Wednesday, February 20, 2013

High Frequencies -- Polysix Adjustments

Continuing from my last post, I've been exploring the high frequency performance of my Korg Polysix and Korg Mono/Poly.  I'm trying to add more sizzle to the Mono/Poly and I'm trying to reduce a bit of upper-treble harshness in the Polysix.  My latest attempt at improving the Polysix was to follow the re-calibration procedure from the service manual, particularly regarding the re-calibration of the filters.  Sadly, it didn't affect the high frequency performance of he synth.  It did, however, bring my resonance and filter frequency control into better consistency between the voices.  As a result, I had fun discovering the unique vibe that comes with actually trying to play the self-oscillating SSM2044 filters...

But, back to the beginning.  After my last post, I received an interesting comment by "terjewinther" from the Polysix Yahoo Group.  He suggested that the calibration can have a strong effect on the sound of the synth.  So, I opened her up again, brought out the multi-meter and oscilloscope, and started in on the calibration procedure as listed in the Service Manual.  I made it through the filter tuning including offset, filter frequency, resonance, and EG intensity.  What I found was that my DAC was a bit off and my EG intensity was way off.  My filter cutoff and resonance were pretty close, but there was some variation from voice to voice.  So, I'd say that the biggest impact of the calibration was to make the 6 voices more consistent with each other, especially at higher resonance settings.

After completing this portion of the calibration (I stopped just before the calibration of the Keyboard Tracking), I closed the lid, plugged in the audio recorder, and took some new measurements of the trusty sawtooth wave.  Did I smooth out the high end harshness??? Did I get rid of that weird bump that I was seeing around 7 kHz???  

Well, the graph below has the answer....and the answer is "no".  There appears to be no change in the frequency response.  I doubt, therefore, that the calibration got rid of the harshness.  (Sure, when I play with the synth over the next few days, my ears will tell me things that this graph can't...but you can't really trust your own initial impression because we humans are so easily prey to confirmation bias...hence, objective measures are best for immediate trouble-shooting and feedback).

Frequency Response After Following the Tune-Up Procedure in the Service Manual
So, calibration did not appear to affect the high frequencies.  I would not, though, consider my effort to be wasted on the calibration.  For example, the EG Intensity range on the filter is so much more usable now.  And, as I mentioned, the 6 voices are much more consistent at high resonance...and this has actually been a bit of an inspiration.  For the first time ever, I found myself actually trying to play the self-oscillating filters in a musical way.  The video at the top of this post shows some of my results.  Sure, the pitches from the self-oscillating filters are not perfectly in tune (that's really really hard to do), but their out-of-tuneness is what engaged me.  Their tone (a fairly pure sine wave) is also strangely engaging to me.  It's a whole type of sound that I didn't know was inside the Polysix.  Now I know.  Thank you calibration!

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