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From the Midi DIN spec schematics we can see MIDI THRU is just MIDI OUT directly at the end of a MIDI IN. Is it possible to just add a midi signal from the arduino TX ("FROM UART") to the midi thru (parallel to the MIDI IN) or do I need to get the MIDI IN signal in arduino and mix them digitally before sending to a regular out? Or is there another simple solution to merge the 2?

It is for a keyboard and a pedal (CC64) so notes won't be the same (if it is an issue).

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    \$\begingroup\$ You'd at bare minimum need to watch the incoming stream to know when its safe to insert something, and the circuit would be simpler if you ran everything through the ATmega. \$\endgroup\$ – Chris Stratton Mar 14 '19 at 17:16
  • \$\begingroup\$ @ChrisStratton it was my first intent but I though maybe it was not necessary to bring these notes in. I guess if a note from the IN is cut in the middle by another note from the TX it would be a problem... I know how to handle that with code, not with electronics. \$\endgroup\$ – mikmikmik Mar 14 '19 at 17:28
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No, combining them in hardware is not possible. Normal way of doing this is to have two MIDI inputs, and two UARTs on a microcontroller receiving messages from both inputs. Then the microprocessor merges the messages so that correct MIDI data bytes belong to correct MIDI status bytes. These merged messages are then sent to one (or several) MIDI outputs.

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  • \$\begingroup\$ This is, of course, the approach one should use, but I disagree that "combining them in hardware is not possible". I'd almost guarantee that some of the stuff available in the '80s didn't use processors. \$\endgroup\$ – Scott Seidman Mar 15 '19 at 21:16
  • \$\begingroup\$ @ScottSeidman Do you have schematics or a link with that kind of build? \$\endgroup\$ – mikmikmik Mar 18 '19 at 21:15
  • \$\begingroup\$ @mikmikmik looked, but can't find one. Honestly, it would be faster and easier for you to do it on an Arduino for $30 \$\endgroup\$ – Scott Seidman Mar 18 '19 at 22:10
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You're trying to build a MIDI mixer! You need to work something out so messages go out one at a time, and don't mix together and get garbled.

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  • \$\begingroup\$ Are you talking about me or the mixer? :) \$\endgroup\$ – mikmikmik Mar 14 '19 at 17:50
  • \$\begingroup\$ Yes but is it possible with components? \$\endgroup\$ – mikmikmik Mar 14 '19 at 17:51
  • \$\begingroup\$ It's possible, but you want a microcontroller taking in the messages, and queueing them to be sent out again. \$\endgroup\$ – Scott Seidman Mar 14 '19 at 18:21

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