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CDTV Conventional definition television (CDTV) is the basic, analog television that has been broadcast since 1953 by all of the major networks. Very few technologies survive over 50 years like entertainment technologies. CDTV, also called NTSC and analog color TV, was developed by the National Television Systems Committee. CDTV is transmitted in a 6 MHz band of frequency that the FCC licenses to networks. You can probably receive multiple television stations in your area. A pair of rabbit ears will pick up the TV stations that are broadcast over the following range of frequencies:
The FCC has carved up the frequencies that television and radio stations can broadcast on. Channel 2 broadcasts in the 6 MHz band from 54 to 56 MHz. The other stations take subsequent bands witha 4 MHz gap between channels 4 and 5. Each 6 MHz channel is broken into the following pieces.
The top half of the image shows how the black and white video carrier (Y) is in the first ~ 4 MHz of the channle while the FM stereo signal is sent separately about 250 kHZ below the upper range of the channel. This FM signal on channel 6 is broadcast at about 87.75 MHz and can be picked up by most FM stereos. The basic black and white video signal is defined by RS-170A.
To be backward compatible with old black and white television sets, NTSC squeezed the color information into the space between the black and white video signal and the audio signal. This kludge ushered in the second generation of color television. The solution is not perfect though. You can see high frequency interference in many televisions when small periodic items are in the picture. Certain fabrics (often ties) or venetian blinds cause a rainbow effect on the TV. This is mostly apparent when you watch an old black and white movie on a color TV and see rainbow effects sometimes. Comb filters separate the color signal from the black and white, but they don't work that well many times. For each MHz of bandwidth that the video signal uses, you usually get about 110 pixels of black and white resolution. Normal broadcast signals have about 440 pixels of B+W resolution and thus take up about 4 MHz of bandwidth. VCRs usually only hold about 320 pixels of resolution and thus only have about 3 MHz of bandwidth. The low quality circuitry of VCRs can't record very well and result in the low video quality of VHS tapes. S-Video was supposed to be the cure for the NTSC quality problems. S-Video separates the black and white signal from the color signal and sends them separately. This way, the black and white signal can be sent over a little more bandwidth and provide a higher quality video signal. The color of S-Video can be expanded more as well, but I usually can't see much of a difference between my cheap VCR and an S-Video VCR. Theoretically, S-Video would improve the quality quite a bit, but I don't see much difference. My miniDV camcorder has an S-Video output, but it doesn't improve my video signal very much in my eye. Here's a quick comparison of the differences. Of course these images would be stretched across the television screen, but this shows the images with equal sized pixels.
VHS is the lowest quality that you'll see on most TVs besides home videos recorded on 8 mm cameras.
CDTV is more commonly known as NTSC. It's the basic signal that was defined in 1953! Over 50 years old and its still going strong. How's that for some technology ahead of its time. S-Video usually provides CDTV resolution.
All of these signals have 480 scan lines that define the vertical resolution. Even DVDs are only shipped with this same vertical resolution so that they are compatible with normal TVs. There are actually 525 scan lines, but only 480 are visible. If you can hear a high pitched signal from your TV, that is the hum of sending 525 scan lines 30 times a second. This produces a hum at 15,750 Hz. Within most people's hearing range. You really start to see the advantage of HDTV when you have a TV that sends out either 720 or 1080 scan lines in HDTV. TV |
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