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 microcontroller. it occurred to me that i could have the microcontroller 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 microcontroller 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 microcontroller. it occurred to me that i could have the microcontroller 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 strippeddown 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 18v is too small for anything besides a monosynth like the Korg MS20 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 opamp. 

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