[sdiy] MIDI notes filter which is also a MIDI converter for Junos ?

Tom Wiltshire tom at electricdruid.net
Sun Dec 29 14:05:55 CET 2013

On 29 Dec 2013, at 11:50, Roman Sowa <modular at go2.pl> wrote:

> I dare to disagree with you.
> Adding an opto isolator in any kind of MIDI-powered translator is highly irrational. It simply replaces plain MIDI cable, so ground at the end of the chain is exactly the same as it was in case of pure cable. Ground loop can appear only if there's some 3rd connection to the device, and there is none in this example.

I agree. Since the specification requires that the MIDI IN you're feeding doesn't have the ground connected, it would be ok for you to connect ground straight through your device. It's perhaps "a bit naughty", but that's about all.
It's a different question if you introduce a power supply to run the box, of course. Then I think the "cable" analogy disappears and I would want to see the job done properly. 

> 5V at MIDI OUT is determined by by 1.2V voltage drop on the opto's LED, 3x220 ohm resistors that are usually found in every MIDI IN-OUT loop, and 5mA standarized requirement for logical zero. I don't think any manufaxturer would risk incompatibility problems using lower voltage at MIDI OUT jack.

I don't agree here. The specification calls for a 5mA current loop, and you can do that at 3.3V as well as at 5V. In fact, I have. Simply adjust the 220R resistors in the MIDI OUT and you're done. As long as the voltage you provide is enough to drive the opto at the far end (e.g. >1.2V, or say 1.5V for safety), the exact level shouldn't be an issue. Isn't that partly the point of using a current loop? MIDI is agnostic about the voltage level - it could be 12V, 5V, or nowadays 3.3V, as long as it's 5mA loop, it's legal.

In fact, I'm a bit surprised you haven't done the same yourself at some point, Roman. Haven't you got some little 3.3V uP-based MIDI gizmo knocking around somewhere?

2 cents,

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