The most common form of video wears the designation NTSC, which stands for National Television Standards Committee, an industry organization formed in the early 1950s to create a single signal standard for color television. At the time, CBS had been broadcasting for over a year with an electro-mechanical color system that essentially spun a color wheel in front of the camera and a matching wheel in front of the monitor. RCA, owner of rival NBC, proposed an all-electronic alternative. The RCA system had the advantage that it was backwardly compatible with black and white television sets while the CBS system was not. The NTSC was formed chiefly to put an impartial stamp of approval on the RCA system.
The RCA system was the prototype for all color television today. Each pixel gets scanned in each of the three primary colors. Although studio equipment may pass along the three colors separately like the RGB signals in PCs, for broadcast they are combined with synchronizing signals to create NTSC video.
The magic is in the combining process. For three independent variables-the three colors-they needed to have three separate signals but not necessarily the three original signals. By transforming the signals mathematically, they found a clever way to package them as one.
First came a transformation of color space. For compatibility with monochrome, they combined all three signals. This produced a signal they called luminance, which encoded all the brightness information in the television image. The luminance signal was essentially a monochrome signal and produced an entirely compatible image on black and white television sets. The name of the luminance signal is often abbreviated as Y.
The other two signals they used encoded difference information-the difference between luminance and the red signal and the difference between luminance and the blue signal-which allowed the reconstruction of the original red and blue signals. Subtract red and blue from the luminance signal, and the remainder was green. This method of encoding colors assured monochrome compatibility. In the NTSC system, the difference signals are called I and Q.
The next step the NTSC used was to combine the two difference signals into a single signal that could carry all the color information, one called chrominance (abbreviated as C). Engineers used quadrature modulation to combine the two signals into one. The result was that colors are encoded into the chrominance signal as a phase angle.
Together the luminance and chrominance signals provided a guide to a map of colors, a polar chart. The chrominance encodes the angle between the color and the X-axis of the chart, and the luminance indicates the distance from the origin to the color.
To fit the chrominance signal in where luminance should only fit, engineers resorted to putting chrominance on a subcarrier. That is, they modulated a carrier wave with the chrominance signal, then added it to the luminance signal. Although the subcarrier had must less bandwidth than the main luminance channel, the process was effective because the human eye is less sensitive to color differences than brightness differences.
The NTSC chose a frequency of 3.58 MHz as the color subcarrier frequency. The chrominance is thus an amplitude modulated signal centered at 3.58 MHz. To avoid interference with the luminance signal, the NTSC process eliminates the carrier and lower sideband of the chrominance signal after the modulation process.
The NTSC process has two drawbacks. The luminance signal must be cut off before it reaches 3.58 KHz to avoid its interfering with the subcarrier. This frequency cap limits the highest possible frequencies in the luminance signal, which means that the sharpness of the image is reduced from what it would be when using the full bandwidth (4.5 MHz for the video signal) of the channel. Chrominance carries even less detail.
The basic frame rate of a video signal is about 29.97 per second. Each frame is made from two interlaced fields, so the field rate is 59.94 Hz. Each frame is made from 525 lines, of which about 480 are visible and the rest devoted to vertical retrace. Ideally, a studio image would have about 640 pixels across a line. Black and white television images may be that sharp. However the 3.58 MHz bandwidth imposed by the NTSC color process constrains the luminance signal bandwidth to 400 to 450 pixels horizontally. Although that might sound paltry, a good home VCR may be able to store images with about half that resolution.
The constraints of NTSC color are required because of the need for backward compatibility. The color signal had to fit into exactly the same bandwidth as black and white. In effect, NTSC gives up a bit of black and white resolution to fit in the color information.
Video signals that never make it to the airwaves need not suffer the indignities required by the NTSC broadcast standard. Studio signals have always transcended broadcast standards-studio RGB signals have full bandwidth, high resolution (640-pixel) images in each of their three colors. To raise home viewing quality, VCR designers came up with a way to get more quality in color signals by avoiding the NTSC process.
The part of the NTSC process that most limits visual quality is the squeezing of the color signal onto its subcarrier. By leaving the video in two parts, separate luminance and color signals, the bandwidth limitation can be sidestepped. This form of video is termed S-video, short for separate video. High-end VCRs, camcorders, and monitors often use S-video signals.
Other than not modulating chrominance onto a subcarrier, the color encoding method used by S-Video is identical to that of NTSC. The three RGB color signals are combined into luminance and chrominance using exactly the same formulae. Although you cannot substitute one signal for the other, the innards of S-Video monitors need not be radically different from those of NTSC displays. The level of quality is often quite visibly different. S-video components may have twice the horizontal resolution as composite video.
Note that once a signal is encoded as NTSC, information is irretrievably lost. There's no point to decoding an off-the-air television signal to S-Video. The only time S-Video helps is when the source of the signals has never been NTSC encoded.
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