I'm trying to send analog video signal through CD4067 analog multiplexer and I have a problem.
Video signal is send by the RCA connector. I have two RCA's, one goes from video camera, second goes to TV. I connected external "border pins" to each other. One of center signal pin is connected to multiplexer OUT. Second center signal pin is connected to one of multiplexer inputs.
5V and gnd and all necessary things are also connected to the multiplexer.
And screen is very dark and have interferences.
When I unconnect GND from Multiplexer there are no interferences :o but the screen is still darker than is should be.
Things around 10MHz are difficult - things above 100MHz are black art. This means you have to know your "physics", it's no longer "logic".
(1) You have a 74HC4067, haven't you? If not, why not?
(2) Anyhow this chip is more tricky than the 74HC4051, as it has no Vee option....
(3) Most likely your input signal is symmetric around ground. You should check this with an oscilloscop. To not loose the "negative" part you have to offset it, this can look a little bit like Figure 13 in the datasheet (cap plus voltage devider plus another cap) to "tunnel" it through the multiplexer.
(4) Also do a lot of decoupling, avoid ground loops!
(5) Doing video on a breadboard is never HiFi, best use PCBs with a lot of copper...
Most likely your input signal is symmetric around ground.
It's a video signal, it's not going to be symmetric around ground!!!
This is what a video signal looks like
Is it a composite video signal? If so where is the DC level?
Most likely you will have to AC couple the video signal input. Then apply a black level clamp or a sync tip clamp. Then you can put it through the multiplexer. Finally AC couple the output with a video driver.
Check the datasheet for the resistance across the multiplexer. I think normal composite video has a 75 ohm resistor on the receiving end. If the multiplexer has significant resistance, the video signal may be degraded. In the past I made a composite video switcher just using common NPN transistors and resistors that operated fairly well.
@deSilve:
Could you do me a favour, and take your keyboard, turn it upside-down and try to free whatever it is that is causing your full-stop key to stick - it's driving me bananas.
No you are thinking of a terminating impedance nothing to do with what is being talked about here.
It may very well have an impact on why the video signal is degraded. If the multiplex chip has any significant resistance across it, then in effect a voltage divider may have been created. On the video capture cards I have, the coupling capacitor that captures the video signal is up stream of the 75 ohm impedance producing gizmo. If the multiplexing chip has signifiant resistance across it (I think the 4051 chips are ~100 ohms), then the signal voltage at the coupling capacitor may well be significantly reduced.
If you actually bother to read what I post I said:-
Finally AC couple the output with a video driver.
A video driver has a high impedance output and a 75R output impedance.
If you are stupid enough not to have a driving output amplifier then yes your video signal will be screwed up. But this will happen what ever resistance you get in your analogue switches.
