CurtPalme.com Home Theater Forum

deronmoped · Joined: 03 Nov 2006 Posts: 1154 Location: San Diego

Which side of the screen is the front porch? Does the front porch side of the image change with the type of display you have? Where would the FP on these displays be when viewing the final image on the screen?

1) CRT monitor.

2) CRT RPTV.

3) CRT PJ.

Thanks, Deron.

**kal** · Posted: Fri Feb 22, 2008 7:38 pm Post subject:

Front/back porch are opposite to what you'd think:

The front porch is the end of the scan, before the electron beam comes around for another pass.
The back porch is at the start of the image, right before the electron beam starts painting the image.

Kal

Ile · Posted: Fri Feb 22, 2008 7:49 pm Post subject:

http://zone.ni.com/cms/images/devzone/tut/a/b2ae74b4554.gif

deronmoped · Joined: 03 Nov 2006 Posts: 1154 Location: San Diego

So if you are looking at a image on your screen and on the left side is some banding, you would want to increase the_____ porch settings to get rid of the banding.

Deron.

blue_z · Joined: 27 Apr 2006 Posts: 63 Location: So Calif

**AnalogRocks** · Posted: Sat Feb 23, 2008 6:30 am Post subject:

Check out this thread: https://www.curtpalme.com/forum_archived/viewtopic.php@t=1710.html

Kai has posted a couple of interesting links.

deronmoped · Joined: 03 Nov 2006 Posts: 1154 Location: San Diego

I was looking at the link Kai posted and it explains the FP and BP real good.

To answer my questions: The front porch starts at the right side of the screen, then you have the sync pulse and then the back porch.

Now if you have banding on the left side of the screen, it looks like too small of a back porch would be most likely to cause that.

It looks like there is almost no need for a front porch.

The sync pulse looks like it is there so the monitor has the proper amount of time to move the scan line down.

Can anyone add their experiences to this? Like what they have found out using different settings for their FP, BP and SW.

Thanks, Deron.

tse · Joined: 03 May 2006 Posts: 1014 Location: Sweatbucket, Fl.

With good old composite video (NTSC) the back porch had to be long enough for the color burst. Three wire RGB had to have a back porch that was long enough for the black level clamp which is called back porch clamp. Like you said the front porch doesn't seem to do alot. Usually video is set up with short front porch, medium sync width, and medium back porch time.

Scott

stefuel · Posted: Mon Feb 25, 2008 1:15 am Post subject:

I'm curious why we CRT front projectionists see such a difference with consumer source devices. Is it because we tend to use 95% of our tubes phospor and very small overscan settings? Consumer display devices don't seem to suffer this problem. If I pop a test pattern DVD in a direct view tv will it show a 10 or more overscan? I usually set my projectors up with a 2-3 overscan.

deronmoped · Joined: 03 Nov 2006 Posts: 1154 Location: San Diego

deronmoped · Joined: 03 Nov 2006 Posts: 1154 Location: San Diego

As for setting up for minimum use of the horizontal porches, I'm thinking that you can go with like in the single digits for the front porch.

The back porch can be set by using 8 to start with, project a image, measure the banding you will get on the left of the screen, take the percentage of the horizontal resolution that the banding makes up and add that to your BP settings. For example, say you have a 100" screen, a resolution of 1000 horizontal pixels and a banding problem of 10" on the screen. That means the banding problem takes up 100 pixels, add that to your horizontal BP and that gives you 108 add 4 to that so you come up with a number divisible by 8. You may even want to add a few more pixels to the BP just to be on the safe side, so you end up with 120 for a minimum BP without the banding problem.

Sync width, I have no idea how you can figure that out, anyone know how?

Deron.

tse · Joined: 03 May 2006 Posts: 1014 Location: Sweatbucket, Fl.

There is a definate requirement with CRT projectors that the horizontal blanking period (front porch + sync +back porch) be long enough to cover the horizontal deflection retrace time. With a magnetic deflection system this means the retrace time will be 15 to 25% of the total line time. 1/horizontal frequency = total line time. The digital displays don't need this as there is no physical retrace involved. The unfortunate thing about needing so much time for retrace is that it puts the picture time in a smaller portion and that raises the required bandwidth for video so there is a benefit from making the blanking time short. So leave generous blanking time for your CRT.

With Marquee projectors leave more than 0.5uS for your horizontal sync time.

Scott

deronmoped · Joined: 03 Nov 2006 Posts: 1154 Location: San Diego

"The unfortunate thing about needing so much time for retrace is that it puts the picture time in a smaller portion and that raises the required bandwidth for video so there is a benefit from making the blanking time short. So leave generous blanking time for your CRT".

Scott, are you contradicting yourself here?

15% to 25% of total line time is quite a bit, I wonder if there are any other ways of getting around this besides cutting the porch and sync times.

Deron.

tse · Joined: 03 May 2006 Posts: 1014 Location: Sweatbucket, Fl.

It takes that much time to move the beam from the right side back to the left for the next line. Leave yourself plenty of time so you can phase the active video over to the right enough to avoid the ringing.

Scott