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Originally Posted by zmix  Hi Sean,
Have you ever listened to a Bode Frequency shifter? |
I have been programming these since 1999 - it was one of my first goals in learning digital audio. I've done a lot with them since then, including putting them in the feedback path of allpass loop reverbs. I didn't like the results that much, at least as far as producing a modulated reverb sound.
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There is a fair example in the Logic built in plugins calld "Ringshifter" if you set the modulation oscillator to 1.5hz or so the effect is very close to the what the modulation on the PCM96 "Hall" and "Random Hall" sounds like.
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Here's a way to test things:
Run sine waves of different frequencies (100 Hz, 500 Hz, 1000 Hz, 2000 Hz) through the algorithms in question. See if the beat rates sound similar. If the beat rates are similar, then the modulation is some form of amplitude modulation (which frequency shifting can be classified as in some weird sense), which produces a beat rate that does not vary with input frequency. If the beat rates get higher with the input frequency, it is probably delay modulation.
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The chorusing LFO in the PCM96 sounds like a square wave and has a maximum rate of 1.5hz. It is not randomized.
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This sounds like the modulation produced by a triangle wave LFO, which produces an up/down modulation when used to modulate a delay line. In most digital systems, a square wave LFO ends up producing different delay lengths with no pitch change, except for a glitch when changing between the LFO output values.
Of course, this is based on one method of calculating delay modulation, while other methods could result in a square wave LFO producing the up/down modulation, and a triangle wave sounding more like a sine wave. It all depends if you are adding the modulation as an offset to your delay address, or doing something else. For BBD-based effects, the delay modulation LFO is usually controlling the clock rate, so a square wave LFO can end up producing an up/down pitch change.
Sean