Film-Tech Forum ARCHIVE: RGB vs. Y,Pr,Pb vs. component

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Topic: RGB vs. Y,Pr,Pb vs. component

Frank Angel
Film God

Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999

posted 07-12-2005 05:28 AM

Can someone with more than my very cursory understanding of video please explain the difference between these two types of signals and the correct use of the term component with regards to them?

We had to set up a Proxima video projector to play a DVD behind a rock band. The DVD player had three video outputs that one of the technicians kept referring to as "component" and proceeded to hookup a cable between them and the "component" inputs of the projector, only to find that the only thing the projector put out was the green image. The cable was one of those that were called "component" and marked with red, green and blue identifiers on each plug on each end. The guy at Radio Shack referred to this as a "Red, Green, Blue Component cable.

After fiddling around for an hour and only getting a green image when RGB on the projector was selected, we tried the DVD's composite output and got full color. We jumped to the conclusion that the RGB (or component) output of the DVD player had to be faulty. A call to the rental house tech and we were told that no, the output of the DVD is fine -- it is not RGB but Y,Pr,Pb and the projector just has to be told that this is what is being fed to it, not RGB as we had originally selected. We were also told that RGB component is not just three signals, Red, Green, Blue, but also needs two other cables, i.e., a five circuit cable. Once we set the projector for a Y,Pr,Pb signal, it was fine.

So here's the question:, on all the consumer stuff they are always talking about component and RGB as if they are interchangeable, yet these inputs on consumer equipment are only three cables so I am assuming from what we were told, they are not RGB at all as laymen refer to them, but they are Y,Pr,Pb, correct? So then is component a generic term that can refer to either RGB and Y,Pr,Pb? And since none of the consumer equipment I've seen actually has FIVE connectors for an RGB (plus chromiance and luminance?) signal, I am assuming that consumer stuff doesn't have real RGB outputs or inputs, but only Y,Pr,Pb.

I am assuming Y,Pr,Pb stands for yellow, and a sum/minus of blue and yellow for green, and then red? Believe me, that is just an out-of-my-butt guess, so any enlightenment would be welcome.

Thanks in advance.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 07-12-2005 06:57 AM

RGB is RGB...if you look at the R signal it is the RED portion...likewise for Blue and Green. In order to display an RGB signal you need a sync signal. This can be done by having the sync signal on the "Green" terminal and is often referred to as "Sync on Green" or sometimes "SOG". Another way of showing that is to write it as RGsB. Heck, some RGB singals are RsGsBs where there is a sync on all colors.

However, most "RGB" sources are now 5-wire as your rental house described. That is, sync is kept off the color signals such that there is a Horizontal and Vertical sync. This sometimes termed "5-wire" or "RGBHV". About 10-years ago they combined the syncs and it was only 4-wire "RGBS".

"Component" is a 3-wire process. It can be called Y, Pb, Pr or Y, B-Y, R-Y or even "YUV." All are component type signals. Unfortunetly, cable manufacturers make the first three coaxes in a in the colors Red Green Blue so it is easy to get confused.

The "Y" portion is the luminance signal and is normally assigned to the green cable though yellow would be more appropriate. When I have the ability to do so, I do color the Y terminal on a jack field Yellow in a component system to tell at a glance that it is Component, not RGB. Note, if you just hook up the "Y" terminal in a component system, you will have a B&W picture.

The color information is contained on the other two signals. Think of it as Dolby Prologic for video. By using the luminance singal and subtracting portions from it, one can derive the rest of the color space.

Component is not as good as RGBHV but it does do a better job than say S-Video where all of the color information is contained in the "Chromanance" signal.

Note too...the voltages are not equal across the board as with RGB (.7V). Component has the luminance at .7V but the color portion at .3V and is subtractive. Thus, when you hooked it up initially, all that the projector saw from an "RGB" signal was Green, and that is what you got.

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