Subject: Re: [linux-audio-dev] [ot] rawmidi && alsa
From: Paul Davis (pbd_AT_Op.Net)
Date: Tue Apr 24 2001 - 17:54:12 EEST
>let's assume a problem case: i have MIDI clock messages (0xF8) to write
>periodically to a device. in addition, a large sysex block needs to
>travel down the same MIDI wire. of course, to be able to send both,
>interrupting the sysex with the clock ticks, our program needs every bit
>of knowledge about the port speed / buffer state. currently i am using
>a userspace buffer for the messages, an assumed port speed and a periodic
>writing function to get this right, but i don't really like this method
>since it is just pseudo-sync by assuming things.
>
>it would be great to be able to do this the right way, ie knowing the
>exact port speed (MIDI, USB, 1394) and being able to make a correct
>assumption on how much data can be written in the current cycle of the
>MIDI scheduling engine. (or, if somebody has a better proposal to make,
>please speak up.)
MIDI 1.0 Detailed Specification 4.2, page 1:
"MIDI, the Musical Instrument Digital Interface, was established
as a <em>hardware</em> and software specification ..."
(my emphasis)
"The hardware MIDI interface operates at 31.25 (+/- 1%) Kbaud,
asynchronous, with a start bit, 8 data bits and a stop bit. This
makes for a total of 320 microseconds per serial byte."
What more information do you need ? There is no way to get faster
transmission speeds without buffering at one end or the other, since
the ultimate target will (for many, many years) be operating at
31.25kb. If this buffering exists (and it must), then the fact that
the bits stream between two points on their journey at 100MB/sec
doesn't do anything to alter the point-to-point speed being
31.25kb. Or I am missing something ?
--p
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