In digital television and digital radio systems, several variable bit-rate data streams are multiplexed together to a fixed bit rate transport stream by means of statistical multiplexing. This makes it possible to transfer several video and audio channels simultaneously over the same frequency channel, together with various services. In the digital television systems, this may involve several Standard Definition
Television (SDTV) programmes (particularly on DVB-T, DVB-S2, ISDB and ATSC-C), or one HDTV, possibly with a single SDTV companion channel over 6 to 8 MHz-wide TV channel. The device
that accomplishes this is called a statistical multiplexer. In several of these systems, the multiplexing results in an MPEG transport stream. The newer DVB standards DVB-S2 and DVB-T2 has the capacity to carry several HDTV channels in one multiplex. Even the original DVB standards can carry more HDTV channels in a multiplex if the most advanced MPEG-4 compressions hardware is used.
On communications satellites which carry broadcast television networks and radio networks, this is known as Multiple Channels Per Carrier or MCPC. Where multiplexing is not practical (such as where there are different sources using a single transponder), single channel per carrier mode is used. Signal multiplexing of satellite TV and radio channels is typically carried out in a central signal playout and uplink centre, such as ASTRA Platform Services in Germany, which provides play out, digital
archiving, encryption, and satellite uplinks, as well as multiplexing, for hundreds of digital TV and radio channels.In digital radio, both the Eureka 147 system of digital audio broadcasting and the in-band on-channel HD Radio, FMeXtra, and digital radio mondiale systems can multiplex channels. This is
essentially required with DAB-type transmissions (where a multiplex is called an ensemble), but is entirely optional with IBOC systems.
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