[fluid-dev] Possible approach to preloading samples / presets

[Marcus Weseloh]

Hi all,

When we implemented the dynamic sample loading, we also discussed if
it would be good to provide a way to preload samples. Back then it was
mainly meant as a way to preload all presets from a MIDI file, so that
loading the samples dynamically wouldn't stall the playback. We
decided that it wasn't worth the effort, as nobody had expressed an
interest or seen a real need for it.

Well, I do have a real need for preloading presets (or rather samples)
now. My use-case is the following: I'm building musical instruments
that make use of FluidSynth for sound generation and those instruments
are used in a real-time performance context. Players have the ability
to switch between different instrument setups at the touch of a button
and want to make use of this feature even in the middle of a
performance. Those instrument setups also change the channel presets
in FluidSynth, and as the SoundFonts reside on an SD-Card, switching
between setups sometimes takes a few seconds. That is way too long for
my users, who would like to switch setups between beats, in the
fraction of a second.

You could say that I shouldn't use dynamic sample loading in that
case, but memory on the instrument is limited and I would rather not
load all samples from a huge soundfont just because the player uses a
single preset from that font. It also makes boot-up of the instrument
*much* faster, so I really want a solution that works with dynamic
sample loading.

So currently I'm using a hackish way to implement preloading of sample
data. I have increased the number of MIDI channels in the synth to 64
and use the otherwise unused channels 16-63 as "preload slots". When a
user triggers the "preload" function, I go through all presets in all
setups and assign each unique preset to the next available "preload
slot channel", thereby loading the samples into the sample cache. That
method works, but it does feel like a hack. It also puts an arbitrary
limit on the number of presets I can preload, regardless of their
actual size in memory.

So I would really like to extend FluidSynth to allow for preloading of
presets / samples via a public API. But before I make a suggestion in
that direction, I wanted to ask your opinion on a a couple of
questions:

1. Would such a preload feature even be interesting to other users? Or
is the use-case so unique to my application that it wouldn't make
sense to add it to FluidSynth?

2. If you think it could be added, is that because you would have a
use-case for it as well? And if so, is it similar to mine (preloading
an externally determined list of presets) or do you need it for
something else (like preloading all presets from a MIDI file, for
example)?

The answers to those two questions would greatly influence the
approach for this feature. Obviously the answer to 1. would be most
important whether to approach it at all. But the answer to 2. might
influence how to go about it. It could be implemented as a preload
(load before use) or as "pinning" (don't unload after use). It might
also affect where to extend FS: a public API for the sample cache,
extending the preset selection functions, extending the soundfont
loader interface, ...

It would be great to hear your opinion about this!

All the best
Marcus