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Date Posted: 22:27:12 02/13/02 Wed
Author: Tom
Subject: Re: AGC sampling added to Rev7t1d2
In reply to: Jain 's message, "Re: AGC sampling added to Rev7t1d2" on 05:27:15 02/13/02 Wed

Digitalman's circuit and code was made to sample the black level during line 23 with the normal switch on. Line 23 in his system is the black level reference and is at the black level for the whole line. He samples that black level and then applies it for the rest of the field. Most systems do not have a black reference level to sample.

The problem with shifting the hor blanking interval(and VBI if necessary) is that it isn't just shifted, it is also attenuated. Suppressed sync means that the RF signal is suppressed or attenuated, usually 6-10db, during the horizontal and vertical blanking intervals. Because it is the RF that is attenuated, this causes the demodulated HBI and VBI to be DC raised and attenuated. If the blanking intervals can be lowered and amplified at the same time, then restoring the original sync is better than manufacturing sync. This can be done by biasing a 733 amp at the zero-carrier point and switching in and out some resistors to change the gain of the 733. Some good reference material on this method can be found at HU's site in the patent information.

DC restoration should only be needed when the AGC is not being corrected. If the AGC voltage is causing the gain of the IF chip to vary, then the DC level of the whole video signal will vary with it. DC restoration will not fix the problems associated with varying gain of the video signal. If the gain varies, then the fixed sync will still cause problems even if the DC level is corrected.

The sample and hold AGC (not DC restoration) circuit will not produce a better picture than a pot but it shouldn't need to be adjusted for every channel with a different RF level.

My cable provider has stopped providing analog scrambled signals so I can no longer test circuits or use my 3chip.

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