USB

Manual

• Axe-Fx II USB features diagram

User contributions

About USB, Drivers, OS X, Windows etc.

• Message from Fractal Audio, June 25 2013: (Source)
  1. The Axe-Fx II is an Audio Class 2.0 compliant device. A class-compliant device requires no drivers. The drivers are provided by the OS manufacturer. Audio Class 2.0 also encompasses MIDI-over-USB.
  2. HOWEVER... Microsoft does not support Audio Class 2.0. Therefore we provide a driver for Windows systems. It works great (flawlessly in my experience).
  3. Apple DOES support Audio Class 2.0, but poorly at this time. Their driver is prone to clock drift. In an effort to mitigate this we now offer the user the ability to increase the buffer size ON THE AXE-FX II END OF THE CONNECTION. This is NOT the same as the buffer size you set in your computer. All peripheral devices also contain buffers to smooth the bursty nature of data transfers from the host computer. Normally this buffer size is fixed but we didn't want to make it unnecessarily large just to satisfy the needs of a poorly designed host driver as we hope the host driver will eventually be fixed.
  4. The driver that we supply for OS-X computers is NOT an audio driver. It is a firmware installer. The Axe-Fx II uses a "soft" USB controller. It gets it's code from the host computer. When you turn the Axe-Fx II on it requests firmware from the host. This is superior to a hard-coded controller in that updates merely require a new host image rather than reflashing the controller.
  5. The driver that we supply for Windows contains both the firmware installer and the audio drivers (as MS does not support AC 2.0).
  6. In beta testing ALL beta testers were able to get artifact-free performance under OS-X by increasing the buffer size. The size of the buffer varied. All beta testers were able to get artifact-free performance under Windows (all versions) using the minimum buffer size.
  7. We have had the OS-X clock problem independently verified by a third party. These results have been forwarded to Apple. At this time all we have heard from Apple is "the issue has been escalated".
• OS X Mavericks was released on October 22 2013. This OS upgrade supposedly resolves issues with USB audio on Macs.

USB driver

• Drivers for Mac and Windows which add USB connectivity are required and available for download here. If the USB driver is not installed, utilities such as Fractal-Bot won't recognize the Axe-Fx.
• Firmware 10.09: "Added USB buffer size select in I/O->Audio menu. Set this to lower values for less latency, set to higher values if experiencing distorted audio. Low values generally work fine with Windows machines. OS-X computers usually need higher values due to poor clock adaptation. You should stop USB audio streaming when changing this value so as to allow the buffer to reset properly. Streaming can be stopped by closing the application sending data to the Axe-Fx or by disconnecting the USB cable."And: "USB buffer level monitoring added to Utility->Status menu. The “USB” bar graph displays the amount of data in the USB FIFO buffer. Ideally the bar should be at around 50%. If the bar sinks all the way to the bottom or goes all the way to the top then the buffer may under/overflow and the USB buffer size should be increased. The number of buffer errors that have occurred since the last buffer reset is indicated above the bar graph. NOTE: The new OS-X driver must be downloaded from our website and installed to ensure proper USB audio operation on Apple computers."

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, using (for example) Fractal-Bot. 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 issues

• 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
• When experiencing USB problems such as dropped connections or an Axe-Fx which freezes after connecting USB, always replace the USB cable first. Example

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

Screen flicker with USB connection

• 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 support

• 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 technical 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
• Cliff: "The hardware is incapable of doing 4x4. The only choices are 3x3 or 4x2 and Logic doesn't work with 3x3. We also had some issues with 3x3 in Windows 7 IIRC." Source