Wet-Dry AM.pd
Hey everyone,

I'm experimenting with trying to get a clean fade between the dry carrier sound and the wet modulated sound when using AM but am running into a couple problems.

1. When I make the modulating signal into a unipolar waveform, I can get a clean transition between the dry sound and modulated tremolo sound, but at faster modulation speeds the carrier can be heard with the other 2 frequency bands
2. When I use the bipolar waveform, the transition between the dry signal and the tremolo signal is sloppy and the rate begins to sound like it doubles

I'm wondering why the carrier sound is present when modulating at high speeds with the unipolar waveform but not with the bipolar waveform? And I'm also curious why the tremolo speed feels like it doubles in its rate when crossfading with the bipolar waveform?

Ideally, I would like to have the best of both worlds in the patch where it crossfades cleanly into a full tremolo, but also where the carrier signal doesn't exist when the modulation is fully wet. Not sure if videos work here, but I'll try and post one.[link text](Amplitude Modulation.mp4 link url)

When you use the bipolar wave form you are modulating twice as fast as when you use the unipolar wave. sin(x) starts at zero and crosses 0 halfway through its period, but the unipolar (sin(x)+1)/2 only reaches zero once during its period. When you use the bipolar wave you are probably hearing both the frequency of the carrier and twice that frequency.

Bit o' algebra:

sig * (mod * 0.5 + 0.5)
= 0.5 * sig * mod + 0.5 * sig

Multiplying your signal by a unipolar sine produces a half-and-half mix between the modulated and unmodulated signals.

I think this will translate to any modulator that is not centered around 0. Any DC offset in the modulator will allow the original signal to leak through.

So you will have to use a bipolar modulator. You could halve the modulator frequency.

I don't understand, though, what "sloppy" means...?

hjh

Couldn't resist playing with this a little more.

This patch demonstrates the double-speed problem. Ring modulator math requires multiplying by a negative sometimes, but of course we don't perceive a negative amplitude. The amplitude we perceive will be the absolute value of the modulator.

To make a smooth transition from an unmodulated signal, I'm using the formula (mod - 1) * mod_amount + 1:

• If mod_amount is 0, then the modulator ranges +1 to +1 = no modulation.
• If mod_amount is 1, then the modulator ranges -1 to +1 = full modulation.

When mod_amount > 0.5, then there is a negative lobe in the modulator, which folds over to positive in our perception. It's this wave-folding function that causes the doubling in speed (as gn said).

21-1021-amp-mod.pd

(The audio source in this patch uses https://github.com/jamshark70/hjh-abs and [sf-play~] in turn depends on cyclone. You can substitute any other audio source.)

I don't have a really good workaround for that. As an experiment, I tried cross fading between a full speed modulator and half speed (substitute this subpatch in place of the [osc~]). It sounds a little awkward (and you should take care that the LFOs stay in phase -- otherwise the linear cross fade will drop 3 dB in the middle).

I'm afraid I don't have a better solution -- just, maybe this sheds more light on it.

hjh