[LAU] Galvanic isolation (was: Re: pianos with built in recorders?)

[Gene Heskett]

On Friday 25 December 2015 09:04:22 Ralf Mardorf wrote:

> On Fri, 25 Dec 2015 13:38:01 +0100, Yves Guillemot wrote:
> >Le jeudi 24 décembre 2015, 11:48:07 Bob van der Poel a écrit :
> >> I believe that the MIDI spec
> >>
> >> http://www.midi.org/techspecs/electrispec.php
> >>
> >> says that you must have a opto-isolator. Devices without one are
> >> probably not compliment ... don't know about direct usb
> >> connections.
> >
> >MIDI uses an unipolar signaling which is very sensitive to
> >electromagnetic noises, in particular those ground loop generates.
> >That's why MIDI standard requires an opto-coupler.
> >
> >USB uses a differential signaling and can works without any galvanic
> >isolation.
>
> Signal routes could be very tricky. A keyboard could be directly
> connected by MIDI to the PC, but audio of the synth might be connected
> to other gear and this gear is connected by audio to the PC. Galvanic
> isolation and an open shielding always could make a difference.
> There's also to keep in mind, that connections could be done by USB to
> MIDI DIN adapters.
>
> In the last 35 years music equipment became much cheaper, electronic
> components became much better, but design of circuits and/or
> carefulness of soldering and assembling became very worse. ISO 9000 is
> just crap to provide good-for-nothing idiots a quality management
> manager job to reduce quality, before that hype the quality of
> circuit design and carefulness of soldering and assembling was much
> better.
>
> 2 Cents,
> Ral

Ralf et all;

Having on several occasions, dealt with venues that were wired by color 
blind electricians, I look at galvanic isolation as a hard and fast rule 
where it can be done.  Musicians with their own gear, routinely flip the 
polarity of ground switch on the front of the amp for least hum. And 
just as routinely break the 3rd pin off their power plugs because the 
venue is too damned old to have ever had a static ground in the duplexes 
anyway.

Or if that worn out duplex has been replaced by a stage hand who never 
saw a copy of the NEC, the 3rd pin may not be connected to anything but 
the handybox its in. And too many times that has led to the whole stage 
lashup being hot enough to kill if a real ground is contacted.

"portable" venues constructed on site by the usual group of "roadies" 
should be tested by a qualified test person and confirmed each time they 
are assembled, with the results posted where the talent can read it as 
they enter.

Having opto-based galvanic isolation in the midi spec should not be 
skipped just because its USB with its differential signalling and 
doesn't stop working when theres a 3 volt+ AC potential to ground.

The fact that this AC potential is even present, should be grounds that 
you write it into the performance contract that if his gear is 
defective, he can't use it on your stage.  By that same contract, the 
smarter musicians who do know about such things can refuse to perform if 
the venues wiring isn't up to code. And he should still be paid in that 
event, as its not his fault.

The quietest studio/control room lashup that ran on 60 hz power I've ever 
not heard, is actually powered by a big (5kw rated) 250 volt center 
tapped transformer, powered by the 127 line applied across the 250 volt 
terminals, with the secondary's center tap on its own deep earthen 
ground so the secondary's 1/1  line voltage from pin to pin was still 
127, but it was a balanced 63.5 volt to ground on each pin.  The 
equipment plugged in didn't know it was specially powered, but the 
board, which had single ended inputs, had a hum level that was about 90 
db down from a live microphone input.  Thats as close to never mind as 
you can get even with true differential inputs, and was about 2 grand 
lower in cost to build.  

Biggest problem?  Convincing the electrician who installed the 
transformer, that it actually met code.  It may not in some locales that 
write their own version of the NEC.

Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>