MOTM

For those of you newer to the list, there was a thread about analog FM about
6 months ago.  Search the archives and I am sure you will find it.  There
was even a list of all the modules you would need to pull it off.  It will
probably take me 10 years to acquire that much.

Paul, any comments on this thread???

--Shemp

> -----Original Message-----
> From: Douglas R. Kraul [mailto:dkraul@...]
> Sent: Sunday, August 22, 1999 8:14 AM
> To: motm@onelist.com
> Subject: [motm] Re: Analog FM
>
>
> From: "Douglas R. Kraul" <dkraul@...>
>
> Here's some more details about "through zero" linear FM.
>
> I can't speak to the circuit mentioned as I have not seen it.
> There was an
> approach floating around about the same time as the this other approach
> (which I will describe) that used two VCOs and a balanced
> modulator in order
> to generate a difference frequency.  This produces proper spectra
> for FM as
> it allows the difference frequency to be negative, which is the
> requirement
> for high modulation index FM.  So I wonder if the mentioned
> approach is the
> same?  BTW if it is this approach it has a set of issues that make it
> somewhat impractical for a precision VCO.
>
> Anyway, the "preferred" approach IMHO is to implement it in the following
> way.  Suppose for the moment that we are starting with a conventional VCO
> design that produces a triangle as its base waveform, as opposed to a
> sawtooth like the 300 does.  The "trick" to making a through zero VCO is
> that the charging current must reverse its direction at the right moment,
> the moment when the modulation has produced exactly zero Hz in the carrier
> and is about to transition through zero.  Reversing the current at this
> point is precisely what happens in a theoretical through-zero VCO as the
> change in charging current is equivalent to a phase reversal of the
> waveform, and reverse phase is equivalent to negative frequency.
>
> Thinking about it another was, if you look at the canonical digital FM
> equations and think about how the digital oscillator is implemented it is
> typically done with an accumulate function.  When the modulation is large
> enough it will actually be bigger then the increment that is
> added each time
> to cause the base frequency in the carrier.  If the modulation waveform is
> negative (as it will be as it completes its cycle) the result will be a
> negative number being added in the accumulate.  This will
> momentarily cause
> the accumulate to "back up".  Again this is just another way at looking at
> the negative frequency thing.
>
> The VCO modified for through zero FM in the above manner is a bit more
> complex circuit-wise in that you need to detect when the linear fm input
> would be going through zero and keep that from happening while at the same
> time cause the charging current to reverse.  There is some hair on the
> circuit to do this but really it is not much worse then any good precision
> VCO design.
>
> As for the motivation behind this, my memory of hearing this 24 years ago
> was that it gave me goosebumps!  The richness of the evolving FM
> spectra was
> a perfect counterpoint to the normal subtractive VCF idiom.  I was estatic
> when the DX-7 first showed up because it was "fm in a box" but the digital
> implementation never left me with quite the same first feeling.  As with
> most things digital it was a bit colder.  That said the Nord
> Modular offers
> "VCOs" with through zero FM and they sound pretty good - for digital that
> is.  Unfortunately, my through zero VCO prototype has long gone to DIY
> heaven otherwise I would post examples.
>
> Douglas
>
>
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