Doepfer

Hi

> Is it?  What's the difference between the effect of having different  
> cores?
The theoretically resulting waveform of FM is only "real" for the core 
wave form. All other waveforms will be that, what the waveform converter 
produces from the fm'ed core-waveform.


I uploaded two graphic files to the files section:
http://launch.groups.yahoo.com/group/Doepfer_a100/files/analog-fm/
These are excerpts from my book.

I some months ago wrote the following text in private mail to Bakis, 
which explains the images:
-----------------------snip------------------

The most relevant thing is: FM with analogue VCOs has nothing to do with 
the theoretical FM, because everything depends on the working principle 
of the core oscillator (saw or triangle), and what the waveformconvertes 
make out of the fm'ed core wave form.

I'll attach two excerpts from my book:
analogue_fm.png explains what really happens in a saw based VCO. Left is 
an log FM, right is the linear FM:

* the first row (rectangle) is the modulator with an amount of +1V to -1V

* the second row (400Hz saw) is the carrier

* the third row shows the two saws which would be created by a CV of +1V 
and -1V. In log FM this will be 1V up/down => 1 Octave up/down, => 
800Hz/200Hz. In lin FM this will be 1V a certain frequency up/down, => 
lets say 200 HZ => 600Hz/200Hz. The higher result(800Hz or 600Hz) is 
light grey; the lower result (200Hz) is dark grey. The two frequencies 
look like two ramps with different steepness.


* Now the fourth line shows what really happens in the VCO: There is a 
thin horizontal line, which is the threshold at which the Capacitor in 
the core-oscillator is unloaded.
The rectangle switches between the two steepnesses. With the bigger 
steepness of the 800Hz at the log FM the resulting wave form will reach 
the threshold faster than with the "600Hz-steepness". As you can see the 
  threshold will be reached at the same time as with the original 
carrier signal.

Thats the one graphic.


The second graphic shows, the same FM, but this time not the core saw 
oscillator, but the output of different wave forms from the waveform 
converter. The Modulator still is the known rectangle.
Line a shows what we would expect for a triangle carrier.
Line b shows what we would expect for a rectangle carrier.
Line c shows what we really will get at the triangle output.
Line d shows what we really will get at the rectangle output.

The latter two show, what the waveform converter will create from the 
originally FM'ed saw from the core-oscillator. Here you can see that 
this has nothing to do with real FM
-----------------------snip------------------