HDTV Bandwidth Management

Many people, especially those with satellite TV receivers, have heard of MPEG. MPEG is a standard for reducing the size of the video signal so that it can be broadcast more easily. This is known as "signal compression". Compressed Digital Video (CDV) is comparable to using MP3 files for music. It makes the file smaller without damaging it, so that it takes less time to download (less bandwidth), and more songs can be stored on your player.

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 files are much larger than standard definition files.

A standard definition DVD movie file size is approximately 3 GB/Hour.
An uncompressed 1280 X 720 (HDTV) movie file size is over 150 GB/Hour.
An uncompressed 1920 X 1080 (HDTV) movie file size is over 350 GB/Hour.
An MPEG-2 (CDV) HDTV movie file size is approximately 9 GB/Hour.
An MPEG-4 (CDV) HDTV movie file size is approximately 5 GB/Hour.

The early use of CDV (for standard definition programming) was pioneered by the satellite TV companies, DirecTV and the Dish Network.

CDV is the process of discarding data for that information that would be invisible to the viewer. However, too much CDV will significantly distort the output. The higher the level of CDV, the more data that is 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 CDV records only the changes within those blocks. This works well if the video is primarily static images like text.

In a basketball game, when Kobe takes it to the basket, more pixels change from one frame to the next. When this happens, the video compression algorithm must work harder. The picture quality after compression depends on the complexity and predictability of the image.

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 standards 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.

About the Author: Brian Bradshaw is a Certified Technical Specialist (InfoComm CTS). Areas of expertise include Video, Audio, Computation, HDTV Satellite Systems, and Communications. He has a communications technology business in Plano, Texas (Dallas). More information can be found at his Website: http://bradshaw-vacuum-technology.com