USB

USB: in the owner's manual

• Axe-Fx II: 3.4 Connection Diagrams
• Axe-Fx II: 3.4.9 Axe-Fx II as a Computer Audio Interface
• Axe-Fx II: 3.4.11 Axe-Fx II- One Possible Big Rig
• Axe-Fx II: 9 Input/Output Parameters
• Axe-Fx II: 10.3 Status Meters
• Axe-Fx II Input/Output Hardware and Parameters Guide

USB: driver for Windows and Mac

• USB drivers for Mac and Windows are required and available for download here. If the USB driver is not installed, utilities such as Axe-Edit, Fractal-Bot and Cab-Lab won't recognize the Axe-Fx.
• When using the USB interface, there's no need for an additional audio interface. An external audio interface is required when connecting the analog Axe-Fx II outputs to a computer (not using USB).
• A MIDI interface is not required to connect the Axe-Fx II to a computer. The Axe-Fx II itself operates as an USB-to-MIDI adapter. MIDI interfaces can still be used, and editing through a MIDI interface also is still possible.

USB: USB audio

• 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 resolves issues with USB audio on Macs.

USB: USB buffer

• Use USB buffer in I/O > Audio 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.
• The “USB” bar graph in Utility > Status 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.

USB: DAW

• See Axe-Fx II: digital audio (USB, S/PDIF, AES)

USB: using the Axe-Fx II as an interface 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 Fractal-Bot or a MIDI librarian. Walkthrough.
• Cliff: "The USB Adapter feature was added for simple USB-to-MIDI functionality, specifically firmware updates for the MFC-101. It is a low priority task since audio processing is the Axe-Fx's primary usage. Latency is not guaranteed. It is not intended as anything more than a no-frills, simple MIDI adapter for non-critical tasks." source

USB: CPU usage

• 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. source and source

USB: audio playback level

• Firmware 18 added the ability to set the level of the USB input signal sent to the main outputs (I/O > USB Level). If you don't hear anything when monitoring the Axe-Fx through a computer, check this parameter. Also verify the USB/DIGI OUT setting.

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

USB: troubleshooting Axe-Fx II and USB

• 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 problems such as dropped connections or an Axe-Fx which freezes after connecting USB, always replace the USB cable first. Example
• 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
• Handy tip for resolving audio dropouts in your Windows DAW

USB: Linux

• Linux and Axe-Fx