[Gllug] OT: Digital Audio Switch

[Pete Ryland]

On Tue, Jun 01, 2004 at 02:54:24PM +0100, Chris Bell wrote:
> On Tue 01 Jun, Pete Ryland wrote:
> > On Fri, May 28, 2004 at 11:44:21AM +0100, Dylan wrote:
> > > Hi All,
> > > 
> > > I know this is rather OT but I figure some of you will have experience 
> > > in this...
> > > 
> > > I've got three audio sources with coax digital audio outputs, but only 
> > > one input on my amp (when will the equipment manufacturers wake up and 
> > > put more inputs on...) I've been looking for a suitable switch for a 
> > > while to avoid the continuous replugging, but have only managed to find 
> > > an optical switch.
> > > 
> > > Anyone know of a decent coax switch? Or would a standard phono switch be 
> > > sufficient?
> > 
> > Better yet, invest in a mixer (expensive) or build your own (cheap).
> > 
> > Pete
> 
>    I have not searched, but are there cheap "mixer" IC's, etc, easily
> available for digital audio streams? I was going to suggest some kind of
> data switch IC, but I am not familiar with the standard signal levels used,
> and did not wish to suggest something that may not work, or even possibly
> cause damage. I would rather be safe than sorry until I could do a quick
> check with the oscilloscope.

A mixer is just an op-amp in the following configuration:

I1 --R1--+--Ro-+
         |   |\|
I2 --R2--+---|-\___
         | +-|+/
I3 --R3--+ | |/
           |
           =

Where each of R1-3 is the pot for input volume and Ro is the pot for output
volume.  Filters can be done by putting variable capacitors in series or
parallel with each resisitor.

So, yes, you can buy an IC with an op-amp in it quite cheaply.

Pete
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