If the popping noise is only on specific voices, then it should be related to
the control voltages that the CPU is writing out for pitch, cutoff, etc.
The Z80 writes these over and over, something like 50-100 times each
second or so. If every so often the wrong value was written out, you would
hear a momentary pop of some kind. Does this only happen when MIDI
data is being received by the CPU
It could be a bug in his code, if so.
If he is clocking the CPU faster than the original design, that could cause
reliability problems, since the circuits were all design with a specific operating
frequency in mind. Some chips may not propagate the information fast enough
to run at higher clock rates. I am not familiar with the Encore MIDI kit,
so I can't say how it was designed. It could also be an issue with the S&H
analog circuits not working just right at the higher frequency. There may
be a decoder that only affects voices 5-8. Good luck.
From:
"richtrix@... [oberheim]" <oberheim@yahoogroups.com>
To:
oberheim@yahoogroups.com
Sent:
Wednesday, February 25, 2015 1:54 AM
Subject:
Re: [oberheim] Finding Clock frequency of my OBXa [1 Attachment]
[
Attachment(s)
from richtrix@... [oberheim] included below]
Thanks Bob,
This EXACTLY what I was looking for. I suppose I should explain. About 9 months ago I got an Encore MIDI kit from Tony Karavidas. There was an issue with voices 5-8 though blipping and popping. His CPU clock he supplies is above that of stock. This was causing problems with my rev of the OBXa but apparently not others. He was very helpful and concerned I even mailed him my LCB and UCB at one point and he replaced an IC on it but when I got it back the error was still there. Finally he mailed me a MIDI kit that ran at "Stock" clock speed. This greatly improved things. The blips still persist though less extreme. He has gone kind of incognito on me recently though. Last I heard from him he suggested that now that it is running at "stock" there should be no issue so it must be my console having an error or damage. I've taken the console back to its stock EPROMs and Z80 and it functions fine though (except the memory banks seem to be not working really, even when saving patches from Manual Mode when I try to play them back they sound nothing like the stored setting. Also my pre-MIDI-kit tape dump loads fine without errors but all sounds are completely F*!$#*, I am not too concerned with this though as the goal is to revert to the MIDI kit). Since the stock controller hardware renders all the voices sounding wonderful (in manual mode) I want to test the stock clock speed and then the Encore clock. If there is a discrepancy maybe Tony can adjust it again and that will fix it. If anyone has any other thoughts or ideas please share. I have attached what the blips with the midi kit sound like.