Why does my composite video signal flicker and how to solve it?
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I have a problem switching between two composite video sources. I am using the composite output on a Raspberry Pi to play video to a small LCD screen that receives composite input. I also have a small composite video camera hooked up to the same screen and I switch the input between the two devices so only one is displayed at any given time. The problem I am having is that whenever I switch the signal from the RPi video to the live camera video the signal repeatedly flickers brightly for a second or two, sometimes longer. When switching back to the RPi signal the video does not flicker, just scrambles momentarily like changing the channel on an old analog television.
The RaspberryPi, camera and LCD screen are installed in a small sculpture, part of which can be seen in the following youtube video that shows the flickering I am talking about. You will see the RPi video of the movie Metropolis for 10 seconds then the live camera video (of my hand) for 5 seconds: http://youtu.be/MvpjgSPL4xM
I need suggestions on possible causes and solutions. I will award the prize to whoever posts a solution that fixes the repeated flickering.
Here are some more details:
The screen being used is: http://www.adafruit.com/products/910
The video sources are switched on/off at a regular interval by solid state relays controlled by the RPi, but the same problem occurs when I remove the relays from the circuit and switch between the signal manually.
If the input to the screen is switched between "no signal" and the camera signal the flickering does not occur.
If the screen is receiving input from the RPi and I disconnect it for several seconds before connecting the signal from the camera the flickering will occur. Subsequent disconnecting and reconnecting the camera signal will not produce the flickering as long as the RPi signal remains disconnected.
The flickering always occurs when switching TO the live video, never when switching to the RPi video output.
The problem is infrequently better than or worse than (lasting less or more time) the sample shown in the YouTube video, but what is shown is typical. The duration of the flicker will stay consistent as long as it is powered on and switching between the two sources. A couple of times I have powered it on and the flickering lasts 3 - 4 seconds before the video stabilizes, and during those 3-4 seconds the video seems to be fading in from a much darker starting point.
I have put the two video sources on an oscilloscope and the problem doesn't seem to be affected by the timing of the vertical sync of the two signals. It does not get better or worse when the vertical sync is lined up closely or significantly off.
I have installed a DC restore circuit on both sources and it made no difference.
Both the camera and the screen are powered from the RPi 5v gpio so the signal ground and the power ground to each are the same line, but I have tried powering each separately from independent power supply and the problem remains.
If I switch from any other composite video source other than (RPi) to the camera source the problem remains.
If I switch from the RPi to another source other than the camera the problem goes away.
If I switch from RPi to the Camera but instead of feeding the signals in to the 1.5" LCD screen I feed them to a crt television I see no prolonged flicker on the television.
To sum it up, it seems like this LCD screen and the Camera don't play nice with each other if the screen has already been receiving a signal from another device.
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