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As a part of the ongoing Arduino Cult Induction workshop series, this month we will be focusing on sound.

In particular I will be going over creating sound using the Arduino’s built in Pulse Width Modulation (PWM) and Direct Digital Synthesis using resistive ladders. We will investigate using the Piezo element
as both a simple speaker and an input trigger. We will review the Musical Instrument Digital Interface (MIDI) from a hardware and software perspective, and look at a couple of useful integrated

I am trying to digest this document.

http://www.usb.org/developers/devclass_docs/midi10.pdf

USB encapsulates midi data into 4 byte chunks and adds the notion of virtual cables.
The first byte in the packet is the cable number and something called the Code Index Number. The remaining 3 bytes are either midi or padding depending on the packet.

Most of the relevant midi status commands have a corresponding Code Index Number.