[alsa-devel] usleep() and nanosleep() timings seem inaccurate using ALSA
subscribermail at gmail.com
Thu Mar 29 11:46:38 CEST 2007
After studying the intricacies of MIDI, I ended up writing an
implementation of the MIDI protocol and file format.
I then studied the ALSA sequencer API to be able to control a synthesizer
keyboard and play MIDI files. I've used aplaymidi, aconnect, arecordmidi,
and all those great ALSA utilities. They're very good!
However, I wanted to have a simple ncurses based, command line MIDI
sequencer meant for small Linux distributions such as DSL or Trustix so I
began to write a command line sequencer using ALSA. I've encountered one
problem about using usleep() and nanosleep() especially in 2.4 kernels.
The core of my problem in my application is that the MIDI files slow down
their tempo when pitch changes are made on a particular channel/track,
while a NOTE-ON event is being played.
For simplicity's sake, I'll give timing examples in terms of seconds and
not bars nor measures.
For example, if I've got a 2 second NOTE-ON playing a guitar note, all
timings are ok. If the NOTE-ON is interspersed with CONTROLLER messages
such as a pitch bend, the part which is supposed to sound in approximately
2 seconds, sounds much longer than that.
The effect of this is that a ritardando is heard everytime short-duration
CONTROLLER messages are sent.
The problem disappears when I use the sequencer queues. However, the
problem manifests itself when using direct output (i.e. not using queues,
using snd_seq_event_output and snd_seq_drain_output one after the other).
The reason why I would not want to use the queue is that I'd want the UI of
my program (such as metronome ticks and counts) to somehow synch with the
sound that is being heard. I've tried removing all UI elements, thinking
that the UI controls might have caused the delay, but still the ritardando
The structure of my code is like this:
for all events; do
The ritardando effect is a bit lesser using 2.6 kernels. If I use queues,
the problem disappears completely in 2.4 and 2.6 kernels. I don't hear
the effect with simple MIDI files, such as with piano pieces.
By the way, I intend to use my app with a 2.4 kernel.
Does anybody have a solution to the delay problem? Please point out to me
if I'm doing anything wrong since I've just been working with alsa-seq for
a few months.
Thank you very much.
Softare Engineer/Network Co-Administrator
Astra Philippines Inc.
UP-Ayala Technopark, Diliman 1101, Quezon City
The Astra Group of Companies
5-3-11 Sekido, Tama City
Tokyo 206-0011, Japan
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