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HOME REVIEWS CONTACT ABOUT April 9, 2009
HDTV
Compressed Digital Video (CDV) For Non-Geeks
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Standard Definition
(SDTV) 1280 X 720 (HDTV) 150 Gb/Hour 1920 X 1080 (HDTV) 350 Gb/Hour MPEG-2 (CDV) HDTV 9 Gb/Hour MPEG-4 (CDV) HDTV 5 Gb/Hour
Technology
Bandwidth Requirement
3 Gb/Hour
Compressed
Digital Video (CDV) is comparable to using MP3 files
for music.
It makes the file smaller without significantly damaging
it (at least
we hope so), so that it takes less time to download
(less
bandwidth), and more songs can be stored on your player.
Similarly,
CDV reduces the signal bandwidth requirements of
a TV
signal to enable its transmission via the Internet, DVD,
cable, or
satellite. This is necessary because an
uncompressed
video
signal is very large. High definition
(HDTV) files are
much
larger than standard definition files.
The
early use of CDV (for standard definition programming) was pioneered by the satellite
TV companies, DirecTV and the Dish Network.
Signal
compression is the process of discarding data for that information that would
be invisible to the viewer. However, too much compression will significantly
distort the output. The higher the level of CDV, the more data that must be
discarded. Discard too much, and the
distortion is very noticeable.
CDV is a
tradeoff between file size and video quality. CDV operates on pixel
neighborhoods, called a macroblock. The compression algorithm records only the
changes within those blocks. This works well if the video is primarily static
images like text.
In a
basketball game, when
The most
common CDV standards are authored by the Moving Picture Experts Group (MPEG).
Applying CDV to video is an intensive computer operation, but uncompressing
(undoing the CDV) MPEG data to video is much less difficult. A C-Cube real-time MPEG-1 video encoder
using 8 custom-designed video processor chips sells for $120,000.
MPEG
standards are not owned by anyone. Various companies and researchers have MPEG
patents, but one does not have to pay a license fee to use MPEG
technology.
DVD and
Blu-ray discs have CDV used during the mastering process (Blu-ray has enough
disc capacity that the CDV is not aggressive, thus does not add significant
distortion). High bitrate video
algorithms with little CDV exist for video post-production work, but these
still create an enormous file, and for this reason are seldom used for the
distribution of video.
There
are several MPEG standards being used but only three are in general use for
commercial video distribution:
MPEG-1
MPEG-1
was the first widely used video (and audio) CDV standard. MP3 is an
abbreviation of "MPEG-1 audio layer 3". MPEG-1
supports a video resolution of 352 x 240 at 30 FPS (frames per
second). It is the standard for the CDV
of moving pictures (and audio). This standard has roots in CD-ROM video
applications, and is a popular for video found on the Internet (.mpg files).
MPEG-2
MPEG-2
can support video resolutions of 720 x 480 and 1280 x 720, at 60 FPS. MPEG-2 is
capable for both standard definition TV and HDTV. The MPEG-2 standard is also used to store
data on a DVD. This is the algorithm
used by DirecTV and the Dish Network.
MPEG-2
is the standard on which Digital Television set top boxes and DVD CDV is based.
It is based on MPEG-1, but modified for use by digital broadcast television.
Different algorithms have been developed and have been integrated into the
standard. MPEG-2 is well suited to HDTV resolution and bit rates. With this technique, CDV can compress video
data by 15 to 25 times while still producing a picture quality that is
generally considered high quality.
MPEG-4
This
standard is the future CDV standard that will almost certainly be used by most
services delivering HDTV. This includes
AT&T, DirecTV, and the Dish Network.
A number of cable operators have also started MPEG-4 trials. This
standard was originally designed for the Internet. Individual objects within a
scene are tracked separately. This is
very efficient.
The move
to MPEG-4 by the satellite companies is not designed to improve picture
quality, but because this algorithm is much more efficient. An MPEG-2 HDTV
channel utilizes the bandwidth of six standard definition TV channels; An
MPEG-4 HDTV channel would only consume the bandwidth of three standard
definition TV channels. MPEG-4 uses half
the bandwidth of MPEG-2, and so your service provider will be able to offer
twice the number of channels.
OpenCable
Application Platform (OCAP)
Consumers
are now able to purchase their own cable box/DVR combos from electronics
retailers. OCAP is a way for cable operators and other service providers to
deploy interactive services like a PC.
This will enable your American Idol voter to cast their vote with the
remote control for their TV.
Consumers
want their HDTV, and service providers who can supply the programming will
thrive. MPEG-4 is the best and most cost-efficient option for cable operators
and other service providers.