video signal switching with relais. Ok/safe?

relais chip

Hello

I'm having a little video switcher project to use 2 camera signals(75ohms RCA) with 1 recorder, NTSC analog kind.
So far it worked with relais, but I wonder if it can create spike of any sort and if I need to add some buffer components?

Next, I would like to have the signal feeded to 2 devices: MiniDV camera and a Viewer. I tried having both devices on the same source and it appear that the MiniDV busted!
So I guess I need some sort of a splitter/repeater.

Does an IC chip doing this whole kind of switching would be better?
like ADA4304-2. but this is a bit too small for my soldering talent!! lol 4x4mm

any IC that could do this job but easier on a hobbyist skills?


thnx much

video switch cd4051

flipper_md said:

Does an IC chip doing this whole kind of switching would be better?

Click to expand...

Yes, go for that.

ep20k

ic from video crossfade

Video signals are continuous, if you use relay switching you will get a brief interruption which will force the signal to re-synchronize. Usually, this shows as a vertical roll or two or tearing in the picture for a moment.

Also, consider that the sort of signals you are using consist of two parts, the video content (the part you see) and synchronizing pulses (that lock the picture). If you have two sources, the chances are that they are not exactly in time with each other. For example, one scan may just be starting at the top of the picture while the second source may be half way down. It is difficult to electronically bring the two in line with each other, a device called a frame store is needed and they are quite complicated. If they are brought into sync, you can not only switch from one source to the other without loss, you can cross-fade between them as well.

There are many video switching chips on the market but for your needs, you can probably get away with a simple CMOS analog switch such as a CD4051. They are not as good at isolating the second signal as specialist device but they are available with real pins and cost pennies.

If you want to split one signal into two outlets, MAXIM and Linear Technology make several devices which are inexpensive and again, have proper pins. Make an amplifier with a voltage gain of 2 (double the video level) then feed its output via a 75 Ohm resistor in line with each connection to the device you are feeding. The 75 Ohm in series and the 75 Ohm input load of the device will make a divide by 2 circuit which will return you to the original video level.

Example at: http://www.atv-projects.com/Video_Amplifier.html

You can omit the voltage supply part if you like but try to keep it running on split positive and negative supplies so it doesn't need an output capacitor. This will preserve its low frequency response which is essential for video work.

Brian.

video signal switching ic

wow! thanks much for the details

I could see the sync problem big time when switching with relais, and crossed my fingers hoping the miniDv recorder you survive that! He did so far.


I'm a bit concerned to have the outputs splitted appart only by 2x 75ohms resistors, from circuit on the linked site.
Also, what happen if only one device is on with that circuit? I need both devices some times but once cameras are setup, I close the viewer.




tnx!

video signal relais

The output of the amplifier IC is very low impedance, and for practical purposes can be seen as a perfect signal source. The two 75R resistors are all it takes to completely isolate the two outputs, nothing (or very very little) will pass from one output to the other so the isolation will be almost complete. If you leave one output disconnected, the other will still work exactly the same, it may even be possible to add a third 75R resistor and split the signal to three outputs although the IC may struggle to produce enough output current with the heavier load. As a rule of thumb, each output will require 15mA of current from the IC. There are devices capable of driving 10 or more loads this way.

Brian.

video fade ic price

very good!

many thanks

Added after 4 hours 18 minutes:

uh, I just saw that the circuit operate out of a 12V DC-DC...

any chance I could make it work with a 5V?? or slightly lower than 9v so I could have margin when the 8AA batteries are fading.




Dom

hobby create a video signal

It might work on a lower voltage but be aware the the video level at the output pin of the amplifier will be 2V peak to peak plus any DC level at the input x2. As you lower the voltage, you run the risk of 'hitting the rails' or at least crushing the top or bottom of the signal. Crushing the top will give the effect of losing detail in bright areas of the picture while crushing the bottom will cause rolling and tearing of the image.

What is important is that you use a negative and positive supply as well as ground. If you want to try a single supply, place a large capacitor (> 470uF) between R2 and ground with its negative end grounded. A much better solution would be to use a split isolated supply generator like the one in the diagram but a version that takes a 5v instead of 12V input. They are readily available and the same price as the 12V versions.

Brian.

video signal tearing

I would try with 5V split supply. hoping that picture remain ok.

...Wish there was a variable voltage DC-DC generator...
I have seen one which appears to output 9V out of 5V input? that's is weird, but sounds interresting to me if not a typo!

https://www.xppower.com/orderPriceList2.php?seriesid=100085&groupid=100032&catuid=2〈=EN

IA0509S
input 5 VDC
ouput ±9V/±55mA

oh! digikey even have some at +-12V outpt! from 5V in

thanks for all Brian. You're the master!

Take a look at www.farnell.com, select your country then in the search bar either put:

1694750 = 5V in, +5V and -5V out,

or

1694751 = 5V in, +12V and -12V out.

Using a split supply from one of these devices is a good idea because the positive and negative voltages track each other. Keeping them equal helps to stop the output of the amplifier drifting from center voltage (0V).

Brian.