Bjoern Roy
07-10-07, 08:57 AM
Hi folks,
in Kenny's recent thread (http://www.avsforum.com/avs-vb/showthread.php?t=870425) about black level problems on his XG, my first thought was 'blanking pedestal level', because this is the ugly step child of the XG series. I have had to correct this problem on dozens of XGs, so this isn't a isolated problem. Actually, i haven't seen a single XG which had this set properly no matter how many hours on the chassis.
Independent of the fact whether this is Kenny’s problem, I think blanking pedestal levels is something not discussed in-depth here yet, so let me give you guys some pointers.
Indicators of the issue:
1.) Common case
You have some funny vertical stripes in your picture which only show up in dark images and a black image. These stripes seem to move when you adjust left and/or right blanking!
Delimitation to other issues:
- Stripes also show up in white field? On the left side mostly? -> Might be: Swing-in of signal. Cause: Projector, Source or a combination of both
- Stripes do not move when adjusting left and right blanking. Heck they even stay in the same place when change width of image through image size! -> Burn in :(
2.) Extreme case
You cannot get your black level low enough. There is always a considerable glow on the tubes after (the hideously painful ;) )G2 calibration. If you watch closely, you might even see some sort of inward bound edges at the top and bottom of the screen. That’s your retrace! Why the heck is this visible here?
Note: The problem having this discussion here is, how much glow is acceptable? What do I mean with ‘considerable’? After proper G2 and black level calibration, there WILL be a glow. Otherwise you crush your blacks. How much is normal? How much is ‘just barely visible’ etc. Don’t have an answer here :(
Solution: Calibrate the blanking pedestal with a scope
The problem with the blanking pedestal is, in my opinion, that the service manual isn't 'real world' enough in its recommendation how to set it. If i remember correctly, it says something like 'Set the blanking pedestal so that it matches the video black +/- 0.xxV'
A) For any sane videophile, this should be '-' only. You do not want to have blanking brighter than your video black, because the blanking level is also the voltage that drives the raster retrace and thus pollutes the image. (See 2 above!)
B) The given range is WAY to small from my experience. I start with at least 10 times the tolerance in mind. The blanking signal just has a LOT of contamination in it, especially in the close proximity of the actual blanking border. Have seen some wild peaks there on some units.
So what i do is set the video level relative to the blanking level so that the video level is just barely above all contamination that i see in the 'visible blanking area'.
Huh? What i mean by that, is that i don't care for peaks in the blanking which are going to be left and right of the picture once you actually USE the blanking. The critical peaks are those which are visible in the retrace!
Definition: ‘visible blanking area’. The retrace part of the blanking signal which is visible behind the actual video signal.
For this to work, i do this adjustment with the target resolution (e.g. 1080p60) and target porches, because these influence which part of the blanking actually the ‘visible blanking area’ is.
Note: How do I correlate the scope with what I see on screen? By increasing brightness and moving left and right blanking around, i get a feeling for which peaks on the scope correlate to which bands on screen. Why do we need to correlate them? Well, the scope is unavoidable for the actual adjustment. But we need the screen for the determination which peaks are in the ‘visible blanking area’
Never mind setting these to the ‘same’ level. Just hide all the ciritical contamination individually on all 3 tubes. These ‘tend’ do be similar, though. Don’t sweat it if they aren’t. Also don’t sweat if your blanking level is WAY above the tolerance mentioned in the service manual. Its written a) without real world signal contamination in mind or b) for use with signals which use NTSC setup pedestal ;)
Since you are changing the balance between the blanking and the video black level with these adjustments, you will need to make sure that you get your desired 700mV peak-to-peak again afterwards. But this is another discussion.
If this is way over your head, consider having someone calibrate your XG. Its one hell of a machine, but a real bitch to master.
