Sandwich
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This is the first thing I would check. Someone else was having the issue they could .begin(), but couldn't .open() anything. Quote from Paul on this thread:

Good point! It was my first project so the details are a little fuzzy, but thinking about it I see what you mean. Either I wasn't using an opto and had it directly wired reversed, or the wiring issue was on the...

EDIT: Check your wiring, I had some MIDI wired wrong on my first project and was receiving my highs/lows backwards. Which resulting in midi messages, just not the ones I wanted. After typing the stuff below here someone...

I figured out that the USB Host library is working for me as well. Synths with a single USB MIDI connection work completely as expected. But synths that have multiple connections available over USB are where I'm running...

Okay, I've made some progress figuring out the issue! But it leads a little further down the rabbit hole. My MIDI equipment is a Korg NTS-1, a Korg Monologue, and a Korg Minilogue. I have figured out that my example...

Hey there, So the optocoupler is easy to explain, so I'll start with that first. Setups will work without one, however they offer protection and circuit isolation between your devices. When you use one, instead of...

This is the simplest code I've been able to come up with to test sending out MIDI notes over the USB Host. From what I gather both functions should be valid to start and to stop notes: #include <Arduino.h>...

I used to have problems trying to use the MIDIUSB library and ended up not using it anymore. Now all I do is select MIDI as the usb type in the programming options pulldown before flashing the teensy and I use ...

To answer your first question, I'm having really good luck using a teensy 4.0 as a usb host for MIDI. Using it to crosstalk between a bunch of USB, USB host, TRS and DIN ports. Getting down to the last hurdle, as with...