sampling frequency Fs. In general, the down-frequency filtering module 140 includes an attenuation compensation filter (Dropcompensation Filter, DCF) 141, a first half-band filter (half band filter) 142, a first double frequency down-converter 143, a second A half-band filter 144 and a second double downconverter 145 . Because the CIC filter will cause the attenuation of the signal energy of the high frequency part in the passband (passband), the DCF141 can be used to compensate for the attenuation of the gain. The first half-band filter 142 is electrically connected to the DCF141 and is a low pass filter (low pass filter, LPF), which is used to filter the digital audio signal output by the DCF141, so as to prevent the signal from passing through the first double drop. After the frequency converter 143, the phenomenon of signal aliasing (aliasing) occurs, which affects the quality of the audio output signal. The first double down-converter 143 is electrically connected to the first half-band filter 142 and used for down-converting the digital audio signal output by the DCF 141 by 2 times. The second half-band filter 144 is electrically connected to the first double downconverter 143 , and the second double-downconverter 145 is electrically connected to the second half-band filter 144 . The operating principles of the second half-band filter 144 and the second down-converter 145 are similar to those of the first half-band filter 142 and the first down-converter 143 . Therefore, the down-frequency filtering module 140 further down-converts the digital audio signal output by the N-fold down-converter 130 by 4 times.

In order to support the aforementioned sampling frequency (48kHz, 44.1kHz or 32kHz), the APLL150 can generate the working frequency Fs*4N of the audio ADC100 according to the sampling frequency Fs. According to the current common specifications in the industry, assuming that the sampling frequency Fs to be generated by the audio ADC100 is 44.1kHz as the sampling frequency Fs of the audio output signal, where N is 32, the frequency generated by the APLL150 is 5.6448MHz (44.1kHz*4* 32). However, although the APLL 150 can generate corresponding sampling frequencies according to different audio signals, the area occupied by the APLL 150 in the integrated circuit is relatively large, so the required cost is relatively high.

Contents of the invention

Therefore, an object of the present invention is to provide a sampling frequency conversion device applied to an audio analog-to-digital converter.

The invention provides a sampling frequency converting device, which is used for converting an analog audio signal into a digital audio signal of a target frequency. The sampling frequency conversion device includes a delta-sigma modulator, a first down-frequency filter module, a down-frequency module and a third down-frequency filter module. The delta-sigma modulator is used for sampling the analog audio signal according to a working frequency to generate a digital audio signal of a first frequency. The first down-frequency filtering module is electrically connected to the delta-sigma modulator for receiving the digital audio signal of the first frequency, and filtering and down-converting the digital audio signal of the first frequency to generate a first frequency. Two-frequency digital audio signal. The up-converting module is electrically connected to the first down-frequency filtering module, and is used for up-converting and down-converting the digital audio signal of the second frequency to generate a digital audio signal of a third frequency. The third down-frequency filtering module is electrically connected to the up-down frequency module for receiving the digital audio signal output by the up-down frequency module to generate a digital audio signal of a target frequency.

Wherein, the frequency reduction module further includes a second frequency reduction filter module, and when the digital audio signal of the third frequency is 2 ⁿ times of a preset frequency, the digital audio signal of the third frequency is output to the third frequency reduction filter module ; When the digital audio signal of the third frequency is not 2 ⁿ times of the preset frequency, the digital audio signal of the third frequency is filtered and frequency-reduced to produce a digital audio signal of the fourth frequency to the third drop frequency filter module. where n is an integer.

The invention provides an audio ADC with a sampling frequency conversion function, which can convert an analog audio signal into a digital audio signal of a target frequency by using an operating frequency generated by a system clock. Since the ADC of the present invention does not need to use an additional crystal oscillator or a phase-locked loop to generate sampling frequency, the circuit area and manufacturing cost can be effectively reduced.

