Modulate one waveform/track with another (FM synthesis)

[steve]

@Thunderbolt1000T

Steve says that the only way this can be done is to put one waveform on the left channel and the other on the right, and combine them to a single stereo track.

I agree with him
Make a stereo track with the wave to be modulated as the lower track and the modulation wave in the upper track. Typically the upper track would have a very low frequency waveform. If you’re clear about how to do that, the next bit is easy.

Select the stereo track and from the Effect menu select “Nyquist Prompt”.
In the text box of the Nyquist Prompt, copy and paste this code:

(defun fm (s-in)
   (let* ((s-in (lowpass8 s-in (* 0.2 *sound-srate*)))
         (s-in (lowpass8 s-in (* 0.2 *sound-srate*)))
         (map (integrate (db-to-linear (aref s-in 0)))))
      (snd-resamplev (aref s-in 1) *sound-srate* map)))

(fm s)

Then press the OK button.

Note that because FM can create frequencies that are twice as high as the source waves, aliasing distortion could occur if the source audio contained frequencies above 1.4 of the sample rate. For best results with this plug-in the audio track should have a sample rate of 96 kHz.
To change the sample rate of a track, use “Resample…” from the “Tracks” menu.

Note 2: Use the latest version of Audacity with this effect (currently 1.3.13).

Note 3: I’ve only tested this on short samples. Long tracks are likely to be slow and as it is little tested I can’t guarantee that it won’t crash Audacity.

I’ll be interested to hear how you get on with it.