MIDI-powered - what the penalty?

Anything about hardware musical instruments.
lfm
KVRAF
4664 posts since 22 Jan, 2005 from Sweden

Post Thu Mar 21, 2013 11:59 am

Looked for some merge boxes and there came up a lot of what they call midi-powered and need no power supplies at all.

Is it special interfaces - like ethernet power - or something?

Or is it picking current from the currentloop in the midi cables?

Never encountered it before - so I wonder.

thanks.
:)

User avatar
BertKoor
KVRAF
10792 posts since 8 Mar, 2005 from Utrecht, Holland

Post Fri Mar 22, 2013 12:28 am

It's simular to phantom power you see on microphones.
MIDI connectors have 5 pins of which just three are used:

1: not connected
2: 0V shield/ground
3: not connected
4: +5V
5: the actual signal

According to the MIDI Electrical Specifications pin 2 should be disconnected also on the receiving side (thus avoiding any ground loops) and an Opto-isolator should be used to read out the current difference between pins 4 & 5.

This opto-isolator is in essence a LED that will pulsate, paired with a light-sensitive transistor (electrical switch) next to it, in a closed box.

Instead of the LED of the opto-isolator, the current supplied on pin 5 could be just enough to power a simple device that doesn't require much power.

You asked in the subject about the penalty. I can think of just one: such splitter/merge boxes cannot send out a stronger signal than they receive. Combinations of several of these in one chain is something I would think about twice. One or two should be no problem though.
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lfm
KVRAF
4664 posts since 22 Jan, 2005 from Sweden

Post Fri Mar 22, 2013 1:38 am

Excellent reply - thank you.

One are afraid of two things basically:

a) that a unit might damage the sending side of the equipment you use.

Who wants a broken synth just because midi broke down?

b) that it might have bad performance due to having to be really low on power consumption.

You want it to merge without timing penalty of any kind.

I had a Philip Rees merge box once - and it was powered from mains directly.

So when I saw these Midi Solutions:
http://www.thomann.de/se/midi_solutions ... 7da2544e8d

.. I thought this must be some special power feed to do this - yet they claim it's not needed.

And to confuse things - they also have power boxes that are fed from a mains adapter. My guess this is not to do my point a) above - damage regular midi equipment.

I'm a bit torn still - but you helped a lot.

I think it might be a better idea to buy an extra midi interface for computer.
:)

User avatar
BertKoor
KVRAF
10792 posts since 8 Mar, 2005 from Utrecht, Holland

Post Fri Mar 22, 2013 4:28 am

Have you read this? http://www.midisolutions.com/faqs.htm#Power
If you are really afraid of damage by powering the box by your own midi device, then you should get their power adapter. The optical isolation makes sure that it behaves just like a regular Midi IN port as far as your own device is concerned.

Since you're interested in the merger, it receives up to four inputs so power consuption is also spread across multiple devices.

But wouldn't you think that if there were a remote chance of damage, they would put up warnings and non-liability statements on their website?

The performance would be my last worry. For example CMOS technology requires next to no power at all to function, yet is fast enough for MIDI. There's just 32000 bits per second to process. For electronics that's an extremely slow data rate.
We are the KVR collective. Resistance is futile. You will be assimilated. Image
My MusicCalc is back online!!

lfm
KVRAF
4664 posts since 22 Jan, 2005 from Sweden

Post Fri Mar 22, 2013 6:27 am

Very good, thanks.
:)

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