DVB-ASI Digital Video Broadcasting - Asynchronous Serial Interface
DVB-ASI is a uni-directional transport mechanism that is capable of transporting any bit rate up to 213 Mbps. The ASI standard was developed by DVB in Europe specifically to transport MPEG video with 188 or 204 byte packets.
The data rate at which MPEG transport streams are encoded vary in time. Even if encoders are set to Constant Bit Rate encoding (CBR), the digital stream generated often shows some small variation in data rate over time. MPEG null packets are used to fill the stream with data so that the total payload rate (and total packet rate) is something constant. The null packets are 188 or 204 byte MPEG packets with the PID set to 1FFF(Hex) which instructs decoders to disregard them.
Once the total payload rate is established, the MPEG packets are encoded using 8B/10B encoding and "K28.5" comma characters are used to increase the clock rate to 270 MHz. In most interface components, a 27 MHz clock is used with 10 parallel lines of data. Comma characters are inserted in two distinct modes, “Byte Mode” and “Contiguous Mode.” To make things more confusing, the terminology to describe the two modes is not always consistent. No one seems to use the same set of terms and "Burst Mode” has actually been used to describe either mode. Regardless of mode, ASI requires that at least two comma characters are inserted between every packet for byte-sync purposes.
"Byte Mode" a.k.a. "Burst Mode"
Comma characters are added between every byte of MPEG data and extra stuffing is not used between packets. When an encoder is set at a total data rate of 27.9 Mbps, the bytes are sent every 289 ns with either 6 or 7 commas between each data byte. The total payload rate is changed by sending a different number of stuffing bytes between each data byte. This method is used to minimize the buffering capacity of the receiving device."Contiguous Mode" a.k.a. “Packet Mode”
Each byte within each data packet is sent at a rate of 270 MHz with no stuffing. 188 byte packets last around 7 ms independent of the total data rate. Stuffing data is added between packets until the rate of 270 MHz can be maintained. When the total data rate is set at 28 Mbps, packets are sent with 47 ms worth of commas. 80 Mbps and 155 Mbps require 12 ms and 3 ms respectively and anything over 200 Mbps requires very few commas. This mode is commonly used when payload rates reach above the 80 Mbps level. At this data rate, Byte Mode does not reduce the need for buffering capacity.|
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