In
digital television and
digital radio systems, several variable bit-rate data streams are multiplexed together to a fixed bitrate 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 one 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 channel 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
SES Platform Services in Germany, which provides
playout, digital archiving,
encryption, and satellite uplinks, as well as multiplexing, for hundreds of digital TV and radio channels.
In digital radio, both the
Digital Audio Broadcasting (DAB) 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 a
DAB ensemble), but is entirely optional with IBOC systems.
