Hello Greg...and all....sadly this seems to be a common
missunderstanding ...and rightly so. FM or frequency modulation is
just that "modulating the frequency" so it's safe to say that ANY
input that modulates the Frequeny is an FM input.
For some reason some synth manufacturers have decided that FM is the
term that is ONLY used to describe inputs with a linear response.
That is to say let's assume you used an LFO squarewave to cause a one
octave jump in frequency of you oscillator. To make a two octave jump
you'd have to double the amplitude of that LFO. To make a three
octave jump you'd have to double it again (four times the original
amplitude). As so on and so on.
NOW.... truth is this isn't EXACTLY what will happen if the LFO is a
VERY low frequency as you'll see below.
Using a 1V/Octave input naturally a one volt square wave produces a
one octave jump, a two volt square wave produces a two octave jump
and so on.
The antenuating inputs respond the same way except the rate of
change is different. Instead of 1V/Octave they can be set to a
fraction of a volt (or volts) per octave based on the depth pot. That
is to say set to 0.5V/Octave....0.5 volts will cause a 1 octave rise,
1 Volt will cause a two octave rise, 1.5 Volts will cause a three
octave rise etc etc.
That's the difference in the CURVE of the response (linear vs
Exponential). The other difference is that the FM (LINEAR) input is A-
C coupled. That is to say D-C signals will not pass only changing
signals (A-C). For example....if you put a static 5 volts into the FM
input of a VCO set to 1Khz you'd hear a fast rise and semi-slow fall
in pitch until if finally comes to rest at 1Khz again. When you
remove that 5 volts the frequency will sharply DROP and they semi-
slowly rise again back to 1Khz. In other words the frequency only
changed when you added applied the voltage and took it away.... but
not inbetween. Give it a try. You MAY find the response opposite to
what I suggested as I can't recall if the FM input is inverting or
not. That is to say applying the 5 volts might cause an initial fall
not rise in frequency and vise-versa.
Hope that helps clear up the difference.
All the best....cheers Dean.
SergeModular@yahoogroups.com, "foleymachine" <foleymachine@y...>
wrote:
missunderstanding ...and rightly so. FM or frequency modulation is
just that "modulating the frequency" so it's safe to say that ANY
input that modulates the Frequeny is an FM input.
For some reason some synth manufacturers have decided that FM is the
term that is ONLY used to describe inputs with a linear response.
That is to say let's assume you used an LFO squarewave to cause a one
octave jump in frequency of you oscillator. To make a two octave jump
you'd have to double the amplitude of that LFO. To make a three
octave jump you'd have to double it again (four times the original
amplitude). As so on and so on.
NOW.... truth is this isn't EXACTLY what will happen if the LFO is a
VERY low frequency as you'll see below.
Using a 1V/Octave input naturally a one volt square wave produces a
one octave jump, a two volt square wave produces a two octave jump
and so on.
The antenuating inputs respond the same way except the rate of
change is different. Instead of 1V/Octave they can be set to a
fraction of a volt (or volts) per octave based on the depth pot. That
is to say set to 0.5V/Octave....0.5 volts will cause a 1 octave rise,
1 Volt will cause a two octave rise, 1.5 Volts will cause a three
octave rise etc etc.
That's the difference in the CURVE of the response (linear vs
Exponential). The other difference is that the FM (LINEAR) input is A-
C coupled. That is to say D-C signals will not pass only changing
signals (A-C). For example....if you put a static 5 volts into the FM
input of a VCO set to 1Khz you'd hear a fast rise and semi-slow fall
in pitch until if finally comes to rest at 1Khz again. When you
remove that 5 volts the frequency will sharply DROP and they semi-
slowly rise again back to 1Khz. In other words the frequency only
changed when you added applied the voltage and took it away.... but
not inbetween. Give it a try. You MAY find the response opposite to
what I suggested as I can't recall if the FM input is inverting or
not. That is to say applying the 5 volts might cause an initial fall
not rise in frequency and vise-versa.
Hope that helps clear up the difference.
All the best....cheers Dean.
SergeModular@yahoogroups.com, "foleymachine" <foleymachine@y...>
wrote:
> hi,both
>
> i am curious about the difference between the fm inputs and the
> attenuating/inverting (i.e. not the 1v/oct) cv input. i tend to use
> them almost interchangeably, except that when i put a low frequency
> voltage (or sequence) into the fm input, there seems to be a fixed
> slew rate. can anyone explain to me what's going on? i am using
> precision vcos and ntos.
>
> thanks,
> greg