Application of voice codec chip MT8965 in ALU

frozenviolet

Application of voice codec chip MT8965 in ALU [Copy link]

Abstract : MT8965 is a powerful voice codec chip. It uses A-law coding and can receive control signals from the processor through the control bus to write control registers, thereby controlling the drive output signal. This article introduces the performance characteristics of MT8965 and its application in ALU.
Keywords : ALU; codec; CODEC; on-hook and off-hook detection; MT8965

　　MT8965 is a powerful PCM voice codec chip produced by MITEL. It adopts the A-rate coding adopted by my country's telecommunications network and European telecommunications, conforms to the CCITT codec standard, and is compatible with the ST-BUS bus standard proposed by MITEL. MT8965 has two registers inside, which can control four output drive signals. It has three buses, one data input bus, one data output bus and one control input bus, so it can receive control signals from the processor. It contains functional modules such as receiving and transmitting filters, input and output registers, A/D and D/A conversion, and control registers. The content of the control register is determined by the data on the control bus, and the drive signal is determined by the content of the control register, so the output signal can be determined by the input data.

1 MT8965 chip description

　　The pin arrangement of MT8965 is shown in Figure 1: The description of each pin is as follows:
　　Pin 1 CSTi: Control signal input bus, which can be used to write control registers A/B through CSTi. Pin
　　2 DSTi: Digital signal input bus, which can receive 8kHz PCM signal. Pin
3 C2i: Bit synchronization clock input, frequency is 2.048kHz. Pin
4 DSTo: Digital signal output bus, which can be used to output encoded PCM signal. Pin
5 　VDD: Input +5V working voltage, the input error does not exceed 5%, that is, 0.25V.
　　Pin
　　6 : Frame synchronization input signal, which can be selected as 8kHz or 16kHz (depending on the working mode). Pin 7 CA: 8kHz square wave signal, used to determine the working mode of the chip and which register the input control word will be written to.　　Pins 8 to 11: SD0 to SD3 chip drive output, its output level is determined by the content of control register B. Pin 12 VEE: -5V working voltage input, its error should not exceed 5%, that is, 0.25V. Pin 13 VX: Analog voice signal input terminal, used to receive analog signals and send them to the transmit filter. Pin 15 VR: Analog voice signal output terminal, used to output decoded analog voice signals. Pin 17 V _REF : 2.5V reference voltage, used for chip encoding and decoding. Pin 14 ANUL: A 0.1μF capacitor should be connected between this pin and GNDA (pin 16).

The MT8965 chip is mainly composed of two parts: codec input and output and control signal input and output. The codec consists of input and output filters and D/A and A/D converters, while the control part includes two control registers (A and B) and an output register, which are mainly used to input control signals from the control bus and output control signals through four drive output pins SD0~SD3. SD0~SD2 are used to control peripheral logic and transistor circuits by switching between high and low levels, such as line ringing and message waiting indication, and SD3 is mainly used to drive peripheral analog circuits, such as switching in and out gain or filtering parts (ringing filter). The functional module diagram of this chip is shown in Figure 2.
Depending on the content in the control register and the CA input signal, this chip can work in three modes:
(1) CA is -5V, CSTi is 0V (GNDD). 　　At this time, the chip is in normal operation, and MT8965, as a codec chip, can convert between PCM signals and analog signals and output them. The drive output of the SD pin is in an active state, and its state is determined by the data in the control register B. When CA is -5V and CSTi is +5V (VDD), the chip is in an energy disabled state. At this time, any input data will not produce corresponding output. The analog signal output pin VR is internally grounded (GNDA), and the output of the digital signal output pin DSTo is in a high-impedance state.

(2) CA is -5V, and CSTi receives an 8-bit control word. 　　At this time, CSTi and DSTi receive digital signals synchronously (that is, when F1i is low, 8 bits of data are received in a 3.9μs time slot and updated every 125μs). Control register A receives the 8-bit control word and performs the following functions, including: controlling input and output gain, controlling energy prohibition, and putting the chip in a self-loop state; while control register B is set to zero, and the drive output of the SD pin is in an inactive state.

