[linux-audio-dev] Quad Chorus for LADSPA plugin

Nick nicktsocanos at charter.net
Fri Apr 11 08:43:01 UTC 2003

If someone would like to take a quad chorus I made and make it a LADSPA
plugin feel free.

The source code is @ www.sourceforge.net/projects/audiostar

I had an implementaion of StereoPhonic-Quadraphonic Matrix processor,
but I don't think the phase is correct, and nor is is 4-channel either.
If you had four channels you would calculate the left rear channel as
90 degrees phase shifted, and then sum it into the formula.

The quadraphonic processor still does make an interesting surround sound
effect even though it is not technically correct.

The algorithm is from an electronics book, the formulas were given as
part of the circuit design. If someone is interested I can post those
formulas to you for implementation, but really the
quadraphonicprocessor.cpp already does it, just the left rear channel is
phase shifted 90 degrees and mixed into the stereophonic stream. Since
it is operating on only stereo channels, I keep a copy of the previous
left channel about n samples behind, and slightly mix it to achieve
something like a phase effect. It does actually sound like surround
sound still.

The Quad Chorus is just a stereo chorus but it has two additional taps.
This is the ultimate in fatting up analog waveforms, but the
implementation suffers from swishing if you set the LFO rate too high.
It also does a deep flange if you set rate all the way down and put
feedback up. I had it go through the quadraphonic, but the way I do
stereo on my new synth, it doesn't sound proper, so I removed it.

I'd make it a LADSPA plugin but I don't know yet how to make them.

I have another chorus design but it is not real time. It is a six tap
chorus, passed through a phaser, and then put through an all pass with a
variable delay length setting. This can make echo effects. It was
actually really cool sounding. 

I also had an FFT chorus that mixed pitch shifted copies of the signal
onto the stream like suggest in The Computer Music Tutorial. I am
working on getting it working with FFTW.

Hrmm, oh, I also had one funny chorus, that was also not real well with
real-time.I took the idea of the QuadraFuzz pedal, split the stream into
four (or more) bandpass (with resonance) and then put them through a
chorus effect. Then I remixed them through comb filters to get a reverb
feel to it. That also sounded pretty cool, but I had too of a hard time
getting it to work in real-time.

IF someone could take those designs and make them real-time they were
REALLY funky and people would like them.

Just some ideas you can experiment with.

Nick <nicktsocanos at charter.net>

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