Hi Michael,
I have responded to your individual questions below:
On 09/19/2011 04:20 PM, Michael Geis wrote:
We were under the (probably naive) impression that all a sampler needs
to do is loop over wave tables and apply envelopes. Seeing that the
soundfont specification actually allows for greater complexity makes
us wonder whether in order to play soundfonts, the sampler needs to be
able to do all the things in the synthesis model. Unless there is a
discrepancy between what the specification allows for and what most
soundfonts look like in the wild. If a substantial fraction of
soundfonts just loop over wave tables and apply envelopes, the sampler
might still be useful for that subset of soundfonts if it just grabbed
their wave tables and envelope parameters. The answer must be trivial
for someone who has used soundfonts for a bit, I must admit it is not
clear to me.
As a SoundFont designer, I personally make use of the majority of the
features in the SoundFont spec to create my instruments. Playing just
the exported samples from most of my SoundFonts will usually sound quite
different than playing the instrument designed within the SoundFont
bank. I can't speak for other SoundFont designers, but I would imagine
the same to be more-or-less true for others as well.
Otherwise (i.e. if soundfont generally make full use of all the parts
of the synthesis engine laid out in the spec), I see 2 options:
1.Implement the entirety of the synthesis model and use the parsed
soundfont parameters as input (That sounds like reimplementing a lot
of what fluidsynth already does).
2.Play the soundfonts via fluidsynth and record the output. The
sampler then loops over that output and applies envelopes. Does 2.
even make sense or is it likely to mangle the sounds? If it is
reasonable, how many notes should I have for each pitch? One per pitch
or one per envelope phase (i.e. 5 for the DAHDSR envelopes since delay
doesn't make a sound) per pitch?
IMO, #2 is not a very good idea for the following reasons:
1. Most sustained samples have loop points which define a section of
the waveform that continues to repeat as a note is held down
indefinitely. Not only will you lose these helpful loop points
when recording the output from a sampler, but your recorded
waveform will often use up more memory than the original sample if
you record some amount of what is actually looped in the original
SoundFont. As an example: the Piano samples in GeneralUser GS are
looped, but the instrument envelope causes each note to gradually
fade out as the sound loops. If you record the FluidSynth output
of one of these notes, you will not be able to recreate this loop
due to the fade-out, and your sample will have to be very long to
capture the entire note (20 seconds to capture middle-C vs.
approx. 4 seconds of original sample data).
2. Sample stretching is used within SoundFonts to play the same
sample over a range of keys. Most instruments will use multiple
samples throughout the range of the keyboard (for example a piano
that has a new sample every 2-3 keys). If you record FluidSynth's
output, you will have to use your ear to determine where the
instrument switches from one sample to another (not to even
mention instruments that have different samples for each velocity
layer). If you get this wrong, you may end up unnecessarily
taking multiple recordings of the same sample (just at different
pitches). This is a very easy mistake to make when some of the
SoundFont's more advanced features such as filters and modulators
are active. Also, the sample sounds most natural at its "root
key", which is the note that the actual sample was taken at.
Recording any pitch other than the root key will result in a small
loss in audio quality from the original due to the interpolation
of sample points when changing the sample's pitch and other
factors. Unfortunately, discerning the root pitch is pretty much
impossible to do by ear.
I guess this might be related to how many wave tables are usually used
for a given instrument in the soundfont format. One per pitch? One for
every envelope phase of every pitch?
This varies from instrument to instrument. Some instruments even use
multiple samples or envelope regions that vary depending on how hard you
hit the key.
My apologies if I am somewhat lacking coherence here, I am still
trying to get a decent grasp on the subject matter.
No problem. You really should consider using Swami or another SoundFont
editor to learn how SoundFonts are built. From there you will come to
better understand what a sampler does and what you will need to do for
your own project. You can also export the waveforms directly from these
editors, which would be much, much better than trying to record them
through FluidSynth.
Good luck, and God Bless!
-~Chris
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