OBERHEIM SYNTH

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? 

Thanks.

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

Paul,
Thanks - this is very helpful.  The first thing I will do next is make sure the Pulse Width/PW CVs are working correctly for the troubled VCO.  A bad signal shape (like no pulse output) might screw up the auto-tune process based on your description of the digital counter.  The second VCO on the same voice board seems to tuneup just fine - so this is isolated to this VCO or the auto-tune components/software for this VOC.  

While I will continue to work this issue on the hardware side, I have this nagging fear that the EPROM code that runs the auto-tune process is corrupted, but that is mostlikely my due to my "what things can I not fix if they break" phobia more than anything else.

Thanks again. I will let you know what happens.    

--- In oberheim@yahoogroups.com, "Paul J. White" <pjwhite@...> wrote:

>
> 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
>