I spent the better part of the day testing trying different methods to reduce the noise emanating from the YM2413 synth chip. I can confidently claim one of two possibilities: the YM2413 inherently introduces "some" noise on the audio outputs, or the YM2413 is extremely sensitive to nearby digital tracks on the PCB. On revision b of the FMPBC, I had placed a ground plane on the top layer beneath the YM2413 and routed the databus on the bottom layer directly underneath the synth chip. On revision c (screen shot to the right), in a last ditch effort to isolate the noise source to the YM2413, I have placed a ground plane on both the top and bottom layers underneath the synth thereby drastically reducing any possible interference with digital tracks. I strongly believe that any noise left after this layout change is simply unavoidable.
This new revision will be ordered in the next few days - on top of the changes listed above, it also adds a routing of the PAUSE signal to the CPLD; the means by which the FM audio is disabled. On that topic, I have tested that FM audio can be enabled/disabled with a Master Everdrive! Video of all known FM games (known to me!) played through the FMPBC coming soon!
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