An improved intercom

Abstract

The present invention discloses an improved intercom, which mainly improves the intermediate frequency demodulation circuit and audio processing circuit of the receiving circuit. In the intermediate frequency demodulation circuit, a DSP demodulator module is used to replace the intermediate frequency amplifier, intermediate frequency demodulator IC, two-way ceramic filters and ceramic discriminator in the prior art; in the audio processing circuit, an audio processing module is used to replace the audio buffer amplifier circuit, audio high-pass filter circuit and audio low-pass filter circuit in the prior art. A digital filter as a second intermediate frequency filter is integrated inside the DSP demodulator module, and no additional ceramic filter is required. Not only are there fewer discrete components and a small size, but the digital filter also has a strong out-of-band suppression capability. A digital filter is integrated in the audio processing module, and the digital filter can filter the sub-audio signals below 300Hz more thoroughly and cleanly than the analog filter, avoiding the crosstalk of the sub-audio, and has a higher signal-to-noise ratio than the analog filter.

Description

Technical Field

[0001] The invention relates to the field of communications, and in particular to an improved intercom. Background Art

[0002] The walkie-talkie includes a receiving circuit and a transmitting circuit. The receiving circuit of the traditional walkie-talkie adopts a single-chip solution, such as Figure 1 As shown, the receiving circuit includes an antenna, a radio frequency low-pass filter, an antenna radio frequency switch, a first band-pass filter, a high-frequency amplifier, a second band-pass filter, a single-chip radio frequency integrated IC, an audio power amplifier unit and a speaker that are electrically connected in sequence, and also includes a microprocessor bidirectionally connected to the single-chip radio frequency integrated IC. Since the current level of IC manufacturing cannot reach the performance of a filter composed of passive components, the receiving circuit of this single-chip solution has the problems of poor reception anti-interference, no out-of-band signal filtering, and easy blockage.

[0003] In order to improve the channel selection capability of the intercom, the existing intercom receiving scheme generally adopts the secondary frequency conversion method. The secondary frequency conversion is because the amplification circuit is divided into multiple stages, and different stages of the circuit amplify signals of different frequencies. The interference between the stages can be overcome by means of notching and filtering, so that each stage of the circuit can work stably and reliably, and the total gain can be made very high. Therefore, the secondary frequency conversion can provide the intercom with higher gain and sensitivity, and more stable operation. Figure 2As shown, the receiving circuit includes an antenna, a radio frequency low-pass filter, an antenna radio frequency switch, a first band-pass filter, a high-frequency amplifier, a second band-pass filter, a first mixer, a crystal filter, an intermediate frequency amplifier, an intermediate frequency demodulation IC, an audio buffer amplifier circuit, an audio low-pass filter circuit, an audio high-pass filter circuit, an audio power amplifier unit and a speaker, which are electrically connected in sequence. It also includes a VCO&PLL frequency synthesizer unit and a microprocessor. The VCO&PLL frequency synthesizer unit is bidirectionally connected to the microprocessor, and the output end of the VCO&PLL frequency synthesizer unit is connected to the first local oscillator signal input end of the first mixer. The intermediate frequency demodulation IC is also connected to two ceramic filters and a ceramic discriminator. When working, the signal received from the antenna passes through the RF low-pass filter, the antenna RF switch and the first band-pass filter and then enters the high-frequency amplifier for amplification. This signal then passes through the second band-pass filter and enters the first mixer. In the first mixer, the RF amplified signal is mixed with the first local oscillator signal from the VCO & PLL frequency synthesizer unit to generate a first intermediate frequency signal. This first intermediate frequency signal is further eliminated by the crystal filter. The first intermediate frequency signal after filtering is amplified by the intermediate frequency amplifier and then enters the intermediate frequency demodulation IC (the intermediate frequency demodulation IC is a second local oscillator, a second mixer, and a second intermediate frequency synthesizer. The second intermediate frequency signal is mixed again with the second local oscillator signal in the intermediate frequency demodulation IC, and the second intermediate frequency signal is amplified and discriminated by the intermediate frequency demodulation IC after filtering out useless spurious signals through a broadband or narrowband ceramic filter to generate an audio signal. This audio signal passes through an audio buffer amplifier circuit, an audio low-pass filter circuit and an audio high-pass filter circuit, which are generally made of operational amplifiers, and is amplified, filtered and de-emphasized in turn, and then enters the audio power amplifier unit for amplification, and finally drives the speaker to broadcast the sound.

