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Ha-ha, I actually have a soundcore prototype with this kind of idea. Never had time to complete it, maybe one day

Powerful!

Just as an illustration: this is what quattropic would sound like, when it is aligned to 8t. As we discussed, the effect of alignment can be subtle, but I hope you can hear that the messy distortion is gone now (I do not even know how to describe it).

Update: looked into xtone, hmm, nah, maybe next time!

Yeah, that would be way to do it
I actually have got several types of PWM coded this year, which are, surprise, surprise, not literally "out (#fe),a : rrca", but ain't far from it. In each case 8 t-state alignment is satisfied. Yes, it can be done. I am pretty much convinced now. The only thing you lose is the possibility to have better than 4-bit resolution for your PWM (e.g. Alone Coder had 5- or 6-bit resolution in his demos, but his demos would not work well on 48K).

I guess my point is, 8t alignment is a pain, but not the worst kind of pain there is out there

Almost no-one is using it. There are two reasons for this.
1. There are differences between 48K Spectrums implementations of the relevant analogue circuitry (see here). Hence, you would have to detect the kind of Spectrum first and them somehow adjust the synthesis. Similar differences between 128K models are not documented. God knows what will happen if you've got a clone.
2. It is actually not worth it. It would have been useful if you'd get a "0.5 voltage" step between 0 and 1. Instead, what you get is 0, 0.1, 0.96, 1.
I believe that some of the Odin titles attempt using this for mixing their channels. It is up to you to decide if this was worth it.

utz, what did you think about my hacks of this engine? I cannot decide if it is worth it, the sound becomes more muffled, but it is also louder. I believe Shiru was messing with the engine to include it into his tracker and this was at some point during the summer, so it may well be that he has some kind of support for it already.

I just popped into a debugger and, no, "Spectone-1" is a very different beast. It is a 4-channel engine which uses short pins, with the code that bears more than a passing resemblance of ZX10. Can you also hear characteristic "pops", just like in Special FX? This is because the sound loop is infinite and tempo is controlled via interrupts. This is the code:

l_fa3b: xor a

    dec c ; channel 1
    jr nz, l_fa67
    ld c, ixl
    or 0 ; volume for channel 1

l_fa43: dec b ; channel 2
    jr nz, l_fa6a
    ld b, ixh
    or 3 ; volume for channel 2

l_fa4a: dec e ; channel 3
    jr nz, l_fa6d
    ld e, iyl
    or 0 ; volume for channel 3

l_fa51: dec d ; channel 4
    jr nz, l_fa70
    ld d, iyh
    or 0 ; volume for channel 4

l_fa58: or l ; previous volume
    ld l, a ; save for next iteration
    jp z, l_fa73
    dec l     ; if non-zero, decrement it
    ld a, 16
l_fa60: xor 31
    out (254), a
    jp l_fa3b

l_fa67: jp l_fa43
l_fa6a: jp l_fa4a
l_fa6d: jp l_fa51
l_fa70: jp l_fa58
l_fa73: jp l_fa60

The mixing is a little bit similar to one of my early sketches for "Octode XL" (which is not very surprising, because I messed with "Octode" and "Octode" was clearly inspired by ZX10, just like this engine). Note how it tries to control the volume by using OR volume, but also note that it is not consistent in that channel volumes are not properly added. It also has a mechanism for carrying pins to next iteration, but once again, not fully consistently.

OK, got you, will try to do it when I have a bit more time.

Just noticed that this needs a tiny correction. This advice is only correct when the sound loop contains only a single OUT command. In the case when there are several OUT commands, this won't be good enough. What you actually need to ensure is that the timings from any OUT command in your sound loop to any other OUT command in your sound loop is a multiple of 8. This automatically ensures the number of t-states in the sound loop is a multiple of 8, but this is, generally speaking, a stricter requirement.

Lovely work, utz. Very, very impressive, esp. the improved quality due to the higher CPU clock. Once my crazy teaching load goes down a little, I will definitely read the player - it is interesting to see what style of mixing you went for here.

Do these TI calcs have a contention of any kind? Some of these calcs seem to come with a 15MHz Z80s; just think about the possibilities in terms of the beeper sound in that case!