Stat-muxing over several channels (or how HDTV might be coming to Freeview)

While we’re all eagerly awaiting OfCom’s expected announcement next month of an ambitious migration path that would transition Freeview from the current DVB-T transmission standard using MPEG-2 compression to the newer DVB-T2 and MPEG-4 standards by 2009, starting with a single multiplex, here’s some more news for those of us for whom HDTV on Freeview can’t come soon enough.
The Digital Terrestrial Television Action Group (DigiTAG) has suggested a spectrum efficiency improvement that could help ease the way for high definition to appear on the DTT platform. Current proposals involve advances in compression technology to help ensure the efficient delivery of HD services on the terrestrial platform. The MPEG-4 AVC compression format is preferred over MPEG-2 for the provision of HDTV services given its enhanced bit-rate efficiency, and further efficiency gains are expected in the next few years, making it increasingly more attractive for HD services. In addition, the DVB Project is in the process of finalising the development DVB-T2, the new terrestrial broadcasting standard that builds upon the improvements to modulation and error-protection made since the development of DVB-T in 1995. DVB-T2 is expected to provide a spectrum efficiency increase of at least 30% compared with DVB-T.
One of the areas in which HD has missed out though is in statistical multiplexing. Standard definition channels are able to make more efficient use of the available bandwidth by statistical multiplexing, which works well where for example 5 channels occupy a 24Mbps multiplex. Until now high definition channels have been unable to take much advantage of statistical multiplexing as it simply isn’t possible to fit many HD channels into a multiplex. DigiTAG’s suggestion is to allow multiplexing over several 8MHz UHF channels, which could give a significant efficiency boost if used for combining several HD channels in one statistical multiplex. This would, of course, add to the cost and complexity of consumer equipment as a tuner would be required for each multiplex, but if it speeds the adoption of HD on Freeview then you won’t hear any of us complaining.

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