(3) CA is 0V, and CSTi receives an 8-bit control word . 　　In this mode, control register A is the same as mode 2. When receiving an 8-bit control word, the function is as described above. However, control register B is not set to zero at this time, so the SD pin output does not change. When CA is +5V and CSTi receives an 8-bit control word: control register B is used to receive the 8-bit control word, while control register A is not affected. At this time, the content of control register B determines the output of the four SD pins. The input and output PCM signals are connected internally, and the PCM signal input from DSTi is not received. The three working modes of the MT8965 chip are listed in Table 1. The working voltage of

　　MT8965 includes +5V and -5V (the error does not exceed 5%, that is, 0.25V). In addition, a 2.5V DC voltage is required as the reference voltage for encoding and decoding. In order to ensure that MT8965 can correctly encode and decode, the accuracy of this reference voltage is required to be high (the error cannot exceed 5%, that is, 0.125V). Therefore, a reference voltage generation circuit is designed inside MT8965. This circuit can generate a standard 2.5V reference voltage from a MC1403A to meet the working requirements of MT8965. The specific circuit is shown in Figure 3.

2 Application in ALU simulation user interface

　　ALU (Analog User Interface) is the interface circuit that connects ordinary analog telephones and digital switching networks. CCITT has specified seven functions for the analog user interface of program-controlled digital switches, called BORSCHT. These seven functions are:

B: Power supply, because analog telephones generally use common power, that is, the DC working current required by the transmitter in the telephone is provided by the switch, and the power supply voltage is generally -48V.
O: Overvoltage protection, the switch interface should protect the internal circuit of the switch from external lightning strikes, industrial high voltage and human damage.
R: Ringing, the interface should be able to send ring current to the telephone, and can cut off the ring current (cut ring) after the telephone is off-hook.
S: Monitoring, the interface should be able to monitor the changes in the DC current of the loop, and output the corresponding off-hook and on-hook signals and dial pulse information to the control system.
C: Codec, used to complete the PCM encoding and decoding of analog voice signals and in-band signaling. H:
Hybrid circuit, which completes the conversion between the 2-wire transmission of the ring line and the 4-wire transmission of the switching network.
T: Test, the interface should usually also provide auxiliary functions for testing the working status of each link of the loop system.
MT8965 can be used to realize the encoding and decoding between analog voice signals and digital PCM signals in ALU; and can be used together with MH88615 to realize the control of the ringing and cutting process of the telephone.
The process of controlling ringing and cutting off is as follows: a two-stage operational amplifier of LM358 is used to generate AC ringing current voltage, and a bias voltage of +5V is added to the output end to meet the requirements of MH88615. The MH88615 chip contains a ringing current amplifier circuit and a ringing current control input pin, which can be used to control the ringing or cutting off of the phone. When a high level is input to this pin, MH88615 can make it ring with the connected phone, and when this pin is set to low, MH88615 can cut off the ringing with the connected phone. Therefore, a drive output pin of MT8965 is connected to the ringing current control input pin of MH88615, and then the drive output level is controlled by filling in the control register B, so that the ringing and cutting off of the phone can be controlled.
The main function of MT8965 is to realize the conversion between voice signal and 8kHz PCM (Pulse Code Modulation) signal, including encoding and decoding and filtering before and after encoding to filter out the out-of-band frequency components and limit the frequency (i.e. the frequency range of voice signal) to between 300Hz and 3400Hz. The sampling frequency of PCM encoder is 8,000 samples/s, and A law is used for encoding and decoding. The specific working process is as follows: the digital signal input from the DSTi pin of MT8965 and output from the switching network passes through the input filter, enters the D/A converter (digital/analog converter) to become an analog signal, and then passes through the output filter to become a voice signal, and then outputs to the analog signal input end of MH88615 through the analog output pin VR of MT8965, and is output to the phone by MH88615; at the same time, the analog voice signal sent by the phone is output to the analog signal input end of MT8965, that is, VX pin, through MH88615, and then sent to the A/D converter (analog/digital converter) through the input filter to become a digital signal, and finally passes through the output filter to become an 8kHz PCM signal, and is output to the switching network through the digital signal output pin DSTo of MT8965. In this way, the mutual conversion between analog voice signals and digital PCM signals can be realized, and the function of analog user interface encoding and decoding can be completed at the same time. The specific circuit diagram is shown in Figure 4. It should be noted that the clock signal in FIG4 is generated by a programmable logic device (EPLD), CSTi and DSTi are respectively connected to two output buses of the switching chip, and DSTo is connected to one input bus of the switching chip.
FIG5 shows a ringing current voltage (RV) generating circuit.

3 Conclusion

　　Using MT8965 in the ALU circuit can not only realize the basic functions of voice codec, but also can conveniently realize ringing and ringing cutoff by writing the control bus. It can be seen that MT8965 is a voice codec chip suitable for analog user interface.

References

1. Zhu Shihua. Principles and Applications of Program-controlled Digital Switching. Xi'an Jiaotong University Press, 1993 2. MITEL. MT8965 Chip Description

lynnlydia

Thanks, I finally found it