BBC report recommends lowering picture quality on Freeview

theDoc — Fri, 14 Dec 2007 01:30:08 +0000

A report by Deloitte for the BBC Trust into efficiency of spectrum use has stated that, in addition to more efficient coding and multiplexing, changes in modulation from 16QAM to 64QAM and future adoption of new technologies such as DVB-T2 and MPEG-4, efficiency improvements in the BBCs Freeview multiplexes over the next five years can also be achieved by reductions in the picture quality of servies.
The report considers reducing the capacity used by individual channels by increasing GOP length and reducing picture resolution to three-quarter screen or less to increase the number of services that can be accomodated within a multiplex. They acknowledge that lower picture resolution will generally look softer as fine detail might be lost. The BBC currently broadcasts BBC1,2,3,4,CBBC,Cbeebies and BBC News24 in full resolution, whereas ITV and Channel 4 only broadcast their primary channels, ITV1 and Channel4, at full resolution. A longer GOP length will increase channel switching times because the decoder must wait longer for the next complete picture frame. The PSB multiplexes (1,B,2) currently use the shortest GOP length, while commercial multiplexes (A,C,D) use higher variable GOP lengths. The report notes that channels on the BBCs Mux1 require at least a third more capacity than channels on other multiplexes. The higher video bit rates on Mux1 are a result of older coding technology, less extensive use of statistical multiplexing (BBC1 is currently excluded from statistical multiplexing due to regional variations) and choices regarding picture resolution and GOP length. The report recommends reducing picture resolution on BBC3 and 4, CBBC, Cbeebies and BBC News24 to 3/4 screen, and varying GOP length from 12 to 36 frames.

The planning process that existed prior to 2007 for the use of spectrum capacity reflected a situation where the BBC had sufficient capacity available for the services that it wished to provide. However, in recent years both the BBC’s and third parties’ service demands have increased. In addition to the BBC’s aspirations to provide HD, several public service broadcasters, including five, S4C and TG4, are expected to be directed by the Secretary of State to be carried in BBC multiplex capacity. The BBC appreciates that available capacity may no longer be sufficient to cover their service aspirations, and therefore the importance of using the spectrum efficiently and effectively. The report recommends that in order to increase viewer value, the BBC needs to consider its service prioritisation, for example by accommodating HD services which the public appear to value highly.

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

theDoc — Thu, 18 Oct 2007 13:56:13 +0000

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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BBC report recommends lowering picture quality on Freeview

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