Description of drawings

1 is a schematic diagram of a sampling frequency conversion device of an audio analog-to-digital converter architecture in the prior art;

2A is a schematic diagram illustrating a sampling frequency conversion device of an audio analog-to-digital converter of the present invention;

2B is a schematic diagram illustrating another embodiment of the sampling frequency conversion device of the audio analog-to-digital converter of the present invention;

Fig. 3 is the relationship table illustrating operating frequency, target frequency and first to fifth parameters;

Fig. 4 is the relationship table illustrating the first to fifth parameters of different operating frequencies;

FIG. 5A is a schematic diagram of a modified embodiment of the sampling frequency conversion device according to FIG. 2A;

FIG. 5B is a schematic diagram of a modified embodiment of the sampling frequency converting device according to FIG. 2B .

Figure number:

100 Audio Analog to Digital Converter

200, 201, 500, 501 Sampling frequency conversion device

110, 210 Delta-sigma modulator

120 filter

130 N-fold down-converter

140 Down frequency filter module

150 Analog PLL

220 The first frequency down filter module

221 The first upconverter

222 first filter

223 The first down-converter

230 Frequency conversion module

231 Second upconverter

232 Second down-converter

233 Low-pass filter

240 Second frequency down filter module

241 Second filter

242 The third down-converter

250 The third frequency down filter module

141, 251 Attenuation compensation filter

142, 252 First half-band filter

143 The first and second down-converter

144, 254 Second half-band filter

145 Second double down frequency converter

253 Fourth Downconverter

255 Fifth Downconverter

A~E The first~fifth parameters

Fs Target frequency

Fs2 Second frequency

Fs3 Third frequency

Fs4 Fourth frequency

Fsc Working Frequency

P1 The first path

P2 Second Path

Detailed ways

Please refer to FIG. 2A . FIG. 2A is a schematic diagram illustrating an audio ADC sampling frequency conversion device 200 of the present invention. The sampling frequency converting device 200 includes a delta-sigma modulator (Sigma-Delta Modulator, SDM) 210 , a first down-frequency filtering module 220 , a down-frequency down-frequency module 230 and a third down-frequency filtering module 250 . Wherein, the frequency reduction module further includes a second frequency reduction filter module 240 . Sampling frequency conversion device 200 utilizes SDM210 to carry out over-sampling (Over-sampling) to an analog audio signal, to generate a digital audio signal, wherein the first down-frequency filtering module 220, up-down frequency module 230, second down-frequency filtering module 240 and The third down-frequency filtering module 250 up-converts and down-converts the digital audio signal according to different parameters to generate a digital audio signal with a target frequency Fs.

The SDM 210 is used for over-sampling an analog audio signal to convert the analog audio signal into a digital audio signal. In this embodiment, SDM210 oversamples the analog audio signal according to an operating frequency Fsc generated by a system clock (for example, the system clock is 24MHz, and the operating frequency Fsc is 6MHz) to output a digital frequency Fsc audio signal.

The first down-frequency filtering module 220 is used for down-converting the digital audio signal with the frequency Fsc to generate a digital audio signal with the second frequency Fs2. The first down-frequency filtering module 220 includes a first up-converter 221 , a first filter 222 and a first down-converter 223 . Before the digital audio signal of frequency Fsc is input to the first filter 222, the first down-frequency filtering module 220 repeatedly receives the digital audio signal according to a first parameter A in the first upconverter 221, so as to perform a digital audio signal of frequency Fsc The signal is up-converted. For example, when the first parameter A is 4, the digital audio signal will be repeatedly input into the first filter 222 4 times, that is, the digital audio signal will be up-converted by 4 times. After the first filter 222 filters the digital audio signal, the first down-converter 223 down-converts the digital audio signal according to a second parameter B to generate a digital audio signal with a second frequency Fs2. For example, when the first parameter A and the second parameter B are 2 and 125 respectively, the second frequency Fs2 is Fsc*2/125. Wherein, the first filter 222 can be replaced by a comb filter (comb filter) or a general digital filter, but is not limited to this embodiment; for example, a general digital filter can be a finite impulse response (Finite Impulse Response, FIR) or infinite impulse response (Infinite Impulse Response, IIR) filter.

