How to make a 1V/oct keyboard controller

Discussion in 'The Projects Forum' started by CiaranM, Jul 28, 2012.

1. CiaranM Thread Starter New Member

Jul 15, 2012
25
2
hello! I found a good site yesterday which has good synthesizer schematics (I'll give you the link if you want). I'm going to try and build a VCO, VCF, and VCA.
My problem is how to get pitch and gate signals. Obviously its the keyboard that does this, but how? What circuit do I need to make to recognise note presses and output CV signals? thanks.

Note: as the keyboard is a 1V/oct synthesizer, I'll need each successive 12 notes (i.e. an octave) to output one volt higher than the last. If one octave ranges up to a volt, then each key should output 0.083333..V .. I think

Apr 5, 2008
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3. CiaranM Thread Starter New Member

Jul 15, 2012
25
2
great, thanks! I've found a keyboard CV on that site; it looks bloody complex though. I hoped it would be simple.. haha

4. #12 Expert

Nov 30, 2010
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Each half-step in music is the 12th root of two times the frequency of previous note. On a 1 volt per octave scale, each voltage should be 1.0594631 times the previous voltage.

5. CiaranM Thread Starter New Member

Jul 15, 2012
25
2
are you sure that's right? I typed '1 x 1.0594631^12' (12 because I'm ascending an octave), and this gave me near enough 2V. However, 1 x 1.0594631^24 is calculated to be 4V instead of the required 3V

6. #12 Expert

Nov 30, 2010
16,665
7,310
The intervals are still 12th root of two for a "well tempered clavier".

If A is 440 hz and the next A is 880 hz, then the third A is 1760 hz. Right?
What's 1760/440?
4
What's the 12th root of 2 to the 24th power?
4
Two octaves: 4x the frequency

If the oscillator doubles frequency for every doubling in input voltage this will work.
If the oscillator doubles frequency for every 1 volt of input, it won't work, and you will have to math out a different formula.

Ediit: Good news. The schematic for the 1v/octave oscillator has been provided by the site Bertus mentioned.

• VCO20090724 pg1_schem.pdf
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Last edited: Jul 28, 2012
7. CiaranM Thread Starter New Member

Jul 15, 2012
25
2
right, I see.
I want to use the 1 volt per octave configuration, so that I don't have to use a high voltage (I haven't got any power supplies, and I'm just using batteries.)
I think that double V = double fq config was used by Yamaha? thanks for your help. that schematic looks good

b.t.w., what's that the bottom right; named PN4391? is that called a MOSFET? what makes these different from BJTs? (in terms of performance)

8. #12 Expert

Nov 30, 2010
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It's an n-channel jfet. You can find these datasheets by searching for, "pn4391 datasheet".
Jfets are very differet from bjt's because they are born in the "on" condition. You use them by biasing them more towards "off". They use very close to zero input current and generally, never pass more current than the idle current they pass at zero volts from gate to source.

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9. SgtWookie Expert

Jul 17, 2007
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Bipolar junction transistors (BJT's) are controlled by current; a small current through the base controls a large current through the collector.

FETs are controlled by voltage; a voltage on the gate controls the drain current.
The PN4391 thru PN4393 appear to be obsolete; the PN4391 is fairly hard to get nowadays. However, Arrow has over 100k of the PN4392 in stock for \$0.053:
http://components.arrow.com/part/detail/1342709S3111758N7769
You could likely get by with that.

As noted in Bertus' link; this really isn't a project for a beginner. A beginner needs to start with less complex projects; as there is much less risk involved. The more complex a project is, the higher the risk of failure. The more small projects you complete successfully, the more likely a complex project will be completed successfully.

We don't want you to fail. At the same time, there is only so much we can do for you remotely.

10. CiaranM Thread Starter New Member

Jul 15, 2012
25
2
I understand now, thanks. so current won't affect a JFET at all? and voltage won't affect a BJT at all (other than making it hot, probably)?
Thanks for the advice. I'm not that much of an amateur, ha ha. I'm able to solder, fault-find (its a surprisingly good thing to be competent at) and I've made a few things... which makes me think, do any of you ever buy project books or anything like that?

11. MrChips Moderator

Oct 2, 2009
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I presume that you are attempting to build an electronic keyboard. Any reason why you wish to go analog instead of doing it digitally using a microcontroller?

12. CiaranM Thread Starter New Member

Jul 15, 2012
25
2
you can use a microcontroller for 1V/octave control?? I'm not really concerned about whether its digital or analogue, just whether it works or not

13. MrChips Moderator

Oct 2, 2009
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You can use an mcu to deliver just about anything you want.
In this case 1V/octave is irrelevant. The mcu will just detect which key is pressed and deliver any note that you desire.
(or any sound, siren, drum beat, bird whistle).

14. CiaranM Thread Starter New Member

Jul 15, 2012
25
2
sounds interesting. would I have to set up resistors or whatever across the keys so that the microcontroller gets different voltages for different keys?

15. MrChips Moderator

Oct 2, 2009
12,624
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You're thinking too much analog. You have to take a systems approach.

Tell me what you are attempting to do. Are you building a keyboard from scratch?
Is it a digital keyboard or more like an analog synthesizer?

A lot has to do with the mechanical keys. If you are building your own that's a huge task.
If you already have a keyboard that you wish to adapt that makes a huge difference.

Digital polyphonic keyboards can recognized any number of keys pressed and are touch/velocity sensitive. The velocity sensing is the difficult part and I am no expert at this.

Once you have this pair of information for each key, the sky's the limit. You just have to use your imagination as to what type of sound you wish to generate. Everything from here on is done in software so you don't have to mess around with hardware VCF, VCA, filters, etc.

Another approach if you wish to create your own sound effects is to use a MIDI keyboard and capture the keys pressed on a computer. Then the computer software can generate the sounds for you. There must be tons of software already out there that does this.

If you wish, you can do away with keys entirely and use a touch sensitive screen, much like an iPad. Then you can have smooth glides from one note to another.

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16. CiaranM Thread Starter New Member

Jul 15, 2012
25
2
I'm going to try and build a VCO, VCF, and a VCA. So its all analogue, for now.. I'm most likely going to buy a cheap keyboard off eBay so that I can take the keys from it. I'm not concerned with velocity or any of those sorts of things (aftertouch? what, you think I'm Bach?! .. I'll be Bach! ..sorry), just gate and CV for pitch. Analogue synths have a limitation of one note at a time, unless you add more oscillators, is that right?
I guess with VC synthesizers, the ability to use any signal for modulation appeals to me. and the sound

thanks for your help, its immensely useful!