brand new YM effect
Many times I hear the tip that all you have to do to speed up YM replay into
half a scanline is to sample the YM register every 50Hz and simply dump them
back to the chip on replay without calculation.
This is simply wrong ;) Sure, it will work for many tunes - but dumb YM sampling
like this will fail to capture some YM effects using techniques developed by
classic chip composer Dark Angel/Aura. This is also why tunes using these
techniques are not reproduced correctly in JAM for Windows, which also ignores
the special YM behaviour used.
MusicMon2 and my up-coming editor maxYMiser use this behaviour to create whats
best described as note start synchronised phasing effects.
Most people know that writing to the YM buzzer waveform register retriggers the
envelope from its start (as stated in the YM2149 datasheet). Whats not often
known is that its possible to retrigger the squarewave in this way as well.
Heres a bit of code that does just that for YM squarewave channel A:
as you see the two YM registers for squarewave frequencies are cleared to zero
shortly, before being immediately set for the required frequency. Obviously this
effect is impossible to record using simple YM2149 sampling.
You can create a combined buzzer/squarewave sound, and by synchronising both
square and buzzer waves at the start of the note you can achieve repeatable
phasing, which great for smooth pad and dirty bass sounds with the right
new YM effect?
You might think, ok so we can sync the squarewave - why not apply this method
to create a kind of syncbuzzer sound, but for the square wave. I also thought
this ;) The problem is that synchronising the squarewave is not
completely reliable. Actually the squarewave is only restarted from the last
time it changed state, and not from a known phase. This gives two sync
possibilities, either where it syncs with the square starting again low(1), or
when it starts again high(2):
If you play with MusicMon2 you'll be able to hear two distinct variations in
phasing, seemingly at random. Its not possible to retrigger the squarewave with
the timer each time with enough precision to give repeatablity, except for very
low bass notes. Normally you might hear some glitchy distorted tones unusable
for melodic instruments.
STeem and SainT emulate this YM squarewave synchronisation such that the wave
restarts at a given phase reliably, but remember on real YM this is not the
case. If you wish to experiment further with this effect, you'll find the code
fragment shown above gives the best performance, or wait for the release of
gwEm Jan 2005