[sdiy] USB/MIDI ground isolation?

[Neil Johnson]

Hi,

johnspeth at yahoo.com wrote: 
> That's only half the view of an interface that is compised of two sides: input and output.
>  Both ends are isolated using an optocoupler.

Incorrect - only MIDI inputs are opto-isolated. The ground connection at a MIDI output only extends along the cable; a MIDI input must not have a ground connection.

> That's true in the case of a USB/MIDI Audio class device to 5 pin DIN MIDI converter.
>  That doesn't apply to devices that don't have 5 pin DIN MIDI connectors; for example,
> a synth with a USB/MIDI connector instead if a 5 pin DIN MIDI In connector.

As others suggested if you mean MIDI over USB, then I guess *technically* its not MIDI since it does not use the standard interfaces and cabling as specified in the MIDI standard.  So in those cases I'm sure there won't be any ground isolation, so the device at the end of the USB cable is in the same ground domain as the PC.

> > Could you elaborate on why you are asking this question?

> Because I can easily see the case where 60 hz hum can sneak across systems connected entirely
> by USB/MIDI but use different power sources.

Of course, you may well get interesting earth loops coupling in 50 or 60 Hz hum.  Such behaviour should not be surprising at all.

>  That was a problem that was elegantly eliminated
> by the decades old 5 pin DIN MIDI, IMO, the best part of MIDI.

Indeed.

>  It seems to me that USB/MIDI has dropped the best feature of old fashioned MIDI.

Yes, because its cheaper. MIDI over USB is a cheap hack designed for low-cost consumer devices, where cost is more important than audio integrity.  If you want to break the ground loops there are several ways, from USB opto-isolators (very expensive), audio transformers (mildy expensive) to cheap tricks such as ground-lift with a lowish-value resistor.

Neil
--
http://www.njohnson.co.uk