The MOS Technologies 6581, or SID, is perhaps the integrated circuit whose sound is most sought-after in the chiptune world. Its three voices and mix of waveforms define so much of our collective memories of 1980s computing culture, so it’s no surprise that modern musicians seek out SID synthesisers of their own. One of these is the MIDISID, produced by [MIDI IN], and in a recent video she investigates an unexpected tuning problem.
It started when she received customer reports of SIDs that were out of tune, and in the video she delves deeply into the subject. The original SID gained its timing from a clock signal provided by the Commodore 64, with thus different timing between NTSC and PAL versions of the machine. This meant European SID music needed different software values to American compositions, and along the way she reveals a localisation error in that the British Commodore 64 manual had the wrong table of values.
Modern SIDs are emulated unless you happen to have an original, and her problem came when switching from one emulated SID to another. The first one used that clock pin while the second has its own clock, resulting in some music being off-tune. It’s a straightforward firmware fix for her, but an interesting dive into how these chips worked for the rest of us.
Image: Taras Young, CC BY-SA 4.0.
One of the SIDs in my 13 C64s also has a slight defect where one of the voices occasionally plays sour notes – out of tune by up to a note and a half. I need to use the SID test program on it and see if there’s irregularities in the sweep on one voice. Kinda odd that this only happens every now and again and only on some tunes, but at least it’s reproducible.
(I also have two where if you use Attack on voice 1, that voice disappears completely – that’s also only noticeable on some tunes)