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I have bought a car radio (AMPrime 4022D) that has ability to connect rear camera to it, so you can see the image on its screen. The problem is that radio doesn't detect the signal (blue screen). But on an old portable TV, video is shown normally. Camera, that I use, was bought separately.

I have tried to connect different resistors (later on trimmer) on signal line of coax cable, and figured out that around 700-800ohm the radio detects video signal but the image is black. Maybe there is slight presence of otherwise bright spots. Every time I wanted to reduce resistance, the screen became blue again - no signal. Then I tried to do the following: when image was black (signal present) I would shortcut resistor with a screw driver and the true image from the camera would momentarily appear on the screen! Then soon after image from camera appeared, the blue screen came back again...

I don't know exactly what is going on, but I connected oscilloscope on an open connector and measured voltages that appear 1V higher than on specification of composite video. Could that be the reason, the signal couldn't be detected. I have also tested this camera on another more "modern" car lcd screen and the symptoms are similar... no video detected, except momentarily when camera is powered on.

What is the cause that this is happening and what could be a solution?

Sync level measured at 1.11V: Sync level measured at 1.11V Voltage at color burst is 1.7V: Voltage at color burst is 1.7V Another level measured at 2.3V: Another level measured at 2.3V Video on an old portable TV: Video on an old portable TV

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  • \$\begingroup\$ Did you terminate the video with 75 ohm resistor before measuring? Also there are no DC voltage specs for the composite interface, it is usually AC coupled but not always. Perhaps all you need is a capacitor in series to block DC? \$\endgroup\$ – Justme Aug 11 at 13:57
  • \$\begingroup\$ It was measured without 75 ohm resistor just open contacts. What kind of capacitor would you recommend, what size? I have noticed now that when I have it connected directly, the gray version of image of camera appears for a moment every 20 seconds or so, then it returns to blue, no signal image. \$\endgroup\$ – GregorK Aug 11 at 15:28
  • \$\begingroup\$ Bizarre.. Video started to work, even when I connect the cable directly. Well sort of work, every second the blue screen shows up for a moment. When there is movement on camera, then this blue screens stops for a while, or are less current. Strange. What I did then is to power camera separately and lower the voltage. When I came to 8.5V there was no blue screens on display, what is over 9V, blue screens shows again. Who would understand this. \$\endgroup\$ – GregorK Aug 11 at 17:04
  • \$\begingroup\$ The video signal peak amplitude and average depend on how bright areas there are in the picture. That and also the used supply voltage can have an effect on the overall DC bias of the video signal, and that's why sometimes the display can and cannot lock on to the signal. \$\endgroup\$ – Justme Aug 11 at 17:17
  • \$\begingroup\$ @Justme thank you for your explanation and advice. It seems that as you said DC bias generated by camera with lower supplied voltage is more acceptable for display to recognize as proper video. \$\endgroup\$ – GregorK Aug 12 at 20:26
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I've solved the problem by supplying the camera lower supply voltage (8.5V) by using DC-DC step down converter. It seems that signal generated by camera is better or more acceptable for display to recognize as proper video signal. @Justme also provided explanation why that could be the the case:

The video signal peak amplitude and average depend on how bright areas there are in the picture. That and also the used supply voltage can have an effect on the overall DC bias of the video signal, and that's why sometimes the display can and cannot lock on to the signal.

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