Re: Real-time category

[Stefan Tiedje <Stefan-Tiedje@addcom.de>]

Travis Hartnett wrote:
> My understanding of the main practical difference for loopers between
> RTOS and non-RTOS is that in the first case there's a guarantee on the
> maximum time the system will take to process an event. 

This is correct.

> In the case of the EDP I seem to recall 1.5ms as the magic number. 

The technical term RTOS in the computer world is used only if it where 
in the range of microseconds, this is 1000 times faster as you claim it 
would be a RTOS. Otherwise an old Apple II with its old operating system 
would have had already a realtime operating system. (I did nice realtime 
Midi stuff on such a machine ;-)

> Everything is read in 1.5ms, all the time. 
> In a non-RTOS, the overall response lag
> may average at 1.5ms (although I seem to recall that Windows and Mac
> OS take slightly longer than this), but there are times when another
> task might have precedence, and so the looping app response drops down
> to something like 20ms.

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 ;-)

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...

The hardware which is on audio interfaces by the way, has its own "RTOS" 
to accomplish the steady stream of samples.

If I have a constant latency, I know it by heart (as being a musician).
Jitter or timing uncertainty would be a problem though, even if its as 
low as 1.5 ms.
But to avoid this is more a question of how I write the software than a 
question of my operating system (Well, I usually avoid Loosedoze as OS, 
so I do not have any expierience with it...)

Stefan

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         Stefan Tiedje
         Klanggestalter
     Electronic Composition
               &
         Improvisation

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