25 Mar
2010
25 Mar
'10
8:49 a.m.
fons(a)kokkinizita.net wrote:
On Wed, Mar 24, 2010 at 11:06:43AM +0100, Ralf Mardorf
wrote:
Hi Fons :)
could you please send me the test programs off-list. I made tests
sending MIDI to a DX7. The DX7 generated an extremely short sinus
impulse. I recorded it and examined the waveforms by Audacity. Btw. the
graphics has access to the main memory, unfortunately it's a shared RAM,
OTOH I used HPET so unwanted interrupts because of a shared RAM
shouldn't be the cause, if I do understand the workings of HR timers
correctly.
Cheers!
Ralf
At the
moment Linux on my computer and on computers of around 30 other people I
know can't use hardware MIDI equipment because of MIDI jitter. On the
same machines there is less jitter for Windows, so using Windows would
solve this problem for most of them.
This made me curious, and as I rarely use MIDI (just
to play piano using Linuxsampler) I wrote two trivial
test programs usin MIDI over Jack.
The first will output a note on or off every 10 ms.
The second just receives midi a prints the time
(number of frames) since the last event for each
one received.
When connected directly via Jack the result is a
boring series of '480', one event each 10 ms.
When connected via a loopback on a HW interface
I expected the worst case to be events quantised
to Jack period (256 frames). Actually it's 10 times
worse - events are bunched into groups, one for
every 10 periods. That 53 ms of jitter or if you
are optimistic, +/- 26 ms.
The interface used is PCI based, no USB problems.
What is going one here ?
Ciao,