Pick  Essential: This article introduces the design and implementation of a single-channel MPEG4 audio and video encoding card based on PCI bus . The card realizes the real-time MPEG4 encoding and compression of audio and video streams , and can realize the recording or network transmission functions of compressed video streams through the general PC platform, which is in line with the development and application promotion of current streaming media technology.
introduction
In today's highly developed network technology, streaming media technology has been widely used, and it is the goal pursued by the development of streaming media technology to achieve low transmission bit rate and high image quality video streaming. The development of streaming media audio and video coding technology is representing the technological development trend in this field. MPEG4 video encoding and decoding technology is favored by the network video transmission field due to its high image quality and low image transmission bit rate. In addition, it realizes variable encoding according to the actual situation of the image, and the encoding efficiency is high, which is in line with streaming media technology. The direction of development.
This design presents a PCI card implementation method that uses hardware ASIC for MPEG4 image encoding . It makes full use of the advantages of the PC platform for network transmission of MPEG4 audio and video streams, hard disk storage, and sound and picture preview functions, making MPEG4 video The compression technology has reached the practical level.
hardware design
The hardware design of the MPEG4 encoding card is shown in Figure 1 . The figure shows the connection relationship between the chips, the function of the main chip in the block diagram in this design is described below.
Video signal sampling chip SAA7114
SAA7114 is the mainstream high-end video sampling chip of PHILIPS Company. The sampling supports 4:2:2 , 4:2:0 , 4:1:1 and 4:1:0 formats, and the A/D sampling conversion accuracy reaches 9bit . The chip mainly completes the digital sampling of analog video signals, and converts the analog color video signals into standard digital video signals conforming to the ITU656 standard output format. The analog video signal input by the front end can be PAL , NTSC or SECAM . The control interface of the chip is an I2C interface, through which the internal registers of the chip can be directly read and written to complete real-time control of the chip. In this design, the analog video signal input terminal provides two input interfaces, namely the video signal interface and the S- video signal input interface to meet the needs of different video input occasions. The digital video output terminal directly utilizes the two ITU656 output interfaces of the chip I and X , one is provided to the video encoding circuit, and the other is provided to the multimedia PCI bridge chip SAA7146 to realize the real-time local image preview function. In this way, a chip is used to complete the image preview and image compression functions at the same time.
Audio signal sampling chip PCM1800
PCM1800 is a stereo audio sampling chip of BURR-BROWN , a TI company , with a sampling accuracy of 20bit ; supported sampling rates are 32kHz , 44.1kHz and 48kHz ; it supports a variety of data serial output modes, including I2S serial bus mode. In this design, the chip mainly completes stereo audio signal sampling, converts the analog stereo audio signal into a standard digital audio signal conforming to the I2S bus standard format, and outputs it to the MPEG4 encoding chip IME6400 for audio encoding and audio and video stream synthesis. At the same time, the audio signal is output through the radio follower, and connected to the input of the computer sound card via an audio cable to realize the audio preview function.
Audio and video compression coding chip IME6400
IME6400 is InTIme 's hardware audio and video coding compression chip that supports MPEG1/2/4 three video coding standards at the same time . The chip has an image compression hard core and works with an external 8M byte SDRAM , which is different from the popular MPEG4 software coding. The point is that the picture of the output video stream can reach 720X480@30fps or 720X576@25fps , which is 4 times the picture encoded by the general DSP chip software algorithm . The internal block diagram of this chip is shown as in Fig. 2 .
Figure 1Â Hardware composition block diagram
Picture 2Â Internal block diagram of IME6400
Figure 3 ITU656 to ITU601 interface conversion diagram
Figure 4 DEBI to Host interface conversion diagram
In this design, the chip mainly completes MPEG4 encoding of video and MPEG1 layer 2 encoding of audio . The realization of different audio and video encoding standards is related to the chip's initial configuration microcode, which is downloaded to the chip before starting the chip to work. Internally, then reset the chip to start working. Because the digital video input interface of this chip is in ITU601 format, which does not match the ITU656 format interface of SAA7114 video output , this design uses a programmable CPLD to convert the interface format. While the present design also utilizes a programmable chip CPLD completes the matching between the main host DEBI Interface Control of the PCI bridge chip SAA7146. The conversion block diagram of these two steps is introduced in the CPLD part of the programmable chip .
Programmable Gate Array CPLD
CPLD is a programmable logic array, and the internal logic circuit is programmed by itself as required. The design of the chip main ITU656 completes the conversion to the interface and the interface ITU601 DEBI IME6400 the Host interface logic and interface logic conversion SAA7146. The logic conversion block diagram is shown in Figure 3 and Figure 4 .
The ITU656 interface is an 8-bit data width interface, which is an internal synchronization method, that is, the synchronization word is embedded in the ITU656 code stream; the ITU601 interface is a 16-bit data width interface, which is an external synchronization method, that is, the synchronization signal is transmitted by a separate signal line. The way to convert from the ITU656 interface to the ITU601 interface is to extract the synchronization signal embedded in the ITU656 code stream, and at the same time separate the data stream representing the brightness and chroma, and arrange them into a 16bit data width ITU601 format for simultaneous output, thereby Complete the format conversion.
Host IME6400 the non-multiplexed address and data interface with a separate control bus, SAA7146 of DEBI interface bus address and control data multiplexing. Here the ALE signal of DEBI is used to separate the address and data bus on the multiplexed bus, and the edge trigger signal of ALE is used to latch the address into the address register.
Multimedia PCI bridge chip SAA7146
SAA7146 is PHILIPS Company multimedia PCI bridge chip, directly provides a rich audio and video interface chip, 5 I2S interface, 2 the D1 interface, a DEBI multiplexed address data interfaces, I2C control interface and general purpose I / O interface Wait. At the same time, the chip has a variety of image processing functions, which is very convenient in hardware design in the multimedia field. The chip design this DMA transfer is completed and cropping preview video stream, the compressed audio and video stream DMA transfer and the control of other chips with its DEBI I2C interface and interfaces.
The above mainly introduces the functions of each key chip. The initialization of the chip has a certain sequence when the system is actually working, otherwise the system will not work properly, which is the result of actual debugging.
Concluding remarks
The MPEG4 encoding card is characterized by a wide range of applications, high image definition, support for constant and variable rate video encoding, and multiple compatible video standards. Different microcodes can support different video encoding standards. At present, the MPEG4 audio and video compression version of this design has been debugged under the Windows 2000 operating system, which has realized the functions of local preview, hard disk storage and network transmission.
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