As you have an oscilloscope first check the signal from the camera is to
specification i.e. 1 volt peak to peak comprising 0.3 volt sync pulses and 0
to 0.7 volt video.

Have you terminated the video camera ?

Regards
Chris

----- Original Message -----
From: "Stephen B Webb" <sbwebb0@SAC.UKY.EDU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, January 26, 2001 6:35 AM
Subject: [EE]: Having trouble with LM1881 / NTSC sync separator


> Breif background:
>
> I have a dual CCD camera module that outputs 2 NTSC composite video
> signls.  The two signals are in sync.
>
> In order that I can view this stereo video feed on a head mounted
> dual-display device, I need to take the two signals and FRAME interlace
> them into one NTSC composite signal.
>
>
> The plan goes something like this:
>
> By using a national semi. LM1881, pull the ODD/EVEN field sync signal off
> of one of the composite signals.  Use this information to switch between
> the two video signals during the vblank period, thus getting a FRAME
> interlaced composite output.
>
>
> The problem goes something like this:
>
> To start with I have only a basic EE background, so I don't necessarily
> understand what happens to video signals as they go through wires, traces,
> capacitors and resistors, etc.  I'm  a software type, so if you find
> yourself thinking "Man, this guy doesn't know what he's doing", you are
> probably right.
>
> So, I start by trying to get the LM1881 to give me a nice steady odd/even
> field signal.  I connect the various components to and let it rip.
>
> First problem:  Nothing happens.  So, I add the optional lowpass filter on
> the signal input, and still nothign happens.  I pick up the camera board,
> and inadvertantly touch the RCA connector; my finger bridges the two
> leads.  The circuit starts working.  So, I add a 1 meg resistor between
> the signal and ground leads on the incoming composite feed.  I get a
> somewhat consistent odd/even signal.
>
> Second problem:  If I expose the camera to more or less light, it tends to
> cause the odd/even output to stop working (for instance if I cover the
> lens of the camera, the odd/even line stays high or low until I uncover
> it)  I'm sure this is related to #1 and my 1 meg resistor hack.
>
> Third problem:  When I have the two capacitors and resistor on the input
> signal, it drastically changes when viewed on my [very crappy] scope.
> (as compared to measuring the signal not connected to anything)
> Since I want to use that signal as an output, I am concerned that it
> doesn't look the same as it used to (I'm not even sure if it contains much
> video information at all any more).  Do I need to run the initial signal
> through some sort of amplifier, or something first?
>
>
>
> Note:  I first put this on a breadboard, and when I ran in to trouble I
> moved it to a Rad Shack style prototype board.  Haven't been able to make
> it do much on the proto board yet. :(
>
>
> Any insight would be appreciated.
>
> -Steve
>
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>
>

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