A low-rate voice communication system based on AMBE2000
1. Introduction Speech signal processing is an indispensable part of modern communication. With the increasing requirements for communication quality, we need to use the lowest possible digital rate to obtain the best possible synthesized speech quality. From the current market situation, the application market of low-speed voice is very wide: voice systems, digital mobile communications, confidential communications, voice mail, video phones, IP phones, voice storage, teleconferencing, telephone shopping and other occasions need to be used.
DVSI's AMBE2000 is such a high-performance, low-power real-time voice encoding and decoding chip, its compression ratio can be adjusted in the range of 2.0-9.6kbps, and it has FEC (forward error correction), VAD ( Voice activation detection) and DTMF signal detection function. This system is to make full use of the excellent features of this new product
1 Excellent voice effect at low transmission rate around 4kbps.
2 System block diagram The block diagram of the voice transmission system is shown in the following figure:
In this system, the microphone inputs a voice signal and amplifies it through the voice module. Then sent to analog-to-digital conversion module for AD conversion, output PCM voice signal. This signal is then sent to the core device AMBE2000 of this system, which performs voice coding, outputs a compressed voice signal, and sends it to the CPLD controller. After receiving the compressed voice data, the CPLD extracts and processes the voice part. The voice data at this time is the low-rate voice data of 2.0k-9.6kbps, which can be transmitted. Because AMBE2000 can encode and decode, this system also realizes the reception of voice. The voice information sent from the previous coding part is fed back, accepted by CPLD, and converted into a format that AMBE2000 can recognize, and then decoded by AMBE2000, restored to a PCM voice signal, sent to an analog-to-digital conversion module for DA conversion, and output analog signal The voice module performs power amplification to the speaker.
3. Hardware design The hardware is the core of this design, including the voice part, the analog-to-digital conversion module, the voice encoding and decoding part and the interface of each module.
3.1 Voice module The function of this module is to amplify the weak voice signal of the microphone to AD and play the voice signal from DA. Including the voice input from the microphone and the voice output from the speaker. The voice input part of the circuit is shown in Figure 2. Here, the LM386 is used for two-stage amplification, using a single power supply 3.3V. The output signal is sent to AD.
The voice output circuit is shown in Figure 3. This circuit uses the power amplifier chip TPA2005D1 to amplify the voice. 
3.2 Analog-to-digital converter and its interface circuit with AMBE AD, DA chip used this time is PCM3500, this is a chip with 16-bit serial AD and DA, the sampling frequency range is very wide, from 7.2kbps to 26kbps, And it has its own loopback self-test mode, which is convenient for circuit function self-test. In addition, the configuration of this chip is simple and does not require complex configuration to work effectively. Part of the circuit between it and the AMBE2000 interface is shown in Figure 4. 
Here, only the active / passive mode selection terminal M / S needs to be configured, and this circuit adopts the active mode. LOOP is to select the loopback detection function port. When it is high, it is equivalent to the output DOUT after AD conversion is connected to DIN. This function of PCM3500 is very convenient for system testing. The communication interface between AMBE2000 and PCM3500 is SPI port, BCK provides the clock, it is the clock signal after the crystal is divided by 512, FS is the strobe frequency, and it is strictly synchronized with BCK.
3.3 AMBE200 part and interface with CPLD 
The AMBE2000 circuit is the core of the entire design. Except for the interface with the PCM3500, the rest of the circuit is shown in Figure 5. From a functional point of view, it can be divided into the following four parts: clock part, function configuration, function mode selection module, and SPI interface part.
The AMBE2000 external 16.384MHZ crystal oscillator can work only after the chip is reset, and the reset time is at least 95ms. The function configuration part can be selected according to the needs of the application, including echo cancellation, voice activation, slip compensation and other functions, which are more flexible to use. In the function mode selection part, the compressed data rate can be selected. In actual circuit measurement, even the lowest
1. 0kbps voice data rate, can also get very good voice quality. Pin75 and pin77 (chann_sel) are used to select the working mode. There are four modes: active with frame, active without frame, passive with frame, and passive without frame. The SPI interface is the part that needs CPLD configuration. After the CPLD provides the clock signal, the compressed voice data is transmitted to the CPLD through the SPI port. At this time, the data can be received for feedback or processing and transmission.
2. Software design
The system software configuration is relatively easy. Provide a clock through CPLD for codec decoding and transmission of compressed voice data. In the active frame format, the frame synchronization pulse is generated by AMBE2000 and is strictly synchronized with the clock signal output by CPLD; in passive mode, CPLD is required to provide the frame synchronization signal.
Each function selection terminal can be configured with peripheral hardware as needed, and can also be configured through CPLD after reset.
5. Conclusion Through the combined use of software and hardware, the encoding, decoding and transmission of voice signals are successfully achieved. At the very low rate of 2.0 kbps, good voice effects are still obtained.
The author's innovation:
This article uses AMBE2000 from DVSI to implement a low-power, high-quality voice communication system with low transmission rate. Based on practice, this article provides a more detailed circuit implementation scheme and a reasonable function configuration. It has high practical value for the increasingly demanding voice transmission system.
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