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

Ralf Mardorf schrieb: > Another stupid question induced by an argument regarding to MIDI > jitter by Daniel James. > >> [snip] I'm sceptical that the realtime kernel is the cause of your >> MIDI problems. If they got this right in the 80's, on computers >> which could not do anything near realtime audio processing, then I >> think it's more likely to be a question of MIDI application design. At that point we should call back, how that whole story with "realtime" started. At the begining was a design mismatch. Many things related to the Linux kernel started out with a kind of "I feel fine" pragmatism. Which, btw isn't to criticise as it is, because this also accounts for the freshness and sometime unconventional new approach to some problems. But with regards to timings, for all of the first decade of Linux development, there seemed to be a completely different mental model, which we could summarise as: permormance == throughput, and timings are only relevant, when you get a network timeout, or a sluggish response in your application's GUI. Thus, if we now consider to use a Linux kernel for making music, we must assess that the whole design isochronously assumed about 1000 times more headroom as there really is. Thus, as writing a new Kernel doesn't seem to be an option, this whole tedious undertaking of the "realtime patches" can be described as an attempt to fix this "problem" (which was never assumed to be a problem in the initial design) by hunting down one by one each individual instance where the existing kernel could possibly be reacting too slow. Thus, we should rather be surprised, how good these realtime kernels work. OTOH, is isn't a surprise the machines from the 80s meet these criteria; their OS software was written with an awareness for a much more limited processing capability right from start. > Why do people (not only me) report jitter for external MIDI > equipment, but I couldn't find any report for real-time audio jitter? > Resp. what's about async and disconnecting clients by JACK? Audio and MIDI are two quite different beasts. Sound is processed in Blocks, where the individual unity (1 Sample) is much more fine grained and way below anything which can be discerned by a human ear. Moreover, Sound as such already exists and 'just' has to be piped through. To the contrary, MIDI consists of events, which immediately trigger a reaction, which could be that a piece of software and at the same time a piece of external hardware starts a processing cycle. You see, thats a completely different situation and thus it's obvious, why for these two media the same problem causes so different symptoms. > OTOH on Windows audio clients don't disconnect, > just MIDI jitter is an issue too. IIRC, this was a design decision for JACK. It never tries to conceal any timeout problem, rather it requires its clients to keep up with a very tight schedule and comply to very strict rules. I don't know the MIDI part of Jack well enough to judge, if it was designed with the same "you're required to comply" policy. And besides, when the MIDI interface is hooked up via USB, we again face a completely different situation. USB is a complicated protocol, which multiple versions and levels and is certainly not designed to get an individual event transfered reliably with less than 2ms jitter. There is even the possibility that the USB peers negotiate to use a lower transfer rate or protocol version transparently, when they determine the connection can't keep up with the higher speed. Cheers Hermann V.

Hi Fons :) fons(a)kokkinizita.net wrote: > On Sat, Mar 27, 2010 at 09:09:38AM +0100, Ralf Mardorf wrote: >> 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? > > Wrong once or twice, if twice in such a way that the two > errors cancel out. > > First note that the test prints the difference between > events. That means that e.g. if *one* note is 100 samples > late you could see 2400 2500 2300 2400. > > The '2300' is just because the previous one was late, > not because this one arrives too early. So you should > divide the jitter as you measure it by two. Aha, okay this is plausible. > Second, 5.33 ms = 256 frames at 48 kHz. But maybe you > are using 96 kHz ?? So you didn't read the attachment ;), yes I did use 96 KHz. [snip]

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

On Thu, Jun 17, 2010 at 10:37:25AM -0400, Gene Heskett wrote: >> At my school we transfered the CAD files per floppy to a DOS box >> that controlled the CNC machine, guess that's for the same reason, >> bad rt capabilities of newer OSes and machines. > > The RTAI works pretty well, I can start a job, switch away from that > window, and talk to the guys on IRC, or browse the web without > hurting the job. That to me is true multitasking. So, that leaves me wondering why no one seems to be trying RTAI for audio work? Or is someone doing that and I'm just not aware?