I've been test driving the demo version of the Arp 2600v and made the following observation with respect to modulating one audible oscilator's frequency with another oscilator running at the same audible frequency (to sort of emulate simple FM sythesis principles). I noticed that when modulation is introduced the timbre changes but the overall pitch of the resultant waveform does not remain constant at different modulation depths, as one would expect when performing FM modulation at matched (or related frequencies).
This leads me to believe that the waveform performing the modulation (a sine wave, to keep it simple) is biased in some way (or effectively becomes so). Perhaps the modulating waveform troughs (lows) are not treated as negatives (being either zero values at their lowest point or perhaps the negative amplitudes are 'flipped' into positive amplitudes). This, at least, appears to be what the results of FM modulation on the 2600v appears to sound like when moving the modulation slider to vary the amount of this type of modulation.
Is this how the original 2600's FM modulation functioned?
Basically, I'd like to know whether on an original Arp 2600 if I were to have one oscilator audible (say oscilator 3) and introduced modulation from another oscilator runing at the same tuned frequency (say oscilator 2), would the resultant's note's pitch remain the same as I moved the modulation depth's slider whilst only the note's timbre and harmonics changed.
This is basically what would happen on a DX7 FM synthesizer. I see no reason why the principle would not remain the same on an analogue system, provided that the control voltages from the modulation source were to fluctuate between -x and +x 'volts' with 0 volts representing the midpoint of the waveforms overall amplitude (the horzontal axis).
Is anyone fortunate enough to have access to a real 2600 to test this out on?
and, am I making any sense? :wink: