How to switch between video inputs

[sinoteq]

OK, let us start with the basic idea of multiplexers. You can look at them like a "One of Eight" switch where one line of the switch stays the same in all cases. 4051 is actually a MUX/DEMUX but let us not complicate this for the moment. Let us look at the datashet for example http://focus.ti.com/lit/ds/symlink/cd74hct4053.pdf PAGE 3


Functional diagram of 4051:
COM In/Out HERE IS WHERE YOU CONNECT YOUR TV
CHANNEL In/Out HERE YOU CONNECT YOUR CAMERAS
ENABLE pin must be low to make enable the chip
S0,S1,S2 are the Select Pins (Adress) and they decide how you switch the signal, look at the diagram under the picture in PAGE 3
S2=0 S1=0 S0=0 >> Connect Pin 13 to Pin 3
S2=0 S1=0 S0=1 >> Connect Pin 14 to Pin 3
and so on


So you see 4051 does not use serial communication, it uses a 3 pin + enable parallell interface. If you want to have many chips on the same Slect Pin bus you have to figure out a way to only enable one chip at the time, this can be done by connecting the enable pins from each 4051 to the PIC or..... Connect the enable pins to a 4051 and control enable for all chips that way. Just at matter of how many pins you have left on your PIC.
You can also connect 2 4051 in parallell, and feed video to one chip and Audio to the other chip, by doing this you always get the audio from the selected camera.

Does this make things more clear?

[The Master]

Does this make things more clear?

Perfectly!

I think i was going wrong because to me "COM" means serial. OK, i know it should be "COMM" but lots of people seem to miss off the last M. It does make perfect sense now since there are 3 address pins (2 states each) and 2^3=8 (8 inputs).

It didnt help having pins labeled "in/out" either. Just a small question on these, does that mean i could use this chip the oposite way? (1 input and 8 switched outputs)

I like the definition of "X" in the datasheet :P "Don't care"

I should have realised how it worked before because when i was at school i actually use a chip that does pretty much the same thing but it was only for LEDs.

One last question. Would i be right in saying that the grounds on the video inputs/outputs should all be connected to each other but *not* to the chip. Then the signal wires go into the correct chip pins

[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/

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