[opendtv] BBC Demonstrates HDTV Broadcasts over SD Channels
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: OpenDTV Mail List <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 20 Jun 2005 08:57:55 -0400
BBC Demonstrates HDTV Broadcasts over SD Channels
June 20, 2005 12:00am
Source: Communications Daily
HDTV broadcasts over standard definition (SD) DTV channels are
feasible as a result of a system developed by the BBC. If adopted,
the system would beam HDTV content to British homes with HDTV-ready
sets before the nation's 2008-2012 analog cutoff opens additional
spectrum.
Unveiling of the HD-over-SD system came at recent broadcast industry
demonstrations at the BBC's R&D labs in Kingswood Warren, Surrey. The
event takes place every 2 years; this year's was held against the
backdrop of BBC plans to eliminate up to 25% of its work force.
The HD broadcast system demonstrated exploits the growing capacity
and shrinking cost of hard-drive PVRs. It uses Britain's existing SD
terrestrial DTV channels -- basically digitized 625i PAL -- to carry
HD content off- hours. During the day and night before the scheduled
SD broadcast, an HDTV version of the program is broadcast to a PVR
using a various frequencies as available. By the time the scheduled
broadcast is transmitted in SD, the complete HD version is stored and
ready to play from the PVR. The scheduled program contains a trigger
signal that's recognized by the PVR, which automatically plays the HD
version. So viewers with HDTV-ready sets see the program in HDTV
instead of SD.
The SD program streams at the usual broadcast rates, up to 4 Mbps.
The HD version stored on the PVR plays at 17 Mbps for MPEG-2 or 12
Mbps for MPEG-4, but can be downloaded at much slower rates and
piecemeal. The PVR stitches it all together before playback. The
system works only with recorded programs, not live coverage. But it
provides a simple way to deliver movies or regularly scheduled TV
series in HD before analog broadcasting is shut down and releases
frequencies to carry digital HDTV live.
In a related area, the BBC's Kingswood engineers have been carrying
out viewer tests to determine the best scanning and display method
for HDTV -- 720 lines progressive (720p) or 1080 lines interlaced
(1080i). Each method was compared with conventional 625 line PAL
pictures, which display 576 lines interlaced on CRTs or 576
progressively on plasma or LCD panels.
Viewers sat at the 2.7 meters distance from the screens typical in
U.K. homes. They agreed that ordinary SD was good enough for screens
up to 27". For larger screens, up to 50", 720p with 1,280 pixels per
line was considered ideal. "It's time to ditch interlacing," said BBC
engineer Richard Salmon. "CCD cameras and flat panel displays are
inherently progressive. Interlacing is analogue compression. It's
yesterday's technology."
The demonstrations gave a useful reminder why flat panels are easier
to watch. As Americans accustomed to 60 Hz NTSC are quick to notice
when they visit Europe, interlaced PAL at 50 Hz on a CRT shows
noticeable flicker, especially when the picture content is bright and
white. Artificially increasing the display rate requires new CRTs and
can introduce image artifacts. But LCD and plasma progressive panels
show no flicker at 50 Hz because the LCD backlights are always on,
and the plasma panel is using "sample and hold" to increase the speed
at which the gas cells fire to emit UV light and excite the color
phosphors.
As for Britain's planned switch-over from analog to DTV, on
technical grounds the BBC stands firmly behind telecom regulator
Ofcom's plan to change terrestrial DTV modulation from 2K to 8K --
that is, increasing the number of carriers for the signal to about
8,000 from the current 2,000. -- Stephen A. Booth, Barry Fox
<<Communications Daily -- 06/20/05>>
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