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

[Martijn Sipkema]

From: "Steve Harris" <S.W.Harris at ecs.soton.ac.uk>
> On Sat, Aug 14, 2004 at 10:07:06PM +0200, Benno Senoner wrote:
> > >UDP also has unbounded transit time. In practice its OK if you dont want
> > >low latencies (just use RTP), but for low latency you really need one of
> > >the non-IP ethernet protocols that can be relaibly used for audio.
> > 
> > I don't think raw ethernet will buy us anything over using UDP. These 
> > few usecs less simply won't matter.
> > (but with ethernet you would have the disadvantage that you loose 
> > routability)
> > On a 100Mbit network the round trip latency between hosts is about 
> > 100usecs so the one way latency of MIDI would be
> > about half of that. and that's form a MIDI point of view instantaneous 
> > because over serial MIDI cable transmitting
> > a NOTE ON event  (3 bytes) takes about 1.1msec which is 20 times slower 
> > than transmitting it over an ethernet cable.
> 
> No, the roundtrip latency is *at least* 100usecs (or whatever), the hardware
> will keep re-transmitting until the packets get through.
> 
> In pratice people dont really demand hard realtime and it will be OK, but
> the maximum time taken to transmit a UDP packet is unbounded, it uses
> exponential backoff IIRC. 

It is only unbounded if the network can't provide it, and if that is the case you
would lose the ethernet frame, which might be difficult to handle for something
like MIDI. Losing packets is not really hard real time either...

--ms