I ran an initial grey scale calibration on my G70 and I'm seeing a hump in Blue around 40 IRE. 20 and 100 IRE are spot on.

I have a Radeon 9800 Pro running with PowerStrip.

Is this the sort of thing gamma correction using a HTPC would help with?

If there is a "how to" thread can someone point me in the right direction?

Blue hump indicates you're overdriving the blue.

Blue CRT phosphor apparently has less light-emitting capacity than R & G. If you run the contrast high enough, you'll drive the blue into a "non-linear" range -- where increasing the input by x% no longer increases the output by x%, but by maybe 60-80% of x%. (That's a simplification, since CRTs don't respond linearly anyway, but hopefully you get the idea.) If you set your color temp at a high IRE, you'll set the blue at the level it takes to get the right temp at that IRE -- but since you're overdriving it to get there, that means the blue is too high at the lower IREs.

(BTW I usually set the temp at 80 IRE instead of 100. 100IRE is not all that common in actual video material. You'll see a lot more 80IRE and I figure it's more important for the colors to be spot-on there, rather than in full-on conditions that are rarely seen outside a test pattern. I think you'll also get more accurate color temps across the IRE range that way, but that's a guess.)

One solution to this blue saturation is to lower the contrast or gain until you get out of the blue-overdrive range. But that might be a lot darker than you want. I just measured a G70 last week and it seemed the blue saturated at a pretty low level.

The other solution is to defocus the blue. Have you ever noticed that, when you focus the blue, you can tell where it's sharpest because it gets *dimmer*? I'm not sure why R & G don't do that, but blue does. So by defocusing the blue you can get higher light output, which means you don't have to overdrive it so much to get the right color temp at 80-100 IRE, which means the blue doesn't run as hot in the lower IREs.

Defocusing the blue won't hurt your PQ. Your eye can't focus very well on blue anyway. You won't notice the defocus except maybe with test patterns.

How to defocus? Some projectors (like the NEC XG) have a "blue defocus" mode that does this automatically. I don't know my way around a G70 -- the one I measured last week was the first one I've played with. I found a "blue defocus" but it didn't seem to defocus nearly enough to flatten out the blue hump. I ended up changing the blue "MAG -- ALL" setting (which controls the full-screen blue focus) from 123 to 153. That got rid of most of the blue hump. I'm not sure if that's the right way to do it on a G70, but it seemed to do the job. (If anybody knows the RIGHT way to do it, I'd love to hear it!)

Maybe Powerstrip can control the output curve of individual colors, so you could flatten out the blue without defocusing it. I dunno, I've never used an HTPC. But defocusing the blue is the "traditional" way of handling the blue hump.

Gary

garyfritz,

I'm sincere when i say this, this is one of the best comprehensive posts on the subject I've seen. A definite bookmark for me. :)

I followed the manual and set blue at max and adjusted red and green around blue. So you are probably right, I'm over driving blue. I'll just need to spend more time at it.

Oh by the way the G70 will defocus both Red and Blue depending on the horizontal frequency. The higher the frequency the less that de-focus is applied. At my resolution 1280-720@72Hz I believe none is applied.

Blue hump indicates you're overdriving the blue.

Blue CRT phosphor apparently has less light-emitting capacity than R & G. If you run the contrast high enough, you'll drive the blue into a "non-linear" range -- where increasing the input by x% no longer increases the output by x%, but by maybe 60-80% of x%. (That's a simplification, since CRTs don't respond linearly anyway, but hopefully you get the idea.) If you set your color temp at a high IRE, you'll set the blue at the level it takes to get the right temp at that IRE -- but since you're overdriving it to get there, that means the blue is too high at the lower IREs.

(BTW I usually set the temp at 80 IRE instead of 100. 100IRE is not all that common in actual video material. You'll see a lot more 80IRE and I figure it's more important for the colors to be spot-on there, rather than in full-on conditions that are rarely seen outside a test pattern. I think you'll also get more accurate color temps across the IRE range that way, but that's a guess.)

One solution to this is to lower the contrast or gain until you get out of the blue-overdrive range. But that might be a lot darker than you want. I just measured a G70 last week and it seemed the blue saturated at a pretty low level.

The other solution is to defocus the blue. Have you ever noticed that, when you focus the blue, you can tell where it's sharpest because it gets *dimmer*? I'm not sure why R & G don't do that, but blue does. So by defocusing the blue you can get higher light output, which means you don't have to overdrive it so much to get the right color temp at 80-100 IRE, which means the blue doesn't run as hot in the lower IREs.

Defocusing the blue won't hurt your PQ. Your eye can't focus on blue very well anyway. You won't notice the defocus except maybe with test patterns.

How to defocus? Some projectors (like the NEC XG) have a "blue defocus" mode that does this automatically. I don't know my way around a G70 -- the one I measured last week was the first one I've played with. I found a "blue defocus" but it didn't seem to defocus nearly enough to flatten out the blue hump. I ended up changing the blue "MAG -- ALL" setting (which controls the full-screen blue focus) from 123 to 153. That got rid of most of the blue hump. I'm not sure if that's the right way to do it on a G70, but it seemed to do the job. (If anybody knows the RIGHT way to do it, I'd love to hear it!)

Maybe Powerstrip can control the output curve of individual colors, so you could flatten out the blue without defocusing it. I dunno, I've never used an HTPC. But defocusing the blue is the "traditional" way of handling the blue hump.

Gary

I'm sincere when i say this, this is one of the best comprehensive posts on the subject I've seen. A definite bookmark for me. :) Now yer gonna make me blush. :D I'm no expert on the subject -- this is just what I've figured out in bits and pieces over the last few years. If you hear different from somebody (Terry, Ken, etc) who actually knows what they're talking about, ignore what I said!!

I followed the manual and set blue at max and adjusted red and green around blue. So you are probably right, I'm over driving blue.From what Terry says, I think that's OK and appropriate for the G70. It doesn't change the fact that in order to get 12 ftL to a 90" wide screen, you have to drive the tubes hard enough that the blue will be overdriving, no matter what your gain/contrast is set at. That's just the way blue CRTs work.

Gary

Setting temp at 80 IRE as opposed to 100 IRE is certainly a nice tidbit I'm glad I read.