The up-converter module 230 includes a second up-converter 231 , a second down-converter 232 and a second down-converter filter module 240 . The second up-converter 231 is electrically connected to the first down-converter 223 of the first down-frequency filter module 220 for receiving the digital audio signal of the second frequency Fs2. The second down-converter 232 is electrically connected to the second up-converter 231 . The up-converter module 230 uses the second up-converter 231 to up-convert the digital audio signal of the second frequency Fs2 according to a third parameter C, and then uses the second down-converter 232 to up-convert the digital audio signal according to a fourth parameter D. The audio signal is down-converted to generate a digital audio signal with a third frequency Fs3.

The second down-frequency filtering module 240 includes a first path P1 and a second path P2. Wherein, the first path P1 has a second filter 241 and a third down-converter 242 , and the second path P2 directly outputs the digital audio signal of the third frequency Fs3 to the third down-frequency filter module 250 . The second down-frequency filtering module 240 is electrically connected to the second down-converter 232 for receiving the digital audio signal of the third frequency Fs3.

When the digital audio signal of the third frequency Fs3 is (2-n) times of a preset frequency Fd (for example, the preset frequency Fd is 48kHz), the second down-frequency filtering module 240 directly outputs the A digital audio signal of the third frequency Fs3, wherein n is an integer. When the digital audio signal of the third frequency Fs3 is not (2-n) times of the preset frequency Fd, the second down-frequency filtering module 240 transmits the digital audio signal of the third frequency Fs3 to the second filter via the first path P1 241. After the second filter 241 filters the digital audio signal, the third down-converter 242 down-converts the digital audio signal according to a fifth parameter E to generate a digital audio signal of a fourth frequency Fs4.

For example, assuming that the third frequency reduction module 250 fixedly reduces the frequency by four times, when the target frequency is 12KHz, 24KHz, 48KHz, 96KHz or 192KHz, the digital audio signal of the third frequency Fs3 is 48KHz, 96KHz, 192KHz, 384KHz Or 768KHz, the third frequency Fs3 is (2-0, 2-1, 2-2, 23 or 24) times the preset frequency (48kHz), the second down-frequency filtering module 240 directly outputs the third frequency via the second path P2 The digital audio signal of frequency Fs3; when the digital audio signal of the third frequency Fs3 is 832KHz, 1.323MHz, 1.664MHz, 2.646MHz, 3.328MHz, 5.292MHz, 6.656MHz, 10.584MHz, 13.312MHz or 21.168MHz and other frequencies, due to The third frequency Fs3 is not (2-n) times the preset frequency (48kHz), so the second down-frequency filtering module 240 transmits the digital audio signal of the third frequency Fs3 to the second filter 241 via the first path P1. After the second filter 241 filters the digital audio signal, the third down-converter 242 down-converts the digital audio signal according to the fifth parameter E to generate a digital audio signal with a fourth frequency Fs4. Wherein, the second filter 241 can also be replaced by a comb filter or a general digital filter (such as the above-mentioned FIR or IIR filter).

The third frequency reduction filter module 250 is electrically connected to the second frequency reduction filter module 240 . The third down-frequency filtering module 250 includes an attenuation compensation filter (Drop compensation Filter, DCF) 251, a first half-band filter (half-band filter) 252, a fourth down-converter 253, and a second half-band A filter 254 and a fifth down-converter 255 . The third down-frequency filtering module 250 can compensate the attenuation of the high-frequency part of the digital audio signal caused by the second filter. In addition, the third down-frequency filtering module 250 further down-converts the digital audio signal of the third frequency Fs3 or the fourth frequency Fs4 output by the second down-frequency filtering module 240 at the same time, and finally generates a digital audio signal of the target frequency Fs. In this embodiment, the fourth frequency down-converter 253 and the fifth frequency down-converter 255 perform frequency down-conversion twice respectively, so the third down-frequency filtering module 250 converts the third frequency Fs3 or the third frequency Fs3 output by the second down-frequency filtering module The digital audio signal of the quad frequency Fs4 is down-converted by four times. The frequency reduction multiples of the fourth downconverter 253 and the fifth downconverter 255 are only used in this embodiment, and users can change the frequency reduction multiples of the fourth downconverter 253 or the fifth downconverter 255 according to actual needs.