[0004] In the existing receiving circuit of the secondary frequency conversion method, the intermediate frequency demodulation circuit is as follows: Figure 3 As shown, this intermediate frequency demodulation circuit has the following problems:

[0005] 1. Wide and narrow band requires two-way ceramic filters, which have many discrete components and large size, which is not conducive to the miniaturization of equipment;

[0006] 2. The out-of-band suppression capability of the ceramic filter is poor; and the ceramic filter and ceramic discriminator are easily damaged, which affects the life of the walkie-talkie.

[0007] In the existing receiving circuit of the secondary frequency conversion method, the audio processing circuit is as follows: Figure 4As shown, in this audio processing circuit, the audio high-pass filter circuit uses an analog filter, which is made of a combination of operational amplifier circuits. If the number of stages of the audio high-pass filter circuit is not enough (below 4 stages), the signal below 300Hz cannot be filtered cleanly, and sub-audio signals are easily strung in. If a multi-stage audio high-pass filter circuit is used, although 300Hz can be cut off, the signal is distorted due to the multiple amplifier circuits, the consistency of components cannot meet the requirements, and the accuracy of components is required to be high. At present, large-capacity ceramic capacitors cannot achieve high precision, resulting in increased distortion of other audio characteristics and poor signal-to-noise ratio. Summary of the invention

[0008] The object of the present invention is to provide an improved intercom, wherein the frequency demodulation circuit has fewer discrete components, a small size, and a strong out-of-band suppression capability; and the audio processing circuit thereof can effectively improve the signal-to-noise ratio and avoid sub-audio crosstalk.

[0009] In order to achieve the above object, the present invention adopts the following technical solution:

[0010] An improved intercom, the receiving circuit of the intercom comprises an antenna, a radio frequency low-pass filter, an antenna radio frequency switch, a first band-pass filter, a high-frequency amplifier, a second band-pass filter, a first mixer, a crystal filter, a DSP demodulator module, an audio processing module, an audio power amplifier unit and a speaker which are electrically connected in sequence, and also comprises a VCO&PLL frequency synthesizer unit and a microprocessor, wherein the microprocessor is bidirectionally connected to the DSP demodulator module and the audio processing module respectively, the microprocessor is bidirectionally connected to the VCO&PLL frequency synthesizer unit, and the output end of the VCO&PLL frequency synthesizer unit is connected to the first local oscillator signal input end of the first mixer; the DSP demodulator module integrates a second mixer, an adjustable gain amplifier, two intermediate frequency filters, an analog-to-digital converter and a digital-to-analog converter which are connected in sequence; the audio processing module integrates an analog-to-digital converter, an adjustable gain amplifier, a digital filter and a digital-to-analog converter which are connected in sequence.

[0011] The DSP demodulator module and the audio processing module are both functional modules on a radio frequency baseband chip of model FD6818, and the DSP demodulator module and the audio processing module share the same radio frequency baseband chip U1; the output end of the crystal filter is connected to the RFIN pin of the radio frequency baseband chip U1, and the AF OUT pin of the radio frequency baseband chip U1 is connected to the input end of the audio power amplifier unit.

[0012] The DSP demodulator module and the audio processing module are both functional modules on a radio frequency baseband chip of model FD6818. The DSP demodulator module and the audio processing module respectively use a radio frequency baseband chip U1 and U2 of model FD6818; the output end of the crystal filter is connected to the RFIN pin of the radio frequency baseband chip U1, and the radio frequency baseband chip U1 outputs through its AF OUT pin and is divided into two paths, one path is connected to the MICIN pin of the radio frequency baseband chip U2, and the other path is connected to the microprocessor through an external sub-audio filtering and shaping circuit, the control output end of the microprocessor is connected to the control end of the audio power amplifier unit, and the AF OUT pin of the radio frequency baseband chip U2 is connected to the input end of the audio power amplifier unit.

