SCI rate control
Real-time video compression with MPEG-2 or H.264 technology can provide two common types of streams with regards to the bit rate control. A bit rate control algorithm in the encoder is necessary to ensure that the buffers at the encoder and decoder do not underﬂow or overﬂow. Constant-bit-rate control is commonly used in the video contribution domain as it provides a constant bit rate that is mandatory for bandwidth planning and video quality assurance. Variable bit rate (VBR) is not commonly used in the contribution domain or primary distribution as it provides streams with bit rates that change, regardless of the targeted average video quality it maintains.
Still, VBR control has the interesting particularity of allowing the change of the elementary stream bit rates while maintaining the consistency of the output MPEG-2 transport stream. The SCI rate control mode is based on this VBR rate control mode, except that the video elementary stream will be changed according to the operator’s request and not as a result of a video encoder algorithm.
This has no impact on the MPEG-TS multiplexing, which combines the elementary streams (video, audio channels and null packets) at the encoder level. The output is a transport stream with a constant bit rate, which contains a video elementary stream running at a new bit rate. Hence, the size of the MPTS at the output of the multiplexer is not changed.
Starting from four HD services running at 12Mb/s each, the operation procedure for adding a fifth channel into the multiplex using SCI would be as follows:
- The operator calculates 48Mb/s ÷ 5 = 9.6Mb/s;
- The operator assigns a new 9.6Mb/s bit rate to each encoder service by using a manual entry (typically using a dedicated Web graphic interface or a network management system that controls the encoders over SNMP). The new service bit rate is applied almost instantaneously;
- Once the four original encoders have been set to their new output bit rates, the operator can enable the fifth encoder to stream to the multiplexer. (See Figure 1.)
Starting from five HD services running at 9.6Mb/s each, a channel is removed as the program line-up changes to four services:
- The operator switches off the unwanted service;
- The operator calculates 48Mb/s ÷ 4 = 12Mb/s;
- The operator assigns a new 12Mb/s bit rate to each encoder service. The new service bit rate is applied almost instantaneously, in this case allowing an enhanced quality for those services that remain in the multiplex. (See Figure 2.)