Furthermore, the sampling frequency conversion device 200 of the audio ADC shown in FIG. 2A is only a schematic diagram of an embodiment of the present invention. Those skilled in the art may make various modifications or changes accordingly. Please refer to FIG. 2B . FIG. 2B is a schematic diagram illustrating a sampling frequency conversion device 210 of an audio ADC of the present invention. The sampling frequency conversion device 201 is similar to the sampling frequency conversion device 200 in FIG. 2A, the difference is that the second down-frequency filtering module 240 in the up-down frequency module 230 can combine the original two paths into one path, and select the second path through hardware. The second filter 241 is a filter with adjustable coefficients, and the second filter 241 can be an all-pass filter or a low-pass filter by adjusting the coefficients.

When the digital audio signal of the third frequency Fs3 is (2-n) times of a preset frequency Fd (for example, the preset frequency Fd is 48kHz), the second filter 241 is adjusted to an all-pass filter by coefficients, The fifth parameter E of the third down-converter 242 is set to 1, so that it does not filter and down-convert the digital audio signal of the third frequency Fs3, and directly outputs the digital audio signal of the third frequency Fs3, wherein n is an integer. When the digital audio signal of the third frequency Fs3 is not (2-n) times of the preset frequency Fd, the second filter 241 is adjusted to a low-pass filter through coefficients, and the second filter 241 filters the digital audio signal , the third down-converter 242 down-converts the digital audio signal according to a fifth parameter E, so as to generate a digital audio signal with a fourth frequency Fs4.

In the embodiment of the present invention, the first to fifth parameters A, B, C, D and E are pre-calculated according to the operating frequency Fsc and the set target frequency Fs. Different combinations of the target frequency Fs and the working frequency Fsc will produce different first to fifth parameters A, B, C, D and E. Please refer to FIG. 2A and FIG. 3 at the same time. FIG. 3 is a table illustrating the relationship between the working frequency Fsc, the target frequency Fs, and the first to fifth parameters A, B, C, D, and E. Referring to FIG. The following will use the audio ADC sampling frequency conversion device 200 in FIG. 2A and the relationship table in FIG. 3 to illustrate how the audio ADC 200 according to the present invention utilizes the operating frequency Fsc generated by the system clock (that is, no additional oscillator or lock is required. phase loop) to convert the analog audio signal into a digital audio signal of the target frequency Fs. Assuming that the working frequency Fsc, target frequency Fs and preset frequency Fd are 6 MHz, 64 kHz and 48 kHz respectively, the first to fifth parameters A, B, C, D and E are (1, 8, 71, 8, 26) respectively. SDM210 first oversamples the analog audio signal according to the working frequency Fsc (6MHz) to output a digital audio output signal with the working frequency Fsc (6MHz). Since the first parameter A is 1, the digital audio output signal will be transmitted to the first filter 222 without changing (Fsc*1=Fsc). After the first filter 222 filters the digital audio output signal, the first down-converter 223 reduces the frequency of the digital audio signal at the working frequency Fsc (6 MHz) by 8 times according to the second parameter B, to generate the digital audio at the second frequency Fs2 signal, and the second frequency Fs2 is 750kHz (6MHz/8=750kHz). Next, the second upconverter 231 upconverts the digital audio signal of the second frequency Fs2 (750kHz) according to the third parameter C, and then the second downconverter 232 downconverts the digital audio signal according to the fourth parameter D to generate A digital audio signal of a third frequency Fs3. Therefore, the third frequency Fs3 is 6656.25 kHz (750 kHz*71/8=6656.25 kHz). Since the digital audio signal of the third frequency Fs3 is not ²ⁿ times the preset frequency Fd (48kHz), the second down-frequency filtering module 240 switches to the first path P1. After the second filter 241 filters the digital audio output signal, the third down-converter 242 down-converts the digital audio signal of the third frequency Fs3 by 26 times according to the fifth parameter E, and generates digital audio of the fourth frequency Fs4 accordingly signal; and the fourth frequency Fs4 is 256kHz (6656.25kHz/26=256kHz). Finally, the third down-frequency filtering module 250 down-converts the digital audio signal of the fourth frequency Fs4 by four times to generate a digital audio signal of the target frequency Fs (256kHz/4=64kHz).

