Removing DC Bias from video cross point TDA8540

J

Joseph Goldburg

Guest
Hi All,

I put CVBS (Composite) Video through Philips TDA8540 Video Cross point and
out the other selected side is CVBS + 2volts.

ny suggestion how I remove the 2V offset and still maintain video bandwidth.

Thanks in advance
Joseph
 
On a sunny day (Sun, 30 May 2004 22:24:42 +1000) it happened "Joseph Goldburg"
<wizard1@SPAMnetspace.net.au> wrote in <c9cju0$bjh$1@otis.netspace.net.au>:

Hi All,

I put CVBS (Composite) Video through Philips TDA8540 Video Cross point and
out the other selected side is CVBS + 2volts.

ny suggestion how I remove the 2V offset and still maintain video bandwidth.

Thanks in advance
Joseph


Depends on what you want to drive with it?
Click to expand...
If you go to a cable, say 75 Ohm, you will like this 2 V, as you can use
directly an emittor follower.
Then you need gain 2x to output the same level you got in.
It is normally NOT needed to have zero volt output at bottom sync.
Any video circuit will likely have a 75 Ohm to ground, followed by an input
decoupling capacitor.
That little extra current will just add to your supply.
JP
 
Hi Joseph,

Strange that a video mux does this. Anyway, if that 2V isn't stable or
drifts with temperature you'd need a black level clamp circuit to
restore. I used to do those discrete back then but there should now be
ICs that really work. In my days there weren't...

Usually though you won't have to worry about the exact DC level since
the monitor restores black level itself. So maybe you could just level
shift with diodes.

Regards, Joerg

http://www.analogconsultants.com
 
Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote in message news:<46c567934be875bc905ed62e06d339a9@news.teranews.com>...
On a sunny day (Sun, 30 May 2004 22:24:42 +1000) it happened "Joseph Goldburg"
wizard1@SPAMnetspace.net.au> wrote in <c9cju0$bjh$1@otis.netspace.net.au>:

Hi All,

I put CVBS (Composite) Video through Philips TDA8540 Video Cross point and
out the other selected side is CVBS + 2volts.

ny suggestion how I remove the 2V offset and still maintain video bandwidth.

Thanks in advance
Joseph


Depends on what you want to drive with it?
If you go to a cable, say 75 Ohm, you will like this 2 V, as you can use
directly an emittor follower.
Then you need gain 2x to output the same level you got in.
It is normally NOT needed to have zero volt output at bottom sync.
Any video circuit will likely have a 75 Ohm to ground, followed by an input
decoupling capacitor.
That little extra current will just add to your supply.
JP
Click to expand...
Real broadcast gear has sync tip at -286 mV for NTSC, -300 mV for PAL.
Joerg mentioned DC restoration chips. I used the Burr-Brown SHC615 and
the Elantec EL2090 in past projects. They were excellent chips, no
doubt why both were discontinued after their respective companies were
bought.

For simple DC offsets there are many ways to change it without causing
bandwidth issues. The simplest is an output blocking capacitor and let
subsequent units restore the DC level. I wouldn't leave the 2V on the
video. Not all video equipment has the input blocking caps and may
malfunction.
GG
 
On a sunny day (31 May 2004 00:47:31 -0700) it happened stratus46@yahoo.com
(Glenn Gundlach) wrote in <acb22b57.0405302347.30858683@posting.google.com>:

Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote in message news:<46c567934be875bc905ed62e06d339a9@news.teranews.com>...
On a sunny day (Sun, 30 May 2004 22:24:42 +1000) it happened "Joseph Goldburg"
wizard1@SPAMnetspace.net.au> wrote in <c9cju0$bjh$1@otis.netspace.net.au>:

Hi All,

I put CVBS (Composite) Video through Philips TDA8540 Video Cross point and
out the other selected side is CVBS + 2volts.

ny suggestion how I remove the 2V offset and still maintain video bandwidth.

Thanks in advance
Joseph


Depends on what you want to drive with it?
If you go to a cable, say 75 Ohm, you will like this 2 V, as you can use
directly an emittor follower.
Then you need gain 2x to output the same level you got in.
It is normally NOT needed to have zero volt output at bottom sync.
Any video circuit will likely have a 75 Ohm to ground, followed by an input
decoupling capacitor.
That little extra current will just add to your supply.
JP

Real broadcast gear has sync tip at -286 mV for NTSC, -300 mV for PAL.
Joerg mentioned DC restoration chips. I used the Burr-Brown SHC615 and
the Elantec EL2090 in past projects. They were excellent chips, no
doubt why both were discontinued after their respective companies were
bought.

For simple DC offsets there are many ways to change it without causing
bandwidth issues. The simplest is an output blocking capacitor and let
subsequent units restore the DC level. I wouldn't leave the 2V on the
video. Not all video equipment has the input blocking caps and may
malfunction.
That would be rotten video equipment in my view.
Click to expand...
Even my DVD player outputs from 0 to 1 V (PAL), consumer electronics OK,
but everywhere.
This input cap is essential in my view.
JP
 
On Sun, 30 May 2004 22:24:42 +1000, "Joseph Goldburg"
<wizard1@SPAMnetspace.net.au> wrote:

Hi All,

I put CVBS (Composite) Video through Philips TDA8540 Video Cross point and
out the other selected side is CVBS + 2volts.

ny suggestion how I remove the 2V offset and still maintain video bandwidth.

Thanks in advance
Joseph
Click to expand...
Google on "Video" "DC Restorer"

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Hi Glenn,

With video equipment you never know. Once I had a monitor, a commercial
one no less, that required the coupling cap to be so large that the
video path didn't cut off below a few KHz but it had to go down to a few
ten Hz. Else the H-Sync would not properly "catch" after the V-Sync,
causing a corner of the picture to wave like a flag. What a poor piece
of engineering that monitor was. I then promptly junked it. Or in NG
speak, I plonked it.

Regards, Joerg

http://www.analogconsultants.com
 
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