To add to this: there is no one true SID. There are two different SID variants (the 6581 and the 8580), with vastly different sounds, especially in the edge cases later C64 music liked to exploit. Since C64s were made with both variants, people who play music, and emulators as well, have to contend with the existence of the differences, since there's nothing stopping a program from running on either SID variant. While it's true that a good number of games on vgmrips have multiple variants supporting wildly different sound hardware (even if audibly they're very similar), if SID were to be added to VGM, then *all* C64 games would need *both* variants ripped, and then compared to see if they sound *identical* or not. Either that, or you add complexity to the VGM player by allowing switching between the two SID formats.
(There are additionally NTSC/PAL differences in SID playback, but that won't be as much of an issue with VGM, as there's usually one correct intended choice on the C64 and the VGM player would be written to be frequency-agnostic. Some people may want to forcibly play a file in the opposite speed, however; I don't know if any VGM player accommodates them.)
There are also some outliers: - dual-SID setups like the SID Symphony cartridge or the sid2sid, but only homebrew ever used these - the SFX Sound Expander (YM3526) and its modern clones (YM3812), but only homebrew ever used these - PET-style userport voices, but I don't think anything ever used this, including homebrew as well as two other computers that used the SID: - various color CBM II-family business computers, whose sound capabilities I don't think was ever used by anyone - the Innovation SSI 2001 expansion for the IBM PC, which also used the SID chip but I'm fairly certain the SID formats have been expanded to support all of these, and even if not, adding them to PSID might be easier (and more beneficial since it would give you PET and VIC-20 and other 8-bit CBMs for free and would appeal directly to the existing userbase instead of trying to move that userbase over to VGM). The only thing I'd be worried about is PC software that uses both the SSI 2001 and some other sound card (or even the PC speaker), but I'd be willing to bet that only homebrew would do this, if they even would.
My only real complaint about HVSC is that they don't separate SIDs ripped from the games from SIDs that are fan-made covers, but I might be missing something.
EDIT Oops I forgot to emphasize that those 6561 and 8580 VGMs would be byte for byte identical except for the header, which is also not true of the other multi-configuration setups we have now.
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