Archive of the former Yahoo!Groups mailing list: MOTM

Sorry, I'm a lurker (But Paul can vouch for me)

Analog FM's limitations are actually more basic then what was discussed.
why all of the points are valid the most fundamental limitation of trying to
accomplish "DX-7 style FM synthesis) with conventional analog VCOs is that
such a VCO is not capable of modulating "through zero frequency". This
limits the modulation index to the less interesting lower range. For
example, using a 1:1 carrier to modulator frequency ratio the maximum
modulation index with a conventional VCO is 1. Unfortunately if any of you
remember your DX-7 experiences the most interesting sideband evolutions
start occurring above this figure.

The reason for this limitation has to do with some pretty esoteric technical
details but in a nutshell most VCOs can not "reverse on themselves". By
this I mean that as the modulating wave drives the carrier's frequency
toward zero Hz it can not force it into "negative frequencies". And yes
these really exist, it just a 180 degree phase reversal on the waveform.

There is a way of making a VCO that is capable of this performance and is
something I discuss if anyone is interested. or you can just scan some old
Electronotes issues in the issues around #63 I think for a good write up.

Back into lurk mode.

Douglas Kraul