Anyway, the main reason you'll see bit 3 being used is because it costs additional cycles to mask it out.

I see. I thought that the 3rd bit is set for reason because I failed to comprehend what's happening in the engine from the readPtn label and up to the playNote.

Thankies! Tho I won't be able to make it without the test track you've made ^~^

I've done some experimenting with using bit 3 for reverb. However, volume levels for bit 3 vary considerably between 48K board revisions (not to mention different models), so it's not really useful. Generally speaking, the output from bit 3 is low enough that it can be ignored for practical purposes. (It does cause problems for engines with a high volume resolution, but that's another story.) Anyway, the main reason you'll see bit 3 being used is because it costs additional cycles to mask it out. Povver is a typical example of this: Use of bit 3 is by no means intentional, but there is at least one location where I couldn't find enough cycles to mask it. Aside from that, some PFM-based engines use bit 3 in sync with bit 4 for a little extra volume boost. Alone Coder's BeepTracker engines do this, for example.

Actually, even if there was a true 2-bit DAC with proper evenly spread levels, I'm not too sure what advantage it could have over 1-bit - a two-channel engine with 'lazy' mixing perhaps, but that wouldn't give a major speed advantage, or extra opportunities for more unusual synthesis techniques. Maybe I'm wrong.

Curious little quirk, nevertheless.

For the Speccy, values: 00, 01, 10, 11 of bits 4 and 3 produces 0.39, 073, 3.66, and 3.79 volts respectively.
For the +3 however, values: 00, 01, 10, and 11 would produce ~0, ~5, ~0, and ~5 volts, so the result level is driven by (bit4 & bit3).

So the question is: what is the reason for using all 4 levels (both MIC and EAR bits)?