How to switch between video inputs

[sinoteq]

Yes, you can connect one camera and eight TV and run it backwards so to speak, that is why I said 4051 is MUX/DEMUX. I would connect all GND to the same place to make sure you get the same potential on all cameras this also includes GND to the PIC and 4051.

[The Master]

Ok, Thanx for the info! Now if only i could find somewhere that sells the correct SCART sockets

[The Master]

Ive just been looking at the datasheet again and ive noticed another pin called "VEE". What does that do? The datasheet doesnt seem to mention it so i assume its another one of those basic things i should have been taught in school

[mister_e]

Vee have to be tied to GND or V- if you're using bipolar signal.

[Darrel Taylor]

If you're going to use the 4051, you will need a video amp after it.

I actually tried it once. And was NOT happy with the results.
Like sinoteq said, the ON resistance is too high.
With a 1V p-p signal, I ended up with .8v p-p. Picture gets darker, and sync isn't great either.
Not to mention the Cross-Talk, ... really bad.

Maxim, has some very nice analog multiplexors and cross-point switches. Many are even less than 1 ohm ON resistance. No Amp required.

http://www.maxim-ic.com/products/switches/

.

[The Master]

If VEE needs to be connected to GND then why isnt it connected inside the chip?

I dont really want to use any extra components (especially a video amp) so ill have a look at that link. I dont think it would matter too much because these are only little £12 cameras and are black and white but i was planning to have my PC and skybox plug into this circuit too and i do want the best possible picture for those.

Im sure i saw "low resistance" in the datasheet somewhere but maybe dropping 20% is classed as "low". I suppose i could always put a resistor in the middle of a wire to test how the picture would look before buying any chips. Im sure the datasheet would specify how much resistance there is

[sinoteq]

Vee is actually for signals that has both a positive and negative part. Look at a normal waveform and you can see that part of it is "under" the GND level. Vcc is feeding the signal during the positive part and Vee feeds the negative part, that would be the simple explanation. For TV this is not something to care about becasue all TV signals are on the + side of GND but to make the 4051 behave you have to connect Vee to GND. The problem with the on resistance is not really that big unless you are planning to load the camera with a low ohm (z) equipment, most TVs are 75ohm. The higher current you use in your cable the more you will loose inside the 4051. One the other hand if you want to place the camera far away from your TV for example make a CCTV system for you house it is possible you have to put amplifiers on the cameras to boost the signal if the cables get really long. How long is really long? Well that depends on how cheap your cameras are, some has really strong internal buffers and some are so weak that even the shortest cable makes things starting to fade. It is possible to make a simple video amp with a simple OP-amp so it is not that hard to fix if you later see you have a need for it.

One more advantage of 4051, cheap... really cheap so you do not risk alot by testing them.