CurtPalme.com Home Theater Forum

[RogerH]

Would it be considered safe to rely on edge blanking to protect the sides of the CRT face from thermal stress due to image/raster overscan?

I am thinking of going to a constant 16 x 9 aspect ratio by creating a second memory that is scaled-up and edge-blanked specifically for 2.40 aspect movies, effectively cropping the sides to keep a constant image height for all movies on my 16 x 9 screen.

My XG-852 is set up closer than spec (intentionally) to the screen and currently has safe margins around the rasters to prevent running off the CRT face. I would have to see where my sides end up on the crt face if I scale a 2.40 movie to fill my screen vertically, but I expect there would be some overrun or at least uncomfortable margin.

It seems that constant aspect ratios have been discussed at length, but I can't seem to find much on the CRT-specific question of relying on edge blanking to protect against potential thermal cracking due to beam runoff. I guess the beam could still be scanning off edge, but I am hoping that if it is blanked there would be no significant heating energy to stress the glass.

My Blu-Ray player (Panasonic BMP-BD85K) seems like it should do this little function for me with the Zoom or Edge Crop functions, which I would think would let me fill the 16x9 screen vertically when showing wider-aspect movies. But these settings seem to make no difference on any movies that I currently have (and the manual says they may have no effect, with no explanation why or when).

Would this be a risky idea?

[virusc]

to a point yes. Their still will be beam current striking close or on tube side surface but at near 0 current. You still could crack tube due to heat and glass expansion though and geometry and convergence components are also overstressed.

[RogerH]

Crap.

I completely forgot about the H convergence and sweep circuits running overboard. I guess that in itself is enough reason to not try it.

Well, I guess I will stay with variable aspect. If and when uneven CRT wear from showing too many wide aspect movies shows up on the 16 x 9 display, I can then decide if constant 2.4:1 aspect is worth it.

Thanks.

[zGman]

Seems like I recall there was an issue with the tube necks
cracking with over-scan on NEC. They use a very thin
neck which leaves no margin for beam clearance.

G

[ecrabb]

Why would you want to do this, anyway? I can't imagine chopping off nearly a full third of the original scope image just to eliminate some black bars. That's a lot of the film frame just gone! It would seriously screw up a lot of really nice scene compositions.

SC

[RogerH]

I gave away my reason in my followup post above. It comes down to making the best choice of what size/shape/aspect ratio that I want my essentially new tubes to wear. I didn't want to start burning in a 2.35 aspect ratio and decide later that a 16 x 9 might have been better.

With my Faroudja as a source I used to set an average gray level in the black areas of non-16 x 9 material, but it will no longer be in the loop, and I still want to keep the wear uniform.

[virusc]

get a video processor that allows you to move image vertically or use raster shift and move the image up and down10% every other movie.

[ecrabb]