Different Sources

The Video Signal

"The composite video signal: The Luminance and Chrominance travel at the same time through the same cable.

· One sole cable is needed (a Cinch plug, otherwise known as an RCA plug, is found at each extremity). It may also travel through a "Scart" cable.
·VHS and 8 mm cassettes and even Laserdiscs are encoded in composite video.
-The limitations: come from all the signals combined, making it hard to attain good quality. The luminance and chrominance sometimes have a tendency to overlap each other, which is what gives the "grill" effect on NTSC. Let’s not go into too much detail on a signal that can’t give the results we want.

The Y/C signal: The luminance and chrominance are separated, and each flows in its own cable.

· A cable composed of two conductors and an earth wire are necessary (a specific connection called a USHIDEN is found at each extremity). It is also able to travel through a Scart cable.
· S-VHS and Hi8 cassettes are encoded with separate luminance and chrominance. On the video tape, a specific place is reserved for each of the two components.
- The limits: the Y/C signal is the logical evolution of a composite signal. The fact that the Y and C signals are separated allows for the passing of more information, and hence to increase the definition of the video image. It’s a little like having designed a motorway with a special lane for heavy vehicles, and another for cars. The risks of major collisions would be greatly limited and the traffic would flow much better.

The RGB signal (Red, Green and Blue): Broken down into the primary colours, the chrominance is entirely decoded and thus no longer is in PAL, NTSC nor SECAM colour format.

· The three primary RGB colours are transported in three different cables, while a separate cable transports the synchronisation information. On the other hand, each colour is mixed with the luminance signal, which retains its own identity of 525 or 625 lines and 50 or 60 Hertz.
· Four cables are required, but the signals can also pass through a Scart plug if it is properly wired. It’s true that depending upon the price of the Scart plug, sometimes only the composite video and stereo audio are cabled. Be sure to choose a 21-pin Scart plug in this case.
- The limitations: The RGB signal allows us to have exceptional colours, but the luminance and chrominance remain combined. In addition, you need four connecting cables, and there’s still no basic coding system generally accepted for the different video standards available to the general public.

The Component Signal: This is an excellent compromise between the advantages of the Y/C signal and those of RGB.

· The luminance is separated from the chrominance, the latter being composed only of red and blue signals. A very simple calculation allows, by subtracting the Red and Green from the luminance, to rediscover the Green value. This is why the system is called Y R-Y B-Y (Luminance, Red minus Luminance, and Blue minus Luminance). CQFD. In professional circles, it’s also called Y U V. Three cables with Cinch plugs at their extremities are necessary for carrying the signal.

Why the Component Signal?
In fact, the principle itself of MPEG video encoding is based on the component signal. All digital satellites (i.e. BSKYB digital) are encoded in MPEG video, as well as the DVD signal. It is thus a shame to have a composite video signal coming out of a DVD player when the original signal is a component signal. The best solution remains that of using an RGB connection. This is why satellite demodulators have an RGB output in their Scart plug. For the moment, only top-line DVD players have component outputs.

What has component inputs?
For the moment, only certain video projectors and line doublers have such inputs.
It’s worth noting that the latest SHARP XV-Z1 video projector is thus equipped, and TOSHIBA is marketing a rear-screen projector in the USA that has such inputs. Shortly, televisions will all be equipped with this type of input, and in the relatively near future, in five to ten years from now, audio/video amplifiers will also have component inputs.

Important: The fact of putting out a YC or an RGB signal from a Laserdisc player will do nothing to improve the image. It’s sure that the code translation in a signal of better quality tends to embellish the image in some cases, but one shouldn’t lose sight of the fact that the original signal is in composite video. "

Related articles

Tutorials