[sdiy] MIDI output current and MIDI-driven gadgets

[Mattias Rickardsson]

Hi folks,

Do any manufacturers intentionally design their MIDI Outs to improve
reliability with MIDI-driven devices?

Even though the MIDI standard only specifies communication, some gadgets
use it for driving electronics by the voltage and current typically
appearing on a MIDI Out. Like CME WIDI Master etc, adding Bluetooth MIDI in
a plug directly on the MIDI port, and in many other hobby projects and
commercial products throughout the years.
https://www.cme-pro.com/widi-master/

The MIDI standard allows quite wide tolerances on everything: Supply
voltage +5V +/-10%, resistors +/- 5%. Hence, the possible voltages and
currents available for a MIDI-driven device can differ quite a lot, at
least in theory and in older equipment. Moreover, the MIDI Out can also run
off a +3.3V +/-5% supply with low-valued resistors, potentially allowing
more current if more than MIDI's 5 mA is sucked out, but also never giving
anywhere near 5 V (or even 4 V) in low-current situations. There seem to be
observations of certain external devices not working on certain MIDI gear,
which is not very unexpected given the out-of-spec application.

For example I'm thinking that it would be possible and fully allowed to
choose 1% resistors near the more current-generous end of the 5% spec
range, and then give slightly more current and/or higher voltage to an
external device, while still being compliant with the MIDI spec (and
hopefully well-working with all MIDI In ports). There are also other
potential tweaks to the MIDI Outs, optimizing its use for certain "illegal"
operations. The driving voltage can be tweaked to give more current or more
voltage at certain non-MIDI-typical usecases, and I guess the MIDI Out
could also be designed more as a true current source than the logic buffer
with output resistors shown in the spec.

Now that the MIDI spec has allowed both 3.3V and TRS since some years back,
it should have triggered more innovation and I figured some of you might
have encountered some interesting concepts and design variations. :-)

Curious regards,
/mr
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