[sdiy] Midi opto isolation

[harrybissell at wowway.com]

Many problems can be cuased by incorrect design in the MIDI
isolation / cables.  The cable "ground" should only be connected at
one end (the transmitter).  The receiver should only have the two
LED pins connected.  A sure sign of this is if one order of modules
a-b-c works but a-c-b does not.

Look for genuine MIDI cables that have onlt two conductors. This will
~not~ work for many drum machines that use the extra pins.

H^) harry



On Tue, 1 Jan 2008 20:24:47 +0100, Sean Ellis wrote
> > MIDI uses a current loop serial interface.  As such, it is very much noise 
> > immune.  The
> > optoisolator ensures that you can't have ground loops due to synth 
> > interconnection because
> > the ground is not part of the current loop.  I don't see how "isolation 
> > lowers the quality
> > of the signal", and the benefit is the noise immunity and absence of 
> > required ground
> > connection.
> 
> That's what I was unsure about. The lower quality is from the 
> led/ldr delay times, but obviously that only applies to certain 
> hardware units.
> 
> > Personally, I've used MIDI for many years and the _only_ problems I've had 
> > have been due
> > to my own errors - cables too long (there is a max length described in the 
> > MIDI spec. -
> > that should be heeded) is an electrical problem that is easy to fix. 
> > Other problems are
> > more ominous, such as MIDI choke (and there are varying degrees of that 
> > depending on how
> > much data you try to push) which can be mitigated by providing more MIDI 
> > busses each with
> > fewer instruments.
> 
> I wasn't implying that there is something inherently wrong with midi,
>  I have used it also for many years without problems except for a 
> few units that frequently suffer hanging or dropped notes. With the 
> live setup I used to use I'm sure some of the problems were my own 
> fault, long cables, up to 4 stages of midi through ports etc...
> 
> > What do you mean by 'complex midi signals'? Electrically, a MIDI signal
> > is just an asynchronous serial stream (8N1) at a 31.25kHz (1MHz/32) baud
> > rate. That data content of the serial stream may be more complex, but
> > the electrical signal won't look any different.
> 
> Complex might not have been the right word :) I am refering to 
> running up to 10 channels of sequences and time code, this is when I 
> got the highest number of errors. The biggest problem modules are a 
> dx-27 (with long midi cable) and a Simmons SDE expander, both of 
> these would drop notes, delay notes and get hanging notes that were 
> not on the same midi channel. Looks like replacing and upgrading the 
> midi inputs on these pieces might reduce the error problems, they 
> were terrible for a few gigs. Every 2 or 3 mins I would have to jam 
> all the keys I could on the dx-27 to stop the hanging, sometimes it 
> was 4 or 5 notes at once hanging!
> 
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Harry Bissell & Nora Abdullah 4eva