Mixxx PFL Switcher

This project contains a pair of simple scripts designed to trigger relays or other audio switch attached to a Raspberry Pi Pico from Mixxx when a PFL control is activated, so that headphone feed can be switched from the output of an external audio mixer, to the Mixxx headphone output. This allows a low-delay headphone feed from an external mixer for live radio operation, with PFL switched to Mixxx when previewing audio from there.

Currently this can control up to 3 switches (only 2 are used for stereo audio switching)

Installation

1. Plug the Device into your Mixxx PC by USB.
2. If your Pi Pico is brand new, install CircuitPython, by copying the latest release from https://circuitpython.org/board/raspberry_pi_pico/ (this was tested with v8.1.0), onto the USB Mass Storage device the Pi Pico presents. The Pi Pico will reboot as soon as you've finished uploading the file. If you want to rename your MIDI device, see these suggestions on how to hack this firmware image with a hex editor before uploading it.
3. Once this has been done (or if you already had CircuitPython), copy the contents of this folder onto the new "CIRCUITPY" USB Mass Storage device that appears (and rename the filesystem if you want).
4. The Pi Pico should flash its LED for 1 second, and then load the code, which will cause all switches to go on for 0.15 seconds, then just left and right for 0.15 seconds, then just the left relay for 0.15 seconds, then off. This is a quick indication that it's booted sucesfully. A red LED lights up next to each relay as it is triggered.
5. Locate the 2 files in the mixxx_mappings folder, and copy them to your Mixxx Controller Mapping Folder
6. Fire up Mixxx, and go to Preferences, Controllers, and select the device (either called "CircuitPython Audio" or whatever you renamed the device to in Step 2), Enable it, and Load Preset of "Mixxx PFL Switcher" from the mapping files you just copied over.
7. Immediately that this preset loads, you should see/hear another quick initialisation sequence, with 0.2 seconds for each of:
   1. Left Switched
   2. Right Switched
   3. Both Switched
8. This will repeat every time you start up Mixxx with this controller attached, showing you quickly that it has been detected and the relevant mapping script has been loaded.
9. Now the switches should be driven by the PFL status on Mixxx:
   1. Try turning PFL on a Deck or Sampler with the Headphone icon, and you should see both switch.
   2. If you hit play on an item in your library to play it onto the preview deck, this should also trigger the switch
   3. If you instead select the "SPLIT" control for the headphone feeds, only the left should switch when PFL/Preview Play is active. This should allow you to listen to Mono PFL in your left ear and mixer out (right leg only!) in your right ear.
10. Now you've tested all this works as expected, it's time to plug some audio up, suggested connections are as follows:
    1. Mixer Headphone Out -> PFL Switcher Normal Input
    2. Mixxx Headphone Out (from a good soundcard) -> PFL Switcher Alternate Input
    3. PFL Switcher Out -> Headphones (it probably makes most sense to do this via a small headphone amp, to give you volume control).

How it works

In Mixx

• Scripted MIDI mapping in Mixxx monitors a set of Channels/Decks, Samplers, and Preview Decks.
• On most groups it monitors the status of the PFL button, but Preview Decks seem to always have PFL active, so on these it activates when they're in play.
• If any of these are active the script sends MIDI commands to the Pi Pico to cause it to switch the relays
• Script also monitors the status of the headSplit parameter to know which relays to activate
• Set of monitored groups is listed in arrays at the top of mixxx-pfl-switcher.js and can be adjusted to add more or exclude some channels

MIDI Comms

The device accepts 3 possible simple 3-byte MIDI command formats:

1. A simple command setting the state of the first 2 switches (ie stereo audio switch):

   • 0x90 - MIDI Note On, Channel 1
   • 0x01 - Note 1
   • 0x0x - Bitmask indicating which relays should be on 1 for left, 2 for right, ie:
     • 0x00 - Both Relays Off
     • 0x01 - Left Relay only On
     • 0x02 - Right Relay only On
     • 0x03 - Both Relays On

2. A more complex command allowing any set of relays to be changed:

   • 0x91 - MIDI Note On, Channel 2
   • 0x0x - Note - Bitmask indicating which relays this command will affect
   • 0x0x - Velocity - Bitmask indicating the state of the affected relays

   So, for example, 0x91, 0x06, 0x02 would affect relays 2 & 3, turning relay 2 on, and relay 3 off.

