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SPU2 is more than just sound!

The SPU2 is the Sound Processing Unit for the Playstation 2, and works a lot like the sound card in your own PC; albeit still quite unique in its approach to mixing sounds/voices and the programmable interface it provides for that. But the SPU2 is more than just sound. It's one of the more reliable timing mechanisms on the PS2 and games tend to use it as such. Without at least basic SPU2 emulation, no games will boot at all. This isn't too surprising if you understand how console hardware typically works, but what might be surprising is realizing how many games won't boot even with what appears to be fairly competent SPU2 emulation.

Until SPU2-X 1.4, no SPU2 plugin had gone the distance on implementing IRQs (Interrupt Requests). IRQs are scheduled via specific SPU2 memory addresses. When a marked memory address is accessed anywhere in SPU2 memory (either read or write), the IRQ is signaled to the IOP. The most important IRQs on DMAs and audible voice playback have been supported for eons; without these no games would boot, period! Meanwhile, many of the lacking IRQ checks were known, but glossed over because of overhead required for the checks (a couple other checks were simply overlooked). The three main culprits for causing emulation errors were as follows:

1) the "free run" feature of SPU2 voices.
2) the write-back areas for each core's mixed output.
3) Reverb Processing, which uses a series of overlapping buffers to generate feedback.

Free Running Voices

The SPU2 has 48 total voices (24 voices for each core), plus two dedicated streaming audio input sources. Each voice can play a sound effect or stream audio, and can either be stopped, looping, or 'free running.' Free running voices typically zero out their volume rather than stopping or looping, and continue to 'play' forever (albeit silently). These free running voices access inaudible areas of SPU2 memory and thus trigger IRQs unexpectedly -- except, of course, some games are cleverly designed to expect these unexpected IRQs!

Because of the overhead required to free-run otherwise silent voices, all other SPU2 plugins (until now!) have opted to ignore processing them. This is the feature that fixes Fatal Frame 2 (Project Zero 2) and a dozen more games.

Output Write-back Areas

The SPU2 defines a handful of special areas of memory where it writes back sound data at various stages of the mixing process. It's perfectly legal for a game to set an IRQ address within these buffers, and then expect it to trigger when the SPU2 does its write-back to that address. The write-back areas are mapped as follows:

0x0400 - 0x05FF  :  Core 0, Voice 1
0x0600 - 0x07FF  :  Core 0, Voice 3
0x0800 - 0x09FF  :  Core 0 Output (Left) [includes Wet/Dry/ADMA sources]
0x0A00 - 0x0BFF  :  Core 0 Output (Right) [includes Wet/Dry/ADMA sources]
0x0C00 - 0x0DFF  :  Core 1, Voice 1
0x0E00 - 0x0FFF  :  Core 1, Voice 3

// Following are results of mixing all 24 voices for the given Core.

0x1000 - 0x11FF  :  Core 0, Dry Mix (Left)
0x1200 - 0x13FF  :  Core 0, Dry Mix (Right)
0x1400 - 0x15FF  :  Core 0, Wet Mix (Left)
0x1600 - 0x17FF  :  Core 0, Wet Mix (Right)
0x1800 - 0x19FF  :  Core 1, Dry Mix (Left)
0x1A00 - 0x1BFF  :  Core 1, Dry Mix (Right)
0x1C00 - 0x1DFF  :  Core 1, Wet Mix (Left)
0x1E00 - 0x1FFF  :  Core 1, Wet Mix (Right)

In specific, some games set an IRQA for Core0's write-back area. The IRQ can either be used as a timing mechanism, or as a synchronization point for post-processing audio effects. Most SPU2 plugins properly handled the write-backs, but overlooked the necessity of doing IRQ checks for them.

Reverb Processing

The SPU2 employs a clever reverberation algorithm that utilizes multiple overlapping read and writeback buffers within SPU2 memory to generate feedback. Each step of the reverb process accesses memory and must test against the IRQ address; for a grand total of 24 IRQ tests per Core. Fortunately, all reverb activity occurs within a specified area of SPU2 memory, so for most games a single simple test can be used to exclude the IRQ test.

And It All Applies to SPU2null!

This is the boring part that I'm going to look to implementing soon: In order for SPU2null to be fully emulation-compliant, it must properly simulate all of these things, which basically means it needs to have a complete sound mixer implemented; including reverb buffering/addressing logic. It probably seems silly, but SPU2null would still be without any platform dependent code or sound drivers, making it an ideal base for emulation analysis and as a base for future plugins.

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