[LAD] Again MIDI jitter - tested with Fons test applications

[Ralf Mardorf]

Regular it shifted between 2395 and 2404, but with a few exceptions, one 
time 2302, three times 2304, two times 2305 and two time 2494. See 
attachment.
What might cause this exceptions? Could it be access to the RAM by the 
graphics? Is there something bad because of the IRQs?

Regular shift 2404 - 2395 = 9 frames of jitter, exceptional maximal 
shift 2494 - 2302 =  192 frames of jitter.

I guess this does mean ...
5.3 ms / 512 frames = 0.010351562 ms/frame
Maximal difference for regular jitter 0.093164062 ms.
Maximal difference for exceptional jitter 1.9875 ms.
... am I wrong?

When I once tested it by recording I got this result for ALSA MIDI on 
Linux, Cubase runs on Windows on the same machine:

      ||Cubase|HR tmr|System|PCM pl|PCM ca
------++------+------+------+------+------
500.0 || 493.0| 504.9| 505.6| 503.4| 503.2
1000.0|| 993.4|1005.4|1005.8|1005.3|1006.4
1500.0||1494.5|1503.6|1506.4|1507.4|1507.3
2000.0||1994.8|2003.8|2007.2|2007.9|2009.5
2500.0||2492.4|2504.1|2504.3|2503.6|2503.2
3000.0||2992.9|3006.0|3006.2|3005.9|3007.6
3500.0||3493.7|3502.7|3505.4|3506.5|3509.5
4000.0||3994.6|4003.1|4003.2|4008.8|4009.9
                        msec +/- 0.1 msec
maxDif||   4.8|   6.0|   7.2|   8.8|   9.9
minDif||  -2.4|  -2.7|  -3.2|  -3.4|  -3.2
--------------+------+------+------+------
Jitter||   2.4|   3.3|   4.0|   5.4|   6.7
                        msec +/- 0.2 msec

It's impossible to make music using external MIDI equipment and ALSA 
MIDI on my machine and for Windows it isn't really much better. While 
the test using Fons apps resulted with 1.9875 ms (if I understand it 
correctly) for JACK MIDI, my tests resulted with 4.0 ms for the system 
timer using ALSA MIDI.

If you play one drum sound after the other using hardware MIDI and you 
record one sound after the other to audio tracks, than jitter needs to 
be less 2 ms. I don't know how much under 2 ms, but more than 2 ms is 
audible and sounds horrible.

spinymouse-sudo at 64studio:~$ hwinfo | grep irq
[snip]
irq:1  0 (      133) "timer"
irq:0  1 (     7480) "i8042"
irq:1  4 (        3) "serial"
irq:1  7 (        0) "parport0"
irq:0  8 (        1) "rtc0"
irq:0  9 (        0) "acpi"
irq:0 12 (   300250) "i8042"
irq:1 14 (    28748) "ide0"
irq:0 16 (        1) "ohci_hcd:usb1"
irq:0 17 (    12747) "ohci_hcd:usb2" "ohci_hcd:usb5"
irq:0 18 (        2) "ohci_hcd:usb4" "ohci_hcd:usb6" 
"radeon at pci:0000:01:05.0"
irq:0 19 (        2) "ehci_hcd:usb3"
irq:0 21 (   246087) "ICE1712"
irq:0 22 (    23094) "ahci" "firewire_ohci"
irq:0 25 (     1925) "eth0"
  used irqs: 0,1,4,7,8,9,12,14,16,17,18,19,21,22,24,25
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