[0013] After adopting the above scheme, an improved intercom of the present invention mainly improves the intermediate frequency demodulation circuit and audio processing circuit of the receiving circuit. In the intermediate frequency demodulation circuit, a DSP demodulator module is used to replace the intermediate frequency amplifier, intermediate frequency demodulator IC, two-way ceramic filters and ceramic discriminator in the prior art; when working, the first intermediate frequency signal first passes through the second mixer located inside the DSP demodulator module to generate the second intermediate frequency signal, and then passes through the adjustable gain amplifier inside the DSP demodulator module for signal amplification, and then the useless stray signals are filtered out by the second intermediate frequency filter inside the DSP demodulator module, and the analog-to-digital converter inside the DSP demodulator module samples and quantizes the filtered analog intermediate frequency signal, converts it into a digital intermediate frequency signal, and then demodulates the digital intermediate frequency signal into an analog audio signal through the digital-to-analog converter inside the DSP demodulator module, and finally outputs the demodulated audio signal. In the present invention, a digital filter as a second intermediate frequency filter is integrated inside the DSP demodulator module, and no ceramic filter is required to be used additionally. Not only are there fewer discrete components and a smaller size, but the digital filter also has a stronger out-of-band suppression capability. In the audio processing circuit, an audio processing module is used to replace the audio buffer amplifier circuit, the audio high-pass filter circuit and the audio low-pass filter circuit in the prior art; when working, the audio signal demodulated by the intermediate frequency demodulation circuit is converted into a digital signal by the analog-to-digital converter inside the audio processing module, and then the signal is sent to the adjustable gain amplifier inside the audio processing module for gain adjustment, and then sent to the digital filter inside the audio processing module for digital de-emphasis filtering to restore the original high-frequency component of the already emphasized transmission signal, digital low-pass filtering to filter out useless signals outside the frequency response band, and digital high-pass filtering to filter out the DC component of the audio processing, and the processed digital signal is converted into an analog signal output by the digital-to-analog converter inside the audio processing module, and the output analog audio signal is amplified by the audio power amplifier unit to drive the speaker. Since the audio processing module is integrated with a digital filter, the digital filter can filter sub-audio signals below 300Hz more thoroughly and cleanly than the analog filter, avoiding sub-audio crosstalk. Moreover, the digital filter performs calculations with digital devices, avoiding the influence of noise generated in the analog circuit, and has a higher signal-to-noise ratio than the analog filter.

[0014] Furthermore, in the present invention, the DSP demodulator module and the audio processing module are both functional modules on a radio frequency baseband chip of model FD6818, and the DSP demodulator module and the audio processing module share the same radio frequency baseband chip.

[0015] Furthermore, in the present invention, the DSP demodulator module and the audio processing module are both functional modules on a radio frequency baseband chip of model FD6818. In order to achieve a better sub-audio decoding effect, the DSP demodulator module and the audio processing module respectively use a radio frequency baseband chip of model FD6818. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a circuit principle block diagram of a walkie-talkie receiving circuit using a single-chip receiving solution in the prior art;

[0017] Figure 2 It is a circuit principle block diagram of a walkie-talkie receiving circuit using a secondary frequency conversion receiving solution in the prior art;

[0018] Figure 3 for Figure 2 The circuit principle block diagram of the intermediate frequency demodulation circuit of the receiving loop;

[0019] Figure 4 for Figure 2 A circuit principle block diagram of the audio processing circuit of the receiving loop;

[0020] Figure 5 It is a circuit principle block diagram of the intercom receiving circuit of the present invention;

[0021] Figure 6 for Figure 5 The circuit schematic diagram of the intermediate frequency demodulation circuit of the receiving loop;

[0022] Figure 7 for Figure 5 A circuit schematic diagram of the audio processing circuit of the receiving loop;

[0023] Figure 8 A schematic diagram of an audio response obtained after filtering the demodulated audio in the present invention and the prior art;

[0024] Fig. 9 The waveform diagrams are of the two intermediate frequency filters in the present invention and the ceramic filter in the prior art. DETAILED DESCRIPTION

[0025] The improved intercom of the present invention focuses on the receiving circuit of the intercom, and other parts of the intercom still adopt existing conventional technical means, so only the receiving circuit is described in detail below.

