Pick   To: 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 function of compressed video streams through a common PC platform, which is in line with the development and application of today's streaming media technology.
introduction
Today, with the rapid development of network technology, streaming media technology has been widely used. Video streaming that can achieve low transmission bit rates and high image quality is the goal pursued by the development of streaming media technology. The development of streaming audio and video coding technology is representing the development trend of technology in this field. MPEG4 video codec technology is favored in the field of network video transmission due to its high image quality and low image transmission bit rate. In addition to its variable coding characteristics according to the actual situation of the image, the coding efficiency is high, which is in line with streaming media technology Development direction.
This design gives a PCI card implementation method that uses hardware ASIC for MPEG4 image encoding . It fully utilizes 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 Compression technology has reached 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 functions of the main chips in the block diagram in this design are described below.
Video signal sampling chip SAA7114
SAA7114 is a 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 analog color video signals into standard digital video signals conforming to the ITU656 standard output format. The analog video signals input at the front end can be PAL , NTSC or SECAM . The control interface of the chip is an I2C interface, through which the registers inside the chip are directly read and written to complete the real-time control of the chip. In this design, the analog video signal input terminal provides two input interfaces, that is, it conforms to the video signal interface and the S- terminal video signal input interface to meet the needs of different video input occasions. The digital video output directly utilizes the two ITU656 output interfaces of the chip, I and X , all the way to the video encoding part of the circuit, and the other way to the multimedia PCI bridge chip SAA7146 to realize the real-time local image preview function. In this way, a chip is used to simultaneously complete the functions of image preview and image compression.
Audio signal sampling chip PCM1800
PCM1800 is the stereo audio sampling chip of TI 's BURR-BROWN company with a sampling accuracy of 20bit ; the supported sampling rates are 32kHz , 44.1kHz and 48kHz ; it supports multiple data serial output modes, including I2S serial bus mode. In this design, the chip mainly completes the stereo audio signal sampling, converts the analog stereo audio signal into a standard digital audio signal that conforms 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 end of the computer sound card via the audio cable to realize the audio preview function.
Audio and video compression encoding chip IME6400
IME6400 is InTIme 's hardware audio and video encoding compression chip that supports MPEG1 / 2/4 three video encoding standards at the same time . The chip has an image compression hard core and works with an external 8M byte SDRAM . It is different from the popular MPEG4 software encoding. The point is that the output video stream can be up to 720X480 @ 30fps or 720X576 @ 25fps , which is 4 times the picture coded by the general DSP chip software algorithm . The internal block diagram of the chip is shown in Figure 2 .
Picture 1 Â Hardware composition block diagram
Picture 2 Â IME6400 internal block diagram
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 audio and video encoding standards of different standards is related to the initial configuration of the chip. The microcode is downloaded to the chip before starting the chip. 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 programmable CPLD for interface format conversion. 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 the 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 circuits are programmed by themselves as needed. In this design, the chip mainly completed the conversion from ITU656 interface to ITU601 interface and the conversion of IME6400 's Host interface logic and SAA7146 's DEBI interface logic. The logic conversion block diagram is shown in Figure 3 and Figure 4 .
The ITU656 interface is an 8-bit data bit-wide interface, which is the internal synchronization mode, that is, the synchronization word is embedded in the ITU656 code stream; the ITU601 interface is a 16-bit data bit-wide interface, which is the external synchronization mode, that is, the synchronization signal is transmitted by a separate signal line. The method of converting from ITU656 interface to ITU601 interface is by extracting the synchronization signal embedded in the ITU656 code stream, and at the same time separating the data streams representing brightness and chrominance, and outputting them into the ITU601 format with a 16-bit data width. 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 address and data bus on the multiplexed bus are separated by the ALE signal of DEBI , 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 the hardware design of 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 chip initialization has a certain sequence when the system is actually working, otherwise the system will not work normally, which is the result of actual debugging.
Conclusion
The MPEG4 encoding card is characterized by a wide range of applications, high image clarity, support for constant and variable rate video encoding, and many compatible video standards. Different microcodes can support different video encoding standards. At present, the MPEG4 audio and video compressed version of this design has been debugged under the Windows 2000 operating system, and has realized the functions of local preview, hard disk storage and network transmission.
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