[linux-audio-dev] LFO Phaser LADSPA plugin in details

Fons Adriaensen fons at kokkinizita.net
Sun Mar 11 23:44:34 UTC 2007

On Mon, Mar 12, 2007 at 12:53:28AM +0300, Andrew Gaydenko wrote:

> Will anybody find a minute ot two to explain me how does the plugin
> work - I mean a user POV rather technical realization details.

(Assuming you mean my plugin from the MCP package)

This is an emulation of an analog phase delay line phaser.

"Input gain (dB)"  Just what is says it is...

"Sections", The number (1..30) of first-order allpass filters that
form the delay line. The phase shift of each section is zero at LF
and goes up to 180 degrees at HF.

"Frequency", (in octaves) the frequency at which each filter section
produces 90 degrees phase shift. The range is 12 octaves. Halfway
is middle C.

"LFO frequency (Hz)" Frequency of the LFO that modulates the "Frequency"
parameter above  (0.01 to 30 Hz).

"LFO waveform". Sets the waveform of the LFO, from falling saw, over
triangle, to rising saw.

"Modulation gain", The amount of modulation by the LFO output.

"Feedback gain". The gain (-1..1) of the feedback from the delay
line output back to the input.

"Output mix". The first half crossfades between the inverted delay
output and the input, the second half between input and the normal
delay output. Mid position is input, i.e. no effect.

So if you set "Sections" to N, the phase shift in the delay line will
vary between 0 at LF and N/2 cycles at HF. The "Frequency" setting will
determine the shape of the phase curve and consequently the set of N/2
frequencies where the delay output is in antiphase with the input.
Setting "Output mix" to +0.5 will produce nulls at these frequencies.
Setting it to -0.5 will produce maxima at these frequencies. "Feedback"
will modify the effect in complicated ways. Finally the LFO makes the
set of frequencies move up and down, producing the phasing effect.


Follie! Follie! Delirio vano è questo !

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