[sdiy] Linux Synth ?

[Linium]

On Mardi 11 Novembre 2003 19:23, john mahoney wrote:

> One limitation with your scheme is that your analog signals can't easily be
> used to affect the CVs coming from the PC. This is fine for some uses but
> is not ideal for modular synth uses. To handle that you'd need some kind of
> card with DC-coupled inputs, plus custom software to use it in your system.

You don't need custom software if you use audio signals since a lot of virtual 
modular synthesizers are already available. No code to write, just a few 
patch to mux the signals of all your virtual generators into 2 audio outs. 

> A sound card may be alright for producing some modulation CVs, or will it?
> First, LFOs are often producing sub-audio frequencies, and sound cards
> can't go that low. Second, the signal levels are all wrong -- they are much
> too weak compared to analog synth voltages. Less than 1V peak-to-peak
> instead of the needed -5 to +5 V. I guess you can use amps on all the
> signals, maybe built into your mux. The mux also needs to have a CV sample
> & hold capability. But the sub-audio LFO problem is not easy to get around
> (with a sound card).

Well i have not well explained the details in the first post. 

First and formost, it is not always possible to use the onboad dac on the 
soundcard for the mux idea because most of the time the DAC has digital 
filter or makes interpolation, the interleaved data won't be preserved. But 
maybe there are some dacs soundcard without such feature  so you just have to 
put a wire straight at the out of the dac before the DC blocking caps. It 
would simplify a little the project specially if you find an old soundcard 
for cheap. (You will have to find some digital signals to synchronize the 
external mux with the soundcard, it is not easy without documentation).

It would be better to use a card with spdif out and use a CS8412 or a similar 
Texas instrument convertor and make an external box for this purpose.
The cs8412 will provide you a I2s signal (SDATA,WS,BCK) and a simple dac (like 
the tda1543) should be ok. A CMOS counter based on WS (which is a square 
signal at 44.1or 48khz for dispatching left right data) give the binary value 
for the mux (like the 4051 or 4067 for 16 outs). It is easy to put several 
counter in serie and manage to do a 8*8 or more mux system, should you use 
some IC logic...or a MCU.
Yes you need after that a bunch of caps to store the data then some OPA to 
produce CV signals to your taste (-/+5V or 0-10V).
You need a bipolar supply as well. 
I made some experiment already and if the computer has no other tasks to run , 
the software synth and the hardware are able to stay in sync..
The last difficulty is to synchronize them together at the boot. I have 
already a MCU with serial output connected to the computer that can talk to 
PureData, so it is not hard to test the outputs to deduce the order of the 
outs. 
 
> > The anti-aliased oscillators (you can find some free as LADSPA
> > plugins) are not so great to my taste :)
>
> Are you really judging all software synths by the quality of free plug-ins?

:) 

> > And i like much analog filters :)
>
> Mmmm, analog! :-)

Currently listening to a Steiner Parker Synthacon filter (Ken Stone) CV driven  
by a soundblaster...

Cheers,

Linium