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 Theremax, Pitch CV, linearity 
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Post Theremax, Pitch CV, linearity
Steve Hobley wrote:

> Thanks - has there ever been any discussion of adding an anti-log function to the output of the Pitch CV in an attempt to create a more linear playing response?
> There's some talk in the Theremin community of different antenna configurations that can do the same thing.
>
> I've been looking into the anti-log opamp designs, some of them seem pretty simple.
> Although I'm not yet sure how to "tune" the response curve.
>
> Steve
>
>

Hello Steve.

I've seen some mention of this on the internet and there is a person that has some changes posted to add circuitry for getting a more evenly spaced hand to pitch antenna response, but it looks rather involved. I haven't tried it because in my experience the response doesn't necessarily need alteration. I might just be accustomed to the way it is too. Just the nature of the theremin too, with it's sensitivity to skin and bones as a conductor to earth ground, make the positioning of the instrument and its proximity to any other conductors or the ground, factors in the control response. The antennae size and shape and their mounts are factors too. As they change much from the Theremax lectern case design, variations in control response can be expected. As an extreme example, the unit can be tuned and tested without any antennae attached as long as the ends are positioned away from the board, its wiring, and any other conductors, and the pitch and volume varied by moving the hand to and from the antennae ends. The range is greatly reduced. I've experimented with attaching a very long (ten to twelve feet) wire and supporting it up off the floor with cardboard cartons (suspending it with string probably would've worked too) and tuning it for this condition, making the pitch sensitive to my walking about the room. Another time I formed a spiral of some of our #22 tinned-solid hook-up wire (supplied for use for printed circuit board jumpers and panel point-point wiring) and twisted this around the end of the usual pitch antenna. Tuned for this different condition, it could be set into motion with just the slightest nudge and would wiggle around for quite a while making a wobbly sound--it caused me to think of wind/breeze sensing...

The pitch change for the hand to antenna change is most evenly spaced when the pitch tuning and Pitch Trim control setting is not at null, but at a lowest anticipated 'idling' frequency, with the hand moving to the antenna working to go up to the desired pitch from there: The hand to the volume antenna works to quiet the output and silence this starting tone. When the pair of oscillators are operating at the same frequency, they have a tendency to want to stay in unison, and this is the null or zero heterodyne frequency tuning. So if the initial tuning is to null, it takes a certain amount of hand to antenna control range to break the pair out of this tendency and then to pass through sub-audio range and into a useful song or melody range. Starting from a low initial setting makes use of all the hand to antenna sensitivity. Note too, there is mention of using coupling to hold the pair at null and this makes the sensitivity of the hand to antenna even less (ie the twisted wires on the emitters or the 10k added to change tone (this occurs as a result of a) weakened pitch heterodyne and b) increased coupling tugging at wave peaks)--I recommend the changes in the Tips and Suggestions I have prepared (3-100pF and 1-6800ohm)).

So, you might just try using some variations in the pitch antennae, with varying lengths and forms, using a 12 or 14 ga. solid wire, using the Theremax lectern case pitch antenna as a point of reference. But also, since you're using a micro-controller and making a conversion to get MIDI Control, you could conceivably tune each step, or put in a 'training' mode where it reads the hand to antenna change and matches this to the desired scaling, be it a linear one with bends at the ends, or an exponential or log function.

Thanks.

--Scott



Steve Hobley wrote:
> Thanks - has there ever been any discussion of adding an anti-log function to the output of the Pitch CV in an attempt to create a more linear playing response?
> There's some talk in the Theremin community of different antenna configurations that can do the same thing.
>
> I've been looking into the anti-log opamp designs, some of them seem pretty simple.
> Although I'm not yet sure how to "tune" the response curve.
>
> Steve
>
>
> Steve Hobley wrote:
>
> Scott,
>
> A while back I asked you about getting pitch tracking to work
> a bit faster than it does on the Theremax - I think you
> replied with some suggestions about changing a cap.
>
> Can you remember what that was?
>
> Steve
>

> On Tue, Aug 25, 2009 at 12:28 PM, Scott Lee <scott@paia.com <mailto:scott@paia.com>> wrote:
>
> Hello Steve.
>
> That was C24. The lower the value, the faster the charge can
> vary. Also related is C21. Again, a lower value will work to
> speed-up the time the voltage can change. Watch the effect of
> less capacitance on the CV output. As it gets too low, the audio
> fluctuations will begin to appear on the dc level which would
> introduce errors in the CV to MIDI conversion.
> Thanks. --Scott
>
>
> Steve Hobley wrote:
>
> Scott,
>
> A while back I asked you about getting pitch tracking to work
> a bit faster than it does on the Theremax - I think you
> replied with some suggestions about changing a cap.
>
> Can you remember what that was?
>
> Steve
>

_________________
Scott Lee
scott@paia.com


Wed Aug 26, 2009 10:05 am
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