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Prophet 6 and OB-6 Tuning Calibration

Exploring the Temperature Tuning and Calibration Logic in the P6 and OB-6

Prophet 6, OB-6, Tuning Temperature Lookup Tables

The Prophet 6, OB-6 and other Analog Sequential Synths store tuning calibration data in lookup tables, indexed to the temperature of the tuning snapshot.

This is a great feature that allows the synth to learn how to stay tuned at a variety of temperatures, but it has some peculiarities that might be of interest.

Tuning Tables Save at a value for every 1 degree C.” - Robot Heart, Seq Forum

After you perform many calibrations for your unit, it learns the offsets it needs on a per oscillator and per filter basis for each voice, in order to be nominally tuned at a given temperature. It stores these offsets in a table / dictionary. When the Prophet 6 temperature sensor reaches a new temperature lookup table index, it automatically adjusts offsets to tuning.

Run Tuning Calibration, and Store a New Index in the Tuning Temperature Table

The Tuning Routine can be performed in about 35-40 seconds... it is very quick.   Once it finishes, the display will report completion with "dOn" (done).   Don't be afraid to re-run the tuning calibration several times during the first few sessions at a new location.  You can also run calibrations while the unit is cold / freshly powered on.   This will allow you to play the unit in tune soon after powering it on... though you should expect tuning performance will change as it heats up.  Also, it's not a bad idea to run it an additional time after unit has fully warmed up and before starting a new recording/playing session.

 > HOLD PRESET, PRESS 0 - Runs Osc/Filter Calibration and Stores an Index in Tuning Table

Auto Correction of Tuning in Real Time

You may notice at times, the oscillators and filter suddenly “correct themselves” while you are playing. .. it is usually unnoticeable, but there are times when oscillators and filter will suddenly jump several cents toward nominal tuning, if the unit detects temperature change, and a more accurate tuning offset table that can be applied. I encountered this in Testing the Vintage Knob Behavior, while holding down notes.

As long as temperatures are not rapidly changing in your playing environment, this should not be an issue... although, it seems like it might be a useful global function to allow users to temporarily turn off the temperature adjustments, or have it based on a timer. For instance, I could see that at an outdoor festival, going from day to night, with temperature changing, there might be value in being able to prevent the unit form adjusting its lookup table offsets during a song, or ideally, have a timer of 10-20 seconds without any playing at which it re-adjusts. Otherwise, it is possible that during a given song, the unit could adjust tuning once or twice while playing.

In a 1-hour test, measuring the tuning of Osc 1 from initial power on, we measured 7 "tuning jumps" on our unit, as the Prophet 6 heated up from 23 degrees celsius room temperature to 58 degrees celsius.    As the oscillator components heat up, they progressively become flatter in tuning.    Whenever a new temperature index (celsius rise) is encountered, where previously a tuning snapshot has been saved, it will jump to the tuning offsets, to be better in tune.  Of these seven tuning jumps in the one hour test, the maximum jump was around 10 cents correction, with most of the jumps being in the 2 cents to 6 cents range.   Note:  This will vary on everyones units, depending on how many tuning calibrations / snapshots have been taken in the period that the unit warms up.   

Short and Medium Term Considerations:
In the short term, upon getting a new unit, you may notice that the unit is not tuned very well out of the box. You should expect to run the Master Tuning Caliration routine several times over the first few days and weeks of operation. The operating temperature of the unit, even in a temperature moderated room, goes up a significant amount over the period of thirty minutes or so. It's not unreasonable to run the Global Tuning routine a half dozen times during the first day of operation... especially if you check with a tuner and see oscillators out of tune more than a few cents (+/-3 cents nominal) In addition, its a good idea to run tuning before starting a new session on a new day. After a period of a few sessions and maybe a dozen or so calibrations at different periods, your unit should have very good tuning performance, without the need for any further tuning procedures.   Remember, you can always run another calibration... there's no downside to having lots of up-to-date, accurate tuning indexes over a short to medium period.  

Long Term Considerations, Occasional Reset of Tuning Table Data:

Over a very long period – for instance, over a year or two of ownership, you may want to reset the Tuning Tables, and rebuild new lookups from scratch. The reason for this is that as the VCO/VCF components age, and acclimate to prolonged usage, some of the Early Tuning Indexes that were stored in the unit may actually be “out of date” and not representative of the tuning offsets required for a given temperature. If tuning tables are stored at 1c increments, and presumably overwritten, then you may be able to get by with just re-running tuning calibrations and overwriting the tuning indexes with current, accurate data. However, if a degree of celsius is missed in modern/current calibrations, and exists from an old index, the instrument might suddenly jump to an inaccurate tuning profile if the ambient temperature of the unit settles in, or passes through this specific temperature range.

To Clear the Calibration Table:
Only do this if you've owned the unit a long time and are experiencing issues with tuning jumps.   This might be a good idea to do once every few years, or if you move the unit into a new location with significantly different humidity/heat.   This could also potentially be a good idea if you were playing at an outdoor venue with rapidly changing weather... though you would want to let the unit warm up and run the calibration at least once or twice before starting your set, and perhaps at some point during the set.  


Tuning Accuracy Measurements and Expectations

We performed several tests after running the calibration routine, and after waiting periods of a couple hours, once the synth has fully heated up. You can expect very good tuning performance from oscillators, and mediocre tuning for self oscillating filter.

Osc 1, Osc 2 Frequency:

+/-2 cents nominal, very well tuned over five octaves, less than 2 cents variance going up keyboard

Cutoff: (tested via Self Oscillating resonance)

+/- 30 cents, highly dependent on octave (usually flat in lower octaves, nominal in higher) Well tuned far up the keybed around C6/C7.


Temperature Measurements

The Prophet 6 and OB-6 have a built in function to check the internal temperature detected by the sensor.  


In a temperature controlled 23 degrees celsius room (74 fahrenheit):

1. after one hour, the internal temperature was measured to rise to 58 degrees celsius (136 fahrenheit)

2. after about two hours powered on, the internal temperature stabilizes around 62 degrees celsius (144 fahrenheit), give or take a couple degrees. 

Once it is fully heated up like this, a tuning index that you store around here will keep the unit very well tuned, with low chance for tuning jumps.    It would be a nice feature if an indicator dot could be included on the temperature readout, to indicate whether a tuning index has been saved at that celsius index.  


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