Stereo compatibility (was: AUDIO FILTERS FOR MANIPULATION)

[Haible Juergen]

	>I am currently building a complex switchable audio filter system 
	>using CEM 3350 State variable filters and VCA's (SSM or CEM).
	>
	>I am desperite for any info that may help in designing this filter 
	>circuit in regards of the CEM chips, good audio design practices, 
	>things to watch out for, its power supply, schamatics etc.etc.
	>
	>The circuit needs input metering, switchable filters and a VCA.  It

	>must also be stereo compatable.

A lot of questions, and you will find a lot of interesting things in
the archives. There's just one point in your question that I will
reply to now, as I think it's not that widely discussed on synth-diy
so far: stereo compatibility.

Here's just a few personal opinions that I gathered over the years,
just take it up for discussion if you like:

(1) Stereo inputs and outputs can be very comfortable on a 
processing device, especially when you run synths thru them
that already have stereo outputs, internal stereo FX, panning
(OB-8!) etc. You don't want to have the processing device
in the signal chain for *all* sounds, so you need stereo input
and output, and a true hard stereo bypass (either with a relais,
or with a switch that has several parallel contacts).

(2) More on hard bypass: Sometimes you want to preserve
a balanced connection thru your processing device. Even when
the FX box is not balanced itself, you want to connect tip to tip
and ring to ring of input and output cables, when the FX box
is bypassed. This means a relais with at least 4 contacts for
stereo (or a 4p2t switch).

(3) Hard bypass again: Sometimes you don't want the 
FX box to load the connected instrument in bypass mode.
There's two main reasons for this: The nominal input load
of your FX box will change the input signal, when the instrument
has a high impedance. This is often the case with guitars,
seldom with synths. The second reason is more important:
You might want to turn the power of your FX box off when you
don't use it for hours. Ok, this will only work if you have hard
bypass in the first place. (A relais solution has the advantage
that the device will go to bypass automatically, when power fails
or is switched off.)
Now think about the side effects when you have hard bypass
from input to output, but the FX electronics input is still connected 
to the signal, and the supply voltage is down. Your input protection
network (you have one, don't you ?!) might cause slight distortion
as diodes will start to conduct and so on. Avoiding this completely
means 4 contacts for a stereo *unbalanced* connection, and 
8 (!) contacts if you want it balanced, too.
(I admit that I have not used more than 4 contacts myself yet; you
can partly avoid this with a very careful input design.)

(4) Forget true stereo filtering. Having identical filters for the left
and right channel is either boring, or will cause trouble when
one side goes more into distortion than the other side. A little
overdrive is everything between harmless and pleasant if it
happens in the *center* of your stereo image. But a sudden distortion
panned to one side is extremely unnatural. It simply doesn't sound
good. And it's very likely when you run a true stereo signal into
two independent filters.

(5) Using two filters (with different modulation) on a mono signal to
*create* a stereo signal is much more rewarding. My suggestion is
provide true stereo bypass, but sum the two channels up to a mono
signal before you do any severe filtering.

(6) I have once experimented with a kind of "matrix stereo" processing:
I created the sum (M) of the two input channels (=mono signal), and the
difference (D). This preserves all information to reconstruct the original
stereo signal from M and D. The idea was to process (filter, distortion)
the M signal only, and then create a stereo output from the processed 
M and the original D. The concept looked great (doesn't it?), but the
result was very disappointing. Not recommended.

(7) Something completely different: When you want to insert an FX
box into a non balanced signal chain, you can be confronted with 
several types of 1/4" connections. (Besides XLR, 1/8" and banana stuff, 
of course): One single Mono cable for mono, two separate Mono cables
for stereo, or a single stereo cable (from a headphone output, for 
example). You can set up a pair of stereo jacks with normalizing
contacts to work with all 3 types of cables: Jack one is "L / Stereo",
and jack two is "R / Mono". If you have a single mono cable, plug
it into "R / Mono", and the (closed) normalizing contact of the 
"L / Stereo" jack will automatically route the signal to the second
channel, too. For two mono plugs it's simple, one to L and one to R,
and a stereo (headphone type) cable will go into L / Stereo and the
"ring" signal will automatically be routed to the other channel over the
(closed) normalizing contact of the (unused) R / Mono Jack.
This setup is very handy, and I use it more and more on new 
projects. You can connect it to a stereo channel on the mixer and
don't have to adjust the panorama setting all the time.
It doesn't work on balanced stuff, though.

JH.