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There's a hacked(?) version of Music Construction Set that does exactly that. I can probably find it somewhere in my collection if you want to reverse-engineer that.

There's also 8088 MPH of course, had some great multi-channel msx in the last section. There's a detailed write-up on the engine tech, which includes a link to the source code of said engine.

Also, luckily archive.org has a copy of my lost pc speaker article. It's quite outdated now, but still there's a few items that are missing in your list

Ha, I wish! I know only a few basic phrases in Polish, this one is courtesy of my friend Google Translate

Radio-86RK does indeed sound scary.
Another thing that used to give me a good headache are those devices that output a fixed (preferably high-pitched) frequency (Channel F, or the Sharp Pocket thingies, for example). Thankfully, Zilog's squeeker algo is great at overriding those. Might be also a good candidate for Radio-86RK, since it doesn't seem to require exact timing.

I actually did write a sound routine for the VIC20 once. Couldn't find anything on how to create a 16-bit pointer, so I wrote some really ugly self-modifying code... then later, I looked it up and realized that I'd written it exactly like it's supposed to be done. I have to admit that kind of shaped my experience with MOS products Aside from VIC20, one machine I really want to tackle though is the Atari VCS, so I guess I'll have to wrap my head around 6502 (or 6507, in this case) at some point.

Love the demo tune!

Hah, that's a great idea!

Results are in: http://events.retroscene.org/dh2017/ZX_Beeper_Music

Pity about temporal

Been thinking about the Earth Shaker method again. As far as I understand, it can essentially be implemented like this:

    di
    ld sp,#292        ;freq divider ch1
    ld bc,#1b7        ;freq divider ch2
;    ld hl,0            ;accu ch1
;    ld de,0            ;comparator/accu ch2

loop
    add hl,sp    ;11
    ex de,hl     ;4
    add hl,bc    ;11
    ex de,hl     ;4
    ld a,l       ;4    ;cp hl,de
    add a,e      ;4
    ld a,h       ;4
    adc a,d      ;4
    sbc a,a      ;4
    out (#fe),a  ;11
    ld a,0       ;7    ;timing
    jr loop      ;12
                 ;80

What I don't understand is how to reliably control the pitch. Usual note dividers often don't seem to work as intended. Is this just a limitation of the algorithm, or is do the dividers need to be adjusted depending on the desired harmonic relation?

Btw using the same divider for both channels with a slight offset gives a sweet phasing effect that I haven't come across earlier, see attachment.

Edit Also tried it with short (12-bit) counters. Turned out pretty lame (no nice detune fx possible either), and the time savings are minimal, just 8t. So it's not worth continuing in that direction, I think.

Garvalf, were you able to solve the issue? If not, you could try forcing gcc into 32-bit mode by adding -m32 to CXXFLAGS and LDFLAGS in the makefile.

Excellent. I think noise is another underexplored area in beeper music, we definately can do more with it.
Btw I have an algo that produces similar quality as the above, but doesn't need DE. I'm using this for the click drums in my recent engines.

     add hl,hl    ;11
     sbc a,a      ;4
     xor l        ;4
     ld l,a       ;4

Then afterwards, the usual cp with duty threshold. Some suitable seeds are 1, #1011, and #1237.

I also tried to do it with just 8-bit operations.

     ld a,d
     add a,e
     rlca
     ld d,a
     rr e

Seed DE with #21. Results are ok-ish, but not great. So I abandoned this idea for now.

Wow, didn't know about Bubble Dizzy either. Seems the music is mis-attributed to Dave Whittaker on WOS.

I agree, we need more vibrato on the beeper. Hey, I did post a vibrato routine earlier in this thread That one takes 96 cycles though, we definately need to do better. The triangle generator in it's current form wouldn't be usable as the algorithm produces an off-by-one error on the downward movement. But perhaps it's possible to derive something from that. I'll think more about it when I'm back from vacation.

Hmm, Sigma 7 has some brilliant glissandos as well. Wonder if those are hard-coded, or if there's a clever algorithm behind it.

Hmm, I believe in most cases, especially with engines with few channels, there is actually at least one change every row. Also, as you know I always use a per-row speed setting. In that case it's something that should be handled on the tracker/converter end - if there is no change, simply increase the previous row length. Nevertheless, I'm sure there will be other cases where this "store only changes" approach is useful. In either way, it's a good reminder to keep thinking about song data structure. Got kind of sloppy with that myself. I'm perhaps too content with my current stack-read/skip-on-flag approach.