Digital audio: USB, SPDIF, AES

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The information on this page supplements the official manuals.

Contents

Digital audio: supported by which Fractal Audio products?

Why use digital audio?

A digital connection skips the analog/digital conversion. The high-quality A/D converters in the Axe-Fx II and AX8 for the analog outputs also deliver high-quality results.

Sample rate: fixed at 48kHz

The sampling rate of the Axe-Fx II, FX8 and AX8 is fixed at 48kHz (24-bit). You need to set digitally connected devices to the same sampling rate. Resampling is better handled by software.

Make sure to set the sampling rate in your DAW software and system settings to 48kHz. Example

Fractal Audio:

Digital audio and latency

When monitoring the audio signal through the computer's output, the latency depends on the computer. When monitoring audio via the hardware, latency is none.

Axe-Fx II and USB

Read this: USB.

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. Microsoft does not support Audio Class 2.0. Therefore FAS provides a driver for Windows systems. The driver for Windows contains both the firmware installer and the audio drivers.

Apple does support Audio Class 2.0, but poorly. To overcome this you can increase the buffer size in the Axe-Fx II's I/O > Audio menu. The driver for Macs is NOT an audio driver. It is a firmware installer. The Axe-Fx II uses a "soft" USB controller. It gets its 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.

Use the USB buffer parameter 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 Level in I/O sets the level of the USB input signal sent to the main outputs. If you don't hear anything when monitoring the Axe-Fx II through a computer, check this parameter. Also verify the USB/DIGI OUT setting.

Linux: read this.

Axe-Fx II and USB Audio

USB Audio rate is fixed at 48 kHz.

Fractal Audio:

USB features.png

Simultaneous use of analog and digital audio connections

You can use digital input together with analog Output 1 (and simultaneous digital out). And you can use Input/Output 2 while using the digital input. source

Disable the analog output signal when using digital audio

When the Axe-Fx II 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. Or set USB/Digi Out Source to Out 2, then route the guitar to FXL only. source

Axe-Fx II as master or slave

Fractal Audio:

Word Clock

Word Clock (Axe-Fx II only) selects the clock source for the A/D and D/A converters as follows:

SPDIF

Axe-Fx II: configure your Axe-Fx II for S/PDIF.

AX8: provides SPDIF output only. The strength of the SPDIF signal level depends on the position of the front panel output knob (unlike the Axe-Fx II). Fractal Audio: "The SPDIF is a digital representation of OUTPUT 1." source

Use the Axe-Fx II or AX8 as an analog-to-digital converter

To use the device as an analog-to-digital converter (source), do this:

Connecting the Axe-Fx II to a DAW drops the volume or puts the Axe-Fx II into Bypass mode

Some DAWs send MIDI commands (intended for other devices) which lower the volume of the Axe-Fx II or put it into Bypass mode. To solve this, adjust the DAW settings. If not possible, force the Axe-Fx II to ignore those commands by setting MIDI CCs 10 and 13 in I/O > Control to "none".

Source: Bakerman

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