HD satellite channels are broadcast by transponders with channels on multiplexers (MUXes). Each MUX has a frequency bandwidth and modulates multiple channels on that MUX. The multiplexing of channels and data are not linear but dynamic, everything is mixed together across the frequency bandwidth, and variable qualities of information including video, audio and data are transmitted on the MUX; this includes multiple programmes and their data; audio, subtitles, encryption, electronic programme guide, forward error correction, etc...
The encoding of each HD programme in high definition (HD) 1080i (i is interlaced) MPEG-4 mode capped variable bit rate (VBR) H.264//AVC, rather a month full to say the least. The output to your TV is 1080 lines, but the picture, audio and data aren't broadcast in straight lines or curves but square blocks of information dynamically in size by intensity, colour, audio blocks and data basically.
The size of square blocks are dynamic and dependant on the how big a coloured area is in the picture and how much it occupies for video; to confuse it further only the changing movement of these square areas are broadcast, if at all possible, to reduce the data transmitted, this process compresses the picture whilst transmitting it along the carrier. Without the compression, we'd be back to transmitting pictures in analogue, and this is why transmitting information digitally uses less overall bandwidth than analogue TV, and therefore more programme channels can occupy the same frequency bandwidth.
Each square block of picture is represented by a compressed identifier referred to as a hash, the whole square is transmitted once and for further transmission of that same information only the hash is sent, thus reducing the transmitted data by large qualities; these square blocks are continually changing in size and dynamics depending on movement in the video and how dynamic the movement is in the video pictures; so more dynamic content like sporting events requires a larger bandwidth and bitrate than most movies or other TV programmes.
But it's unlikely that all the channels on the MUX are all highly dynamic concurrently, so this enables the intermixed programme content to occupy even less space, because bitrate programme content can be high on one programme whilst low on another programming channel on the MUX, but if they are all dynamic at once then the picture quality will suffer on one or more programme channels on the MUX. For improving this situation information is buffered, so there is a slight delay before the video is received at the receiver and the picture is displayed on the TV, so the dynamic nature of MUX channels is almost always okay.
Some video and audio information is removed from the stream to improve the compression with HD programme broadcasts, for the codec (video encoder) is lossy, but this usually cannot be perceived by one's eyes or ears easily and is acceptable to most people, even DVD's use lossy encoding to reduce the size of the video and audio data when recording on media or transmitting programmes along a medium.
Broadcast HD TV technical details in plain language
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