It should be noted that under the same combination of the target frequency Fs and the working frequency Fsc, different combinations of the first to fifth parameters A, B, C, D and E may still be generated. For example, please continue to refer to FIG. 3 , when the working frequency Fsc and the target frequency Fs are respectively set to 6 MHz and 192 kHz, the first parameter A can be 16, 8 or 4. When the first parameter A is 16, when the digital audio signal with the frequency Fsc is input into the first filter 222, the frequency will be up-converted by 16 times to 96 MHz. Under the existing system architecture, adopting a higher operating frequency of 96 MHz in the first down-frequency module 220 will increase the cost of the circuit design; such a high-speed operating clock does not necessarily exist in the system. Therefore, the user can select the first parameter A to be 8 or 4 to reduce the operating frequency of the first down-frequency module 220 , and the third parameter C is adjusted to 2 or 4 accordingly. This adjustment does not affect the quality of the digital audio at the output Fs.

Please refer to Figure 4. FIG. 4 is a relational table illustrating first to fifth parameters A, B, C, D and E of different working frequencies Fsc. The audio frequency ADC 200 of the present invention can also be used for different operating frequencies Fsc, such as 1.5MHz, 3MHz, 6MHz, 12MHz or 24MHz. Taking the working frequency Fsc, the target frequency Fs and the preset frequency Fd as 12MHz, 96kHZ and 48kHz respectively as an example, the first to fifth parameters A, B, C, D and E are respectively (4 or 2, 125, 1 or 2 , 1, 1). SDM210 firstly oversamples the analog audio signal according to the working frequency Fsc (12MHz) to output a digital audio output signal with the working frequency Fsc (12MHz). Since the first parameter A is 4 or 2, the digital audio signal of the working frequency Fsc (12MHz) is up-converted four times (Fsc*4=48MHz) or doubled (Fsc*4=48MHz) by the first upconverter 221 according to the first parameter A 2=24MHz). As mentioned above, the first frequency reduction module 220 adopts a higher operating frequency, which will increase the cost of the circuit design, and such a high-speed operating clock does not necessarily exist in the system. Therefore, selecting the first parameter A to be 2 can reduce the operating frequency of the first down-frequency module 220 when the first parameter A is 4, that is, the third parameter C is also adjusted to 2 correspondingly. The digital audio output signal is transmitted to the first filter 222 under the condition of up-converting twice (Fsc*2=24MHz). After the first filter 222 filters the digital audio output signal, the first down-converter 223 reduces the frequency of the digital audio signal at the working frequency Fsc (24 MHz) by 125 times according to the second parameter B to generate the digital audio at the second frequency Fs2 signal, and the second frequency Fs2 is 192kHz (24MHz/125=192kHz). Next, the second upconverter 231 upconverts the digital audio signal of the second frequency Fs2 (192kHz) according to the third parameter C, and then the second downconverter 232 downconverts the digital audio signal according to the fourth parameter D to generate A digital audio signal of a third frequency Fs3. Therefore, the third frequency Fs3 is 384kHz (192kHz*2/1=384kHz). Since the digital audio signal of the third frequency Fs3 is (23) times the preset frequency Fd (48kHz), the second down-frequency filtering module 240 switches to the second path P2 to directly output the third frequency Fs3 via the second path P2 digital audio signal. Finally, the third down-frequency filtering module 250 down-converts the digital audio signal of the third frequency Fs3 by four times to generate a digital audio signal of the target frequency Fs (384kHz/4=96kHz).

