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

[John Check]

On Saturday 14 August 2004 08:27 pm, Martijn Sipkema wrote:
> 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

A) Forest for the trees
B) Good Enough ® is good enough