introduction The voice codec chip CMX618 introduced by the British CML company can perform codec processing at a lower bit rate and ensure a high voice quality. Here, a digital voice communication system is designed and implemented based on CMX618. The system has a simple structure, but powerful functions, and its working voltage is very low, and its power consumption is very small. l CMX618 functions and features 1.1 RALCWI algorithm CMX618 is a half-duplex voice codec chip close to the long-distance level. It uses a new data rate algorithm technology-RALCWI technology to encode and decode voice. RALCWI is a robust advanced complexity waveform insertion technology. Unlike other speech codec technologies, it uses unique signal decomposition and parameter encoding methods to ensure good speech quality at higher compression rates . In the vocoder, the speech quality achieved by the RALCWI technology is basically consistent with the voice quality of the standard vocoder with a coding bit rate above 4 Kb / s. Its MOS (mean opinion score) is between 3.5 and 3.6, and its performance is quite excellent. The RALCWI vocoder transmits on a frame-by-frame basis. At a sampling rate of 8 kHz, the speech signal is framed, and each frame of speech contains 160 sampling points, forming a 20 ms meta-speech frame. The speech encoder performs speech analysis with a higher timing resolution (8 times / frame), and generates a series of evaluation parameters for each speech segment. Then, using different vector quantization (VQ) methods, these estimated parameters are quantized to generate 41 b, 48 b, or 55 b frames. It is worth mentioning that these vector quantization values ​​are mixed and arranged on the basis of multilingual speech, which includes speech samples of multiple languages ​​in East and West. 1.2 Main functions and features of the chip CMX618 voice codec chip is small in size, high in performance and low in power consumption. Its specific characteristics are as follows: (1) When encoding, there are three bit rates to choose from (2 050 b / s, 2 400 b / s or 2 750 b / s). In the case of selecting forward error correction coding (FEC), a 3 600 b / s bit data stream (60 ms / 216 b data packet or 80 ms / 288 b data packet) can be formed through channel coding and interleaving. (2) During decoding, a forward error correction (FEC) decoder can be selected to deinterleave and channel decode the input encoded speech bit stream (216 b / 60 ms or 288 b / 80 ms packets) to generate correction The bit rate of the encoded speech after the error is 2 050 b / s, 2 400 b / s or 2 750 b / s, and the rate depends on the selected module. When the FEC decoder is used, the "soft decision" method can be used to enhance the decoding function and reduce the occurrence of bit errors. (3) An integrated voice compression / decompression (CODEC) is included to realize the compression / decompression process from analog voice to low bit rate coding. (4) Most functions of the chip can be realized by configuring internal registers through software programming, which is simple and convenient. (5) With auxiliary functions such as discontinuous transmission detection (DTX), comfort noise generator (CNG), voice activation detection (VAD) and dual-tone multi-frequency signal detection (DTMF), etc., to optimize voice performance . 1.3 Working principle of CMX618 The internal structure of CMX618 is shown in Figure 1. As can be seen from the structure diagram, CMX618 is mainly composed of audio compression / decompressor (CODEC), RALCWI codec, forward error correction codec and other special function modules. When encoding, the input analog voice must first go through the audio compression / decompressor (CODEC) module, adjust gain, A / D conversion, filtering and compression processing, and then enter the encoder to start encoding. After encoding, if you choose to use the forward error correction (FEC) function, error correction will be performed on the encoding to try to eliminate bit errors. In this way, the encoded speech data, according to the selected bit rate and frame structure to generate data packets, using the C-BUS serial bus, transmitted to the microcontroller LPC2138. Decoding is the inverse process of encoding. The digital voice transmitted through the C-BUS serial bus enters the decoder (optional FEC function) to start decoding, and then after decompression, filtering, D / A conversion, gain adjustment and other processing, it becomes audible analog voice. In addition, during encoding and decoding, if you choose some auxiliary functions, such as discontinuous transmission detection (DTX), voice activation detection (VAD) or dual tone multi-frequency signal detection (DTMF), you need to deal with it separately. 2 System design and implementation 2.1 Microcontroller ARM microcontrollers have the advantages of low core power consumption, strong functions, and low cost. They are now used in many fields such as wireless communication, GPS, and smart phone development. Here select LPC2138 of PHILIPS Company as the main controller of the digital voice communication module. LPC2138 is a microcontroller chip based on a 32-bit ARM7TDMI-SCPU that supports real-time simulation and embedded tracking. The small package and low power consumption make the LPC2138 particularly suitable for small systems. In addition, because the LPC2138 integrates ROM, RAM, A / D and multiple peripheral modules, such as general I / O ports, timers, serial ports, etc., it is very suitable for communication gateways, protocol converters, software modem , Speech recognition, low-end imaging and other occasions, provide large-scale buffers and powerful processing functions for these applications. 2.2 System hardware design and implementation The voice communication module based on CMX618 is mainly composed of voice codec CMX618 and LPC2138, as shown in Figure 2. The voice communication system uses the CODEC module built in CMX618, which integrates A / D and D / A conversion, channel filtering, gain adjustment and other functions, which is enough to meet the index requirements for analog voice sampling and quantization. Therefore, there is no need to connect an external chip, and a lot of physical space is saved, which is very important in the actual development and design. The microcontroller LPC2138 is connected to the CMX618 through the C-BUS serial bus. C-BuS is a four-wire interrupt one-drive serial system, which can transmit data, control or status information between the main controller and CMX618 internal registers.
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