Distribution channels A broadcast teleproduction usually is created
Distribution channels A broadcast teleproduction usually is created in a single format. If other formats are needed for distribution, then standards conversion will be used, but as a separate downstream process. In contrast, when encoding media for streaming it is common to produce multiple output formats. You may be called upon to encode up to 20 different versions of a video clip. These are files at different stream rates, each rate repeated for a number of different streaming codecs. Television has an allocated bandwidth, and the MPEG-2 compression is configured to fit. It may be variable bit rate, but within narrow limits; for standard definition it could be 2 to 6 Mbit/s. The channels used to deliver streams vary from 3G wireless, dial-up modems, to E-1/T-1 lines. Video encoding 155 Table 8.1 Typical Distribution Channels Channel data rate Guaranteed Notes bandwidth 601 270 Mbit/s Yes Uncompressed, standard definition video Digital television 4 6 Mbit/s Yes Variable bit rate T-1 (E-1) 1.5 Mbit/s (2 Mbit/s) Yes DSL 144 kbit/s 8 Mbit/s No Cable modem 128 kbit/s 1 Mbit/s No Dual ISDN 128 kbit/s Yes ISDN 64 kbit/s Yes Dial-up modem 56 kbit/s No Typical capacity 35 Kbit/s In Table 8.1, the digital television example is the standard definition television that is broadcast to the DVB standard, and uses MPEG-2 MP@ML coding. It is very similar in quality to DVD video. Some channels offer a guaranteed bandwidth; others are dependent upon the capacity of the local loop, plus the contention ratios within distribution nodes. Typically the local ends offered to the consumer do not have a guaranteed quality of service (albeit they are at a much reduced cost compared to the traditional telecoms circuits). To encode video for all these different applications, the video is delivered with a range of basic parameters. Table 8.2 shows some typical data rates for the video alone, excluding audio and control data.
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