Furthermore, the sampling frequency conversion device 200 and the sampling frequency conversion device 201 shown in FIG. 2A and FIG. 2B are only schematic diagrams of an embodiment of the present invention. Those skilled in the art may make various modifications or changes accordingly. Please refer to FIG. 5A and FIG. 5B , wherein FIG. 5A is a schematic diagram of a modified embodiment of the sampling frequency conversion device 200 according to FIG. 2A . The sampling frequency conversion device is similar to the sampling frequency conversion device in FIG. 2A, and the difference is that the up/down frequency module 230 may further include a low-pass filter (Low Pass Filter, LPF) 233, electrically connected to the second up-converter 231 and the second down-converter 232 , used to filter the image signal of the digital audio signal of the second frequency Fs2 before up-converting and down-converting the digital audio signal. Similarly, please refer to FIG. 5B , which is a schematic diagram of a modified embodiment of the sampling frequency conversion device 201 according to FIG. 2B . The sampling frequency conversion device 501 is similar to the sampling frequency conversion device 201 in FIG. Between the up-converter 231 and the second down-converter 232 , it is used to filter the image signal of the digital audio signal of the second frequency Fs2 before the digital audio signal is up-converted and down-converted.

In the prior art, although the APLL150 of the sampling frequency conversion device 100 of the audio ADC can generate the sampling frequency corresponding to the analog/digital conversion according to different audio signals, an APLL needs about 500*400 (20k) logic gates. Under the condition of not degrading the output quality of digital audio, the sampling frequency conversion device 200 of the audio ADC of the present invention uses a frequency reduction module 230 and a second frequency reduction filter module 240 without using the APLL 150 . The up/down frequency module 230 needs about 0.3k logic gates, and the second filter needs about 2K logic gates, so the sampling frequency conversion device of the audio ADC of the present invention only needs about 2.3k logic gates. Compared with the prior art, the sampling frequency conversion device of the audio ADC of the present invention can save about 17.7k logic gates, that is, save about 88% of the number of logic gates. Therefore, the sampling frequency conversion device 200 of the audio ADC of the present invention occupies a small circuit area, which can effectively reduce the cost.

To sum up, the present invention provides an audio ADC with a sampling frequency conversion function, which can convert an analog audio signal into a digital audio signal of a target frequency by using a working frequency generated by a system clock. Since the ADC of the present invention does not need to use an additional crystal oscillator or a phase-locked loop to generate sampling frequency, the circuit area and manufacturing cost can be effectively reduced.

The above descriptions are only preferred embodiments of the present invention, and all equivalent changes and modifications made according to the claims of the present invention shall fall within the scope of the present invention.

Claims (9)

