Re: Real-time category

[Matthias Grob <matigrob@terra.com.br>]

>I don't think Wikipedia is a great source of definitive technical
>data, but in the RTOS entry therein I do see where the two examples it
>cites are in range of microseconds.  The matter that I was discussing
>was actually related to the period that elapses between when a MIDI
>event is received by the EDP (a "Record" message for example) and when
>the EDP can actually begin recording.
>
>Again, my understanding is that the EDP was designed so that the lapse
>between command and execution is constant, and I believe that constant
>is 1.5ms.

hm, no, thats the jitter. I poll for MIDI and switches every 64 audio 
samples = 1.5ms if you press right then, it only takes about 10 
samples to execution, if you press right after it, it takes about 74 
samples.
In case of going to a recording mode, we still have to wait for the 
fade in which happens in a analog VCA chip.
So if you want to record a full attack, you need to press the button 
about 7ms before you play it - which does not seem to be a problem 
for anyone.

>
>  > You might refer here to computer keyboard or mouse clicks, I don't 
>know,
>>  I never assume these as part of my instrument, they are only an aid for
>>  navigation, not for playing the music.
>>
>>  If the timing jitter of an EDP is really 1,5 ms, then its much, much
>>  less accurate than my software looper. If I need, I could have it 
>sample
>>  accurate (trigger with audio), this would be more than 50 times more
>>  accurate than that specific hardware looper. But I guess other loopers
>>  are not that bad ;-)

you are exagerating. to trigger with audio is not that simple. Andy 
and me spent quite some time to figure it out for the Mathons 
plugins. Its never within a sample. And we want foot operation, and I 
doubt that a PC can garantee 1.5ms for that. But there is no need. 
Praxis counts!

I just had a chat with Tom, an old engineer friend and keyboard 
player (most who played at Zurich festival know him) and he confirmed 
that MIDI latency and jitter is just not a problem for keyboard 
players, only audio latency.
so much less for loopers, because the loop lenght is still less 
critical than the response to a note played, and if the delay is no 
problem, the variation of it lesser.

He also said that the keyboards and effect units built arround a PC 
board use regular Linux, no RTOS.

>  >
>>  Before using technical terms it might be a good advice to check the
>>  Wikipedia to be sure what you are talking about. Here is a quote from
>>  the RTOS entry:
>>
>>  "On a 20MHz 68000 processor, task switch times run about 20 
>microseconds
>>  with two tasks ready. 100 MHz ARM CPUs switch in a few microseconds."
>>
>>  Thats the time it needs to switch from an interupt (incoming Midi event
>>  for example) to deal with this event. 20 microseconds on a 20MHz 
>68000....
>>  I run a 1,5 GHz Powerbook with a PPC, I guess its probably more in the
>>  range of nano seconds...

I dont think so. the 20us go for a RTOS when its clear that changing 
the task is needed. On the Powerbook there are loads of threads 
running already and there must be some priority avaliation that 
finially decides to care for the MIDI event, which then resides in 
some buffer and so on... I have no idea how long it takes, but I 
doubt you get to nano seconds.

-- 


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