In the transmission interface of current DVB-C system equipment, there are two MPEG2 video stream transmission interface standards: asynchronous serial interface ASI and synchronous parallel interface SPI.
There are a total of 11 useful signals in SPI. Each signal is differentiated into two signals to improve the anti-interference of transmission. DB25 is used for transmission on the physical link. Therefore, the connection is more and complicated, the transmission distance is short, and it is prone to failure. However, SPI is a parallel 11-bit signal with simple processing and strong scalability. Therefore, the output of the current general MPEG2 video encoder and the input of the video decoder are standard SPI interface signals.
ASI uses serial transmission, only one coaxial cable is needed for transmission. The connection is simple and the transmission distance is long. According to the advantages and disadvantages of SPI and ASI, it is often necessary to convert between SPI and ASI interfaces when transmitting signals.
1. SPI signal structure
The parallel transmission system SPI includes a one-bit clock signal, 8-bit data signal, one-bit frame synchronization signal, and one-bit data valid signal. The frame synchronization signal corresponds to the sync byte 047H of the TS packet, and the data valid signal is used to distinguish the length of the TS packet from 188 bytes or 204 bytes. When the TS packet is 188 bytes long, the data valid signal is always high, and all signals are synchronized with the clock signal.
2. ASI interface ASI transmission streams can have different data rates, but the transmission rate is constant at 270Mbps, so ASI can send and receive MPEG2 data at different rates. The ASI transmission system has a layered structure. The top layer and layer 2 use the MPEG2 standard ISO / IEC 13818-1 (SYSTEM), and layer 0 and layer 1 are FC fiber channels based on 1SO / IEC CD 14165-1. FC supports multiple physical transmission media, such as coaxial cable transmission.
First convert the 8-bit codeword of the packet-synchronized MPEG2 transport packet into a 10-bit codeword; then when parallel / serial conversion is required, when a new word is required and the data source is not ready, a K28.5 synchronization should be inserted Word to achieve ASI's fixed 270Mbps transmission rate. The formed serial bit stream will be sent to the coaxial cable connector through the buffer / drive circuit and the coupling network. When reaching the receiving end through the coaxial cable, when receiving the received data, it must first go through the connector and the coupling network, coupled to the circuit that recovers the clock and data, and then perform serial / parallel conversion; in order to restore byte synchronization, the ASI decoder The K28.5 sync word must be searched first. Once the sync word is searched, the boundary of the subsequent received data is demarcated, thereby establishing the correct byte arrangement of the output bytes of the decoder; finally, 10 / 8bit conversion is performed to recover Packet synchronized MPEG2 TS stream data. But the K28.5 sync word is not valid data, so it must be deleted when decoding.
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