Misc notes on Commando, by Rob Hubbard

What can I say?  Rob Hubbard was a true master -- someone who understood
SID, how to manipulate it and use it to advantage.

This tune has a primary purpose of showing that Iseq is capable of
doing complicated stuff.  Since this is just a cover of the tune, I took
some editorial license in not exactly duplicating the tune, but I think
it captures the original well enough.

(I'll also fess up and say that there are a lot of areas in Iseq which
need major improvement, and also several outright bugs.  Guess that's what
version numbers are for!)

Several things are different in the iseq cover from the original.  The
original tune does a gating three frames before the end of the note,
by setting AD and SR to zero and turning off the gate bit, e.g.

$d406 = 0
$d405 = 0
$d404 = $40

Currently, Iseq can only gate $d404, and that functionality will have to
be added in the next version (well, will probably be added!).  To get
around this, I used different ADSR settings.

Another thing he does is have instruments like

AD = 09
SR = 09
CR = 41 80 80 40

Without resetting AD and SR, the above is pretty tricky (if the SR cycle
doesn't complete by the next note, the AD cycle doesn't begin and there
are 'holes').  Again, I implemented it using different ADSR settings, and
keeping the gate bit on.

Hubbard also uses different frequencies for the notes, so the iseq version
is slightly higher in frequency.  I assume these are some sort of PAL
frequency settings (e.g. A-4 = 440Hz on a PAL machine), but didn't feel
motivated enough to calculate it out.

To do the slides, I did my original plan for slides, and implemented extra
instruments.  It works OK, but obviously a better way is needed.