Clamp width & position settings?



Clamp width, Clamp position

PAL DVD player connected via component.
My DLP Projector has adjustable settings for clamp width and clamp position.

Changing the width of the clamp while looking at the DVE test image of the models face, it alters how smooth - harsh the texture - skin tones look. Changing the clamp settings means I then need to adjust the brightness and contrast settings to keep the 2% above black bar visible and the higher than white bar indistinguishible from the 100% white.

I have a vague idea of what it does.
With analogue sources the signal wanders, and one end needs to be clamped down to 0 volts. If this is not done then image black level will not remain constant.
Clamping is done during the blanking period, the part of the signal - time allocated to display going down to the next line or next frame. This part of the signal also contains sync information, if the clamp is applied when black is being set it is know as Black Level Clamping. PAL black is 0 volts.

If the clamp is set too wide or in the wrong position, the capacitor reducing the voltage to 0, is not big enough and the image will get darker left to right, top to bottom, and may also cause spatial distoration.

1: Does Black Level Clamping also work with NTSC since this has a step up.
2: Is there a correct setting for the clamp position and width.
3: If there is a correct setting, is there a test pattern or tool that should be used.
4: How does it relate to bias / brightness settings.
5: Is it having a indirect effect on the displays gamma tracking by altering the part of the gamma curve being used - reason I see a difference in the models face?


Distinguished Member
I'm not 100% sure what everything you mention means, but black is 0v and 100% white should be 0.7v - could that be the clamp width?


Distinguished Member
Could well be that, as NTSC has a voltage of 54mV for black (7.5 IRE), whereas PAL is 0v (0 IRE).

If it's a black level thing, then you can calibrate using the standard black level test patterns.

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