Multi-rate voice communication terminal module based on AMBE-2000

Abstract: Introduce the principle and implementation of a voice communication terminal module based on AMBE-2000 and DSP chips. The terminal module has many advantages such as simple design, variable voice coding rate, excellent sound quality, high cost performance, and low power consumption. It can be widely used in satellite communications, short wave, microwave communications, and military secret communications.

Keywords: vocoder AMBE forward error correction coding (FEC)

AMBE-2000 is a monolithic vocoder chip launched by DVS (Digital Voice System Inc.). The chip uses an improved multi-band excitation (MBE) algorithm, which can achieve variable rate low bit rate, high voice quality voice compression coding. This paper designs and implements a voice communication terminal module based on AMBE-2000. The module has many advantages such as simple design, variable voice coding rate, excellent sound quality, flexible interface, low operating voltage, low power consumption, etc. It can be widely used in satellite communication, short wave, microwave communication and confidential communication, etc. Practical value.

1 Overview of AMBE-2000 functions

1.1 Introduction to the MBE algorithm

MBE (Multi-Band Excel) speech coding technology is a low-rate speech coding and decoding technology with high speech quality and strong robustness that was proposed in the late 1980s. Its improved IMBE algorithm has become the voice coding standard for the International Mobile Satellite Organization (INMARSAT) and AUSAT mobile satellite communications. EIA / TIA also chose MBE as the coding standard for North American land mobile communication systems. The MBE speech codec algorithm is widely used in the fields of satellite communications, digital voice storage, and secure communications due to its superior performance at low rates (<4.8kbps).

The MBE coding method divides the frequency spectrum of a frame of speech into multiple frequency bands in the frequency domain according to the fundamental harmonic frequencies, and makes clear and voiced (U / V) judgments for each frequency band. For the voiced band, the pulse sequence with the pitch as the period is used as the excitation signal; for the unvoiced band, the white noise is used as the excitation signal. The total excitation signal is formed by adding the excitation signals of each band. The excitation signal is used to excite the channel filter, and finally the speech with higher naturalness is synthesized. The MBE model makes the synthesized speech spectrum and the original speech spectrum fit well in the detailed structure. Therefore, in the case of low bit rate, the sound quality of the synthesized speech can still maintain a high naturalness.

1.2 Functions and features of AMBE-2000

The AMBE-2000 chip is an improved product of AMBE-1000. Compared with AMBE-1000, its voice compression algorithm is more optimized, the voice quality is higher, and the minimum coding rate is also reduced from the original 2.4kbps to 2.0kbps; in terms of hardware and interface, several improvements have been made and improved The efficiency and reliability of its compression coding and forward error correction coding (FEC). AMBE-2000 mainly has the following characteristics:

(1) Multiple speeds and high voice quality: with eight compression rates of 2.0k, 2.4k, 3.6k, 4.0k, 4.8k, 6.4k, 8.0k and 9.6kbps. When the rate is above 4.0kbps, the voice quality close to long-distance calls can be obtained; when the rate is 2.0kbps, it still has a high degree of intelligibility and naturalness.

(2) Variable-speed FEC function: The rate of FEC can be flexibly selected according to the channel conditions. The rate selection of voice and FEC can be set by hardware pin or software.

(3) Convolutional encoder and Viterbi decoder are integrated in the chip.

(4) Low power consumption and low complexity.

(5) Able to generate and recognize dual tone multi-frequency (DTMF) signals.

(6) It has the functions of voice activation detection (Voice Activity Detection), echo cancellation (Echo Cancellation) and comfort noise (Community Noise) generation function, etc.

(7) The serial channel interface can be set to active and passive modes, and the transmission data can be set to frame structure or non-frame structure.

The functions of the main pins of the AMBE-2000 chip are shown in Table 1. The working frequency of AMBE-2000 is recommended to use 16.384MHz. It has two connection methods with the crystal. As shown in Figure 1 and Figure 2, it is the connection method with active crystal and passive crystal.

Table 1 AMBE-2000 main pin functions

Both methods can be used, depending on whether the crystal used is active or passive.

2 System hardware design and implementation

2.1 System block diagram and principle

Figure 3 is a block diagram of a voice communication terminal module based on AMBE-2000. The communication terminal module is composed of a micro-control circuit, AMBE-2000, Flash, timing generation circuit, A / D, D / A circuit, etc. The control circuit uses TI's TMS320VC5402? Eye 2? Performance, used to control and read and write AMBE-2000, and exchange data with the outside; Flash is a TE28F008 produced by Intel Corporation, constitutes 1M & TImes; 8bits storage space for storing programs And initialization data; the timing generation circuit is completed by the programmable logic device EPM3032S, which is used to generate various timings required by the system; the A / D and D / A circuits are composed of the TLV320AIC10 and its peripheral circuits. All devices work at 3.3V.

The A / D and D / A chips are connected to the CODEC interface of AMBE-2000 to exchange PCM data. The serial port of AMBE-2000 is connected to the multi-channel auto-buffered serial port of MC5402 (Muchanical Buried Field Serial Port) to exchange coded data; VC5402 can also be connected to the outside world via HPI (Host Port). The data is exchanged through the asynchronous serial port (XF, BIO); the external parallel 8-bit BOOT mode is used to load the program between the VC5402 and the Flash device.

Figure 3 Block diagram of voice communication terminal module based on AMBE-2000

2.2 Hardware design

The TMS320VC5402 as a microcontroller is TI's low-power, high-performance fixed-point DSP chip. Its main features include:

(1) Fast speed and strong function.

(2) It has an optimized CPU structure with 32K bytes of on-chip RAM inside. VC5402 can be used not only to implement complex DSP algorithms and operations, but also as a microcontroller for intelligent control.

