Linux SDR – Configuring the Sound

At their core, software defined radios like the Elektor SDR and SoftRock are sampled detectors operating at 4x the frequency of interest.  The output of the detector consists of two audio streams containing in-phase and quadrature (I-Q) components of the RF signal which are fed to the PC sound card input through the left and right channels.

This places at least two requirements on the sound card.  First it must have a stereo input to handle the I-Q samples.  Second, line inputs rather microphone inputs are typically used since the microphone input can be easily overloaded and the amplifier can add noise.  Unfortunately, the sound card on the EEE PC 701 satisfies neither requirement.  Therefore, it was decided to use an external USB sound card.

Based upon previous experience, the M-Audio Transit USB sound card was chosen.  This is a line-powered 24 bit card capable of sampling at 96 kHz simplex (48 kHz duplex) with a line input.  Just as important, it works under Linux.

Before the Transit is configured for for Linux operation on the EEE PC 701 the internal sound card must be disabled.  To do so, reboot the EEE PC and press F2 at the startup screen.  This will bring up the BIOS Setup Utility.

Scroll over to the Advanced menu and down to the Onboard Devices Configuration, then press Enter.

Scroll down to the Onboard Audio entry.  Press ‘+’ to disable the Onboard Audio if it is not already disabled, then F10 and OK to save the changes and restart the boot process.

Once the EEE PC finishes booting, open a terminal (Applications : Accessories : Terminal) to make the necessary changes.

First, install the m-audio transit firmware loader

sudo apt-get install madfuload

Next it’s necessary to fix the device so it doesn’t steal index 0.  To do so, edit the file ‘/etc/modprobe.d/alsa-base.conf’ and add the following line if it’s not already there

options snd-usb-audio index=-2

Finally, it’s necessary to create a new madfuload rules file for madfuload.  Open the file using the following command:

sudo vi /etc/udev/rules.d/42-madfuload.rules

and enter the following lines

# madfuload.rules - udev rules for loading firmware into
# M-Audio DFU devices

# Transit
ACTION=="add", SUBSYSTEM=="usb", SYSFS{idVendor}=="0763",
SYSFS{idProduct}=="2806", RUN+="/usr/sbin/madfuload
-l -3 -f /usr/share/usb/maudio/ma006100.bin -D $env{DEVNAME}"

ACTION=="add", SUBSYSTEM=="usb", SYSFS{idVendor}=="0763",
SYSFS{idProduct}=="2006", RUN+="/sbin/modprobe snd_usb_audio"

Save the file, plug in the Transit and reboot the EEE PC.  With a set of headphones you should hear the boot-time sounds.  Once boot is complete, open a terminal and enter the following command:

aplay -L

You should see the Transit USB card listed.

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