[0026] An improved intercom of the present invention, such as Figure 5As shown, the receiving circuit of this intercom includes an antenna, a radio frequency low-pass filter, an antenna radio frequency switch, a first bandpass filter, a high-frequency amplifier, a second bandpass filter, a first mixer, a crystal filter, a DSP demodulator module, an audio processing module, an audio power amplifier unit and a speaker that are electrically connected in sequence, and also includes a VCO&PLL frequency synthesizer unit and a microprocessor. The microprocessor is bidirectionally connected to the DSP demodulator module and the audio processing module respectively, and the microprocessor is bidirectionally connected to the VCO&PLL frequency synthesizer unit, and the output end of the VCO&PLL frequency synthesizer unit is connected to the first local oscillator signal input end of the first mixer; the DSP demodulator module integrates a second mixer, an adjustable gain amplifier, two intermediate frequency filters, an analog-to-digital converter and a digital-to-analog converter that are connected in sequence; the audio processing module integrates an analog-to-digital converter, an adjustable gain amplifier, a digital filter and a digital-to-analog converter that are connected in sequence.

[0027] The present invention mainly improves the intermediate frequency demodulation circuit and the audio processing circuit of the receiving loop.

[0028] The intermediate frequency demodulation circuit in the present invention adopts a DSP demodulator module to replace the intermediate frequency amplifier, intermediate frequency demodulator IC, two-way ceramic filters and ceramic discriminator in the prior art. When working, the first intermediate frequency signal first passes through the second mixer located inside the DSP demodulator module to generate the second intermediate frequency signal, and then passes through the adjustable gain amplifier inside the DSP demodulator module to amplify the signal, and then the useless stray signal is filtered out by the second intermediate frequency filter inside the DSP demodulator module, and the analog-to-digital converter inside the DSP demodulator module samples and quantizes the filtered analog intermediate frequency signal to convert it into a digital intermediate frequency signal, and then the digital-to-analog converter inside the DSP demodulator module demodulates the digital intermediate frequency signal to convert it into an analog audio signal, and finally outputs the demodulated audio signal. In the present invention, the DSP demodulator module has integrated a digital filter as the second intermediate frequency filter, and no additional ceramic filter is needed. In this way, not only are there fewer discrete components and a smaller volume, but the digital filter also has a stronger out-of-band suppression capability.

[0029] In the audio processing circuit of the present invention, an audio processing module is used to replace the audio buffer amplifier circuit, audio high-pass filter circuit and audio low-pass filter circuit in the prior art. During operation, the audio signal demodulated by the intermediate frequency demodulation circuit is converted into a digital signal by the analog-to-digital converter inside the audio processing module, and then the signal is sent to the adjustable gain amplifier inside the audio processing module for gain adjustment, and then sent to the digital filter inside the audio processing module for digital de-emphasis filtering to restore the original high-frequency component of the already emphasized transmission signal, digital low-pass filtering to filter out useless signals outside the frequency response band, and digital high-pass filtering to filter out the DC component of the audio processing. The processed digital signal is converted into an analog signal output by the digital-to-analog converter inside the audio processing module, and the output analog audio signal is amplified by the audio power amplifier unit to drive the speaker. Since the audio processing module is integrated with a digital filter, the digital filter will filter the sub-audio signal below 300Hz more thoroughly and cleanly than the analog filter, avoiding the crosstalk of the sub-audio. Moreover, the digital filter performs operations with digital devices, avoiding the influence of noise generated in the analog circuit, and has a higher signal-to-noise ratio than the analog filter.

[0030] In the present invention, the DSP demodulator module and the audio processing module are both existing functional modules in the FD6818 RF baseband chip. In the embodiment, the FD6818 RF baseband chip is directly used to implement the functions of the DSP demodulator module and the audio processing module. However, the present invention is not limited to the FD6818 RF baseband chip, and other types of RF baseband chips with the same / similar functions may also be used.

[0031] As an embodiment, in the present invention, the functions of the DSP demodulator module and the audio processing module are implemented by the same FD6818 radio frequency baseband chip. Figure 6 As shown, the functions of the DSP demodulator module and the audio processing module are realized by the same FD6818 RF baseband chip U1. When working, the received RF signal (RF IN) after high-frequency amplification enters the first mixer Q1, and the amplified RF signal is mixed with the first local oscillator signal (1LO) from the VCO&PLL frequency synthesizer unit through the first mixer Q1 to generate a first intermediate frequency signal. The first intermediate frequency signal is further eliminated by the crystal filter XF1 to eliminate the clutter signal of the adjacent channel. The filtered first intermediate frequency signal enters from the RFIN pin (pin 15) of the RF baseband chip U1, and the DSP demodulator module and audio processing module inside the RF baseband chip U1 perform the corresponding processing as described above on the input signal. Finally, the demodulated audio signal is output from the AFOUT pin (pin 9) of the RF baseband chip U1 to the audio power amplifier unit, which amplifies it and drives the speaker to broadcast the sound.

