Re: Multiplexing and mixing

On Wed, 1 Jul 2009 12:36:43 -0700 (PDT), "Bob.Jones5400@xxxxxxxxx"
<bob.jones5400@xxxxxxxxx> wrote:

On Jun 29, 11:13 am, whit3rd <whit...@xxxxxxxxx> wrote:
On Jun 27, 1:44 pm, "Bob.Jones5...@xxxxxxxxx"

<bob.jones5...@xxxxxxxxx> wrote:
I want to design a system, unless something similar exists, that
multiplexes digital audio signals but also allow for mixing of those
signals in arbitrary ways. This should be relatively cheap because I
want to use a lot of them.

The signal is arranged Sample1_audio1, sample1_audio2, etc...

This is the domain of sound programs (like Logic Pro on a Macintosh).
Smaller  numbers of channels are do-able with the nearly-free
software, like GarageBand and iDVD.

In any case it should not be difficult to "mix" to signals. I would
just extract a sample, multiply it by a the "mix" value, and add it to
a some a buffer.

But how difficult will this be to implement using discrete components
(IC's but not fpga's or uP's)?  The number of signals is fixed around
20(the more the better). The audio rate is ~200khz @ 24-bit.

Ahh, you don't want to use a standard audio encoding.   If your
samples
are all the same, it's easier than if they aren't, but you WILL  have
to
reinvent the wheel.   It'll be harder if you insist on 'using
discrete'
than if you stick with massproduced computing machinery.

If it were an analog signal, you could buffer all the inputs with
high output impedance gain-controlled amplifiers (LM13700 is
a dual of one such amp), and just connect all the outputs together
(which is allowed because they have high output impedance).
The 'control' function would then just be generating slow waveforms
to program all the gain-control pins, which is relatively easy.

This is an embedded solution and does not work with a pc or similar
device. Because it is actually a mixture of digital and analog I need
some way to pass around signals easily as I do not want to hardwire
the signal chain.

At which point the method of choice becomes a PC embed. Something
like PC104. There is sufficient processing power, USB interface
support (for the digitizers and optional DACs), and a minimal OS and
carefully tuned interface and you are golden. If you include GPL
portions be careful of the licenses and clearly segregate which is
your stuff and which is GPL. Commercial OS and SW licenses can kill
you on cost (per unit) instead.
.

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