Correcting video offset and amplitude

Discussion in 'General Electronics Chat' started by RobeeJ, May 10, 2013.

1. RobeeJ Thread Starter New Member

Mar 19, 2013
24
1
Hi all!

Okay, short short story, I'm trying to work out the best way to correct a composite video signal, but I'm sort of snowed under by options and unsure of which direction I should be taking or if indeed any of them would actually work.

The signal is AC (about 1.1v pp) with a DC offset of around +1.8v. I need to set the DC offset to 0, with a 0.7v pp.

One way I thought of doing this was AC coupling, then an op-amp to correct the output level. Is that the route I should be investigating, or should I be looking at something else?

I'm not really looking for a "here's the circuit you need" kind of answer, more "do X and Y" as I want to understand it all, not copy it.

2. kubeek Expert

Sep 20, 2005
5,193
975
AC coupling will be needed anyway. But since you have such nice numbers and want the amplitude to go down, you can use a capacitive divider made of two same caps. This will give you AC coupling and half the voltage in one go.

3. crutschow Expert

Mar 14, 2008
18,633
5,201
By "set the DC offset to 0" do you mean clamp the negative peak to zero, as a DC restore circuit would do, or do you mean the average DC value should be zero?

4. RobeeJ Thread Starter New Member

Mar 19, 2013
24
1
Yes, I mean clamp the negative peak to zero, sorry should have made that a bit clearer.

Composite video signals should run from 0v - 0.7v, well that's the best range I can find anyway!

5. chuckey Well-Known Member

Jun 4, 2007
75
10
I thought that composite video went from -.3 -> 0 for the sync pulses then 0 -> +.7 for luminance. just AC couple it then DC restore it with a diode, should put the sync tips at -.6V (Si diode) or - .3v for a Ge diode.
Frank

6. SgtWookie Expert

Jul 17, 2007
22,202
1,785
You can hardly find Ge (germanium) diodes anymore. Instead, use a Schottky diode; a 1N5817 should work just fine for the sync tip. Use a 1N914/1N4148 for the luminance.

7. RobeeJ Thread Starter New Member

Mar 19, 2013
24
1
According to the Video Demystified book, unless I'm mis-reading it, the blank level is 0.321V, the sync level is 0.02V, and the white level is 1.02V. So I guess that's what I need to make it, well for PAL anyway, it's slightly more complicated for NTSC as the black and blank levels differ, but I'm not sure that makes any real difference for these purposes.

I guess also this is a more generic question, of how do you take an AC signal with a DC offset that runs between A and B volts pp, and make it run between X and Y volts pp.

(thanks for the replies so far!)