Digital info of (possible) interest to analog people

[Sean Costello]

Hi all:

The newest release of Csound (3.55) was made availabe today. It features the first opcodes that I
have developed, two of which are directly inspired by analog ideas I found out about on this list:

- svfilter. This is a digital implementation of the state-variable filter, as described by Hal
Chamberlin in "Musical Applications of Microprocessors." Has simultaneous lowpass, highpass, and
bandpass outputs available. I just found out that it "blows up" when the cutoff gets close to 20
kHz; apparently this is a flaw in the Chamberlin's code. I am going to try to implement a
state-variable design where the integrators are replaced by allpass stages - I think that there was
an Electronotes issue that described such a circuit.

- hilbert. This is a IIR digital implementation of a Hilbert transformer. It is a DIRECT translation
of Bernie Hutchin's phase differencing network from the Musical Engineer's Handbook, based on two
parallel 6th order allpass networks. I have had WAY too much fun with this opcode. It can be used to
produce all of the classic frequency shifter effects, including the "barberpole" effects obtainable
at low shift frequencies and high amounts of feedback. I am working on a ssbmod opcode that contains
the quadrature oscillator needed to make this work, as well as a "barberpole" generator that
incorporates 12 additional allpass stages for a digital recreation of Bode's barberpole phaser. For
now, it is easy to write a Csound orchestra that incorporates the two oscili units needed for the
frequency shifter.

Anyway, if you want to check these out, you can download the latest version of Csound at 

ftp://ftp.maths.bath.ac.uk/pub/dream/newest/

Csound works on pretty much any computer you could imagine - Windows, Mac, Linux, most other Unix
flavors, etc.  If you want to check out the source code of my new ugens, download csound_src.zip
(for Windows), or Csound.tar.gz (for Unix).  I also created two new flavors of resonator filters,
but they don't have any analog counterparts, so I won't get into them here.

If you don't have any experience with Csound, I would highly recommend downloading it and giving it
a try. It is a somewhat steep learning curve at first, but the results can be very cool. It is a
GREAT environment for testing out new signal processing ideas - think of it as a modular synth,
limited only by how long you are willing to wait for a sound to process, and how much work you wish
to put into it. I could see using it as a way of prototyping analog circuits before constructing
them - for example, want to know what a frequency shifter sounds like with a 24-pole Dome filter?
Write a Csound orchestra, and see if it is worthwhile!

I'll be writing more ugens this summer, and will keep this list posted on any analog-related ones. I
also am going to get back to the analog circuits, as I still like the physical world as a place to
make music (Csound is great for testing out sounds, but it doesn't inspire me like a panel full of
knobs...).

Sean Costello