Re: [LAD] [64studio-users] MIDI jitter

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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