Interlaced video fields of unequal brightness

I apologize, I am a video signal processing newbie.

I am using a NI 1410 board to capture images from an interlaced RS-170
video source.  When the images are captured, the odd field is about 3%
brighter than the even field.  The noise is also higher in the odd
field.

Has anyone experienced this?  Could it have something to do with the
clamp timing?  Is it possible that interference on the video cable
could cause this since it's uniform across the entire field?  The video
is coming from an X-Ray machine so high voltages are present.  The
interference would basically would basically be at a rate of 30 hertz,
if I'm not mistaken, since it is every other field.

Thanks in advance,
Mike

Mike wrote:
> I apologize, I am a video signal processing newbie.
>
> I am using a NI 1410 board to capture images from an interlaced RS-170
> video source.  When the images are captured, the odd field is about 3%
> brighter than the even field.  The noise is also higher in the odd
> field.
>
> Has anyone experienced this?  Could it have something to do with the
> clamp timing?  Is it possible that interference on the video cable
> could cause this since it's uniform across the entire field?  The video
> is coming from an X-Ray machine so high voltages are present.  The
> interference would basically would basically be at a rate of 30 hertz,
> if I'm not mistaken, since it is every other field.
>
> Thanks in advance,
> Mike

To narrow down the problem, I would use an oscilloscope or TV waveform
monitor to examine the source video stream...  is the problem there
too?

Is it always the odd field that is brighter or does the change slowly
drift through the video and sometimes the even field is brighter?  In
other words is the 30 Hz intererence "phase locked" to the video or is
it just "close to" the video sync frequency?

Mark
  



Mark

Thanks for your response.  Good point.  I wasn't able to put it on a
scope, but I did confirm that it isn't always on the odd field.    I
zoomed in and watched the live video feed and it seems the difference
is there on both the odd and even fields.  But it seems to be close to
the timing.  It may actually be much faster than 60 hertz  (every other
line) in each frame.

I don't know much about it, but from my research on the internet, it
seems that this would be classified as "hum bars".   They are vertical
and roll though the picture.  (Although they roll so fast it is hard to
notice.  I can actually see fast waves and one big peak wave come
through every 3 seconds or so if I zoom in on the live video.)

Many sites said that hum bars come from electrical interference --
harmonics on the electrical line and/or ground loops.  The camera and
the computer are both on the same electrical circuit as the x-ray
transformer and equipment.  The interference seems to vary within the
cycle based on what's going on with the machine.   Based on this
tomorrow we are going to try to put both the camera and the computer
with the frame grabber on a UPS and then run an extension cord to an
isolated "normal" circuit (i.e. no equipment).

Hopefully this is a plausible theory. If it doesn't work, I'm out of
ideas...

Thanks again for your help.

Mike

if the interference rolls through slowly, then it is probably due to
power line related interference.... the power line (in the US) is 60 Hz
and video vertical sync rate is 59.94 Hz.  (someone check me on this
number not sure right now)  the point is is it a little off from 60 Hz
so when you get interference from power line (hum bars) they roll
through the picture slowly vertically.  Note hum bars are HORIZONTAL
bars that roll slowly through the picture vertically.

You may be a ground loop or some other power line related intereference
on the video cable that is interering in a sugtle way with the sync
circuit.

You should try to get a scope to look at the video....

Mark

Well, the NTSC field frequency is 59.97 IIRC.  Another possibility for
this uneven brightness on odd/even fields can come from the light
source.  Whether it comes from a fluorescent or incandescent light
source, it fill flicker at the same frequency as the power source (60Hz
in the US).  One quick test I would do to verify this is your actual
source of the problem is to leave the power to your computer and camera
as they are, and darken the door and illuminate an object with a good
flashlight.  If the uneveness is still there then it is the power
supply (I hope is not one of those awful wall-warts).  If it is gone
you need to change your light source.  If you use incandescent you can
use a DC power source and your problem shoudl go away.

Paolo

Thanks for your reply.  The image is actually coming off of a camera
inside an industrial x-ray machine, so I can't test it that way, but I
have seen that effect you mention before under mercury vapor lights...

Mike

Mark-

Thanks for the reply.  The test we tried today (isolating on a separate
circuit) didn't work, but I'm not sure we got it truly isolated.
Basically we can't see the wiring and we have sketchy documentation, it
seemed when we did it that the camera and the control unit were on
different power supplies (we previously thought the camer was fed from
the control unit)...

I'm not an electrical engineer or very experienced.  I have used a
scope once and do have access to one, but I'm not sure what I'd be
looking for in this case.  I've read about NTSC standards and seen the
wave form though.  Do you know of any sites that have directions?

BTW, the video signal seems to be NTSC Composite Video 1 wire with a
"setup component" that puts the black ref level at 0.054 volts.  (Saw
it in the manual for the port of the monitor that it is going into.)
I'm not sure if it has a color component or not.  (We have no English
documentation and can't physically look at the camera.)  I read in the
NI manual that if the clamp timing is set wrong it showed an image that
looked very similar to what we see, but I haven't been able to find
clamp values that take care of it.  Right now it's set to 106 through
116.

Maybe tomorrow, I'll try the scope and see if I can figure anything
out...

Thanks again for your help.

Mike

Mike wrote:
> Mark-
>
> Thanks for the reply.  The test we tried today (isolating on a separate
> circuit) didn't work, but I'm not sure we got it truly isolated.
> Basically we can't see the wiring and we have sketchy documentation, it
> seemed when we did it that the camera and the control unit were on
> different power supplies (we previously thought the camer was fed from
> the control unit)...
>
> I'm not an electrical engineer or very experienced.  I have used a
> scope once and do have access to one, but I'm not sure what I'd be
> looking for in this case.  I've read about NTSC standards and seen the
> wave form though.  Do you know of any sites that have directions?
>
> BTW, the video signal seems to be NTSC Composite Video 1 wire with a
> "setup component" that puts the black ref level at 0.054 volts.  (Saw
> it in the manual for the port of the monitor that it is going into.)
> I'm not sure if it has a color component or not.  (We have no English
> documentation and can't physically look at the camera.)  I read in the
> NI manual that if the clamp timing is set wrong it showed an image that
> looked very similar to what we see, but I haven't been able to find
> clamp values that take care of it.  Right now it's set to 106 through
> 116.
>
> Maybe tomorrow, I'll try the scope and see if I can figure anything
> out...
>
> Thanks again for your help.
>
> Mike

if you have a ground loop this may help:

http://www.epanorama.net/documents/groundloop/video_isolation.html

or search on video ground loop

also search on video waveform
 
Mark