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The original Lawmate Audio coupling cap is quite small. This works well since the input impedance of most A/V devices is about 10K ohms. The new audio output's suggested 0.47uF is fine and gives as good (if not better) fidelity than the stock output.

BTW, there is no need to defocus the camera lens when tweaking the Video levels. It is comforting {to me} to see the bright light source move around, which would probably be much harder to see if not focused. But, everyone has their own tricks, so use what is best for you. Just be sure that when you are done, the full white levels are 1.0Vpk-pk. Not higher or lower.

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That is a surprise. I'll go ahead and change it to 4.7uF or larger. It certainly does not hurt to have a bigger value. Thanks.

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Not a problem. Its the smallest 4.7UF cap I've ever seen. If I hadn't looked twice, I'd have called out a .47UF too.

What do you think the drive capability behind the cap is? I wonder if would handle a 27 ohm headset directly if the cap is increased even more?

Best,

T.I.

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Cameras are pretty dumb for the most part and still use a simplified "18% grey" view of the world.

By removing the lense and exposing the camera to bright light, you fool it into thinking you are looking at a very well lit 18% grey card and it will scale down the exposure accordingly, which means the resulting video signal does not represent full white.

By picking a scene where you can fool the camera into giving you a perfectly white object you'll stand a better chance of seeing what the true peak is.

A large black field with a medium size white dot should do the trick

Just a thought....

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What do you think the drive capability behind the cap is? I wonder if would handle a 27 ohm headset directly if the cap is increased even more?

I am often surprised at the abuse we can subject these sort of circuits to and still get what we want. If you cannot get satisfactory operation with a larger coupling cap, then an amp (with handy volume control) is probably best. An LM386 based solution is simple to toss together.

It looks like you are actually installing the A/V jack feature, so that would mean you have the 200mW Lawmate system. Did you happen to measure your Rx's full white amplitude using the procedure? If so, what voltage did you find?

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Yb2normal is right. Hmmm...I wonder what voltage 18% falls at if the white clip is 1V?

I'm not installing one right now, but I am going to have dual A/V jacks in the near future. Like many in the past, have found that one set is not enough and I appreciate your notes! I was just musing over the drive capability. I don't really need to drive my headset direct. If I have a moment, I'll check out the drive capability and post it. I do have some LM4881's around.

T.I.

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Using a 18% gray card is not the standard way of setting up the NTSC or PAL signal level of a video source. The goal is to ensure the full (saturated) whites are at the 1.0Vpk-pk level when driven into a 75ohm termination. In absence of a precise video generator, this can be accomplished with a bright white light source, using the procedure discussed in the web page.

Mapping the grayscale region of the video signal is useful for linearity performance tests. But grayscale or color purity testing is not the intent of the dynamic range tweak discussed in the new project page. Besides, the tweaking that I discuss comes first. Once the level is properly set, you can perform a battery of other tests. :)

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???

Not sure where to go with the brief comment. If there is an easier way {than I describe} to adjust the video levels then let me know. I'm all for making things simple.

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