On a sunny day (Wed, 02 Jul 2014 21:26:44 +0100) it happened Mike Perkins
wrote in :

On 02/07/2014 20:36, Jim Thompson wrote:
Just did a video DC restorer function (in CMOS) for a UTC camera chip,
and realized that it would be trivial to do the same function with
bipolar discrete devices.

On the S.E.D/Schematics page of my website check out...

Video_Restorer_Discrete_NPNs.png

...Jim Thompson


I have used a similar idea before, but its general usefulness is limited
by the black level being dependent on the sync height, and to a small
extent, the average video level.


Right, the circuit is a DC resorter, but not a black level stabilizer.

A black level stabilizer will also restore DC level.
The black level stabilizer is *required* to keep the black level in the display device constant,
and usually all the way at the end of the 'chain' of amplifiers [1].
Sync amplitude changes, and cannot be used for that, and should not be dependent upon for display.

[1]
It is done by clamping at the black level, just after the sync,
and needs a special correctly timed pulse for that, as there is also the color burst there in a composite PAL or NTSC signal,
so it should be done where there is no burst, just before the CRT or whatever, and that is where you want the black level constant anyway.

You can make a good DC restorer by clamping on the sync tip to keep your signal in the linear amplifier range,
but that is ALL it is good for.
Not much accuracy is needed in such a case, as usually the amplifier range is much bigger than the composite signal,
so a simple diode will do in most cases.






I'm surprised,

I'm not :-)