San Diego Rust April 2018 Challenge

Write a Rust app to control the Behringer XR-12 audio mixer! The XR-12 is a WiFi-enabled audio mixer that can be controlled using Open Sound Control (OSC) over UDP.

OSC quick description

OSC is a simple binary messaging protocol used by many audio devices. Here is the hex dump of an example OSC message to call a method "/play/note" with the arguments "Guitar", 12, and 1.0:

2f706c61792f6e6f74650000  2c73696600000000  4775697461720000  12000000  3f800000
|-> /play/note            |-> ,sif          |-> Guitar        |-> 12    |-> 1.0

• /play/note is the address pattern, a URI-like string that identifies the method to execute.
• ,sif is the type tag, a string describing the types of the method's parameters. The type tag starts with a , and each following letter indicates the type of one parameter. ,sif describes 3 parameters:
  • s: a null-terminated string
  • i: a 32-bit signed integer
  • f: a 32-bit float
  • An OSC message with zero parameters should still have a type tag (a comma followed by three zero bytes).
• The parameters follow the type tag. Note that all strings must be null-terminated, and all numbers are stored in binary big-endian format.
• All OSC fields must be aligned to 4 bytes. Extra zero bytes must be inserted to pad each field to a multiple of 4 bytes. For example, the command string /play/note has two zero bytes on the end to pad it to a length of 12 bytes.

Consult the OSC specifications for more detailed information.

The challenges!

1. Create a UdpSocket and connect to the mixer on UDP port 10024. Send the /info command to the mixer, and inspect the response. The mixer will return an OSC response with version info about the mixer. Remember that all fields in an OSC command must be aligned to 4 bytes. /info is 6 bytes (including the trailing null) and will require an extra 2 null bytes to pad it to 8 bytes.

2. An audio input is connected to channel 1 of the mixer, but channel 1's volume is turned all the way down. The OSC command /ch/01/mix/fader can be used to control the channel's fader (volume knob). It takes a single float parameter between 0.0 to 1.0. You will hear some audio if you are succesful.

3. Bonus Hard Challenge: Display an audio level meter for Channel 1. The meter represents the loudness of the music as it's playing.

   • Use the /meters.,s../meters/1 command to request periodic updates of the audio levels for the next 10 seconds (. represents a zero byte). You will receive OSC commands from the mixer containing updates in this format:
   • /meters/1...,b..<binary blob>
   • The binary blob is in this format:
     • Length of blob (32-bit big endian)
     • The number of volume values in the blob (32-bit little endian)
     • An array of 16-bit values (16-bit little endian)
     • Read the first 16-bit value because it represents Channel 1; you can ignore the rest.
     • The value is an integer, usually in the range of -32768 to 0. 0 reprsents max volume (clipping).
   • Each time you receive an meters message, output a row of asterisks based on the loudness of the audio.

Helpful docs + links

• Rust UdpSocket docs
• Rust byteorder crate, useful for writing binary data
• Open Sound Control 1.0 specification
• Behringer X-series OSC protocol