MIDI Out normally carries messages generated by a device, while MIDI Thru passes incoming MIDI data onward to another destination. Soft Thru muddies the labels because software can make an Out port retransmit data that arrived at an In port.
You can build a working rig without caring about the distinction until the signal comes back to its source. Then one note becomes two, a knob starts fighting itself, or a burst of repeated messages makes the setup behave as if something has failed. The cables may be fine.
Most trouble comes from assuming every MIDI Out behaves the same way. It does not. Knowing how MIDI data travels between devices gives you the foundation, but routing depends on what each device chooses to generate, copy, merge, filter, or return.
MIDI Thru starts somewhere else. It takes data received at MIDI In and passes that stream onward, traditionally without changing it. A controller can therefore feed one synth, whose Thru port feeds a second synth, while both instruments receive the controller's performance.
The second synth does not need the first synth to regenerate every note. Thru exists specifically to continue the incoming path. Hardware designs and MIDI documentation have long treated In, Out, and Thru as separate jobs rather than three interchangeable sockets.
Modern gear often combines Out and Thru on one physical connector. A menu setting may switch the jack between the two functions, while another device may offer a software option that merges received data with locally generated data before sending everything from Out. Reading the exact manual matters because the word Out no longer guarantees one fixed routing behavior.
A combined design can be useful in a compact rig. Your keyboard can receive notes from a sequencer, generate controller moves of its own, and send a merged stream toward another device. The same convenience also creates a path back toward the sequencer if both directions are connected.
Problems appear when the synthesizer is also playing its own keyboard locally. One key press reaches the internal sound engine directly, then travels into the DAW, through Soft Thru, and back into the synth as another Note On. You hear two attacks even though you pressed one key.
Local Control is designed to break one of those paths. With Local Control off, the keyboard can still transmit MIDI outward while its internal sound engine waits for returned MIDI from the sequencer. The DAW becomes the routing authority instead of the instrument sounding both the local and returned copies.
Turning Local Control off is not a universal cure. If the DAW is not returning the track, the keyboard may suddenly make no sound at all. A better diagnosis is to trace the path first and disable only the connection responsible for the duplicate.
The distinction appears in separate MIDI routing paths where MIDI In, Out, and Thru are treated as distinct connections and incoming commands can continue to another instrument without alteration. Real instruments add software choices on top, which is where simple diagrams stop being enough.
Notes are not the only messages caught in the circle. Controller changes, pitch bend, aftertouch, program changes, clock data, and SysEx can also be retransmitted depending on the equipment. A dense controller stream can make the failure look far stranger than a simple echo.
Start by identifying every route that can retransmit received data. Check the DAW's monitoring or MIDI-thru behavior, the synth's Soft Thru or merge setting, Local Control, and any hardware router between them. Disconnecting one cable temporarily is often revealing because the loop disappears the moment its return path is broken.
A clean bidirectional setup needs deliberate ownership of each return route. The synth can send performance data to the DAW while the DAW sends sequenced data back, but received data should not keep circling through another enabled Thru path. Once the return path is singular, doubled attacks, runaway controllers, and mysterious stuck notes usually stop at their actual source.
You can build a working rig without caring about the distinction until the signal comes back to its source. Then one note becomes two, a knob starts fighting itself, or a burst of repeated messages makes the setup behave as if something has failed. The cables may be fine.
Most trouble comes from assuming every MIDI Out behaves the same way. It does not. Knowing how MIDI data travels between devices gives you the foundation, but routing depends on what each device chooses to generate, copy, merge, filter, or return.
MIDI Out and MIDI Thru have different sources
A conventional MIDI Out sends data produced inside the device. Play a keyboard, move a modulation wheel, turn a mapped knob, or run the internal sequencer, and those messages can leave through Out. Incoming data does not automatically belong there.MIDI Thru starts somewhere else. It takes data received at MIDI In and passes that stream onward, traditionally without changing it. A controller can therefore feed one synth, whose Thru port feeds a second synth, while both instruments receive the controller's performance.
The second synth does not need the first synth to regenerate every note. Thru exists specifically to continue the incoming path. Hardware designs and MIDI documentation have long treated In, Out, and Thru as separate jobs rather than three interchangeable sockets.
Modern gear often combines Out and Thru on one physical connector. A menu setting may switch the jack between the two functions, while another device may offer a software option that merges received data with locally generated data before sending everything from Out. Reading the exact manual matters because the word Out no longer guarantees one fixed routing behavior.
A combined design can be useful in a compact rig. Your keyboard can receive notes from a sequencer, generate controller moves of its own, and send a merged stream toward another device. The same convenience also creates a path back toward the sequencer if both directions are connected.
Soft Thru can quietly create a second note path
Soft Thru is routing performed by software or device firmware rather than a dedicated hardware Thru connection. A DAW can receive a note from your synthesizer and immediately send it back to the synthesizer so you can monitor the instrument through the current track routing. Many sequencer workflows depend on exactly this behavior.Problems appear when the synthesizer is also playing its own keyboard locally. One key press reaches the internal sound engine directly, then travels into the DAW, through Soft Thru, and back into the synth as another Note On. You hear two attacks even though you pressed one key.
Local Control is designed to break one of those paths. With Local Control off, the keyboard can still transmit MIDI outward while its internal sound engine waits for returned MIDI from the sequencer. The DAW becomes the routing authority instead of the instrument sounding both the local and returned copies.
Turning Local Control off is not a universal cure. If the DAW is not returning the track, the keyboard may suddenly make no sound at all. A better diagnosis is to trace the path first and disable only the connection responsible for the duplicate.
The distinction appears in separate MIDI routing paths where MIDI In, Out, and Thru are treated as distinct connections and incoming commands can continue to another instrument without alteration. Real instruments add software choices on top, which is where simple diagrams stop being enough.
Feedback loops are worse than ordinary doubling
A doubled note has two finite paths. A feedback loop has a circular one. The synth sends MIDI to the DAW, the DAW returns it to the synth, Soft Thru sends the returned message outward again, and the DAW receives another copy to repeat the trip.Notes are not the only messages caught in the circle. Controller changes, pitch bend, aftertouch, program changes, clock data, and SysEx can also be retransmitted depending on the equipment. A dense controller stream can make the failure look far stranger than a simple echo.
Start by identifying every route that can retransmit received data. Check the DAW's monitoring or MIDI-thru behavior, the synth's Soft Thru or merge setting, Local Control, and any hardware router between them. Disconnecting one cable temporarily is often revealing because the loop disappears the moment its return path is broken.
A clean bidirectional setup needs deliberate ownership of each return route. The synth can send performance data to the DAW while the DAW sends sequenced data back, but received data should not keep circling through another enabled Thru path. Once the return path is singular, doubled attacks, runaway controllers, and mysterious stuck notes usually stop at their actual source.