(3) Low working voltage and low power consumption. It can work under 3.3V or 2.7V voltage, and can reduce power consumption in low power consumption mode (IDLE1, IDLE2 and IDLE3), especially suitable for wireless mobile devices.

(4) Intelligent peripherals. Provide intelligent peripherals such as multi-channel automatic buffer serial port McBSP, HPI interface for communication with external devices. The McBSP port provides 2K bytes of data buffer read and write capabilities, which can reduce the extra overhead of the processor. When the instruction cycle is 20ns, the maximum data throughput of McBSP is 50Mbps. Even in the IDLE mode, McBSP can work at full speed.

In addition to being a microcontroller, VC5402 can also be used to complete other functions, such as channel coding, encryption and decryption, etc., which can be added flexibly according to the specific requirements of the system. AMBE-2000 can interface with a variety of A / D and D / A chips, select different A / D and D / A chips, and pin CODEC_SEL needs to be set differently, as shown in Table 2.

Table 2 A / D, D / A type settings

In this system, A / D and D / A chips use TLV320AIC10. TLV320AIC10 is a linear PCM codec chip of TI company. The chip has ADC analog anti-aliasing filter and digital FIR filter. It has good anti-aliasing and anti-noise capabilities. It has programmable input and output gain control, programmable ADC and DAC conversion rate control, the working voltage is 3.3V ~ 5V, also has a low power consumption working mode.

The connection between AMBE-2000 and TLV320AIC10 is shown in Figure 4. CODEC_SEL? Eye 1 ~ 0? Is grounded on the hardware, set the A / D mode to 16-bit linear PCM, its crystal frequency is 16.384MHz, frame synchronization clock and bit synchronization clock are generated by TLV320AIC10, and provided to AMBE-2000 .

The programmable logic device EPM3032S is used to generate other various timings required by the system (such as the channel serial timing of AMB-2000, etc.) and chip select signals. EPM3032S is a PLD device based on the E2PROM process. The scale of the device is about 2000 doors. With in-system programmability (ISP, In-System Program Memory), easy to use. During design, the download interface should be reserved.

28F008 is 1M byte Flash memory of Intel Corporation, used to store the program and initialization data of VC5402. The connection between 28F008 and VC5402? Eye 3? In the picture, 28F008 is used as the external data storage of VC5402. The address bus and data bus are connected to the external bus of VC5402. CE is connected to the DS pin of VC5402. The R / W pin is connected to MSTRB or later to WE and OE pin. To a jumper, when the jumper is connected low, the Flash is in the reading state. When the jumper is connected high, the Flash can be erased or programmable. At this time, the program can be written to the Flash.

3 software programming

The working process of the entire system is as follows: After the system is powered on and the TMS320VC5402 is reset, the programs and data stored in the Flash (28F008) will be moved to the internal RAM by the internally self-booting program (BOOT); then the VC5402 resets the AMBE-2000 chip To start encoding and decoding voice. AMBE-2000 completes the encoding and decoding operation of a frame of voice data every 20ms and exchanges data with VC5402. VC5402 outputs the encoded speech, and at the same time sends the data obtained from the channel or other equipment to AMBE-2000 for decoding.

Software programming is mainly to program the TMS320VC5402. The program is divided into the main program module, the communication and control module with the AMBE-2000, the data communication module with the outside world and the BOOT program loading module.

Main program module. This module executes the initialization of the program, including the initialization of the interface and registers of VC5402; the reset and rate setting of AMBE-2000, the setting of voice activation detection, the echo cancellation function; the initialization of TLV320AIC10, etc. Scheduling control of service programs. The initialization of TLV320AIC10 is mainly to set up its 4 control registers, of which the function of control register 1 is to enable operational amplifier, enable or bypass ADC analog anti-aliasing filter, enable or bypass digital FIR filter, activate Software reset, etc .; the function of control register 2 is low-power mode control, frequency-division register control (determines the clock frequency and sampling period of the filter); the function of control register 3 is to select analog signal self-loop and digital signal self-loop, control Continuous data transmission mode, etc .; the role of the control register 4 is to control the input and output gain (by controlling the input and output programmable gain amplifier to achieve).

The exchange of data between VC5402 and AMBE-2000 takes place in a standard serial manner. VC5402 McBSP interface can be divided into a data channel and a control channel in structure. The data channel completes the sending and receiving of data; the control channel completes the functions of channel selection and control. When writing the serial communication subroutine, the interrupt mode is used. Each time an interrupt occurs, data transmission and reception are performed.

VC5402 is connected to the outside world through an HPI port. HPI is an 8-bit parallel port, through which the data exchange between VC5402 and external devices can be completed. 8-bit parallel mode or 16-bit parallel mode can be used. The HPI interface mainly realizes data movement through three registers, including control register, data register and address register, and the programming is more convenient. In addition, as an alternative solution, data can be exchanged with external devices through the asynchronous serial port and only through XF and BIO.

VC5402 and Flash devices use external parallel 8-bit BOOT mode for program loading. VC5402 provides a variety of BOOT methods, including: parallel I / O port BOOT, serial port BOOT, HPI port BOOT, external parallel BOOT, etc. These different BOOT methods can meet different application occasions of users. When writing the BOOT program, it is necessary to set the VC5402 to the microcomputer working mode (MP / MC \ pin set low). In this way, after the reset of the DSP, the program starts to run the BOOT program from the internal ROM, and the program stored in the Flash device is moved to the internal RAM for execution.

4 system implementation

The rate setting of AMBE-2000 adopts the software mode, and the command word setting is sent by VC5402 through the serial interface. According to this design, the system can work stably in an environment with a channel error rate of 4% when the voice rate is set to 2.4kbps and FEC is set to 1.2kbps, and the system can be easily Change the voice rate and FEC rate to achieve variable rate voice communication.