[sdiy] Midi opto isolation

[Sean Ellis]

> 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!