[0032] There are two usage modes of walkie-talkies: normal mode and sub-audio mode. The normal mode is a one-to-N usage mode, that is, the walkie-talkie can respond to a hundred calls, and the receiver can receive signals from any transmitter at the same frequency; in some specific usage scenarios, if the receiver does not want to receive so many signals, the sub-audio mode can be set. After setting, the transmitter is required to set the same sub-audio mode so that the transmitter's signal can be played by the receiver.

[0033] In the prior art, if the sub-audio mode is to be realized, the audio signal demodulated by the intermediate frequency demodulation IC is sent to the audio processing circuit in one way, and the other way needs to be connected to the microprocessor through a sub-audio filter shaping circuit. The components other than the sub-audio are filtered out by the sub-audio filter shaping circuit, and then the sub-audio in the received signal is obtained by the microprocessor through sampling and calculation. The microprocessor compares the sub-audio with the sub-audio set by the machine. If they are different, the microprocessor keeps the mute instruction to the audio power amplifier unit, keeps the mute closed, and prohibits the audio power amplifier unit from outputting audio; if they are the same, the microprocessor sends an instruction to the audio power amplifier unit to enable the audio power amplifier unit to output audio, thereby realizing sub-audio decoding.

[0034] In the present invention, the FD6818 radio frequency baseband chip used has a sub-audio decoding module integrated therein. If the intercom is to work in the sub-audio mode, it can be directly implemented on the same FD6818 radio frequency baseband chip in theory. However, the existing radio frequency baseband chips (including the FD6818 radio frequency baseband chip) integrate many functions of radio frequency transmission and reception, which makes the resource allocation of each function particularly important under the limited resources and power consumption of the chip. Sub-audio decoding, as an additional function of intercom, cannot be regarded as a priority support for the radio frequency baseband chip, and the resources allocated are relatively small. This also determines that the sub-audio decoding in the radio frequency baseband chip considers using an algorithm with a small amount of calculation data, and the data accuracy is relatively poor.

[0035] If the data accuracy requirement for sub-audio decoding is not too high, the cost can be reduced by directly using the sub-audio decoding function of the FD6818 RF baseband chip itself. When working, the audio signal demodulated by the DSP demodulator module inside the RF baseband chip U1 is divided into two paths, one of which is output to the audio processing module inside the RF baseband chip U1, and the demodulated audio signal is output to the audio power amplifier unit by the AFOUT pin (pin 9) of the RF baseband chip U1; the other is sent to the sub-audio decoding module inside the RF baseband chip U1, and the sub-audio in the demodulated audio signal is compared with the sub-audio set by the local device through the sub-audio decoding module inside the RF baseband chip U1, and the comparison result is transmitted to the microprocessor. If they are not the same after comparison, the microprocessor keeps the mute instruction to the audio power amplifier unit, keeps the mute closed, and prohibits the audio power amplifier unit from playing. If they are the same, the microprocessor outputs the control instruction to the audio power amplifier unit, enables the audio power amplifier unit to output audio, and drives the speaker to play sound, thereby realizing sub-audio decoding.

[0036] As another embodiment, if a better sub-audio decoding effect is to be achieved, a better solution is to use a microprocessor to perform sub-audio decoding. Since the RF baseband chip has only one audio output port, in this embodiment, in order to implement the microprocessor to perform sub-audio decoding, the DSP demodulator module and the audio processing functions are respectively implemented using an FD6818 RF baseband chip.

