Using the Griffin Knob as a Musical Controller
I thought I'd document my investigations with the Griffin NA10629 PowerMate USB MultiMedia Controller.
A good high-resolution knob is the sort of thing I've been looking for for a while. In the MIDI world, they simply don't exist; 7-bits of resolution ain't enough. Two weeks ago, I googled "USB Knob" in the hopes that a generic peripheral would pop up. And this thing did. So I bought it.
In past, I've parsed data from the Logitech F310 game controller and the Huion 680s tablet. I feel hopeful in being able to read data from the Griffin.
Decoding the byte stream
The first step in hacking any peripheral device is to figure out how it speaks to the computer, which is some sort of binary message.
When I plug it in, the Griffin seems to light up. Off to good start.
Running "dmesg | tail" shows that the Linux kernel is able to know it exists:
[ 2426.585121] usb 1-3: New USB device strings: Mfr=1, Product=2, SerialNumber=0 [ 2426.585128] usb 1-3: Product: Griffin PowerMate [ 2426.585130] usb 1-3: Manufacturer: Griffin Technology, Inc. [ 2426.585325] usb 1-3: ep 0x81 - rounding interval to 64 microframes, ep desc says 80 microframes [ 2426.585339] usb 1-3: ep 0x2 - rounding interval to 64 microframes, ep desc says 80 microframes [ 2426.595289] input: Griffin PowerMate as /devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3:1.0/input/input9 [ 2426.595389] usbcore: registered new interface driver powermate [ 2426.597317] hidraw: raw HID events driver (C) Jiri Kosina [ 2426.598455] usbcore: registered new interface driver usbhid
Linux can treat every device as a filehandle, and this device resides in /dev/input/by-id/usb-GriffinTechnologyInc.Griffin_PowerMate-event-if00. The raw binary data can be sent into a hex reader in the following way:
cat /dev/input/by-id/usb-Griffin_Technology__Inc._Griffin_PowerMate-event-if00 | xxd
With a little trial and error and looking the hex output, I am able to figure out that the message size is 24 bytes long. The hex can be grouped in the following way:
cat /dev/input/by-id/usb-Griffin_Technology__Inc._Griffin_PowerMate-event-if00 | xxd -c 24
This produces some hex that looks like this:
0000300: 051d bf57 0000 0000 88f6 0b00 0000 0000 0200 0700 0100 0000 ...W.................... 0000318: 051d bf57 0000 0000 88f6 0b00 0000 0000 0000 0000 0000 0000 ...W.................... 0000330: 051d bf57 0000 0000 c809 0e00 0000 0000 0200 0700 ffff ffff ...W.................... 0000348: 051d bf57 0000 0000 c809 0e00 0000 0000 0000 0000 0000 0000 ...W.................... 0000360: 051d bf57 0000 0000 0629 0e00 0000 0000 0200 0700 ffff ffff ...W.....).............. 0000378: 051d bf57 0000 0000 0629 0e00 0000 0000 0000 0000 0000 0000 ...W.....).............. 0000390: 051d bf57 0000 0000 4648 0e00 0000 0000 0200 0700 ffff ffff ...W....FH.............. 00003a8: 051d bf57 0000 0000 4648 0e00 0000 0000 0000 0000 0000 0000 ...W....FH.............. 00003c0: 051d bf57 0000 0000 8667 0e00 0000 0000 0200 0700 ffff ffff ...W.....g.............. 00003d8: 051d bf57 0000 0000 8667 0e00 0000 0000 0000 0000 0000 0000 ...W.....g.............. 00003f0: 051d bf57 0000 0000 c986 0e00 0000 0000 0200 0700 feff ffff ...W....................
At this point, I look for patterns in the data. From experience, I can infer that most of the chunks refer to timestamp data, and that the rightmost bytes contain the data I need.
From the data, I am able to figure out how button toggle message worked. I am also able to conclude that the Griffin does not store any type of internal value like a typical potentiometer would. Instead, it sends messages about when it is turned, and the direction it is going in.
The message format of the Griffin adds an extra complication. The end goal is to get the Griffin data inside of Sporth, but there is no built in way to actually handle this step-based approach to incrementing data. Soundpipe and Sporth code has to be written.
Creating a Soundpipe Module and Sporth UGen
A new Soundpipe module is made specifically for the Griffin called incr. When triggered, the module will increment up or down by a specified step. Other parameters to incr are minimum, maximum, and the initial value. It is a pretty trivial module. The source code can be found here.
As it turns out, incr is the first soundpipe module with a trigger that takes sign into account. When it is negative, it increments downwards. when it is positive, it increments upwards.
Once the incr module is created and documented properly. , it can then be quickly turned into a Sporth ugen thanks to a code generator I wrote in lua.
Now that there is way to handle the Griffin data, the Griffin data needs to be brought into sporth.
Creating a Sporth Plugin
The Sporth plugin to built will allow the Griffin to be parsed inside of Sporth. Luckily, a good chunk of this code can be recycled from my previous work with the huion tablet.
The way it works is pretty straight forward: the Sporth plugin will spawn a thread which will open the Griffin as a file. It will parse it at 24 bytes at a time. Every time a knob message is recieved, the sporth plugin will send out either a positive or negative trigger for incrementing up or down. To prevent the loop from taking up too much CPU, the while loop has a usleep();
Since triggers are exactly one sample long, some careful thought has to given with regards to creating the trigger signal with the asynchronous threads. The thread therefore doesn't set the trigger directly; it sets a switch that tells sporth to trigger. Once triggered, sport shuts that switch off.
The final code can be found here
With the parsing figured out, it's now time for the hard stuff: making musical sense out of the knob. In the Griffin repository, I've taken some stabs at some Sporth patches, but there is still plenty to be done. I am still in the process of incorporating this into my musical workflow, but thats for a future post.