I was wondering, if it is possible to modify these old televisions to have component video input. When I see flowcharts that show how NTSC is decoded and always see it going to Y, R-Y, B-Y, I can't help but notice that these old TVs seem to use component video internally, since component video is Y, Pr, Pb, which is basically the same.

Is this doable, or is this not as simple as I think it is?

Generally do-able, but you need to use a bit of caution.

Essentially, you would inject the composite video in just after the video detector stage. Couple it in through a capacitor, to keep any DC present from damaging your DVD player or whatever.

Unless you use some kind of isolation circuitry in the video line (wideband transformer or optical isolator), DO NOT attempt this modification on a series string "AC/DC" type chassis.

You asked about component, not composite - so, the answer depends on the particular set circuitry. You would inject the Y signal the same way as injecting a composite signal - following the video detector stage. Besides isolation, you may need some amplification if the normal video detector output is greater than 1 V pp.

Doing the Pr and Pb would depend on having appropriate R-Y and B-Y points. You could probably do this in most tube sets that use either R-Y/B-Y or X/Z demods, but you may need some amplification and/or adjustable levels to get the color saturation normal.

A set using separate R-Y, G-Y, B-Y demods (usually diodes) would not take the Pr, Pb directly unless you are willing to accept strange results from not having G-Y.

As N2IXK said, do NOT try this on a hot chassis set.

[Edit - we're talking tube sets or solid state with discrete components. Later sets that used IC demodulators may not have any accessible point to inject Pr and Pb.]

Hmm, I didn't know there was a G-Y.. On my research from a few years ago, this is basically the NTSC flow chart I drew up. I thought it was accurate.

http://img.photobucket.com/albums/v395/Evilweredragon/video-1.png

Am I wrong?

"Am I wrong?"
Depends on the set. Color tube sets usually have three cathodes and three grids thus often the three cathodes are tied to the Y signal and the grids tied to R-Y,G-Y,and B-Y respectively. The Y signal is composed of a mix of RGB so if you have the R-Y and B-Y you can add them then subtract their sum from Y to get G-Y(which is what most tube R-Y/B-Y sets did).

But I believe that some solid state sets just converted to pure RGB and fed only three video signals to the CRT due to having only one CRT grid in most inline tubes.

And if I remember correctly Y is a weighted grayscale, for example, green is brightest, followed by red, followed by blue.

I tried doing this with a Philco 17MT80, but there are some issues.

First, there is a luma delay in sets to make up for the time shift associated with processing the color signal. It must be matched to each set because each decoder is unique, or the color information will not correctly overlay the luma in the picture- it will be offset to one side or the other.

Because of this if you inject Y after the detector and R-Y/B-Y after the decoder, you must also make a custom Y delay so the signals coincide at the CRT.

To add to the problem, there seemed to be a big difference in the level of signal coming out of my DVD player. Red was a much stronger signal than blue was, which meant there will be some amplification required in order to get the colors correct. But if you do that, there must also be another delay line (on the red signal) because the amplification of the blue information will cause it to arrive slightly later than red if you don't.

So as you can see, it's not just a simple matter of hooking up some cables. Not that it couldn't be done, just that it would be a lot more involved than you first think.

Hmm, I didn't know there was a G-Y.. On my research from a few years ago, this is basically the NTSC flow chart I drew up. I thought it was accurate.

http://img.photobucket.com/albums/v395/Evilweredragon/video-1.png

Am I wrong?

You're not showing an internal part of the matrix circuit that derives (G-Y) from proportions of -(R-Y) and -(B-Y).

Think about this example - to produce a full green color (G=100%), you have 59% from the Y signal going to all three outputs R, G, and B. In this case, both (R-Y) and (B-Y) equal minus 59%, so the R and B outputs go to zereo. But you still need a (G-Y) of 41% to add to Y and make G=100%.

Without the (G-Y), greens will look too dark.

Now suppose the transmitted color was magenta. In this case Y= 41%, R and B will come out to 100% as they should, but without (G-Y) of -41% to make G = 0, G will still be 41% and the magenta will be too bright and pale.