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    Copyright (C) 2003-2008 Fons Adriaensen <fons@kokkinizita.net>
    
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

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--------------------------------------------
cs_phaser.so  -- version 0.4.0 - 5/4/2008
--------------------------------------------

Code cleanup.

--------------------------------------------
cs_phaser.so  -- version 0.3.0 - 10/4/2004
--------------------------------------------

Assured central frequency is middle C for zero control voltage

--------------------------------------------
cs_phaser.so  -- First release - 20/5/2003
--------------------------------------------

This is similar to the CSound module 'phaser1'. It's a series
connection of 1 to 30 first order allpass filters with feedback.
For 'Output mix', the range -1 to 0 crossfades between the inverted
output and the input, and the range 0 to 1 crossfades between the
input and the non-inverted output. Without feedback, the maximum
effect is at +/- 0.5. For both 'Feedback gain' and 'Output mix',
the best polarity depends on whether the number of sections is
even or odd.

Phaser1+LFO is the same thing, but the external modulation has
been replaced by a built-in LFO. The 'LFO waveform' control ranges
from a falling saw wave (-1), over triangle (0), to a rising saw
wave (+1). 

Note: CSound's 'phaser2' is in the pipeline.


--------------------------------------------
cs_chorus.so  -- version 0.4.0 - 5/4/2008
--------------------------------------------

Code cleanup and triple output version added.
The third one is as the second, but has three
separate outputs. Pan L,C,R for a nice stereo
effect.

--------------------------------------------
cs_chorus.so  -- version 0.3.0 - 10/4/2004
--------------------------------------------
--------------------------------------------
cs_chorus.so  -- First release - 20/5/2003
--------------------------------------------

These two plugins are based on a CSound orchestra file by Sean Costello.
There are two low frequency oscillators, each having three outputs
that are 120 degrees apart in phase. The summed outputs modulate three
delay lines. Make sure the static delay (first parameter) is at least
equal to the sum of the two modulation depths.
The two plugins are functionally identical. The second one upsamples
the input to the delay lines in an attempt to mitigate the errors
produced by the linear interpolation at the output. If you hear any
difference please let me know which one you prefer.



--------------------------------------------
mvclpf24.so  -- version 0.4.0 - 5/4/2008
--------------------------------------------

Code cleanup.

--------------------------------------------
mvclpf24.so  -- version 0.3.0 - 10/4/2004
--------------------------------------------

Assured central frequency is middle C for zero control voltage

--------------------------------------------
mvclpf24.so  -- version 0.1.3 - 22/8/2003
--------------------------------------------

Renamed parameter "Output" in Mvclpf-4 to "Filter poles".

--------------------------------------------
mvclpf24.so  -- version 0.1.2 - 28/7/2003
--------------------------------------------

Added fourth version, offering 0, 6, 12, 18, 24 dB/oct outputs.

Changed filenames, plugin names and labels to avoid any possible
trademark related problems. I'm happy Bob Moog is back in business,
and surely do not want any trouble with him.

Added a quick hack solution to stop FP denormalization. This will
be improved on in future versions.

--------------------------------------------
mvclpf24.so  -- First release - 25/4/2003
--------------------------------------------

Description
-----------

The mvclpf24 plugin contains four versions of a digital implementation of
the voltage controlled lowpass filter as used in vintage Moog synthesizers.
Many software versions of this filter already exist. Most use some limiting
mechanism in order to keep the self-oscillation amplitude under control, but
as far as I know, none of them attempt to accurately emulate the non-linear
circuit elements of the original analog filter. To do this has been the 
main reason for developing this plugin.


The Moog VCF
------------

The basic architecture of the Moog VCF is quite simple: it consists of a
chain of four first order RC filters and a feedback path. In the clever
design by Robert Moog, the four filter sections are current driven 
parallel RC combinations, the current flowing in the resistor of one
section also being the input signal for the next. The 'resistors' are
each formed by the series connection (as far as the signal is concerned)
of two forward-biased base-emitter junctions, the impedance of which is
controlled by the bias current coming from an exponentially controlled
current source.
Whereas for a normal resistor we would have I_R / V_R = constant = 1/R,
in this scheme we find I_R = w * tanh(V_R), where w is proportional to
the bias current. Thus the impedance increases with signal level, which
explains why the resonance frequency rises (by about one semitone) when
the filter is taken out of self-oscillation by decreasing the feedback.


The four versions
------------------

Mvclpf-1 is a fairly simple design, and it does not even pretend to come
close the 'real thing'. It uses a very crude approximation of the non-linear
resistor in the first filter section only. I retained it in this distribution
because it's a cheap (in terms of CPU usage) general purpose 24 dB/oct lowpass
filter that could be useful, and for sentimental reasons since this was my
first ever LADSPA plugin.

Mvclpf-2 uses five non-linear elements, in the input and in all four filter
sections. It works by using the derivative of the nonlinearity (for which
1 / (1 + x * x) is reasonable approximation). The main advantage of this is
that only one evaluation of the non-linear function is required for each
section. The four variables that contain the filter state (c1...c4) represent
not the voltage on the capacitors (as in the first filter) but the current
flowing in the resistive part.

Mvclpf-3 is based on the previous one, with two differences. It uses the
the technique described by Stilson and Smith to extend the constant-Q
range, and the internal sample frequency is doubled, giving a better
approximation to the non-linear behaviour at high frequencies.
This version has high Q over the entire frequency range and will
oscillate up to above 10 kHz, while the two others show a decreasing
Q at high frequencies. Moogvcf3 is reasonably well tuned, and can be
'played' as a VCO up to at least 5 kHz.

Mvclpf-4 is the same as the previous one, but adds a selection of 0, 6, 12,
18 or 24 db/oct output.

All four versions subsample the audio rate controls to 1/16, with linear
interpolation in between. This could be changed at the expense of more
CPU cycles.


--------------------------------------------
mvclpf24.so  -- version 0.4.0 - 5/4/2008
--------------------------------------------

Code cleanup.

--------------------------------------------
mvchpf24.so  -- First release - 10/4/2004
--------------------------------------------

This is based on the voltage controlled highpass filter by Robert Moog.
again with some attention to the nonlinear effects. This are quite 
different from the lowpass filters. When you 'overdrive' the filter,
the cutoff frequency will rise.

This first version is really very experimental.


-------
Thanks
-------

Many thanks are due to Dr. Matthias Nagorni of SuSE AG, who first prodded
me gently to undertake the development of the lowpass plugin, later of the 
cs_phaser and cs_chorus, and who provided invaluable feedback and comments on
the early versions. The higpass filter was also developed following a request
from Matthias, and he has already made some spectacular AMS patches with it.

