Monitoring radio audio through the computer

Mon, Jan 16, 2017

I typically listen to my FT-991’s audio out using headphones plugged directly into the phones jack on the radio, but it’s occassionally helpful to pipe that output through the computer’s speakers and monitor audio that way. Here’s how to do that using the pulse audio tools on a Linux box.

Credit where it’s due: I found this answer from falconer over on the askUbuntu forums.

First, make sure the FT-991 is connected via USB to the computer, and then turn on the radio so the USB audio is available as an input device. Now use pactl list sources to list the available devices. We’re looking for the source # associated with the devices named ‘alsa_input.usb-Burr-Brown_from_TI_USB_Audio_CODEC-00.analog-stereo’ Here’s a quick one-liner to find it:

~$ pactl list sources | grep -B 3 "alsa_input.usb-Burr-Brown"

Source #8
        State: SUSPENDED
        Name: alsa_input.usb-Burr-Brown_from_TI_USB_Audio_CODEC-00.analog-stereo

So in this case we want source #8. Keep in mind this will need to be done every time we want to setup this configuration because the source number will likely change. By the way “State: SUSPENDED” just means nothing running on the computer is actively listening on the input device.

Now use pactl list sinks to find the Source number of the output you want. For example, on my system I want to output to the PC speakers:

Sink #1 State: SUSPENDED Name: alsa_output.pci-0000_00_14.2.iec958-stereo Description: Built-in Audio Digital Stereo (IEC958)

Other computers are likely to have a different audio device listing, so go through the listing of ‘sinks’ to find the output device you want.

So for my example config, here’s the command I’ll need to establish an audio loopback from the input to output:

~$ pactl load-module module-loopback latency_msec=1 source=8 sink=1

If the source and sink numbers are correct you should now hear the radio’s audio out through the PC’s speakers.

As falcon suggested in his post it’s helpful to open up the PulseAudio Volume Control gui to view the new loopback device. Open pavucontrol and go to the ‘Recording’ tab; then select ‘All Streams’ from the ‘Show:’ drop-down menu at the bottom of the window. This should show you the Loopback device and allow you to view the current audio level passing through the loopback device. If you’re having trouble hearing the audio make sure the audio level of the loopback device is set to 100% then use the ‘Output Device’ and ‘Input Device’ tabs in pavucontrol to adjust the audio levels of the output and input devices associated with the loopback device.

When you want to discontinue this configuration, it’s as simple as unloading the loopback module:

~$ pactl unload-module module-loopback

It’s been my observation that piping the radio’s audio over USB through the computer results in a lower quality signal than plugging directly into the radio’s phones jack or using the radio’s built in speaker. Nonetheless, there may be times when this configuration is handy.

Cheers and 73!

Series: Moving ham ops to Linux