1. A sampling frequency conversion device, applied to an audio analog-to-digital converter, is used to convert an analog audio signal into a digital audio signal of a target frequency, wherein the sampling frequency conversion device comprises: A delta-sigma modulator, used to sample the analog audio signal according to an operating frequency to generate a digital audio signal of a first frequency; A first down-frequency filtering module, electrically connected to the delta-sigma modulator, used to receive the digital audio signal of the first frequency, and perform up-conversion, filtering and Down-converting to generate a digital audio signal of a second frequency; A frequency reduction module, electrically connected to the first frequency reduction filter module, used for frequency up and frequency reduction of the digital audio signal of the second frequency to generate a digital audio signal of a third frequency, when the When the third frequency is ²ⁿ times the preset frequency, output the digital audio signal of the third frequency; when the third frequency is not ²ⁿ times the preset frequency, the third frequency The digital audio signal is filtered and down-converted, wherein n is an integer; and A third down-frequency filter module, electrically connected to the up-down frequency module, used for compensating, filtering, down-frequency, re-filtering, and re-down-frequency of the digital audio signal output by the up-down frequency module, so as to output the digital audio signal at the frequency of interest. 2. The sampling frequency conversion device according to claim 1, wherein the first down-frequency filtering module comprises: a first up-converter, electrically connected to the delta-sigma modulator, and used to up-convert the digital audio signal; a first filter for receiving the up-converted digital audio signal and filtering the digital audio signal; and A first down-converter, electrically connected to the first filter, is used for down-converting the up-converted digital audio signal to generate a second-frequency digital audio signal. 3. The sampling frequency conversion device according to claim 2, wherein the first filter is a comb filter, a finite impulse response filter or an infinite impulse response filter. 4. The sampling frequency conversion device according to claim 1, wherein the frequency-decreasing module comprises: A second frequency up-converter, electrically connected to the first down-frequency filter module, for up-converting the digital audio signal of the second frequency; a second down-converter, electrically connected to the second up-converter, for down-converting the digital audio signal to generate a digital audio signal of the third frequency; and A second down-frequency filtering module, electrically connected to the second down-converter, receiving a digital audio signal of a third frequency and judging whether the digital audio signal of the third frequency is 2 ⁿ times a preset frequency, Wherein when the third frequency is ²ⁿ times of the preset frequency, directly output the digital audio signal of the third frequency to the third down-frequency filtering module; when the third frequency is not the When the preset frequency is 2 ⁿ times, the digital audio signal of the third frequency is filtered and down-frequency to generate a digital audio signal of a fourth frequency and output to the third down-frequency filtering module, wherein n is a integer. 5. The sampling frequency conversion device according to claim 4, wherein the second down-frequency filtering module comprises: a first path and a second path, wherein the first path has a second filter , for receiving the digital audio signal of the third frequency, and a third down-converter, electrically connected to the second filter, for down-converting the digital audio signal to generate a fourth frequency digital audio signal; wherein the second path directly outputs the digital signal of the third frequency to the third down-frequency filtering module. 6. The sampling frequency conversion device according to claim 4, wherein the second down-frequency filtering module comprises: a second filter for receiving the digital audio signal of the third frequency and filtering the digital audio signal according to a coefficient setting; and A third down-converter, electrically connected to the second filter, for down-converting the digital audio signal according to a parameter setting; Wherein when the third frequency is 2 ⁿ times of the preset frequency, adjust the coefficient to set the second filter as an all-pass filter, and set the parameter to 1 to make the first Three down-converters directly output the digital audio signal of the third frequency to the third down-frequency filtering module; when the third frequency is not 2 ⁿ times of the preset frequency, adjust the coefficient to be The second filter is set as a low-pass filter for filtering, and the parameters are set to make the third down-converter down-frequency the filtered digital audio signal of the third frequency to generate a fourth The frequency digital audio signal is output to the third down-frequency filtering module. 7. The sampling frequency converting device according to claim 4, wherein the frequency-up/down-frequency module further comprises a low-pass filter electrically connected to the second up-converter and the second down-converter Between, used for filtering the digital audio signal after the digital audio signal of the second frequency is up-converted. 8. The sampling frequency conversion device according to claim 2, wherein the first upconverter repeatedly receives the digital audio signal of the first frequency according to a first parameter, so that the digital audio signal of the first frequency Digital audio signals are up-converted. 9. The sampling frequency conversion device according to claim 4, wherein the third down-frequency filtering module comprises: An attenuation compensation filter, electrically connected to the second down-frequency filter module, for compensating the digital audio signals output by the first down-frequency filter module and the up-down frequency module; A first half-band filter, electrically connected to the attenuation compensation filter, for filtering the digital audio signal; A fourth down-converter, electrically connected to the first half-band filter, for down-converting the digital audio signal; a second half-band filter, electrically connected to the fourth down-converter, for filtering the digital audio output signal; and A fifth down-converter, electrically connected to the second half-band filter, for down-converting the digital audio signal to generate a digital audio signal at the target frequency.

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