Last October, I attended the AVS Forum Home Theater Cruise for the
first time. One panel in particular covered the emerging
high-definition DVD standards. If you haven’t heard by now, there are
two proposed formats: Blu-ray Disc (BD), which is led by a consortium
of 13 major manufacturers including Sony, Panasonic, HP and Dell; and
HD DVD, led by Toshiba and NEC (recently joined by Sanyo).
To be sure, both sides made compelling cases for their proposed
standards. Blu-ray, which uses a short-wavelength blue-violet laser
pickup (hence the name) will offer 27 GB of capacity on a single-layer
DVD and 54 GB on a dual-layer DVD. That’s enough for 2+ hours and 4+
hours of HDTV programming, respectively, assuming 9 GB per hour and
MPEG-2 encoding.
The HD DVD format also uses a blue laser, but employs much of the
existing manufacturing process for regular DVD. It claims capacities of
15 and 30 GB for read-only and rewriteable versions and (supposedly)
offers considerable manufacturing cost savings over the
create-from-scratch Blu-ray format.
However, the differences between the formats and the alliances forming
to support one or the other aren’t what I found interesting. Rather, it
was the news that both DVD formats will support MPEG-2, MPEG-4, AVC
(H.264), and VC-1 (Windows Media 9) video encoding.
MPEG-2’s Final Hour
For those readers who are still getting their feet wet with HDTV, one
of the biggest problems (if not the biggest) is juggling the bit rate
with MPEG-2 encoding to achieve a reasonable compromise between HD
picture quality and transmission efficiency. MPEG-2 has certainly done
a yeoman’s job with standard-definition video (480i or 576i), but
there’s plenty of evidence it’s straining to keep up with HD content.
Consider this: The pipes for MPEG-2 are already tight as is. Off-air
HDTV can’t use any more than 18.5 - 19 Mb/s data rates; the 6 MHz
channel size is capped at 19.39. Empirical results show that
multicasting HD with other SD programs and chopping the HD bit rate
down has a noticeably adverse effect on picture quality, particularly
on larger (40 inches and up) TV screens.
Cable system operators use a different modulation scheme to pack two HD
programs into a single 6 MHz digital cable channel with a 38.8 Mb/s
data rate, adding a pair of SDTV channels to fill out the package. But
they have finite channel capacity and can’t afford the luxury of higher
bit rates. Satellite operators are also constrained by the cost of
transponders, so precious bits are used for as many programs as
possible, not for enhancing picture quality.
JVC’s languishing D-Theater (D-VHS) HDTV tape format has a maximum bit
rate of 25 Mb/s for showing 1080i encoded movies, but requires special
tape players and has not been able to penetrate the mainstream consumer
market.
The answer is a more efficient codec, and that’s where MPEG-4, AVC and
VC-1 come in. Both systems are relatively new and both promise to cut
the required bit rate for HD in half.
That means an HD program could be encoded at 8-9 Mb/s and SD programs
at 1-2 Mb/s, yet still provide the same image quality that MPEG-2 is
capable of. Public demos of VC-1 have already been going on for several
months—both AVC and VC-1 were demonstrated at NAB 2004—and several VC-1
high-definition DVDs have been released by Microsoft, including Step
Into Liquid, Terminator 2: Judgment Day, To the Limit and Stormchasers.
Given that MPEG-2 is over a decade old but universally supported,
there’s a bit of a dilemma here. For a still-evolving high-definition
disc playback and recording format, the decision to support more
advanced codecs is a win-win for everyone. But the existing
broadcasting/cable/satellite industry is still stuck with an MPEG-2
infrastructure for the time being.
And Now, the Hard Part
The Advanced Television Systems Committee (ATSC) is currently
investigating AVC and VC-1 for digital TV broadcasting. SMPTE recently
standardized VC-1, and there has been talk of HD satellite broadcaster
VOOM moving to VC-1 encoding to free up transponder space. But giving
up tried-and-true MPEG-2 will be a tricky, time-consuming and expensive
process.
To be sure, MPEG-2 picture quality issues would never have come up if
television screens had stayed in the 20-inch to 30-inch range—it’s a
lot harder to spot compression artifacts on smaller screens,
particularly with interlaced programming and the typical viewing
distances of seven feet or more.
If there are quantization errors such as macroblocking and compression
problems such as mosquito noise, it’s a sure bet cable and satellite
operators (not to mention TV stations) will get plenty of phone calls
complaining about picture quality. Anyone who has watched
standard-definition digital cable on a 50-inch rear-projection LCD or
DLP TV knows very well what I’m talking about.
Right now, HD program content providers are faced with a Hobbesian
choice and must decide whether bit rate, bandwidth, or bucks (as in $)
is most important to them. If it’s the last two (and it usually is),
picture quality is inevitably sacrificed.
Why Bit Rates Matter
By halving the bit rate, content providers get more breathing room and
can add additional channels/program streams on top of all that.
The Blu-ray and HD DVD camps won’t have to make these hard decisions;
they’ve already future-proofed their systems for advanced codecs. Plus,
they have plenty of breathing room with the proposed 36 Mb/s data
rates—twice that of the ATSC standard.
Toshiba is already announcing it will release HD DVD players in early
2005 with software expected to follow. Blu-ray member Sony says a
player/recorder won’t be available until late 2005 or early 2006,
though Sony and Panasonic already have models available for sale in
Japan.
As we went to press, Toshiba won another coup: Warner Bros., Paramount,
Universal and New Line Cinema all pledged support for the HD DVD
format. A few days later, Disney backed Blu-ray.
Either way, the door has been opened for the introduction of more
advanced, HD-friendly video codecs into the mainstream consumer market.
How much longer before cable, terrestrial, and satellite HDTV
programmers follow suit?
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