CurtPalme.com Home Theater Forum

[nathan]

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My PJ has a bit of trouble scanning correctly for 1080p. I was able to pick up a VP50 which I was told could fix the porch settings that aren't adjustable on this PJ directly.

The question is, how is this actually done? I don't actually know exactly what these settings are. I've actually got the picture fitting my screen using the underscan adjustment, but I am thinking this is just using less of the picture.

From another thread:

[gjaky]

For 1080p-60 try:
H front: ~100
H sync: ~120
H back: ~250
These are only estimated numbers, but they should work right away, later you might want to reduce porches and sync to optimize the bandwidth requirement of the signal. You don't have to set the V paramters.

[nathan]

Thank you that worked close enough for starters.

Sounds like I really ought to figure out the meaning behind the settings and experiment intelligently.

[gjaky]

Hello Nate,
It's not that hard to comprehend, I'll try to explain it for you.
The projector draws the picture with electron beams as you may know. The electron beam's position is controlled mainly by the vertical and horizontal deflection yokes -you probably also know this too. After the beam is finishing a scan line horizontally or a full frame vertically, it has to be placed in starting position (begining of a new line, or begining of a new frame). The minimal time that is needed for the electron beam to travel from the end of the line to the begining of a new line is usually found in the given projector's technical specifications under the name horizontal and vertical retrace time.

So if you want to display 1920x1080 pixels at 60Hz refresh you will have to add the retrace times to the resolution as a dead time, to not loose any infromation from the video content, since the projector is not able to project information during the retrace times. This dead time is presented in the video signal by the sync time (tsync), front porch (tfp) and back porch (tbp), and all these together called blanking time (see bellow).



In horizontal plane the ratio of (tsync+tfp) and tbp gives how the image is shifted horzontally (H-phase), these numbers are given in pixel count, so my above mentioned timing is actually producing 2390(=1920+100+120+250) total pixels horizontally, the more the count of total pixels the more bandwidth you need for the signal, as our CRT projectors are extremely bandwidth limited we want to keep down the total pixel count (related to active pixels of course), but as you already found out too little total/active pixel ratio causing horizontal clipping. So your goal is to find the least total pixel count for your projector that still shows all adressed pixels. I bet this will be in the 2400(+/-50) pixel range, that's why I suggested those numbers.

[nathan]

Gjaky, I just realized I never thanked you for this detailed explanation at the time. I read it and immediately ran off to go tweak things and never came back.