[sdiy] Midi opto isolation

[Scott Gravenhorst]

"Sean Ellis" <tensiontype at hotmail.com> wrote:
>Over the years I have picked up some modules that have trouble with complex 
>midi signals. I'm considering ways to fix them like improving the midi 
>inputs stage but I'm curious, why have the signal isolated? Surely using the 
>isolation lowers the quality of the signal with very little benefit, is it 
>really needed? 

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.  When MIDI was designed, signal quality was, I'm sure, quite important. 
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.

-- ScottG

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-- Scott Gravenhorst
-- GateMan-III - FPGA Based Monophonic MIDI Synthesizer with SVF
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