Sunday, April 28, 2013

Mono/Poly - Amount of Detuning

In the process of adding detuning to my Korg Polysix, I spent a bit of time looking in detail at the detuning on my Korg Mono/Poly.  I've always liked the sound of the Mono/Poly's 4-voice unison with the Detune knob cranked up a bit.  So, as I was setting up the detuning functions on my Polysix, I looked at the detuning on my Mono/Poly to act as a guide.  This is the story about what I found...

Mono/Poly VCO2 with Maximum Detuning -- Flat by 30-35 cents.

I started my investigation experimentally.  I put the Mono/Poly in Unison mode and locked in a "C" using the Hold button.  Using each voice's Level knob, I listened to the pitch of each voice by itself.  By turning the Detune knob, I could hear what the detuning did to each voice.  To my surprise, the Detune knob had no effect on VCO1 or VCO3 -- it only affected VCO2 and VCO4.  At this point, if you look at the block diagram in the Mono/Poly service manual (see the figure below), it clearly shows that Detuning only affects voice 2 and 4.  I had never noticed this before.  I found it to be an interesting design choice by the Korg engineers.

Block Diagram from Mono/Poly Service Manual.  Detuning is only on VCO2 and VCO4.

My next goal was to quantify the amount of detuning that is generated by the Mono/Poly.  Looking at the schematic (see the bottom of page for the KLM-357 PCB), the circuitry involved with distributing and scaling the detuning control signal is too complicated for me to figure out quickly.  So, instead, I returned to my experimental approach and just measured the out-of-tuneness generated by the detune knob.

Mono/Poly VCO4 with Maximum Detuning -- Sharp by 20-25 cents.

I found that with the Detune knob set to maximum, VCO2 was driven flat by 30-35 cents and that VCO4 was driven sharp by 20-25 cents.  While it's somewhat interesting that the pitch deviations are slightly different for the two VCOs, I'm thinking that the difference is not purposeful and is actually just due to imprecision in the circuit components.  What I think is more interesting is that they chose to do a balanced de-tuning with one voice driven sharp and another voice driven flat.  Taken as an ensemble, therefore, the overall pitch of the four oscillator cluster is basically unchanged for any setting of the Detune knob.

Compare this approach to your more typical 2-oscillator-per-voice monosynth or polysynth.  Whether it's an old Minimoog or a Prophet-5, it is my impression that detuning is usually effected by changing the pitch of just the 2nd oscillator while leaving the 1st oscillator at the original pitch.  If this is true, it means that the ensemble of the two voices together is always a bit sharp or a bit flat.  Since these other synths are wonderful instruments and people love them, this unbalanced detuning must be an acceptable approach...but I've always felt that the detuned sound of the Mono/Poly just felt more "right".  So, in adding detuning to the Polysix, I chose to use the balanced detuning approach.

Due to the limited number of knobs and switches on the Polysix that are seen by my replacement Key Assigner (ie, my Arduino), I don't have a knob that I can dedicate to the Detune function.  Instead, I'm going to use a push button to turn the Detuning "on" with some default detuning factor.  I did include a method to adjust the amount of detuning (a complicated press-the-button-while-turning-the-knob combination), but I'd like to get the default detuning correct so that adjustments are usually unnecessary.

How much detuning to I want to be my defualt?  On the Mono/Poly, when i'm really getting the party started, I like to set the Detune knob to 5-6, which is 50%-60% of the total amount of detuning available on the Mono/Poly.  Given that full detuning is (take the average of VCO2 and VCO4) is about 27 cents, my target detuning amount for the Polysix should be about 14-16 cents.  The Polysix has six voices to detune, so I chose to detune two voices by 14-16 cents (one sharp and one flat), two voices by 7-8 cents, and two voices were left unchanged.  I then coded these values into my detuning routines on my Arduino and, as you can see below, I got 15-20 cents...pretty darned close to my 14-16 cents target!

Detuning Amount on my Polysix...about 15-20 cents.  Just what I wanted!
Like you can see in my demo video, the detuning works well on the Polysix -- I find the sound very engaging.  My experience with the Mono/Poly was really helpful in tweaking the detuning so that it sounds right.  Thank you Mono/Poly!

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