3. A simple command to change the state of a single relay:

   • 0x92 - MIDI Note On, Channel 3
   • 0x0x - Note - The index of the relay you want to change (ie 1,2 or 3)
   • 0x0x - Velocity - 0 indicates turn the relay off, any other value indicates turn it on

   So, for example, 0x92, 0x03, 0x01 would turn on relay 3

Pi Pico

• CircuitPython code which loops around polling for MIDI input, and enables GPOs 20 (left), 21 (right) and 19 (spare) to cause the relays to close/audio switches to activate, as instructed by the host.

Changing the code

You can tweak the code that runs on the Pi (in code.py), but more likely to be insteresting is tweaking the Mixxx-end script in mixxx-pfl-switcher.js. Changes people may want to look at:

• Only switching headphones when PFL is selected on a channel and it's playing.
• Adding a small (100ms?) delay before switching the relays off, so that you don't get a brief off/on again as you play different items on the Preview list.

The Pi Pico will perform a soft reboot every time you save a file to it, reloading its code.

Notes on very cheap onboard headphone outputs

When experimenting with a very cheap onboard headphone out, I found odd noise introduced onto the other outputs, presumably via weird crosstalk through the ground, so these are probably best avoided. These problems could probably be mitigated by adding a 3rd relay to also switch grounds instead of commoning them between all inputs/outputs and not allowing split headphones.

Building your own (relay switches)

Parts

The basic parts are just:

1. A Raspberry Pi Pico (or compatible)
2. A cheap 2-channel relay board, 5V powered, able to trigger off 3.3V logic (eg the 2-channel version of https://www.aliexpress.com/item/33038634587.html which turns relays on with a signal between 0 and 1.2V, and off at higher voltages)
3. 4 lengths of jumper wire
4. If you want to be posh:
   1. A project box to put it in
   2. Jack sockets or RCA sockets to attach to the outside of the box
   3. Some wire to connect the sockets to the relays
   4. Some M2 screws/nuts to secure the boards down.

Wiring

1. Connect 4 jumper wires from your Pi Pic to your Relay board:
   • VBUS (Pi) - VCC (Relay Board)
   • GND (Pi) - GND (Relay Board)
   • GP20 (Pi) - In1 (Relay Board)
   • GP21 (Pi) - In2 (Relay Board)
2. Connect together the grounds of all audio inputs/outputs
3. Connect all left channel inputs to Relay 1, and right channel to Relay 2:
   • NC (Normally Closed) (Relay) - Normal (Audio In)
   • COM (Common) (Relay) - Out (Audio Connector)
   • NO (Normally Open) (Relay) - Alternate (Audio In)

Building your own (electronic switch)

Parts

The basic parts are just:

1. A Raspberry Pi Pico (or compatible)
2. A +/-10V DC-DC Power Supply board, such as https://www.aliexpress.com/item/1005003197384707.html
3. A CD4053BE IC
4. A 4 channel logic level shifter, such as https://www.sparkfun.com/products/12009 or https://www.aliexpress.com/item/1005004225321778.html
5. 0.1 uF ceramic capacitors
6. Plenty of lengths of jumper wire
7. Veroboard or Breadboard to mount everything on
8. If you want to be posh:
   1. A project box to put it in
   2. Jack sockets or RCA sockets to attach to the outside of the box
   3. Some wire to connect the sockets to the IC
   4. Some M2 screws/nuts to secure the boards down.
   5. Some LEDs to indicate the status of the switches
   6. A 1kOhm resistor for each LED

Wiring

Connect up as shown in this Circuit Diagram