Abstract: AMBE-1000 is a low bit rate vocoder chip with good voice quality. A specific implementation scheme for developing a voice communication vocoder using this chip is proposed. The telephone user interface circuit, PCM voice digitization coding circuit and AMBE-1000 supporting circuit in the voice communication system are given.
Keywords: AMBE-1000 Vocoder Voice Communication
AMBE-1000 is a mature duplex vocoder chip developed by DVSI of the United States and produced by Lucent. The chip adopts the AMBE speech coding algorithm with a coding rate of 2.4 to 9.6 kb/s. The AMBE (Advanced Multi-Band Excitation) algorithm is an improvement and expansion of the MBE (Multi-Band Excitation) algorithm. The MBE speech coding algorithm divides the speech spectrum into several bands according to the fundamental frequency, processes the unvoiced/voiced sounds (V, UV) in each band separately, and finally superimposes the signals of each band to form a full-band synthetic speech. The AMBE-1000 vocoder has relatively good speech quality under low rates and strong background noise, making it widely used in mobile satellite voice communication systems on vehicles and ships. Inmarat (International Maritime Satellite Organization) has applied AMBE-1000 to its various generations of satellite voice communication systems. The chip can also be used in voice compression and storage systems [3]. This paper applies AMBE-1000 to voice communication system, proposes a specific implementation plan, and gives its telephone user interface circuit, PCM voice digitization coding circuit and AMBE-1000 support circuit 
.
1.1 Main features of AMBE-1000
(1) A full-duplex encoder with high voice quality and low bit rate. The coding rate is variable from 2.4 kb/s to 9.6 kb/s. The comparison of voice quality with other vocoders is shown in Figure 1 [1]. 
As can be seen from Figure 1, at a coding rate of 4.8 kb/s, AMBE-100 has very good voice quality; at a coding rate of 2.4 kb/s, the voice generated by the chip is better than GSM voice.
(2) It has strong resistance to background noise, FEC function, and good resistance to channel interference, as shown in Figure 2[1].
As can be seen from Figure 2, the ability of the AMBE-1000 algorithm to resist background noise is significantly higher than that of other algorithms such as CELP.
(3) It has the advantages of low power consumption and can detect, identify, generate and send DTMF signals, as well as voice activation, comfort noise insertion and echo cancellation [2]. 
1.2 Basic working principle of AMBE-1000
The AD/DA voice interface signal of AMBE-1000 can be a standard μ- law or A-law compressed and quantized PCM signal, or a 14- or 16-bit linearly quantized PCM signal. The transmission interface of compressed voice data can be set to active or passive mode, and the data can be transmitted in serial or parallel mode. AMBE-1000 provides a series of pins for setting the default working state of the chip. When the chip is powered on, it automatically enters the default state set by the pins. These settings include AD/DA conversion format, voice coding rate, FEC rate, active/passive mode, parallel/serial data mode, VAD enable, echo cancellation enable, etc. These states can be set by hardware or changed by software through control words [2]. 
The data format of AMBE-1000 can be frame format or non-frame format, and frame format is usually used. For frame format, AMBE-1000 works in full-duplex parallel with a period of 20ms. After 20ms, AMBE-1000 compresses the digitized voice sent by the A/D converter, packages it according to its frame format and sends it to the encoding output buffer, and decompresses the data packet in the decoder input buffer and sends it to the D/A converter, thus completing the encoding and decoding of the digital voice.
2 Application of AMBE-1000 in Voice Communication System
Based on the advantages of AMBE-1000, the overall block diagram of the system designed in this paper is shown in Figure 3. The system uses a telephone to input voice, provides a standard RJ11 interface, and the interface circuit is implemented with MC3419-1L. The digital PCM encoding of the voice is implemented with MC14LC5480, and finally AMBE-1000 is used to compress the voice. 
2.1 User loop interface circuit
The subscriber loop interface circuit SCIL (Subscriber Loop Interface Circuit) mainly provides the "BORSHT" function for the user telephone loop and is implemented by MC3419-1L.
MC3419-1L is a user loop interface circuit interface chip produced by Motrola. Its basic performance includes: feeding DC power to the user loop; using current mirror, operational amplifier and external balance network to complete 2/4 line conversion; having the function of detecting the user line status and outputting the corresponding level information. MC3419-1L circuit uses current mirror to realize various main functions. The most important feature of current mirror is that it can decompose one input current into several output currents, the input is low impedance, the output is high impedance, and the output and input current have a strict proportional relationship. MC3419-1L uses two of the six current mirrors to form a DC feeding circuit. In order to improve the power feeding capability and reduce the chip function, an external TIP125/111 is required as the output current power amplifier tube; the 2/4 line conversion function of the chip is also realized by using the operational amplifier and current mirror. The two-line user loop signal (balanced signal) can be transmitted to the four-line transmission output terminal (unbalanced signal), and the signal at the four-line receiving input terminal is not only transmitted to the user loop, but also offsets the signal returned to the transmission operational amplifier input terminal through the balanced network, thus realizing the isolation between reception and transmission; the user on-hook and off-hook detection of the chip is also realized through the internal current mirror comparison [5]. 
