OBERHEIM SYNTH

At 11:31 AM 10/15/2009, you wrote:
>Does anyone have a good (engineering level) understanding on how the auto tune works on the OB-X?
>
>I have one OSC on one voice that is getting a strange auto-tune control voltage causing it to be out of tune. After swaping around op-amps to no avail, my next guess is a bad cap supporting the voltage at the op-amp.
>
>However, I don't understand what goes on during an auto-tune. I assume it compares the OSC frequency to a reference signal. I don't understand who the comparison in made, or how the software turns off voices, gates the voice, etc...
>
>Thoughts? 

I don't have the schematic handy, but I'm pretty sure the auto-tune process is something like this:

1. Mute all the voices at the audio output (so you don't hear the auto-tune process).

2. Set the VCAs to turn on one voice at a time.  The filter is set wide open and the oscillator pulse output signal is shaped to a logic level and mixed on the OSCMUX bus which gates a high frequency digital counter on the control board.  This counter is capable of measuring the oscillator period to within a microsecond or two.

3. The computer uses the digital count to determine the period of the waveform and adjusts the oscillator control voltage until the count corresponds correctly to the desired ideal frequency.

4. Store the correction voltage in memory, to be used in choosing the right control voltage for any given note pitch during normal operation.

The process is repeated for each oscillator and each voice, probably across a range of at least two different control voltages in order to establish a linear relationship between desired frequency and control voltage.

-- Paul White