From a Skype conversation between @MJoergen and @sy2002:
MJoergen:
At some point we might want to look into the C64 core's SID implementation. While debugging my audio controller I noticed some strange effects from some of the games I played. Specifically, what should be a smoothly varying signal from the SID had some mysterious discontinuities, i.e. large jumps from one sample to the next. I have a slight suspicion that there is something wrong with the audio filters.
I therefore added the option to generate a simple sine wave (test tone) of a fixed frequency, and that works like a charm. I even fed the analog output into my desktop machines microphone input and fired up the program audacity and did a spectral analysis! The waveform looks smooth as a baby's bum :-)
So I'm confident my audio controller works as expected.
sy2002:
AARGH: A SID problem? Hopefully it is "just" the filter: Can you do this test again and switch-off "Audio-improvements"? This "Audio-improvements" is some strange MiSTer "thing" that we never looked-at more closely.
Next steps:
First of all, @MJoergen should re-do the very test with the test signal but with DEACTIVATED MiSTer filters, so that we can see: Is this a SID bug or a MiSTer audio filter bug?
If it is a SID issue:
We need to rename this issue and then probably move to a future release (post 5.1). Sorgelig of MiSTer completely re-did large part of the SID in late 2022, see this list: https://github.com/MiSTer-devel/C64_MiSTer/commits/master There are five SID related commits between November 16 and November 25. Back in the days, we (AmokPhaze, deft and sy2002) tested using audio recordings of MiSTer and our current SID version for various songs and both SID variants (6581 and 8580) if and what sorgelig's updates did to the sound: WE WERE NOT ABLE TO HEAR ANY DIFFERENCES between "before and after".
But maybe the refactoring sorgelig did, which includes using code from the this project: https://github.com/daglem/reDIP-SID, nevertheless fixes the issue that @MJoergen detected.
If it is NOT a SID issue, then let us NOT touch the SID implementation we currently use.
If it is a MiSTer audio filter issue:
Let us find out what this audio filter actually does
Let us decide if we can/want to fix it or if the filter needs to go away (or at least is DISABLED by default)
sy2002
commented
8 months ago
From a Skype conversation between @MJoergen and @sy2002:
MJoergen:
At some point we might want to look into the C64 core's SID implementation. While debugging my audio controller I noticed some strange effects from some of the games I played. Specifically, what should be a smoothly varying signal from the SID had some mysterious discontinuities, i.e. large jumps from one sample to the next. I have a slight suspicion that there is something wrong with the audio filters.
I therefore added the option to generate a simple sine wave (test tone) of a fixed frequency, and that works like a charm. I even fed the analog output into my desktop machines microphone input and fired up the program audacity and did a spectral analysis! The waveform looks smooth as a baby's bum :-) So I'm confident my audio controller works as expected.
sy2002:
AARGH: A SID problem? Hopefully it is "just" the filter: Can you do this test again and switch-off "Audio-improvements"? This "Audio-improvements" is some strange MiSTer "thing" that we never looked-at more closely.
Next steps:
First of all, @MJoergen should re-do the very test with the test signal but with DEACTIVATED MiSTer filters, so that we can see: Is this a SID bug or a MiSTer audio filter bug?
If it is a SID issue:
If it is a MiSTer audio filter issue: