Implementation of H.324 Terminal Based on DSP

　　At present, two methods are generally used in the research and development of H.324 systems: one is to develop a pure software H.324 system, that is, a PC-based system; the other is to use a programmable video signal processor to implement the functions of the H.324 system. The latter method has great flexibility and can develop different applications on the same hardware platform. The solution proposed in this article is an implementation method based on a programmable digital signal processing chip. This solution can develop an independent model of videophone terminals, and the system can be applied to communication, monitoring and other fields.

　　1 Functional elements of H.324 terminals

　　The H.324 recommendation specifies some functional elements of the system. The H.324 terminal configuration and its peripheral elements are shown in Figure 1, including terminal I/O devices, modem, GSTN network, MCU and other system operation entities. The implementation of H.324 does not require every functional element to be required.

　　The H.324 videophone terminal framework recommendation includes the following five aspects:

　　H.263/H.361 video codec

　　H.223 channel multiplexing/demultiplexing

　　H.245 system control

　　G.723.1 audio codec

　　V.34 Modem

　　The multimedia information flow in the H.324 terminal is divided into video, audio, data and control flow:

　　?1?Video stream is a continuous code stream that transmits color moving images. During the transmission process, the bit rate of the video stream changes according to the needs of the audio and data channels.

　　?2?The audio stream is real-time, but it may be delayed appropriately when processed at the receiving end to maintain synchronization with the video stream. To reduce the average bit rate of the audio stream, voice activation can be used.

　　?3?Data streams can represent still images, faxes, documents, computer files, undefined user files and other data streams.

　　?4?The control stream transmits control commands and instructions between peers. Terminal-to-modem control complies with the V.25ter recommendation (using an external modem connected through a separate physical interface). Terminal-to-terminal control complies with the H.245 recommendation.

　　2 H.324 Terminal Implementation Solution

　　H.324 is just a framework protocol, and there are many ways to implement the system. A dedicated circuit can be designed according to the H.324 standard. Although this method has the characteristics of fast processing speed and convenient application, it has great limitations. Because a dedicated circuit can only be designed for a specific function, it has the disadvantages of high development cost and difficulty in improving functions. In the existing or under-development products, pure software, that is, computer-based methods, or programmable digital signal processors are generally used to develop and implement H.324 systems. The system developed by the author of this article is mainly implemented using the W90K series chips, that is, it is implemented using a programmable digital signal processor. Figure 2 is a principle block diagram based on the W90210 terminal.

　　The development board is divided into video codec module, audio codec module, code stream multiplexing and system control module, video display module, modem module. These modules, together with peripheral devices such as video display and sound input playback, complete the H.324 videophone function.

　　Video codec chip W9960 This chip mainly completes the video codec function. This chip is particularly suitable for completing H.263/H.261 functions, and can complete CIF, QCIF, SQCIF and other formats of encoding. The chip is solidified with the standard H.263 video codec program, which can work in unrestricted motion vector and PB frame mode. The multiplexing control chip on the development board controls the start, stop and codec mode conversion of the codec. W9960 has a digital video interface and can support digital camera input. In addition, the chip also has a PCI interface for exchanging data with the development board main CPU and display module.

　　Audio codec chip CT8020 This chip completes the audio codec function. The chip contains G.723.1 microcode, which can encode the original audio data (16bit?8kHz) into G.723.1 data frames and decode G.723.1 data frames into original audio data. The audio codec module with CT8020 as the core and its peripherals can receive voice input encoding and receive G.723.1 code streams for decoding and playback. The module output code stream can work at 6.3Kbit and 5.3Kbit. The module's initialization, work start and stop, and work mode change can be achieved by calling its API function.

　　Video display module This module is mainly composed of W9971 and its peripherals. This module receives the input digital image signal to display the local image, or receives the decoded data of W9960 to display the remote image. The W9971 chip can also manage the display mode, such as image size, position, hue, contrast, etc.

　　· Video input module This module is mainly composed of a digital camera or an analog camera and SAA7111 chip. The function of SAA7111 is to convert the input image signal of the analog camera into a digital image signal, which is input to the video display module for displaying local images. At the same time, the signal is also input to the video codec module, and the signal is encoded and multiplexed and transmitted to the remote end.

　　Modem module This module is mainly used to complete voice input and modulation functions.

　　·Multiplexing control module This module is the core of the entire terminal, mainly composed of chip W90210 and its memory. The main function of this module is to complete the initialization of each functional module on the board, receive the code stream output by the audio codec module and the code stream output by the video codec module, and multiplex these code streams and system control information according to the H.223 standard and transmit them on the GSTN network after being modulated by the modem. The more important function of this module is to control the operation of each functional module on the entire board. The functions of the H.229 and H.245 protocols specified in the H.324 system are also implemented in this module.

　　3. Terminal Software Development

　　This terminal is developed and implemented under the embedded real-time operating system Supertask. The relationship between the hardware, operating system and developed application is shown in Figure 3.

　　Figure 3 shows the relationship between software and hardware of the H.324 terminal in this article. The hardware of the development hardware platform is at the bottom of the system and is managed by the SuperTask real-time operating system. Based on the operating system, hardware driver functions are developed, such as the driver functions of audio and video codecs and the driver functions of input/output modules. These functions can be called when developing application systems. The software in the above terminal is developed based on these driver functions and uses the services of the operating system.

　　H.324 terminal application system software

　　In the SuperTask real-time operating system, the H.324 terminal system is functionally divided into several independent tasks, and the tasks are connected as a whole to form a complete system. Figure 4 shows the relationship between the tasks.