YM2149 and AY-3-89xx volume/amplitude steps?

[kool kitty89]

From what I understand the AY8910/YM2149 (rather like the SN76489) uses logarithmic (decibel scale) volume/amplitude control with 5-bit resolution (32 steps -unless 4-bit on the SN76489).

What I'd like to know is what the actual decibel step size used is (the data sheets seem to say nothing on that -not sure if the manual does either, perhaps it was only found by testing). I'd assume this was at least established once PCM hacks started being used on the various AY/YM based platforms (including the ST obviously), but I haven't been able to find anything online about this.

 

I also want to know the same thing for the SN76489 if anyone on these boards knows (for comparison purposes).