[sdiy] Midi opto isolation

[Eric Brombaugh]

Sean Ellis wrote:
> Over the years I have picked up some modules that have trouble with 
> complex midi signals.

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.

> 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?

Optical isolation provides an effective barrier to ground loops and RFI. 
That prevents nasty buzzes & radio interference from getting into 
complex MIDI cabling systems. It also provides a nice standard interface 
between various logic voltage standards that may be employed in 
different MIDI devices.

The optoisolators used in decent-quality MIDI systems are plenty fast 
enough to pass the serial data without reducing reliability. If you have 
devices that aren't reliably receiving or transmitting MIDI then either 
they were built with poor-quality isolators, your isolators have 
degraded with age (some LEDs do get dimmer over time), or your MIDI 
devices have poor firmware or parsing logic.

Since it's likely that every device is built differently there aren't 
any catch-all fixes for this. Firmware updates, hardware patches or 
replacing old isolators are the only solutions. Certainly removing or 
bypassing the optoisolators will not improve reliability and will 
probably introduce other problems that will degrade performance or 
damage other equipment.

Eric