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Composite signals...

Discussion in 'General Electronics Discussion' started by cardholderone, Jan 3, 2014.

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  1. cardholderone

    cardholderone

    3
    0
    Jan 3, 2014
    Hi all,

    new member here, working on a project that I have been thinking about for quite a while. I wanted to build a Infrared night-vision system, like so many seen on the web, using a composite ir cam and old camcorder viewfinder... well, it works, but not too well. The cam I was using was a very cheap cam. Now, I decided to upgrade the cam. I found a new one and really liked the image it put out. However, when connected to the composite input of my viewfinder, the image is, how to put it..., overly rich... or just plain awful. I noticed the composite output of my working cam was +/- 0.6V and the output of this new cam is about 3v! I cannot know why this cam puts out such a high signal. I tried using a resistor to ground on the output to try to knock this down some, and it did. Picture is a little better, but don't know what the long term implications of such a rogue resistor in the output may do to the rest of the cam. Long story short, (too late), I need to know it this is a common output for some composite sources, and if sinking some of that signal will actually hurt the cam.

    Cam: Ir off brand 8v setup...
    Viewfinder: Jvc crt type, also 8v
    powered entire setup using lm317 regulator putting out about 8.2v from 12v
     
  2. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    From what I remember, but I might be wrong is that the composite signal has defined voltage levels for different parts of the waveform. This then tells the receiver all it needs to know about the signal. White level Black level, brightness etc. You have to remember that it will have a 75 Ohm impedance. An unloaded video signal will be about 2V pk-pk and loaded about 1V. Built up of sync pulses which are about -0.3V leaving 0.7V for your fields making 1V pk-pk loaded with 75 Ohms. So 3 Volts does seem a bit high.
    Thanks
    Adam
     
  3. cardholderone

    cardholderone

    3
    0
    Jan 3, 2014
    Thanks for your reply.

    A sticker on the cam said 8v, however, i believe this is not the working voltage. I made a quick vari voltage reg and swept it from 4v to 9v and it seems this cam works better on about 6v. I believe i may have misunderstood the voltage requirements of the camera. When I lowered the voltage to it, the picture it produced become better and clearer, and the output signal fell to within expected range. The iR led's didn't put out as much after the voltage adjustment... :) It had enough light before to shine across the street! I know it would probably reduce their life to runn them so high, but boy did they put out!

    Thanks again, friend.
     
  4. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    No probs
    Adam
     
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