15 декабря 2015

                 AY music with digital samples
                        by Alone Coder

Playing digital samples in CPU free time is easy. Combining AY
with General Sound was also a known (yet unreleased) idea. But
until now, there wasn't any editor that allowed digital samples
combined with on-int AY music.

For this, I used the most straightforward method: I added a
special command 8snn in Pro Tracker. "s" is sample number, "nn"
is for note. (General Sound plays only 3 Amiga octaves with note
values #24 to #4A.) If you use this command in AY's channel A,
the sample will play in General Sound's channel A. B to B.
C to C. So you have 9 channels in this editor: 6 AY channels
(TurboSound) plus 3 digital channels. Of course, if you want to
use your music in AY only release, you can limit your channels.

So what is the workflow in this experimental version of Pro
Tracker?

1. Prepare a MOD file with your samples (uncycled).
2. Put it on TRD as "filename.i".
3. Run Pro Tracker.
4. Load your MOD in it (nothing happens apparently but the
samples will load in General Sound).
5. Write your song :)

Note, if you don't load your MOD, General Sound will buzz at
8snn command. That's because in General Sound "Play FX" command
doesn't work if you use it with non-existing sample. Maybe I
must clear the samples at the start of Pro Tracker? Contact me
with your ideas.

                             * * *

If we play music on AY and GS at the same time, we need to
synchronize the replay speed between ZX and GS (defined by
framerate), and match the pitch (defined by AY clock). So we
need a standard for these values.

If I remember correctly:

48K: 50 Hz, 1750000 (AY is an add-on)
128K: 50 Hz, 1773400
Scorpion/KAY: 50 Hz, 1750000
ATM Turbo: 50 Hz, 1750000
ZX Evo default: 48.83 Hz, 1750000
ZX Evo 48K: 50 Hz, 1750000
ZX Evo 128K: 49.34 Hz, 1750000
Pentagon (f=14318000): 49.94 Hz, 1789750 (AY is an add-on)
Pentagon 2.6ббLE: 50 Hz, 1792000
Unreal Speccy default: 50 Hz, 1774400

50 Hz is the most popular variant without doubt (a lot of
programs count time with this). The only trouble is AY clock. GS
was initially developed for Scorpion and KAY that have AY at
1750000 Hz. Maybe this will be the best option. ZX Evo can
emulate it in 48K timing mode. Default AY clock in Unreal Speccy
is strange anyway, but there is an option for 1750000 Hz in it.
No 128K is known to have GS attached. So the only machine that
has problems with playing such songs is Pentagon. Maybe tone
values must be shifted in realtime to play there? Changing
frequency tables will affect slides; changing finetune in GS
samples will affect sample replay time (that's not that
noticeable though).

If we play samples on the same AY we can only care about correct
number of t-states per frame, because AY is generally
synchronous with the CPU. Pentagon is far from ideal 70000
t-states, 128K is somewhat closer. And turbo mode might be
switched on... So our replayer must detect the number of
t-states and change the frequency table for digital samples