06.22.2005 12:00 AM
BBC Demonstrates HD over SD Channels
It is now feasible for the BBC air HD broadcasts over SD channels via a new system it has just tested, according to the public broadcaster. If adopted, the system would beam HD content to British homes with HD-ready sets prior to Britain's analog cutoff that will subsequently open additional spectrum. The cutoff is scheduled to begin in 2008 and span at least four years.

The new HD-over-SD system was unveiled recently at biennial industry-wide demonstrations at the BBC's R & D lab near London, according to published reports. It uses a unique approach to HD capture and viewing, in effect, since it requires consumers to set up their own HD recording schedules for eventual simultaneous HD-SD playback at specific times.

Basically, this is how it works. The HD broadcast system takes advantage of growing capacity and shrinking price points of hard-drive digital video recorders (DVRs). It uses Britain's existing SD DTV channels (mostly 625i PAL) to transmit HD content during non-peak hours.

Prior to a scheduled SD broadcast, an HD version of the same program is transmitted and stored on a DVR using an instantaneously changing variety of available frequencies. When the scheduled broadcast is transmitted in SD, it contains a trigger signal that's recognized by the DVR, which then automatically plays the HD version on HD-ready sets.

The BCC reports that SD programs will stream at its usual broadcast rates of up to 4 MB, while the HD versions stored on the DVR play at 17 MB for MPEG-2 (or 12 MB for MPEG-4) although the HD content is downloaded at much slower rates and intermittently. (The DVR pulls the content together chronologically at playback.) The BBC system works only with recorded programs, not live coverage, and proponents say it skillfully provides a simple way today to transmit motion pictures and TV series in HD before analog broadcasting is shut down for good.

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