Archive of the former Yahoo!Groups mailing list: eml synthesizers

Hi Matt,

 

Amplitude modulation is exactly that. Acontrol signal changes the amplitude, or the volume of the signal. If thecontrol signal is a low frequency, like from the LFO, it changes the volume upand down at the LFO speed. This is known as tremolo.

 

 

When you use an audio signal to modulatethe volume, Interesting things happen. In amplitude modulation, you hear theoriginal signal plus the sum and difference of the original and modulatingsignal. So, if the original signal is 1000 hz and the modulating signal is 100 hz,you would hear three tones. One at the original frequency of 1000 hz, thedifference at 900 hz ( 1000-100) and the sum at 1100 hz. (1000 + 100) This isexactly the same process as full blown broadband AM radio (Guess what the AMstands for) only, for audio, it’s a LOT slower. Since the tones aren’tharmonic equivalents, they sound clangy and bell-like.

 

If you leave the sum and differencefrequencies there and remove the original signal, you have what is commonlycalled ring modulation. In radio terms, it would be called suppressed carrier broadband,or double-sideband AM.

 

If you could look at a spectrometerconnected to the VCA output, you would see the original signal. As you turn themodulation control, you would see the sum and difference frequencies come inand get more prominent. As you turn the control toward ring, the originalsignal starts dropping out. At the ring position, the original signal is goneand the two sum and difference signals remain.

 

Remember. FM is the same process only, you’remodulating the frequency, or pitch of the signal, not the volume. At highmodulation rates, they can sound similar but are very different processes.

 

The very fact that you CAN morph from AMinto ring modulation on the 101 proves that they were innovative beyond theiryears. Most other systems can have AM or ring modulation but, nothing inbetween.

 

George Mattson