PICTURE FORMATS

There are 18 formats embraced by the ATSC system. The most common ones appear below:

480i

Equivalent to analog TV, and therefore sometimes called "compatibility mode." In the analog world, TV is said to have 525 horizontal lines, but 45 of them are used for a vertical sync interval, leaving 480 visible lines. Since the aspect ratio (horizontal to vertical dimensions) is 4:3 for 480i, there are 640 pixels in each line. Analog TV, however, rarely is able to manifest that quality of resolution after passing through the vestigial sideband filters of a TV transmitter.*

The "i" stands for Interlacing, because 480i only paints 30 complete frames per second and does so by first painting every even numbered line and then every odd numbered line. The intent is to avoid flicker by having 60 frames per second, even though each of those frames is incomplete.

480p

This is identical to 480i except that there is no interlacing. 60 full frames are painted per second. "Progressive scan" DVD players intended for HDTV displays produce this format; some also can produce some of the HD formats listed below.

720p

This is a HDTV standard comprised of 720 pixels vertically and 1280 pixels horizontally. That is a 16:9, "wide screen" aspect ratio, 60 full frames per second.

1080i

This HDTV standard is comprised of 1080 pixels vertically and 1920 pixels horizontally. This format is only 30 frames/sec, however, to keep the bandwidth within the allowed spectrum. Interlacing is applied in the receiver to eliminate flicker. It has the same 16:9 "wide screen" aspect ratio as 720p.

1080p

This format is regarded as the "holy grail " of digital TV, because it provides the resolution of 1080i and the frame rate of 720p. It is not part of the ATSC specification and is not expected to be available for broadcast because of its huge bandwidth demands. It is availble on Blu-ray disk players, however, so if you want to be ready for the next round of TV innovation, look for a HDTV set with an ability to display 1080p.

COMPUTER MODES

A comparison of digital TV display modes with the standard modes used on computer displays may be helpful. Only 720p and 1080i/p are wide screen:

*Analog TV's picture signal is amplitude modulated in a 6 MHz channel with a a carrier 1.25 MHz above the lower edge. The audio signal, centered 4.5 MHz above the video carrier, establishes the upper limit for all video information, so 4.25 MHz was about the maximum video bandwidth possible for broadcast black-and-white TV. When color was added, picture resolution dropped due to the addition of color information centered at about 3.6 MHz above the video carrier. To make color broadcasting work with black-and-white sets, the original monochrome video was maintained, becomming known as the "luminance" part of the picture video signal. The color information modifed the luminance signal to provide color. The result was a further reduction in resolution to make room for the color (called "chroma") signal. When all this is taken into account, the horizontal resolution of broadcast 480i is barely over 400 pixels rather than the 680 that the 480i format allows. But color resolution is much worse because the signal is very severely bandwidth starved. First, the color signal is split into two "quadrature" components, each of which is band limited differently. These are then de-multiplexed into the green-red-blue color primaries so that the colors can be displayed. To fully appreciate the severe color bandwidth limiting on analog TV (as bad as 80 pixels horizontally), see if you can find a picture with red lettering on a green background or vice-versa: it will be so blurry that it will be unreadable (content providers minimize these problems with care in set decoration and costume design). Also, look for color smudging between objects in the picture. These problem necessarily occur whenever video passes through analog modulation and de-modulation.

The poor horizontal resolution and picture artifacts of broadcast 480i TV can't be improved because they result from the NTSC video standard itself, but you can take steps to reduce these problems when passing video between the components of your entertainment system. Here is the pecking order, from worst to best, of the various methods used to pass signals from various sources to your TV:

TV Tuner (coax and Channel 3)

Picture horizontal resolution limited to around 400 pixels, color much worse, because the video and audio must be modulated and demodulated.

Composite Cable

Depending on the signal source, better picture detail is possible because the bandwidth limitations of the 6 MHz TV channel do not apply. Color resolution does not improve, however.

S-Video Cable

This breaks the luminance and color information into separate cables. Picture detail improves because there is no color information on the video signal. Color information improves because the bandwidth limits on color no longer apply.

Component Cable

The three primary colors are essentially passed separately to the video monitor. Horizonal resolution is limited only by the bandwidth limits of the source and the monitor itself, whichever is lower.

HDMI Cable

This doesn't really belong in the list because it is a digital transmission medium. Where available, HDMI is preferred because all information is transferred on only one cable and the digital transfer means that there is no information lost.

Digital TV

The term "Digital TV" can be applied to so many configurations that it can only be understood as "non-analog" TV. It includes any system that transmits TV images as a series of numeric values rather than as a continuum of voltage or current. As such, it demands different "modulation" methods (ways to put information on a radio or cable channel) than analog TV.

Analog TV has been the standard in the USA for decades. Technically called "NTSC", it was actually quite an accomplishment for the days of vacuum tubes. In a 6-MHz span of RF bandwidth (one third the size of the whole FM band), broadcasters could put picture and sound on a single signal -- and later would add color, stereo, captions, secondary audio programs, and other information. It was a process with many technical shortcomings, but it was good enough for a whole industry to emerge.

NTSC's days are numbered, however. All analog TV will stop broadcasting on June 12, 2009. In the end, it was not the technical shortcomings (poor resolution, poor tolerance to a number of transmission problems, low chroma bandwidth, etc.) that would bring NTSC down. It was its terrible inefficiency. The RF spectrum is too valuable an asset to have so much of it dedicated to an inefficient broadcast technology that could barely manage a "480i" picture in a 6-MHz band. And there are many private and government services coveting that bandwidth for other uses.

Because of the demand for more channels, higher quality movies and high-definition, Lucas Valley Cable has been adding digital TV options since the Spring of 2004 when we introduced two completely separate digital formats on the cable, each requiring different processing at your TV. You are free to choose either, both, or none at all. The following table is intended to help distinguish the two options: