APU Problems

[rdanbrook]

I have noticed you have the exact same problem I do in my emulator, which is that during the Zelda title screen, the pulse waves cut out too quickly; there is a bit of a gap that should not be there. Just thought I'd point it out in case you can figure out the root cause before I do; I suspect we may have used the same docs to write our APUs, and maybe there is a problem in the documentation. I have determined that in my emu, the pulse length counter hits 0 too quickly, but not sure why.

[fogleman]

Yeah, I've been suspicious of subtle APU bug(s)... let me know if you figure it out.

[rdanbrook]

// mode 0:    mode 1:       function
// ---------  -----------  -----------------------------
//  - - - f    - - - - -    IRQ (if bit 6 is clear)
//  - l - l    l - l - -    Length counter and sweep
//  e e e e    e e e e -    Envelope and linear counter

This is incorrect for "mode 1" which is actually the 5-step mode. The correct timings are here: https://wiki.nesdev.com/w/index.php/APU_Frame_Counter

It appears your 5-step mode is simply backwards. I rewrote my frame sequencer code to reflect this, and now all of the problems have gone away.

[fogleman]

What do you mean "backwards"? Can you explain further?

[rdanbrook]

    case 5:
        apu.frameValue = (apu.frameValue + 1) % 5
        switch apu.frameValue {
        case 1, 3:
            apu.stepEnvelope()
        case 0, 2:
            apu.stepEnvelope()
            apu.stepSweep()
            apu.stepLength()

Cases 1 and 3 should step Sweep and Length, cases 0 and 2 should not.

I can't figure out how to recompile your application to test (I used "go get github.com/fogleman/nes" and I'm not really familiar with the Go programming language). But this is what fixed my APU, as I also originally used blargg's apu_ref.txt.

[chrisd1100]

I actually think I've found the root of the problem:

https://github.com/fogleman/nes/blob/master/nes/apu.go#L5

When in mode 1 (5 step sequence), the steps happen at a lower frequency (see https://wiki.nesdev.com/w/index.php/APU_Frame_Counter). Simply divide the CPU frequency by 192 instead of 240 when in mode 1 and it will slow down the pulse.

[fogleman]

@chrisd1100 192 is 4/5 of 240. Note that nothing happens in one of the five cases in stepFrameCounter. So I think that's correct.

@rdanbrook I think I notice a difference on the Zelda title screen with that change. So I guess I'll commit it.