Figure 4 is a simplified application circuit of MC3419-1L. In Figure 4, TIP125 and TIP111 are both current-fed power amplifier output tubes; the diode bridge and resistor between the -48V power supply and the user line mainly play the role of overvoltage and overcurrent protection, and the capacitor plays the role of shock protection; the external resistor and capacitor connected to the CC pin are mainly used to suppress the power frequency common mode interference; the connection part between MC3419-1L and PCM is a balanced network circuit.
2.2 PCM Circuit
PCM encoding of speech is to convert analog speech signals into digital speech signals. It is the first step in speech digitization and the basis of speech compression.
MC14LC5480 is a μ/A -law PCM chip produced by Motorola . It has the following features: low power consumption; low-noise fully differential analog circuit design; integrated transmit bandpass filter and receive lowpass filter; RC pre-filter and post-filter; μ/A- law selectable. 
Figure 5 is the principle block diagram of MC14LC548[5].
In Figure 5, RO+, RO- and TI+, TI- are the differential output and input of the PCM analog voice signal respectively; PI, PO+, PO- are used to amplify the analog signal to drive the analog voice device; the synchronization control part is mainly used to control frame synchronization and bit synchronization, and the control pin is mainly used for the selection of μ/A law and the selection of low power consumption mode.
Figure 6 is the specific application circuit diagram of MC14LC5480[6].
In Figure 6, the 8kHz and 2.048MHz clock sources can be generated by Motorola's MC74HC4060. The specific implementation is shown in Figure 7.
2.3 AMBE-1000 Circuit
The AMBE-1000 circuit consists of three parts: the PCM interface part; the chip control pin setting part; and the compressed data input and output part[1][2].
2.3.1 AMBE-1000 and PCM interface circuit
AMBE-1000 requires PCM voice data to be input and output in serial mode. The key to this interface circuit is the frame synchronization and bit synchronization of PCM voice data. The specific implementation circuit is shown in Figure 8. 
Among them, the input 8kHz clock source is used for bit synchronization, and the 2.048MHz clock source is used for frame synchronization. The D flip-flop and inverter are used for the synchronization of the two clock sources.
2.3.2 Chip control pin settings
The pin settings of AMBE-1000 mainly include the settings of the transmission interface, that is, the settings of the transmission interface modes of serial and parallel, active and passive, frame and non-frame, and also include the settings of chip-specific functions.
The serial and parallel settings of the transmission interface are to select whether the compressed voice data is input and output in 8-bit parallel mode or in serial mode. The active and passive modes of the chip refer to whether the strobe pulse signal for the compressed voice data input and output is provided by AMBE-1000 or provided by the outside. The frame and non-frame mode refers to whether the input and output data are provided by the outside. The frame and non-frame mode refers to whether the input and output data are encapsulated in the fixed data frame format of AMBE-1000. In this system, AMBE-1000 is set to work in the main branch, serial, and frame format.
The settings for specific functions of AMBE-1000 include AD/DA conversion format, voice coding rate, FEC rate, VAD enable, CNI enable, echo enable, DTMF processing, and low power mode. The pin setting sets the chip to the default state automatically when it is powered on. Some of the above settings can also be changed through the software control word of AMBE-1000. The circuit for pin setting can be implemented with jumper sockets and jumper caps to facilitate the change of hardware settings. 
2.3.3 Input and output circuit of compressed voice data
The input and output circuit of compressed voice data mainly refers to the setting circuit of the transmission interface and the connection circuit with the standard serial port DB-9. The transmission interface of this system is set to active, serial, frame format mode, that is, CH_SEL2, CH_SEL1, CH_SEL0 (98, 99, 2 pins) are set to 0, 1, 0. Figure 9 is a simplified connection circuit diagram of the chip and DB-9.
Among them, CHS_I_CLK (serial input clock), CHS_O_CLK (serial output clock), CHS_I_STRB (input data strobe), CHS_)_STRB (output data strobe), and CHS_SYNC (serial synchronization) in AMBE-1000 are used for input and output clock synchronization.
提升卡
变色卡
千斤顶
京公网安备 11010802033920号