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DESIGN
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KMidi Design
Greg Lee, Nov. 1999
Processes
KMidi is the user front end which handles the screen display and button
presses, etc. It executes as a separate process from TiMidity, which
reads midi files and plays them.
Synthesis Model
TiMidity has a tone generator, two envelope generators, three low frequency
sine-wave oscillators, a low pass filter, and an effects unit. (The low pass
filter is used only when KMidi's fourth interpolation option is chosen and
the "filt" button is depressed -- at present it doesn't work well.)
The tone generator resamples instrument patches at the basic frequencies appropriate
for specific notes being played. (The interpolation algorithm used is selectable
using the four checkboxes on the upper panel.)
The envelope generators modulate each note with successively: an attack phase, a
hold phase, a decay, and a release. One of the generators affects amplitude,
and the other affects pitch and/or the cutoff frequency of the low pass filter.
The low frequency oscillators modulate amplitude (for tremolo), frequency (for
vibrato), and the low pass filter cutoff frequency (XG "rezo sweep").
The effects unit supplies echo, detuning, reverberation, chorusing, celeste, and
phaser effects. Echo (done by generating extra echo notes) and reverbration
(done with a filter) implement midi reverberation. Detuning (done by generating
extra pitch bent notes) and chorusing (done with a filter) implement midi
chorusing. (The effects filters are active only when KMidi's "eff" button is
pressed on. Echo and detuning can be controlled with the middle two checkboxes
on the right of the lower panel and from the Chorus and Reverb menus.)
Instruments may have either one or two tone elements, and for each of these,
a patch set may provide separate patches for different note-velocity ranges
(commonly for pianos) and for different note-pitch ranges. GUS patch sets,
however, provide only one tone element and one velocity range per instrument.
(If you click the rightmost checkbox at the right of the lower panel, KMidi will
display in its info window the names of patches as it loads them, a "(2)" if
they have two tone elements, and the velocity range.)
Midi Implementation
Almost all midi messages and controllers that I'm aware of are implemented somehow
(meaning that they cause something to happen, if not necessarily the right thing).
NRPN controllers for setting attack and decay time are not implemented, because
I haven't found any midi files that use them. The flanging controller is not
implemented (though flanging requested in an XG sysex dump is implemented). Only
a few of the hardware oriented controls in Roland and Yamaha system exclusive
dumps are implemented.
TiMidity can handle midi files meant to play on up to four synthesizers, keeping
separate the four sets of 16 channels for each synth. However, in the midi file,
the synths have to be selected using port commands (in either Roland-GS or
Yamaha-XG sysex dumps). Some midi files intended for multiple synths don't have
port commands. (The info window shows port commands when present.)
The KMidi vertical-bar display of channel activity is limited to two sets of 16
channels. The channel chart of patches with horizontal bars for controller values
is limited to just one set of 16 channels.
Volume
I've reduced the volumes levels TiMidity generates quite a lot, as compared with
the original implementation. This minimizes clipping and gives an increased
dynamic range. (There may be some artifacts now at low volumes, unfortunately.)
If you need more volume, I suggest you turn up the volume somewhere else, either
use KMix to turn up the line volume, the mixer volume for /dev/dsp, or turn up earphones,
amplified speakers, or whatever.
Computation Issues
TiMidity can fall behind in computing data to send out to the sound driver, and
then there are dropouts in the music. TiMidity tries to anticipate the possibility
of a dropout by monitoring the state of its output buffer and those of the
sound driver, and when there is not much in the buffers, it tries to catch up
by minimising the calculations it has to do: it uses a cruder interpolation
routine in resampling, stops doing extra echo and detuned notes for reverb and
chorus effects, and starts terminating notes early, or even skipping notes
altogether. So, depending on midi song, patchset, how fast your system is,
even if you don't hear dropouts, you will notice a loss in quality as TiMidity
gets busier. KMidi's front panel lights give some picture of how desperate
TiMidity is becoming in its effort to keep the music playing. (Signs of
desperation: the buffer led goes from healthy greenish to busy orangish,
the interpolation led turns off (meaning linear interpolation is being used
for resampling), the echo led goes off, or the detuning led goes off (meaning
no extra echo or detuned notes are being played).
Monitoring the output buffer involves calling the sound driver to find out how
much data it has buffered up. I know how to do this for Hannu Savolainen's OSS
driver (Linux and some other systems), perhaps for older Suns (it used to work),
but not for other drivers.
You can lighten TiMidity's computational burden by various maneuvers: (1) reduce
the permitted polyphony (spin box on lower panel), (2) press off the "eff" button,
(3) choose cheaper resampling interpolation (one of the checkboxes on the upper
panel further to the left), (4) turn off echo, detuning, or stereo using the left
three checkboxes on the lower panel, (5) choose the "dry" setting on the Reverb
menu. Of course, there is generally some attendant loss in quality. Having the
"filt" button on doesn't cost anything unless you've also chosen the "cspline+filter"
interpolation option.
Memory Issues
It's possible for TiMidity to use an excessive amount of memory for patches. Symptoms
are your window system becoming very sluggish, or crashing. This is probably only
a problem when using big sf2 soundfonts of 30+ megabytes. For instance, the
piano patches in Personal Choice's PC400 soundfont can occupy 25 megs each. After playing
a list of midi files which use three of these pianos, well, that's 75 megs of
ram right there. In its configuration dialog, KMidi lets you set a maximum number
of megs to be used to keep patches used in playing previous midi files; when this
maximum is exceeded, TiMidity will unload least recently used patches, returning
ram to the system. The default maximum is 60 megs, which works ok on my system (I
have 48 megs ram and 196 megs cache). Setting this to a lower value means that
more patches have to be reloaded from patch files as you play through a list of midi
songs, which slows things up. If you're ram rich, you can set it to "no limit".
If you allowed enough room, entire soundfont files are read at once, instead of
patch by patch. Setting the maximum too low can be counterproductive, because
soundfont files occupy much less memory in toto when the entire file is read than when
the patches are read individually (I don't know why).
For little-endian systems like x86, rather than being read in, sf2 soundfonts are
mmapped. This is much faster on startup. I haven't figured out a way to do this
for big-endian systems.
After playing each midi file, KMidi will report in the info window on how much memory
is currently occupied by patches.
If you don't have much ram or cache memory, setting a low maximum for patch memory
might not prevent KMidi from crashing your system. So be careful using big soundfonts.