Autotune for everyone

[analogmonster]

As already announced some time ago I decided to develop a module for auto tuning VCO modules which don't have an integrated autotune function built in as the Memorymoog or some Oberheim synths have. Inspiration came from the discussions here in the forum concerning tuning stability of modular systems, especially the Moog reissues and the 901 VCO. And I shared this experience with the 901 as I cloned a bank of them and had to retune them from time to time, especially the octave spreading (scale).

So I dedicate this module to VCO for many fruitful discussions!

The task was to find a way to control a lets say 901 analog VCO digitally. My current solution sends the NOTE ON sequences of the notes C0 to C5 to a MIDI2CVGATE module (in my case my midified Moog 950 keyboard clone T950, but any interface or device will do) and scans the triangle output of the 901. It measures the time of one wave cycle and compares it with a time table. Dependant from the result a CV offset is created, which fine tunes the 901. This is done in an iterative process until the output frequency of the 901 matches the time table perfectly. The resulting CV offset is stored in an internal note-dependant list for reuse during your play.

Here a short demo video of two auto tune runs: First after system reset, second with taking the results of the first scan as iteration base for each note. You see that runs much faster. Apologies for the poor audio quality as I recorded with my smartphone. Better demo recordings and videos will follow. Youtube Demo T908 prototype

In the video you can hear the module coming closer to the desired result before switching to the next note during the first run. In the next run this does not happen as the CV offset is taken from the list. In the lower frequencies there is a new correction done though, as the 901 drifts permanently a bit and the measuring process for lower frequencies takes more time as the scanning of the higher frequencies.

Any questions and comments welcome.

[ummagumma]

That is quite amazing!

Did I see an "h" in there? Lol

a b c d e f g....h?

[analogmonster]

Yes, LOL, my fault

I use German notation for displaying the notes. I've not "internationalized" the software yet

[analogmonster]

Current development progress of the T908 firmware:

Yesterday I completed version 1.0 - Now you can select and invoke three different run modes / functions of the T908:

1.) "P1": AUTOTUNE Full Run:
That's what you see in the demo. 5 Octaves (c0 - c5) are scanned and compared to the internal frequency table, a CV offset is derived from the deviation and stored in a program-internal note list.

2.) "P2": AUTOTUNE Online:
Auto correction is done during your play, and each note you play is rescanned and checked again, the CV offset list is adapted and the correction is corrected In this mode the warm-up phase of an analogue VCO like the 901 or your studio door suddenly being opened by your little daughter who wants to tell you that she ate two pieces of cheese cake does not have to be compensated by doing another full run.

3.) "P3": AUTOTUNE Offline:
Auto correction is done during your play according to the CV offset list of the full run. This is the way the Memorymoog and some Oberheim synths work, AFAIK.

For some reason I can't upload images any more, so I will put this on my web site probably next week.

[VCO]

Congratulations good work. You an excellent and gifted engineer Analog Monster.

[ummagumma]

Really neat!

I guess the "h" could be a ghost note....

a b c d e f g host

[analogmonster]

I've just uploaded the next demo video: Playing the 901 VCO with and without support of the T908 Autotune module. As I ran out of space in my modular cabinets I opened my third modular system (the T908 looks a bit lonesome in it )

The lonesome rider

Anyway, it controles my untuned and unscaled 901 VCO as a post-installed remote stand alone module.

T908 Autotune Full run and octave play

How to integrate it into an already existing modular system and more technical background info can be found on my new T908 website

Have fun...

[ummagumma]

Very cool!!

[analogmonster]

New firmware version 1.06 for the T908 AUTOTUNE released. The module provides two additional program functions now:

"P4": SAVE EEPROM: The control voltage offset list which is currently in use for an e.g. 901 VCO can be saved in the module permanent storage EEPROM.

"P5": LOAD EEPROM: The control voltage offset list to be used can be loaded from the module permanent storage EEPROM.

With these functions another AUTOTUNE FULL run after a module RESET or POWER DOWN can be avoided.

[analogmonster]

New firmware version 1.08 for the T908 AUTOTUNE released. I use all 12 bit of the DAC now. Now you can (theoretically) catch VCOs which are +/- 2.5 octaves out of tune and out of scale - but this is rather academic, I'd say. No VCO is that much out of tune and out of scale in a modular environment. But the question is: How far can I go with it.