[0037] Specifically, Figure 6-7As shown, the DSP demodulator module is implemented by an RF baseband chip U1 of model FD6818, and the audio processing module is implemented by another RF baseband chip U2 of model FD6818. During operation, the received RF signal (RF IN) after the previous high-frequency amplification enters the first mixer Q1, and the amplified RF signal is mixed with the first local oscillator signal (1LO) from the VCO&PLL frequency synthesizer unit through the first mixer Q1 to generate a first intermediate frequency signal. The first intermediate frequency signal is further eliminated from the adjacent channel clutter signal through the crystal filter XF1, and the filtered first intermediate frequency signal enters from the RFIN pin (pin 15) of the RF baseband chip U1. The audio signal demodulated by the DSP demodulator module in the RF baseband chip U1 is not processed in any way and is transmitted through the AF of the RF baseband chip U1. After the OUT pin (pin 9) is output, it is divided into two paths. One path is connected to the MICIN pin (pin 11) of the RF baseband chip U2, and the demodulated audio signal is sent to the audio processing module in the RF baseband chip U2 for audio processing. Finally, the demodulated audio signal is output to the audio power amplifier unit by the AFOUT pin (pin 9) of the RF baseband chip U2; the other path is connected to the microprocessor through an external sub-audio filter and shaping circuit (not shown in the figure), and the control output end of the microprocessor is connected to the control end of the audio power amplifier unit. The components other than the sub-audio are filtered out by the sub-audio filter and shaping circuit, and then the sub-audio in the received signal is obtained by the microprocessor through sampling and calculation. The microprocessor compares the sub-audio with the sub-audio set by the local machine. If they are different, the microprocessor keeps a mute instruction to the audio power amplifier unit, continuously closes the squelch, and prohibits the audio power amplifier unit from outputting audio; if they are the same, the microprocessor sends an instruction to the audio power amplifier unit to enable the audio power amplifier unit to output audio to drive the speaker to broadcast sound, thereby realizing sub-audio decoding.

[0038] The audio response obtained after filtering the demodulated audio in the present invention and the prior art is as follows: Figure 8 As shown, it can be seen that, on low frequency signals (below 0.3 KHz), the filtering effect of the present invention is better than that of the prior art.

[0039] The waveform diagrams of the two intermediate frequency filters in the present invention and the ceramic filters in the prior art are shown in FIG. Fig. 9 As shown, it can be seen that the out-of-band suppression capability of the two intermediate frequency filters (digital filters) of the present invention is better than that of the prior art.

[0040] The above is only a preferred embodiment of the present embodiment, and all equivalent changes and modifications made within the scope of the claims of the present invention should fall within the scope of the claims of the present invention.

Claims

1. An improved intercom, Features: The receiving circuit of this intercom includes an antenna, a radio frequency low-pass filter, an antenna radio frequency switch, a first band-pass filter, a high-frequency amplifier, a second band-pass filter, a first mixer, a crystal filter, a DSP demodulator module, an audio processing module, an audio power amplifier unit and a speaker which are electrically connected in sequence, and also includes a VCO&PLL frequency synthesizer unit and a microprocessor. The microprocessor is bidirectionally connected to the DSP demodulator module and the audio processing module respectively, and the microprocessor is bidirectionally connected to the VCO&PLL frequency synthesizer unit, and the output end of the VCO&PLL frequency synthesizer unit is connected to the first local oscillator signal input end of the first mixer; the DSP demodulator module integrates a second mixer, an adjustable gain amplifier, two intermediate frequency filters, an analog-to-digital converter and a digital-to-analog converter which are connected in sequence; the audio processing module integrates an analog-to-digital converter, an adjustable gain amplifier, a digital filter and a digital-to-analog converter which are connected in sequence; The DSP demodulator module and the audio processing module are both functional modules on a radio frequency baseband chip of model FD6818, and the DSP demodulator module and the audio processing module share the same radio frequency baseband chip U1 of model FD6818; The output end of the crystal filter is connected to the RFIN pin of the RF baseband chip U1, and the AF OUT pin of the RF baseband chip U1 is connected to the input end of the audio power amplifier unit; or, The DSP demodulator module and the audio processing module are both functional modules on a radio frequency baseband chip of model FD6818. The DSP demodulator module and the audio processing module respectively use a radio frequency baseband chip U1 and U2 of model FD6818; the output end of the crystal filter is connected to the RFIN pin of the radio frequency baseband chip U1, and the radio frequency baseband chip U1 outputs through its AF OUT pin and is divided into two paths, one path is connected to the MICIN pin of the radio frequency baseband chip U2, and the other path is connected to the microprocessor through an external sub-audio filtering and shaping circuit, the control output end of the microprocessor is connected to the control end of the audio power amplifier unit, and the AF OUT pin of the radio frequency baseband chip U2 is connected to the input end of the audio power amplifier unit.

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