At Fri, 30 Mar 2007 09:53:33 +0800, Carlo Florendo wrote:
Takashi Iwai wrote:
At Thu, 29 Mar 2007 17:46:38 +0800, Carlo Florendo wrote:
Good Day!
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.
You cannot get a small sleep usually on user-space processes. Usually, usleep() is implemented with select/poll and its timeslice is defined by HZ in kernel config. In most cases, it's HZ=100, 250 or 1000 while 2.4-i386 kernel supports only HZ=100. That is, the least sleep time is 10ms no matter what value you pass to usleep().
This can be overcome by using a realtime schedule class and priority like JACK does.
Ok. I will take that seriously. I've heard about JACK and know it's popular but I've never used it.
Well, I don't mean JACK handles the MIDI in that way but as an example of real-time (audio) application. Basically you have to just set process scehduler class to SCHED_FIFO and raise the priority. Then your process would get invoked as soon as triggered, such as return from poll() events.
In any case, how does aplaymidi (or even timidity) produce the proper timings using ALSA? (In other words, how does the queue output sounds with correct timings?)
The ALSA sequencer queue is implemented on kernel, so it can have and use more accurate timer sources.
Takashi