[LAD] [64studio-users] MIDI jitter

[Adrian Knoth]

On Wed, Jun 30, 2010 at 05:28:05PM +0200, Clemens Ladisch wrote:

> >> latency distribution:
> > ...
> >   3.1 -  3.2 ms:        1 #
> > ...
> >   3.9 -  4.0 ms:        1 #
> >   4.0 -  4.1 ms:     9903 ##################################################
> > ...
> >   5.0 -  5.1 ms:       95 #
> 
> The default parameters of this tool are unrealistic; the next MIDI
> command is always sent immediately after the previous one has been
> received, which tends to align everything to USB frames.  Please use
> the -w and -r options.

Absolutely. I already suspected 4ms being correlated to USB frames
(granularity 1ms, right?)

With random wait, it looks like this:

> latency distribution:
...
  3.0 -  3.1 ms:       86 ####
  3.1 -  3.2 ms:      986 ##################################################
  3.2 -  3.3 ms:      958 #################################################
  3.3 -  3.4 ms:      967 #################################################
  3.4 -  3.5 ms:      985 ##################################################
  3.5 -  3.6 ms:      927 ###############################################
  3.6 -  3.7 ms:      966 #################################################
  3.7 -  3.8 ms:      938 ################################################
  3.8 -  3.9 ms:      980 ##################################################
  3.9 -  4.0 ms:      904 ##############################################
  4.0 -  4.1 ms:      906 ##############################################
  4.1 -  4.2 ms:       45 ##
  4.2 -  4.3 ms:       44 ##
  4.3 -  4.4 ms:       38 ##
  4.4 -  4.5 ms:       28 #
  4.5 -  4.6 ms:       37 ##
  4.6 -  4.7 ms:       34 ##
  4.7 -  4.8 ms:       54 ###
  4.8 -  4.9 ms:       34 ##
  4.9 -  5.0 ms:       42 ##
  5.0 -  5.1 ms:       40 ##
...
  5.2 -  5.3 ms:        1 #


Probably still acceptable, given that the majority of packets deviates
by only 1ms.

Does USB have a notion of "ticks" (clock?), that is, frames are sent at
isochronous distances? Or is the host controller free to send a packet
whenever it wants to? Without the latter, 1ms jitter could be the result
of queueing up the data for the next time slot.


TIA

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