SPI and ASI interface

SPI has a total of 11 useful signals, and each signal is differentiated into two signals to improve transmission anti-interference. It is transmitted by DB25 on the physical link, so the connection is many 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 general MPEG2 video encoder and the input of the video decoder are all standard SPI interface signals.

ASI uses serial transmission, which only needs a coaxial cable for transmission, which is simple to connect and has a long transmission distance. According to the respective advantages and disadvantages of SPI and ASI, SPI and ASI interfaces are often converted between SPI and ASI interfaces when transmitting signals.

1. SPI signal structure

Parallel transmission system SPI includes a clock signal, an 8-bit data signal, a frame synchronization signal and a data valid signal. The frame synchronization signal corresponds to the synchronization byte 047H of the TS packet, and the data valid signal is used to distinguish the length of the TS packet as 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 transport stream can have different data rates, but the transmission rate is constant, 270Mbps, so ASI can send and receive MPEG2 data at different rates. The ASI transmission system is a layered structure. The highest layer and the second layer use the MPEG2 standard ISO/IEC 13818-1 (SYSTEM), and the 0th and 1st layers are FC fiber channels based on 1SO/IEC CD 14165-1. FC supports a variety of physical transmission media, such as coaxial cable transmission.

First, convert the 8Bit codeword of the MPEG2 transport packet with packet synchronization into 10bit codeword; then in parallel/serial conversion, when a new word is required to be input, and the data source is not yet ready, a K28.5 synchronization should be inserted Word, in order to reach ASI's fixed 270Mbps transmission rate. The resulting serial bit stream will be sent to the coaxial cable connector through the buffer/drive circuit and coupling network. When it reaches the receiving end through the coaxial cable, when receiving data, it must first pass through the connector and 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 must The K28.5 synchronization word is searched first, once the synchronization word is searched, the boundary is demarcated for the subsequently received data, thereby establishing the correct byte arrangement of the decoder output bytes; finally, the 10/8bit conversion is performed to restore the packet Synchronous MPEG2 TS code stream data. But the K28.5 sync word is not valid data, so it must be deleted during decoding.