it has come to my attention (with invaluable debugging help from Robert LaPorte) that the default pin definitions for the AltSoftSerial library ( TX = 5, RX=13) used in this project are not the same as the ones that are routed on the hardware ( TX = 9, RX = 4 )
(D4 = 4 & B5 = 9)
this is because the sparkfun pro-micro does not expose pin 13 on the ATmega32U4 ...
however inside the AltSoftSerial/config/AltSoftSerial_Boards.h L95-L105 we can see an alternative setup using timer1 and TX = 9, RX = 4
i can see that my local version of AltSoftSerial has this config switched around so the correct pins are sending serial, however this is in no way reflected in this repo - so anyone attempting to modify code for transcribe will pull the default mappings and their MIDI_DIN5_SERIAL ports will not work as expected (although they will work when flashed from my computer)
the solution to this is to fork AltSoftSerial and make these changes here - cyberboy666/AltSoftSerial
then in the platformio.ini in this project update the dependancies to point to this version of the serial library:
lib_deps =
felis/USB-Host-Shield-20 @ ^1.3.2
blokaslabs/USBMIDI @ ^1.1.5
https://github.com/cyberboy666/AltSoftSerial
mathertel/OneButton@^2.0.2