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1v/oct and hz/v
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author 1v/oct and hz/v
axolotl
i've been pondering the usage of exponential conversion, and how i don't understand schematics where it's implemented

i'm designing a synth that generates CV values from a micro-controller. it occurred to me that i could have the micro-controller generate the CV values i need, rather than relying on an analogue exponential converter

i did some research, and looked into the differences between the 1v/oct standard and v/hz. here's an easy to read example of the two, tuned to A:

[1v/oct CV]

freq * (2^v) = note

55hz * (2^0v) = 0055hz A1
55hz * (2^1v) = 0110hz A2
55hz * (2^2v) = 0220hz A3
55hz * (2^3v) = 0440hz A4
55hz * (2^4v) = 0880hz A5
55hz * (2^5v) = 1670hz A6

----------------------------

[hz/v CV]

freq * v = note

55hz * 1v = 055hz A1
55hz * 2v = 110hz A2
55hz * 4v = 220hz A3
55hz * 8v = 440hz A4

----------------------------

clearly there are differences in resolution. my design so far uses a frequency divider, and i thought of a way to use it for an octave switch. i'm wondering if using the micro-controller for the linear values is a bad idea

i don't know. i'm still trying to sort out in my head how to implement key tracking for a VCO and VCF. it's been difficult. i considered the usage of DCOs, as mentioned in the topic i made about designing oscillators. i'm inexperienced w/ circuit design really. i'm currently ahead in logic tho

people like to send me schematics where it says how to make the oscillator, however does not state the logic, math, ranges, example values, etc. thanks to anyone who tried to help me, but it's been discouraging

-alexa
nigel
axolotl wrote:
i'm designing a synth that generates CV values from a micro-controller. it occurred to me that i could have the micro-controller generate the CV values i need, rather than relying on an analogue exponential converter


That seems like a sensible idea. You will need good resolution in your D/A converter to keep the tuning accurate in the low frequencies.

axolotl wrote:
people like to send me schematics where it says how to make the oscillator, however does not state the logic, math, ranges, example values, etc. thanks to anyone who tried to help me, but it's been discouraging


J3RK posted a very stripped-down circuit, but you still need some understanding of linear electronics to make sense of it. Maybe I can expand on the explanation in that post:
U13 just provides constant +5V and -5V signals.
U2:A adds together all the control voltages. (Most oscillators have multiple frequency controls, which are just added together.)
U2:B and U3 are the exponential converter. Really, all you need to know is that if you connect up a transistor in the right way, you get an output voltage which is the exponential of the input voltage.
U4 generates the waveform. The input voltage (from the exponential converter) charges up capacitor C3 - the higher the input voltage, the faster it charges. Q1 is basically a switch - once the capacitor charges to 5V, U6:A turns on the switch, which quickly discharges the capacitor and then turns off again. The result is that you get a sawtooth waveform at a frequency which is exponentially related to the sum of the input control voltages.
axolotl
after thinking about the possibility of using hz/v for an entire day, i concluded that the ~3 octave range provided by 1-8v is too small for anything besides a monosynth like the Korg MS-20 that uses hz/v

i actually discourage the idea that it be used, but it helped me get to the next point of clarity in my mind. i'm attempting to properly understand exponential conversion, and i'll make a topic about it for more discussion. thanks, nigel

-alexa
Starspawn
Remember that you can start earlier, on the Yamaha CS range a value of 0.250V is C0, 0.500V C1 1V is C2 etc so you get 5 octave range, that stays in tune as well.
axolotl
starspawn,

that's an excellent point. that actually redeems this a bit! i thought the 0.25v 0.5v ranges were too low, but yamaha tho

i thought of another option for 5 octave range as well. this one is considerate of 12v in eurorack

0 - 00.375v
1 - 00.750v
2 - 01.500v
3 - 03.000v
4 - 06.000v
5 - 12.000v

i was writing about compressed exponential conversion in my other post. i don't understand it yet, however looking at this clears things up a bit

-alexa
loki
axolotl wrote:
i've been pondering the usage of exponential conversion, and how i don't understand schematics where it's implemented

-alexa


I wish I could sit down for an hour with a pencil and paper to explain it all to you. You are being confused by the different elements of the schematics. The answer is much simpler. You only need to look at the behavior of a single transistor. All by itself it is an exponential converter. A small voltage is applied to the base - emitter junction and if there is a suitable voltage on the collector a current will flow through the transistor and out the emitter.
That collector current is exponentially related to the voltage on the base - emitter junction. Rather than being complicated, it is the simplest possible thing.

It is complicated by the fact that there are two properties affected by the absolute temperature. You can worry about that once you get your head around the fact that the transistor is inherently an exponential device. I suggest you go to the library and check out a book on basic transistor theory. I'm going to attach a paper here that takes you rapidly through the background. Don't be confused by the fact it is about a logarithmic circuit. That is achieved by putting the exponential transistor in the feedback loop of an op-amp.
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