[LAD] discussing HZ (kernel timer frequency) and midi timing

Mulyadi Santosa a_mulyadi at telkom.net
Wed May 2 21:16:26 UTC 2007

hello everyone

    One thing that draws my attention is MIDI playing. So far, by using
Timidity and ALSA, I tried to play several MIDI files and it is hard for
me to notice any lag. I only noticed delayed notes when system is under
busy load (I run CPU bound program with nice level above -9).

    Things I'd like to discuss are (I am sorry if they sound dumps
because I still study all these stuffs):

1. Is it me just who is not professional enough to notice any difference
when using various kernel HZ setting (smooth playing on HZ=x, not so smooth
when HZ=y and so on)? Or, does the delay fall in non-human-perceptible
range thus normal human being can not see the difference?

2. Is it correct that recent ALSA sequencer use RTC as source timer thus
kernel HZ is no longer relevant in the sequencer's point of view?

3. Related to the system load I mentioned above, I think I was seeing
scheduling latency and not really a timing latency. If that's correct,
could you kindly suggest what I must do to show that MIDI timing does
have any relation with HZ?

4. I am also thinking about MIDI recording as a way to show HZ effect.
Since I don't have any MIDI devices (but I plan to buy one if it's
really neccessary), I'd like to explore the correctness of this idea
first. Do you think it is really a proper experiment in HZ test context?

    Thank you in advance for your help and attention and I am looking
forward to read your feedback. Sorry if it has been discussed before, 
but I fail to completely research it until today.



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