[sdiy] MIDI over Ethernet (was Microchip DSP kit $60 > AVR32)

[Gene Stopp]

I remember the days when MIDI was just being deployed. I used to sit in
at the MMA meetings at the NAMM shows, when people like Chris Meyer,
Lucky Westfall, and Jeff Rona were talking about the pros and cons of
GM, for example. MIDI was an attempt to walk the fine line between
complexity and cost, so that the synth manufacturers would buy in to it
and play together. The connectors were cheap, the electronics were
cheap, the bit rate was chosen as a divisor of 1Mhz so that common CPU
clocks could be used as a source, and the signalling was based on the
then-ubiquitous RS-232 Async UART technology. The messages were as
simple as possible to still get the job done, what datacomm people call
a "best effort delivery" protocol. Electrical isolation was favored
because all these audio people were well aware of stage ground loop
issues.

Ethernet was black magic at the time (half duplex, and 10BaseT was just
coming out to replace the "garden hose and vampire taps") and still
expensive. Nowadays, it's used everywhere, full duplex, and the parts
are as cheap as DIN connectors and optoisolators, if not cheaper. IP and
TCP stacks are in the silicon, and can provide stateful guaranteed
delivery at Gigabit rates. The minimum framesize is 64 bytes, so there's
plenty of room for complex datagrams, even with IP/TCP headers added.
Larger frames can give even more throughput.

There is a big danger, however, which is if musical instrument control
can be mixed in with everything else, congestion can lead to higher
latencies and jitter. To keep this in check you'd have to isloate your
instrument control from everything else. I don't mean using VLANs or
some other domain restrictions, I mean keeping everything else out of
the switches (forget hubs, those are so passe).

The big question is - what really is the demand? Is MIDI really that
crummy? Does it get the job done? How many instruments need to be
controlled at the same time, from the same source? How big does the MIDI
orchestra need to be? What subtle additional performace nuances are
currently missing? Well, MIDI actually is kind of crummy given the state
of the art in everything else. You *can* hear the difference between a
performance on a "real instrument" and something controlled over MIDI.

I can see migration of instrument control to Ethernet. Ethernet is here
to stay, for better or worse. It's funny how it bubbled to the top, how
it survived the battle of the networking protocols. At first it was the
trashiest of all the protocols - at 25% utilization everything starting
to go to hell, what with all the collisions. Token Ring could do line
rate with no loss! But alas, it was way too IBM, too complex to survive.
Ethernet still has the same framesize rules (except the max framesize
has been pushed out to >9k bytes), and it still has a preamble, even on
fiber, useless like an appendix.

I still see Lucky Westfall all the time, he's the coach on my daughter's
fastpitch softball team :)

- Gene

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Neil Johnson
Sent: Tuesday, March 25, 2008 1:27 AM
To: Tom Wiltshire
Cc: Synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] MIDI over Ethernet (was Microchip DSP kit $60 >
AVR32)

Hi,

On 22 Mar 2008, at 15:13, Tom Wiltshire wrote:
> On 22 Mar 2008, at 15:00, Jim Patchell wrote:
>
>> The new XMega AVR is also looking very promising.  It will have  
>> the ability to do a lot of things in hardware ( DMA, Event  
>> Routing) that will help a lot in increasing the amount of work the  
>> processor can do...plus the 32MIPs processing speed.
>>
>> There are other free tools available for the AVR such as  
>> Ethernut.  It is a combination RTOS/TCP-IP Stack that works really  
>> nice.  I am in the process of writing a server (running on Windoz)  
>> and client (running on an Ethernut board) that will do MIDI over  
>> Ethernet.
>
> Sounds interesting, Jim, but I've got to ask "Why?"
> I thought everyone was moving from 'traditional' MIDI via the old  
> DIN plugs to MIDI over USB? What's the advantage of ethernet as a  
> connection, particularly if it's non-standard?
>
> Please don't think I'm trying to knock it. I'd just like to  
> understand your thinking.

One other aspect that I haven't yet seen mentioned is that Ethernet,  
like MIDI, has electrical isolation on inputs - opto-isolator for  
MIDI, transformer for Ethernet.  Even with Power-over-Ethernet you  
still maintain the isolation between input and device via the  
switcher magnetics.  Ethernet also adds isolation on the output as  
well just for completeness.

Unless you spend $$$ on special boxes you don't get isolation on USB,  
so ground loops and noise-sharing via common grounds start to be a  
problem again.

Neil
--
http://www.njohnson.co.uk



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