Best regards
Bjoern
P.S:
http://www.tfw2005.com/boards/images/smilies/this_thread_is_useless_without_pics.gif
Yeah, i know. Some drawings, or pictures of the tube or scope would be awesome. Maybe in a future XG install. Am not doing nearly as much calibrations these days due to time constraints, though.
in Kenny's recent thread (http://www.avsforum.com/avs-vb/showthread.php?t=870425) about black level problems on his XG, my first thought was 'blanking pedestal level', because this is the ugly step child of the XG series. I have had to correct this problem on dozens of XGs, so this isn't a isolated problem. Actually, i haven't seen a single XG which had this set properly no matter how many hours on the chassis.
Independent of the fact whether this is Kenny’s problem, I think blanking pedestal levels is something not discussed in-depth here yet, so let me give you guys some pointers.
Indicators of the issue:
1.) Common case
You have some funny vertical stripes in your picture which only show up in dark images and a black image. These stripes seem to move when you adjust left and/or right blanking!
Delimitation to other issues:
- Stripes also show up in white field? On the left side mostly? -> Might be: Swing-in of signal. Cause: Projector, Source or a combination of both
- Stripes do not move when adjusting left and right blanking. Heck they even stay in the same place when change width of image through image size! -> Burn in :(
2.) Extreme case
You cannot get your black level low enough. There is always a considerable glow on the tubes after (the hideously painful ;) )G2 calibration. If you watch closely, you might even see some sort of inward bound edges at the top and bottom of the screen. That’s your retrace! Why the heck is this visible here?
Note: The problem having this discussion here is, how much glow is acceptable? What do I mean with ‘considerable’? After proper G2 and black level calibration, there WILL be a glow. Otherwise you crush your blacks. How much is normal? How much is ‘just barely visible’ etc. Don’t have an answer here :(
Solution: Calibrate the blanking pedestal with a scope
The problem with the blanking pedestal is, in my opinion, that the service manual isn't 'real world' enough in its recommendation how to set it. If i remember correctly, it says something like 'Set the blanking pedestal so that it matches the video black +/- 0.xxV'
A) For any sane videophile, this should be '-' only. You do not want to have blanking brighter than your video black, because the blanking level is also the voltage that drives the raster retrace and thus pollutes the image. (See 2 above!)
B) The given range is WAY to small from my experience. I start with at least 10 times the tolerance in mind. The blanking signal just has a LOT of contamination in it, especially in the close proximity of the actual blanking border. Have seen some wild peaks there on some units.
So what i do is set the video level relative to the blanking level so that the video level is just barely above all contamination that i see in the 'visible blanking area'.
Huh? What i mean by that, is that i don't care for peaks in the blanking which are going to be left and right of the picture once you actually USE the blanking. The critical peaks are those which are visible in the retrace!
Definition: ‘visible blanking area’. The retrace part of the blanking signal which is visible behind the actual video signal.
For this to work, i do this adjustment with the target resolution (e.g. 1080p60) and target porches, because these influence which part of the blanking actually the ‘visible blanking area’ is.
Note: How do I correlate the scope with what I see on screen? By increasing brightness and moving left and right blanking around, i get a feeling for which peaks on the scope correlate to which bands on screen. Why do we need to correlate them? Well, the scope is unavoidable for the actual adjustment. But we need the screen for the determination which peaks are in the ‘visible blanking area’
Never mind setting these to the ‘same’ level. Just hide all the ciritical contamination individually on all 3 tubes. These ‘tend’ do be similar, though. Don’t sweat it if they aren’t. Also don’t sweat if your blanking level is WAY above the tolerance mentioned in the service manual. Its written a) without real world signal contamination in mind or b) for use with signals which use NTSC setup pedestal ;)
Since you are changing the balance between the blanking and the video black level with these adjustments, you will need to make sure that you get your desired 700mV peak-to-peak again afterwards. But this is another discussion.
If this is way over your head, consider having someone calibrate your XG. Its one hell of a machine, but a real bitch to master.
Best regards
Bjoern
P.S:
http://www.tfw2005.com/boards/images/smilies/this_thread_is_useless_without_pics.gif
Yeah, i know. Some drawings, or pictures of the tube or scope would be awesome. Maybe in a future XG install. Am not doing nearly as much calibrations these days due to time constraints, though.