[linux-audio-dev] Audio synchronization, MIDI API

[John Lazzaro]

On Aug 18, 2004, at 12:38 PM, 
linux-audio-dev-request at music.columbia.edu wrote:
>>   -- There are tools for synchronization (RTCP mappings of NTP
>>       and RTP timestamps), tools for security (SRTP), tools for
>>       all sorts of things someone might need to do someday.
>
> this does seem very useful. there's no way to transport time between 2
> jackd instances right now, and it would be wise to reuse existing
> technology whenever this is added. otoh, it has to be bit more
> extensive since we need music and/or smpte time rather than just
> wallclock time.

One way to do this is to have a multi-stream session, with one of
the sessions being RTP MIDI, that uses System Exclusive commands
to do MTC, or MIDI sequencer, or MMC to do your timing.  So,
this would recreate the current hardwired world, but using RTP MIDI
to do pseudo-wire emulation of the MIDI cable carrying MTC ... the
RTP RTCP stream would have NTP, as would all the audio RTP streams,
and the receiver uses these common NTP timestamps to derive
cross-sync between the MTC sync information in RTP MIDI and the RTP
timestamps on the audio stream.

Of course, this only works as well as your NTP sync ... in an ideal 
world,
a single server generates these streams off of a single NTP clock, or
at least you have a very good NTP daemon keeping things in sync.
Roger Dannenberg gave a good talk on these issues at the OSC-fest
here at Berkeley last month ...

---
John Lazzaro
http://www.cs.berkeley.edu/~lazzaro
lazzaro [at] cs [dot] berkeley [dot] edu
---