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 Forum index » DIY Hardware and Software » Musical Interfaces
Absolute values for 1v/octave ?
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Mongo1



Joined: Aug 11, 2011
Posts: 411
Location: Raleigh NC

PostPosted: Wed Aug 17, 2011 7:18 am    Post subject:  Absolute values for 1v/octave ? Reply with quote  Mark this post and the followings unread

Hi,
I'm getting ready to convert an old Kimball organ into a dual 1v/Oct controller. I plan on using the controller boards Ray Wilson at MFOS makes for the project, but had a question.

Is there any standard for what voltage represents a given note? In other words if I press the Middle-C key, should it generate 3.23 volts (or something), or is it all just relative?

The reason I ask is that I'm planning on making my VCOs use octave switches instead of the more common Coarse tuning knob. I'd like to be able to calibrate the whole thing so I know that if I place the coarse knob at X, and the Fine tune in the center of its range, and press middle C, that I am actually close to middle C, instead of some totally random note.

Is there a standard that would help me calibrate this whole thing?

Thanks
Gary
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Mongo1



Joined: Aug 11, 2011
Posts: 411
Location: Raleigh NC

PostPosted: Thu Aug 18, 2011 1:40 pm    Post subject: Well, I guess I'll propose an answer Reply with quote  Mark this post and the followings unread

Since there have been no responses, I assume either the question is just too dumb, or nobody knows the answer.
Either way, I guess I'll propose a solution - if anyone has any feedback I'd appreciate hearing from you...

Well, here's what I'm thinking. I have a table below of the frequencies of 'C' in various octaves.
A piano's lowest note is A0 (27.5 Hz), and the highest is up at C8. I notice that on the Buchla complex waveform generator,
the Principal Oscillator can go from A1 through A9, while the Modulation Oscillator can go from A0 through A8, and with the flick of a switch (LFO mode) , can go from .25 Hz up to 64 Hz

So - it seems like assigning C0 as 1 volt would be a reasonable thing to do. That would allow me to set up the fine tuning control with a range of +/- 1 octave. If the same oscillator has to go into a low range, a change could made to the integrator to knock the frequency down several more octaves.

Unless anyone knows of an alternate approach, I guess that's the thing to do. I realize that tracking over this whole range will probably not be possible, so I'll focus on the C1 to C8 range. If I can get that to work, I'll be pretty happy.
Gary

C-1 8.175 0V
C0 16.35 1V
C1 32.70 2V
C2 65.41 3V
C3 130.8 4V
C4 261.6 5V
C5 523.3 6V
C6 1047 7V
C7 2093 8V
C8 4186 9V
C9 8372 10V
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jordroid



Joined: Jan 17, 2010
Posts: 193
Location: ithaca, new york

PostPosted: Thu Aug 18, 2011 7:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think for the DIY stuff we are up to there is no standard for this, and you are on the right track with your approach of going for what makes sense to you.

I like the idea of using octave switches and fine tuners too, a lot of people like the minimoog approach, where the fine tuner will swing a major sixth (nine semitones) in either direction, allowing you to hit any interval you want with finer resolution than if you had it swing a whole octave up and down. Note that you can hit all the notes with six semitones of swing either way, for slightly finer resolution, but it may make more intuitive sense for a musician to tune up to a fifth, rather than switch up an octave and then tune down, a perfect fifth is probably a common interval to use, which makes the minimoog sixth seem like a pretty well thought out choice.

Sounds like you are on the right track, keep us posted Smile

jordan

(it sounds like you have a foundation in music and descriptions like "major sixth" will make sense to you, apologies if this is incorrect)
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Clack



Joined: Aug 08, 2005
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PostPosted: Fri Aug 19, 2011 2:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey,

I think you have to remember that input voltage to frequency out is a relative measurement and is dependent on the VCO settings. The VCO is tuned by a voltage at the same input in the VCO input from a trimmer usually which quite often holds it at a funny place like -2.35 (made it up) and this point is not going to be the same on all VCO's even of the same design because of tolerance.

The Middle C output voltage is also relative to the size of the keyboard (# of octaves) On a bus style keyboard (the MFOS one) the lowest key is 0v because that is the way it is designed, it would be pointless to shift it anywhere else with extra circuitry - unless you have a lot of octaves and it maxes out the input!

If you want your middle C to be standard middle C (by Hz)

1.make sure the bus keyboard is calibrated properly and so is the octave select switch ( check it is putting 1v/oct out )

2.make sure the VCO is responding to 1v/oct (set keyboard at 0v, set VCO to 100hz then press each octave to see if it is correct, it should double each octave)

3.press middle C set octave switch to its middle setting ( 8'? 4'? ) http://en.wikipedia.org/wiki/File:Pitch_Table.png

4.tune to whatever it should be 8' = 65.4Hz

(maybe 2 sould be after 3?)

The only thing you have to be wary of is how far your CV will wane from its initial tuning point because of the accuracy of your 1v/oct calibration.

but as we have tuned both to their middle points it wont wane far! if both do 4 octaves that will be -4v +4v max from that tuning point

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Antimon



Joined: Jan 18, 2005
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PostPosted: Fri Aug 19, 2011 4:58 am    Post subject: Reply with quote  Mark this post and the followings unread

I was going to respond earlier, but since I don't really know the true answer I thought I'd wait for someone more knowledgeable to post a number.

Anyway, when I've handcoded chuck programs that send CV through my MOTU soundcard, I've taken a MIDI note in the middle of the range (#64 - E0) on zero volts, and then gone from there, lower notes than E0 on negative voltages and higher on positive. I have no idea how this will work with other CV gear - I don't have any proper CV controllers.

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Mongo1



Joined: Aug 11, 2011
Posts: 411
Location: Raleigh NC

PostPosted: Fri Aug 19, 2011 5:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks to all of you - that's some really good input.

I'll process this input and see where it leads.

I did want to address this point that Clack made:
The Middle C output voltage is also relative to the size of the keyboard (# of octaves) On a bus style keyboard (the MFOS one) the lowest key is 0v because that is the way it is designed, it would be pointless to shift it anywhere else with extra circuitry - unless you have a lot of octaves and it maxes out the input!

That's really one of the root issues here. The organ I'm retrofitting is a dual keyboard spinet. The keyboards start on different notes.
My thought is that it makes the world a much simpler place if I use the MFOS controllers to generate a range of 0 to N volts, and then add a fixed offset which corresponds to the actual starting note for each keyboard. That way hitting middle C on one keyboard will generate the same voltage as middle C on the other. Otherwise it starts getting really confusing.
At least that's what I'm thinking right now....

Thanks again!

Gary
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factus10



Joined: Jun 20, 2007
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Location: Buffalo, NY

PostPosted: Sun Jan 01, 2012 7:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Did you see this wikipedia article:
http://en.wikipedia.org/wiki/CV/Gate

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Mongo1



Joined: Aug 11, 2011
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PostPosted: Sun Jan 01, 2012 9:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah - thanks - I'd looked around on Wiki before but hadn't seen that one.

Gary
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