Re: [Audacity-nyquist] Problem with this plug-in in stereo

[<edg...@we...>]

Hi David,

The bug is caused anyhow by the offset calculation in the rectify function.
With stereo tracks one channel is shifted upwards and the other one downwards.

I assume you use the offset of 0.475 to compensate the direct current
component caused by the rectification. Unfortunately a value of half
maximum peak only works with square wave signals. To calculate the offset
needed for a complex signal is pretty complicated. If you find a good
formula for this you can make a lot of money.

In electronics this is usually solved by using highpass filters because
a constant signal or a slow changing signal cannot pass a highpass filter
and a highpass filter is easy to calculate. I suggest a cutoff frequency
of 20Hz or something around this.

But I first have to have a closer look at the code to find out wether
or where exactly a highpass filter could help to solve this.

I will write you more in a couple of hours.

Cheers,

- edgar




the transcript of your original request follows


aud...@li... schrieb am 27.11.05 20:13:15:
> 
> Hi,
> 
> Attached is rectimod.ny , Rectifier with Modulation under the
> effects menu. (Explanation below.)
> 
> This plug-in works great with mono signals, but when applied
> sequentially more than once to a stereo signal (panned to center),
> the signal gradually separates into two separate signals in the
> left and right channels. Is it something with my code or Audacity
> Nyquist?
> 
> Thanks
> 
> David
> 
> * rectimod.ny : Rectifier with Modulation
> 
> A rectifier takes a signal (such as a sinewave or guitar track)
> which goes above and below the zero line, and either returns only
> the positive portion of the signal ("half rectification"), or it
> also returns the negative portion of the signal which has been made
> positive ("full rectification"). The result of such rectification
> is added higher frequency harmonics not in the original signal.
> 
> Regular rectification makes the entire signal positive, so re-
> applying a rectifier adds no further higher frequency harmonics.
> However, this plug-in takes an audio signal, applies half or full
> rectification, then converts the all-positive result into another
> signal which crosses the zero line. this means the plug-in can be
> re-applied to the result, adding additional high-frequency
> harmonics with each application.
> 
> this plug-in also has four options for modulating the resulting
> rectified signal and mixing it back with the original signal. For
> instance, the default settings have the original selected signal in
> Audacity fade out while the rectified signal fades in.
> 
> Variables:
> 
> 1. Rectification type - half or full rectification.
> 
> 2. Modulation type - 
> up (fade in), 
> down (fade out), 
> constant (same level over time), 
> LFO (low frequency oscillator, a sine wave which rises and falls
> below 20 cycles per second).
> 
> 3. LFO frequency - frequency of the LFO signal, when LFO modulation
> is chosen.
> 
> 4. Rectified volume - maximum volume of the rectified signal, in
> percent.
> 
> 
> Many guitarists use rectifiers as part of their 'effects suite'.
> the noisier the original guitar signal (from fuzz boxes, distortion
> pedals, etc.), the noisier will be the result from this plug-in.
> 
> Works on mono and stereo audio, best results occur with mono input
> signals.
> 
> 
> Written by David R. Sky, November 27, 2005.
> 
> Thanks to Steven Jones for a Nyquist patch to make the rectifier
> possible.
> Released under terms of the GNU Public License
> http://www.opensource.org/licenses/gpl-license.php
> 
> Other Nyquist plug-ins are freely available for download from:
> 
> http://audacity.sourceforge.net/download/nyquistplugins/
> 

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