Re-multiplexing
The straight forward broadcasting involves a transport stream carrying a number of programs. However, in several applications, a provider may take in a number of streams from several sources and select programs from among them to be carried by a newly created transport stream. Furthermore, a broadcaster may wish to include an advert within a trans- port stream. To support such activities, re-multiplexing, also known as transmultiplexing is used. Nominally, a re-multiplexer does not alter bit rates while constructing a new multiplex out of the input streams. It is of course possible, with statistical multiplexing that the sum of the bit rates of the selected programs would exceed the bit rate of the transport stream. To avoid this, transcoding or re-compression has to be used. Transcoding is the technique by which a compressed video stream is translated to a lower bit rate strictly within the compressed domain. It can reduce the bit rate of MPEG-2-compressed video without fully decoding and re-encoding a bit- stream. It is in partial compression in that it involves identifying the DCT coefficients and re-quantise them to ensure the totality of the bit rate remains within the limit set for the transport stream. This results in occa- sional or moderate reductions in the average bit rate of individual video streams with little noticeable degradation in picture quality.
While insertion of MPEG-2-coded advertisements into a channel with stat-multiplexed video requires bit rate transcoding of the advertisement stream, MPEG-4 coding schemes not only allow use of coding tools that are more attractive for advertisements (such as natural video merged with synthetic video with lots of scene changes), they allow for lowering of bit rates to values that are far lower than MPEG-2 stat-mux stream rates. Advertisements could be authored at rates lower than 500 kbps with qual- ity that matches that of the MPEG-2-coded video.
With the use of MPEG-4 coding, multiple advertisement streams can now be inserted into a stat-mux channel instead of a bitrate-transcoded, single advertisement stream using MPEG-2.