Rebuilt 15GP22's?

[Pete Deksnis]

Sounds like a nightmare to me. They almost certainly would use 'new' phosphors. The new tube would be driven by a CT-100 designed for the 'old' phosphors. So you'd have to modify the matrix to match the 'new' phosphors. Doubt that you'd ever match the '53 NTSC colors. 'course, it'd be better than a universe of dark 1954 15-in. roundies, I guess...

[old_tv_nut]

Originally Posted by Pete Deksnis

Sounds like a nightmare to me. They almost certainly would use 'new' phosphors. The new tube would be driven by a CT-100 designed for the 'old' phosphors. So you'd have to modify the matrix to match the 'new' phosphors. Doubt that you'd ever match the '53 NTSC colors. 'course, it'd be better than a universe of dark 1954 15-in. roundies, I guess...

You're right about the color. Some early rear-projection TVs went to P1 green because the modern sulfide green saturates at high beam current, giving magenta highlights. However, the modern matrix with P1 green makes flesh tone too sensitive to hue variations. The fix could have been to go back to the NTSC matrix, but usually it was to introduce a rare-earth/P1 or other mixture that matched the sulfide green. If you put a modern green in a CT-100, you will have the opposite effect - greens, yellows, oranges and flesh tones will move closer together and toward red, and the hue variation over this range will be less than nominal. The major tweak to the matrix for sulfide green would be to increase the R-Y drive, but there are related minor tweaks, so all the matrix resistors would require a change.

By the way, modern blue phosphors are also considerably off the mark from the NTSC phosphors (more towards violet, which moves yellows more towards green, since yellow has to be complementary to the blue to make the proper white). This means that modern sets have improved saturation in the magenta region compared to the NTSC gamut.

Such matrix corrections are always approximate due to being in the wrong place in the system for the gamma characteristic of the picture tube. In modern NTSC sets, this results in reds being overly bright (and cyans being dark) when the hue variation is corrected.

All this explains why HDTV colors are more accurate than current NTSC, even though the green area is more restricted than true NTSC - the HD cameras and receivers actually have color characteristics that are matched to each other, producing colors that are the correct saturation, brightness and hue. PAL has had this since they started, because they standardized on the newer phosphors for their primaries.