[sdiy] Linear/Log FM (was "Living VCOs" PCB in 2009 ?)

[Simon Brouwer]

Tom Wiltshire schreef:
>
> On 10 Dec 2008, at 11:19, Simon Brouwer wrote:
>
>> "Linear" on that type of modulation input means that the frequency
>> deviation has a linear relation to the modulation voltage. *This
>> deviation
>> is proportional to the unmodulated frequency*.
>>
>> In contrast, Jürgens "linear detuning" control provides an independent
>> frequency offset.
>>
>> So with the linear modulation input (if it is not AC coupled!), or
>> with
>> the normal fine tuning control, you could get a frequency deviation of
>> e.g. 1 Hz but this deviation would be 2 Hz that when you play a
>> note one
>> octave higher.
>>
>> With the "linear detuning" you could set a 1 Hz deviation across
>> the pitch
>> range. This way you can get a pleasant rate of beating between
>> VCO's at
>> lower notes without them sounding too out of tune at higher notes.
>
> Is this right? That wasn't my understanding of a FM linear modulation
> input. I thought the folllowing;
>
> If it's linear, a signal of a given level produces a modulation of a
> given depth, say 200Hz. What this represents musically will vary with
> pitch.
> If it's logarithmic, a signal of a given level produces a modulation
> of a given interval, say 2 semitones.
>
> So a linear detune control is just a small DC offset fed to a linear
> FM input.
>
> Or am I hopelessly wrong about this (like lots of other stuff)?

I wouldn't say hopelessly ;)

The linear FM modulation in most (exponential control) VCO's works by
addition of the modulation signal as a current to the reference current of
the exponential converter. The exponential converter multiplies this
reference current by a factor determined by the exponential control
voltage, and the VCO frequency is proportional to this resulting current.

See for example http://www.elby-designs.com/asm-2/vco/vco1-cco-asm2-cct.pdf

Because of this you get a modulation index (the ratio of deviation to
modulation frequency) that is independent of the pitch of the note you
play. If this were not so, the effect of FM would be much stronger for
lower notes than for higher ones.

See http://en.wikipedia.org/wiki/Frequency_modulation_synthesis

-- 
Vriendelijke groet,

Simon Brouwer
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