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

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Subject: Re: [linux-audio-dev] Audio synchronization, MIDI API
From: Steve Harris (S.W.Harris_AT_ecs.soton.ac.uk)
Date: Sun Aug 15 2004 - 01:47:27 EEST


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.

This is why latency sensitive application have used token ring networks
traditionally.

- Steve


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