AES67 USB Receiver and Transmitter

This document describes how to use the AES67 Daemon on a board to operate as AES67 USB Receiver and Transmitter.

The board doesn't need to have an audio codec but a USB client or OTG connector that is able to operate in device mode.

Eventually you will connect your card to your PC ( Windows / Linux / MAC ) via USB and use the PC as a player or recorder and the card to relay audio from USB to Ethernet and vice versa.

Kernel configuration

You need to enable support for USB gadget and compile and enable the USB Audio Gadget 1.0 module of the Linux Kernel. For this make sure you have the following kernel config options enabled:

  CONFIG_USB_F_UAC1=m
  CONFIG_USB_CONFIGFS_F_UAC1=y
  CONFIG_GADGET_UAC1=y

In the Kernel menu config the location of these option is:

  -> Device Drivers 
    -> USB support (USB_SUPPORT [=y])
      -> USB Gadget Support (USB_GADGET [=y])

Transfer the new kernel and modules to the board. Also make sure your board is properly configured to use the USB connector in device mode.

NOTE: The reason for configuring USB Audio Gadget 1.0 instead of 2.0 is that 2.0 does not currently work properly with Windows.

Install the USB Audio Gadget 1.0

Install the Audio Gadget module with:

  sudo modprobe g_audio

If the negotiation with the connected PC is succefull on the board a new virtual audio card shows up. To verify run:

  cat /proc/asound/card

The expected output is:

   0 [UAC1Gadget     ]: UAC1_Gadget - UAC1_Gadget
                        UAC1_Gadget 0

By default this new audio codec works with a sample rate of 48kHz, 2 channels and sample format S16_LE.

On the PC new audio input and output devices show up. The name assigned to these depend on the OS. On Windows a new speaker and microphone named AC interface get configured.

NOTE: If the USB gadget module loads but the USB audio device doesn't show up check in the kernel ring buffer for the following message:

udc-core: couldn't find an available UDC - added [g_audio] to list of pending drivers

This means you have other USB gadgets currently active (for example g_serial). To proceed disable the automatic load of these modules and configure g_audio instead.

AES67 USB Transmitter

Before starting make sure that the AES67 daemon basic setup is done.

On the board run the following command to relay the audio from the USB to the AES67 Source:

  sudo nice -n -10 alsaloop -c 2 -r 48000 -f S16_LE -C plughw:UAC1Gadget -P plughw:RAVENNA

On the PC start the audio playback on the new device.

NOTE: The ALSA RAVENNA driver will perform the format conversion from S16_LE to the Source format.