On 03/07/2014 15:19, Jim Thompson wrote:
On Thu, 03 Jul 2014 10:16:08 +0100, Mike Perkins
wrote:

On 02/07/2014 23:41, Jim Thompson wrote:
On Wed, 02 Jul 2014 21:26:44 +0100, Mike Perkins
wrote:

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,

Everything is ratiometric in "IEEE Units". In my particular case the
video is fixed amplitude from sync tip to UTC (Coaxitron) peaks.

[UTC (Coaxitron) are the control bits for the camera.]

and to a small
extent, the average video level.

Wrong

I see you have now removed the offending circuit from your website.

Huh? It's still there.

Yes - forgot it was in a lower directory.


You "had" a 100k resistor down to ground to bias the circuit. The
current flowing through this, and the clamp will be dependent on video
level.
The clamp will have an impedance and so there will be a small variation
of clamp level with video level.

Careful there, sonny. You're displaying your ignorance of circuit
design.

It depends on what you want to do. If you want a crude video restore
then fewer components could be used.


Ordinarily you would need a buffer after this to ensure the clamp is not
affected by anything downstream.

I live in a hi-Z world... it's called CMOS. But it's not hard to have
a "downstream" bipolar comparator to do the UTC slicing and NOT load
the clamp.

Most things have to interface to the real world and composite video is
generally associated with 75 ohm impedance.



I'm surprised, given this video signal is of known origin, why would you
need such a circuit? Plus you can hardly integrate the 100nF on the CMOS.

Video is commonly AC-coupled. A DC restorer "clamps" the sync tips to
ground, from which all measurements are referenced.

Agreed, but why would your customer specifically introduce AC coupling?

Why do you have a hair up your butt?

Is your abuse to cover being unable to answer a very simple question.

Most CMOS and integrated camera provide a DC output with no real need
for DC restoration. Very old ones, yes.

At most a DC shift in output is all that would be needed, and that
doesn't cause complications like line droop.



Who said the 100nF was integrated?

...Jim Thompson


...Jim Thompson


--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk