USB

Manual

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Axe-Fx II USB features diagram

User contributions

USB driver

• Drivers for Mac and PC which add USB connectivity are available for download here.
• It's not an USB driver in the conventional sense. It's a daemon that waits for the Axe-Fx USB port to appear and then downloads software to the port to make it USB class-compliant. The plist-file is located here: /Library/LaunchDaemons/com.fractalaudio.axefx2load.plist.
• The USB driver control panel should not be used for configuration. Ignore error messages when clicking on the controls. Source1 Source2
• The names of the Axe-Fx channels on the computer are assigned by the computer, not by the Axe-Fx. Source
• If the USB driver is not installed, Axe-Edit won't recognize the Axe-Fx.
• To install the driver in Windows 8, use Compatibilty Mode. 

MIDI interface and audio interface not required

• When using the USB interface, there's no need for an additional audio interface. An audio interface is still required when connecting the analog Axe-Fx II outputs to a computer instead of through USB.
• A MIDI interface is not required to connect the Axe-Fx II to a computer. USB suffices (MIDI-over-USB). The Axe-Fx II itself operates as an USB-to-MIDI adapter. MIDI interfaces can still be used and editing with the help of a MIDI interface is also still possible.

Using the Axe-Fx II to update the MFC-101 firmware

• The Axe-Fx II can be used as an USB-to-MIDI interface to update the firmware on the MFC-101. Here's a walkthrough.

USB connection increases CPU utilization

• Creating an USB connection increases CPU utilization. This is by design. If a preset uses much CPU, you may run into CPU overload problems when connecting the Axe-Fx II to a computer through USB. Source1 Source2

Latency

• When monitoring audio via the Axe-Fx II, latency is none.
• When monitoring the audio signal via the computer's output, the latency depends on the computer.

USB noise

• High-pitched noise may be caused by a ground loop. Cliff: "The USB spec calls for grounding the cable at both ends (mistake). This can create an USB ground loop. Make sure everything is plugged into the same outlet strip." Source

Disabling the Axe-Fx output signal when monitoring through a DAW

• When the Axe-Fx is connected to a DAW through USB, you're using monitors connected to the Axe-Fx and you're recording, you may want to monitor just the DAW signal, not the signal from the Axe-Fx. To accomplish this, turn down the preset level in Axe-Edit. Or set USB/Digi Out Source to Out 2, then route guitar to FXL only. Source

USB storage media

• The Axe-Fx II cannot connect to an USB stick. Source

Screen flicker

• Cliff: "An occasional flicker when hooked up to USB is not unusual. This is due to the screen redraw being interrupted by the USB thread." Source

Linux

• Linux is not officially supported by FAS. Nevertheless all USB features can be obtained under Linux, too.
• The Axe-Fx II is fitted with a Cypress USB chip (FX-LP2) which needs to be loaded with firmware (this is not the Axe-Fx II firmware but a dedicated bootloader) to achieve its functionality.
• The firmware hex file can be extracted from the Mac OSX driver package (Axe-Fx_II_USB_Driver_Setup.dmg from [1]).
• With fxload ([2]) you could upload the USB firmware manually, which would be pretty inconvenient. But with a proper udev rule executing fxload, it is also possible to upload the USB firmware file automatically to the Axe-Fx II.
• You may write your own udev rule or use the already existing bash script ([3]) to setup a Linux system to recognize the Axe-Fx II each time it is connected to a USB port and then upload the USB firmware.
• After executing the script or applying your own udev rule and then switching on or rebooting the connected Axe-Fx II lsusb will report a new USB device with VID 2466 for Fractal Audio Systems and PID 8003 for the Axe-Fx II. If you like to have a named output you can add adequate entries to the file called usb.ids which is usually located under /usr/share.
• Linux recognizes the Axe-Fx II as a generic USB audio and midi device. As such it is handled by the snd-usb-audio respectively the snd-usbmidi-lib module. Unfortunately only MIDI over USB works out of the box but not the USB audio interface. The latter needs some handwork:
• Since the Axe-Fx II (at the moment) neither responds to set sample rate requests nor get sample rate requests, it is necessary to patch the files clock.c and format.c (located under /usr/src/linux-kernelversion/sound/usb/) as described here [4]. After applying the patch the snd-usb-audio module has to be recompiled. Once the previous version of this module has been removed from the kernel (rmmod) and the newly built module has been added to the kernel (modprobe), playback and capturing devices of the Axe-Fx II appear and are functional.

USB details

• Cliff: "The Axe-Fx II USB is 24 bits. This is 144.7 dB of dynamic range. Full-scale is about +20 dBu. So even if your guitar is -20 dBu (-40 dB re. FS) you still have over 100 dB of dynamic range. A typical single coil pickup can easily exceed -20 dBu. A humbucker can easily exceed 0 dBu. Full-scale of 20 dBu gives you a few bits of headroom in case of very hot pickups. The self noise of a guitar pickup and associated electronics limits its dynamic range to less than 100 dB typically." And: "The digital bit depth on the USB and Digital I/O exceeds both the dynamic range of the Axe-Fx itself and certainly that of any guitar. Furthermore the bit depth is sufficient to fully capture the dynamic range of a guitar while still maintaining +20 dBu as full-scale." Source