Subject: Re: [linux-audio-dev] rtlinux, MIDI timecode
From: Paul Barton-Davis (pbd_AT_Op.Net)
Date: Thu May 04 2000 - 20:54:04 EEST
>On Thu, 4 May 2000, Paul Barton-Davis wrote :
>
>>As I've mentioned before, MIDI timecode seems very problematic for
>>general purpose computers. Sending 2 bytes every 1.3 msec (for 30fps
>>SMTPE) is just not something that Linux (or Windows, or MacOS) was
>>really designed to make it easy to do.
>
>MIDI timecode (MTC) requires only 2 bytes (MTC Quarter frame message : F1
>nd) per quarter frame. Therefore, 30fps requires 2 bytes every 8.3 ms (not
>every 1.3 ms). The reception of 8 Quarter Frame messages completely define
>the SMPTE time every two frames.
Once again, I blow my timing numbers in a public forum. Why don't I
have the page open when I write this stuff down ? I even gave the
necessary information in my post to demonstrate that I was wrong.
*blush*
Anyway, the problem is still that if the MTC receiver does not
tolerate timing jitter, then we can't use the RTC, because of the
power-of-2 problem. David Woodhouse's suggestion of switching the use
of the RTC and the 8253 is a nice one, and its even been partially
written up on the source already. Quite promising.
>Concerning performances, MIDI is very demanding in terms of interrupts. But
>an old Mac II was able to deal with 2 full MIDI Thru + a 1 ms timer
>interrupt (representing 2*2*3125 + 1000 = 13500 interrupts / second)
>without problems.
Well, Linux can do that without problems as well. Its not the number
of incoming interrupts that is a problem, its getting precise timing
for sending timecode.
--p
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