[sdiy] Linear Analog Synth Portamento

[Scott Gravenhorst]

David G Dixon <dixon at mail.ubc.ca> wrote:
>> What I hear from my Fatman sounds natural enough to me.
>> 
>> So what am I missing?  Is "linear portamento" bad?  What 
>> won't "linear portamento" do that I need or want?
>
>A pot-and-cap gives exponential portamento, which is actually constant-time
>portamento (because the cap-charging current which defines the glide time is
>the integral of the voltage-time curve, and this integral is proportional to
>the initial voltage difference for an exponential decay function), which is
>the normal kind of portamento (because it's so damn easy to do).  In this
>case, the initial glide rate is very fast, but the final approach to the
>destination is asymptotic (i.e., much slower).
>
>With linear portamento, the glide rate is constant (because it is determined
>by the rail voltage of a comparator), but the glide time is proportional to
>the initial voltage difference.  It gives a different sound, and is perhaps
>less useful for keyboards and sequencers, because the time of arrival is not
>uniform -- i.e., of the glide time is to large, some notes will not reach
>their destination voltages before the next note is played.
>
>It is possible to have constant-time linear portamento, but (AFAIK) this
>requires storing the voltage difference and using it to program a
>variable-current source to the Miller integrator.  This would be a very cool
>effect, and it is something I'd like to try to build in the near future
>(probably with linearized 2164s).  (Consider: three VCOs in tune playing
>random 3-note chords, and each time the chord changes, all three notes snap
>(not in an asymptotic manner, but abruptly) to their destinations at exactly
>the same time, no matter how big their individual intervallic jumps are.
>That would be awesome!)  If someone here is sitting on such a circuit, I'd
>love to see the schematic!

That's interesting, using an actual linearly changing voltage - yes, that would give
different times for VCOs with pitch voltage differences that are, uh, different.

In the Fatman and in my digital synths that use this RC modeled method, the time is
equal.  And to have more fun with that, in my digital synths, I can give each NCO it's
own portamento time setting so that (in a 4 NCO per voice synth) when the note changes,
the NCOs go out of tune and then come back together.  It's a nice sound...

-- ScottG
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-- Scott Gravenhorst
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