T3D Re: A good chip for multiplexing video.

[Robert Cruickshank <robcruic@xxxxxxxxxxxx>]

Actually, I haven't drawn it all up yet,(I'll post something when I do) 
it's just a collection of
scribbles and notes, it's not all in one piece (except maybe in my head,
and that's not doing anyone any good...) However ther's nothing really
strange about it, and if you have the data sheets for the chips, you're
90% there. The data sheets for the clc411 op amp show a recommended
configuration for a non-inverting amp- you'll want two of those, built
exactly as shown. The input impedance of each stage should be 75 ohms, and
the voltage gain equal to two (since you will be driving a 75 ohm load
through a series resistor, also 75 ohms.) The outputs of the CLC411s are
simply connected together, and go to the shared 75 ohm output resistor.
To derive the field switching information, I used an LM1881 sync seperator
chip. Note that it needs a single sided 5v power supply. This chip
provides an odd/even field output, thats the only output you need. (the
input is fed from one of the video inputs, it's high impedance, so i
didn't use a buffer, but you could, if you wanted.

To connect the output of the sync separator to the op amps, you will have
to do some level translation, since the 5v  out of the sync seperator is
not high enough to switch the control pins of the op amp. I used a
ULN2803, mostly because I had one handy, but also because it is an
inverting buffer, one of the op amps must be fed a the inverse of the
odd/even pulse, so it will be on when the other is off. There are pull
up resistors on either side of the ULN2803, and a few resistors and caps
associated with the sync sep, but thats it in a nutshell. 
Like I say, I'll draw it up soon...

Rob Cruickshank      Toronto, Canada 
www.interlog.com/~robcruic
                


On Mon, 5 Apr 1999, Larry Berlin wrote:

> 
> 
> ****  Could you share more information about your circuit here? The
> simplicity in yours is very attractive and might make a worthy home style
> project. Maybe you could post a circuit diagram on the web?
> 


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