[sdiy] Bunching of MIDI clock messages

[rsdio at sounds.wa.com]

Great link, René.

In this case, I would expect that the problem is Cubase and Windows.  
Under Mac OS X with a CoreMIDI compatible MIDI interface, it should  
be possible for any decent CoreMIDI application to schedule the  
timing of MIDI messages on the MIDI interface itself, rather than in  
OSX. This doesn't work with strict USB-MIDI Class Devices, because  
that standard has no timing information at all, but it does work with  
proprietary USB Class Devices that happen to handle MIDI. The  
CoreMIDI driver establishes a timing reference between OSX and the  
MIDI hardware, and then CoreMIDI allows the software sequencer to  
send MIDI event data in advance, so that the hardware can control the  
precise timing of the MIDI transmission. Of course, this only works  
with MIDI sequences, not with live MIDI. It would be very interesting  
to repeat Jörg's tests with a decent software sequencer under OSX  
with CoreMIDI to see whether the actual timing lives up to the  
promise of this API.

But the lesson is good: Don't depend upon the timing accuracy of your  
MIDI software unless you know everything that's going on.

Brian Willoughby
Sound Consulting


On Sep 11, 2013, at 05:28, René Schmitz wrote:
> Jörg Schmitz has analysed the timing of some software sequencers
> some years ago:
> http://www.analog-synth.de/avr/timing_en.htm
>
> The trouble seems to be the time-slices of the OS, as can be seen  
> from the distributions.
>
> Cheers,
>  René
>
>
> Am 11.09.2013 13:32, schrieb rburnett at richieburnett.co.uk:
>> How common is "bunching up" of MIDI clock messages due to erratic
>> transmission out of sequencers?
>>
>> For example, if I check for the reception of a MIDI clock message  
>> every
>> 2 millisconds in a receiving instrument is that sufficient?  (In  
>> theory
>> 2ms polling rate should be sufficient to support tempo up to 1250 BPM
>> provided the clock events arrive with even spacing!)  Or do you  
>> think I
>> should cater for the possibility of receiving as many as seven MIDI
>> clock events transmitted "nut to butt" over my 2 millisecond polling
>> period!?!?
>>
>> I know the MIDI specifications says that MIDI clock messages are  
>> to be
>> transmitted with even spacing at the average tempo rate, but I don't
>> think i've ever seen a maximum jitter specification anywhere?  (I can
>> just imagine sequencer software getting preempted in a Windows
>> environment, and later transmitting all of the overdue MIDI data  
>> in an
>> intense burst.)
>>
>> Any thoughts/experience on this?
>