Akai APCmini Launchpad

Headsup: Some of this stuff I’ve implemented in https://github.com/TomasHubelbauer/web-midi

The launchpad sends a 40 byte payload over USB on each event.
These payloads are MIDI messages.

The first two bytes determine the event type:

• 09 90 button press
• 08 80 button release
• 0b b0 slider move

The third byte identifies the button or the slider causing the event.

The fourth byte represents the value of the button or slider:

• Button press or release: always 7f
• Slider move: 00 through 7f with an increment of 1

The remaining 36 bytes is always all zeroes.
When received as MIDI messages (e.g. WebMIDI), the zeroes are trimmed.

Prior Art

Someone already discovered what I document in the rest of this document.
Credit goes to David Morrill
over at getsatisfaction.com.

Grid Buttons

Vertical Buttons

Horizontal Buttons & Sliders

With Ableton

Ableton seems to be able to instrument the launchpad to give the button a wider range of values.

When it starts, it sends the following to the launchpad:

• 04 F0 7E 7F 07 06 01 F7
• 0b b0 xx 6c for some of the sliders
  • Is this perhaps a new range/increment for the slider?
• 09 90 xx 00 to reset all of the launchpad buttons
• I open a set in Ableton Live
• 04 F0 7E 7F 07 06 01 F7 again
• The launchpad responds with:

04 F0 7E 7F 04 06 02 47 04 28 00 19 04 01 00 00
04 00 7F 00 04 00 00 00 04 00 00 00 04 00 00 00
04 00 00 00 04 00 00 00 04 00 00 00 06 00 F7 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

• 0b b0 of random values to some/all? of the sliders
• Resets slider buttons again
• Sends 09 90 xx 05 to light buttons yellow or 00 for black
• 04 F0 7E 7F 07 06 01 F7 again
• The response above again
• Some more lighting buttons

0B B0 35 59 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

• I press a yellow lit launchpad button to play the set
• 02 might be blinky green and 01 might be still green
• For horizontal buttons 01 would be still red

So for each unlit grid button press:

• Launchpad sends 09 90 xx 7f to indicate button press
• Ableton sends 09 90 xx 02 to blink until playing
• Launchpad sends 08 80 xx 7f to indicate button release
• Ableton sends 09 90 yy 01 to light the corresponding column button
• Ableton sends 09 90 xx 01 to light the button when it is playing

When the button in the grid column is already on when pressed:

• Launchpad sends “pressed”
• Ableton sends “be blinky”
• Launchpad sends “released”
• Ableton sends “be lit”

When a button is already on in the grid column and another is pressed:

• Launchpad: X is pressed
• Ableton: Blink X
• Launchpad: X is released
• Ableton: Light X
• Ableton: Dim Y (to 05 - yellow not black because if black would not be pressable)

When a button is pressed which is not part of the set:

• Launchpad: pressed
• Launchpad: released
• Ableton doesn’t care or respond

When column button pressed with lit button:

• Akai: column button pressed
• Ableton: blink it
• Akai: it was released
• Ableton: dim it
• Ableton: dim (to yellow) the button that was on

When column button has no lit buttons and gets pressed:

• Akai: column button pressed
• Ableton: keep it off
• Akai: it was released

When a slider is moved:

• Akai: spams move messages
• Ableton: “confirms” last value

When a grid row button is pressed:

• Keeps existing row buttons lit green where they were
• Blinks new row buttons green
• Dims existing row buttons to yellow when switched
• Handles changes to the column buttons depending on if they have buttons

Row buttons can able be lit but they are only ever blunk, never lit continuously.

The shift button is never blunk by Ableton but might be blinkable/litable using libusb.

To-Do

Build a driver to make the launchpad show graphics on the buttons

The marketing photos show even grid buttons glowing red:

https://www.akaipro.com/apc-mini

Perhaps all buttons have multiple and all the same color states.

Play around with this to see what all colors are possible and animate some graphics.

https://stackoverflow.com/q/17239565

https://github.com/dcuddeback/libusb-rs/issues/20

libusb-rs seems abandoned, but has a maintained fork: https://github.com/a1ien/rusb

Maybe also use a C# wrapper.

It seems as though there are two options: first make the device recognizable to the OS
so that it can select a generic driver for it. This is done using “descriptors” and there
is a general HID framework which is cross-OS and then there is WinUSB which is Windows
specific, but more capable. Perhaps the Akai launchpad’s string descriptions already make
it qualify for one such generic driver. Ableton doesn’t make me install a driver for it and
can use it from the get-go and I have not had to install a driver for it manually either,
so this might be the case. The other option is to develop a kernel USB driver, which is
much more involved but might not be needed since Ableton can use the launchpad without it,
so it must be possible. Or, and this is also something to check, perhaps the launchpad,
despite using USB, is actually a serial over USB kind of a device like many other are. I
don’t know how to validate if this is the case or not, but if it is serial over USB, I might
want to look in ways of determining the serial port name and other serial parameters
(mainly the baud rate) and then use C# APIs for serial communication which can work straight
from user space.

When connected to a Windows machine, the Akai launchpad has its driver installed automatically
by the OS. This tells me that the device is HID/WinUSB compatible and thus does not need its
own driver.

When inspected in Device Manager, the device appears in a group called Software Devices.
It exposes two entries there: APC MINI [0] and APC MINI [1].

It also appears in a group called Sound, video and game controllers. There, it has one
entry named APC MINI.

I have scraped the USB device properties data in akai.csv.

The next steps are:

• Trace OP-1 USB calls to see if they rememble these ones, if so, it is generic MIDI comms
• Compae a mobile USB properties to see if they resemble these ones, if so, RS-232 maybe
• Check BusDog again to see what device ID it was that I was connected to anyway