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Directly access hard-of-hearing (Comtek) FM audio using Software Defined Radio

Posted: Tue Dec 14, 2021 11:15 am
by samhbilodeau
As both my WTS and STS I've been running my ward and stake broadcasts for over a year, and have been using the Comtek receivers as my audio receiver for the bulk of that, as noted as an option on this forum. Our buildings don't have audio outputs so I haven't been able to patch in directly, and the receiver's let me be mobile and position the camera equipment at the best place in the room without doing enormous cable runs.

Recently I've been frustrated with radio interference drowning out the Comtek's reception, and so I started looking for a different solution.

What I've switched to is a Software Defined Radio, or SDR. It's a little USB device with an antenna that is a radio receiver that I can use on our ward's laptop to pick up the Comtek's FM radio transmission directly and pass it right into zoom. Since it's made with modern radio equipment and modern digital filters, the sound quality is PHENOMENAL compared to what we had been getting.

I bought a cheap SDR dongle on Amazon, made by a company called Nooelec:
https://www.amazon.com/dp/B009U7WZCA

To make it work on Windows, I needed four software packages.
  1. The open source driver package from Zadig: https://support.nooelec.com/hc/en-us/ar ... tion-Guide
  2. The radio tuning software package SDR# from Airspy: https://airspy.com/download/
  3. A virtual audio cable package from VB-Audio: https://vb-audio.com/Cable/
  4. Process Lasso https://bitsum.com/ which fixes a choppy/stuttering audio problem with VB-Audio virtual cables that shows up on some computers, on some versions of Windows, as outlined in this video: https://youtu.be/71HrZfR_Fro
Setup
  1. Attach the SDR USB dongle to the computer.
  2. Install the driver for Bus 0 ONLY using the Zadig Software.
  3. Install VB-Audio Virtual Cable (free version)
  4. Download and unzip SDRSharp.
  5. In the SDRSharp folder, run the batch file "install-rtlsdr.bat". This lets the software see your dongle.
  6. Create a shortcut to SDRSharp and put it on your desktop or in your taskbar for easy access
  7. IF YOU HAVE STUTTERING AUDIO, Install Process Lasso, and use it to set audiodg.exe's CPU Priority to High and CPU Affinity to once processor core.
To Use (see pictures below)
  1. Plug in the USB dongle, and mount the antenna.
  2. Open SDRSharp
  3. Set the SDR# source to RTL-SDR USB
  4. In the SDR# settings, turn on RTL AGC and Tuner AGC. Keep the sampling rate to 2.4MSPS (higher numbers can cause audio distortion / popping sounds)
  5. Set the radio settings to WFM and the Bandwidth to between 100,000 and 175,000 Hz. This helps minimize stray RF interference. If you hear loud sounds being distorted, increase the bandwidth.
  6. Set the Audio Output to MME Cable Input (VB-Audio Virtual Cable).
  7. Click the play button in the top left corner to start capturing the radio. You should now see a visual representation of what's happening on different frequencies in the frequency viewer, and a visual representation of decoded audio in the audio viewer.
  8. Tune the radio to the base frequency of the Comteks used in your building. Most chapels use the Channel 1 (72.1 MHz) but there may be other FM transmissions for multi-chapel stake centers, or buildings with language translations. Use the frequency guide in the Comtek PR-75a manual to look up the channels. (https://img1.wsimg.com/blobby/go/414496 ... 1900586456, Page 6.)
  9. Enlarge the frequency graph using the Zoom slider on the right. Fine tune the KHz and Hz so that you're as aligned as possible with the middle of the spectrum. When the mics are silent, this should appear as a sharp vertical line. For my chapel, my final tuning is 72.103.202. In the frequency pane you can save this as a tuning shortcut.
  10. In Zoom, set your audio input to be CABLE Output (VB-Audio Virtual Cable), and turn on Original Audio.
  11. If you're using OBS to feed media to the Zoom meeting, such as for Stake Conference or a Fireside, you can set up OBS to also use CABLE Input (VB-Audio Virtual Cable) to mix OBS into the same audio stream.

    I've run this setup for the last couple of weeks and have found it to be fully resilient to RF interference in my ward meetings, though I haven't tested it for any stake events yet. The audio quality is MARKEDLY improved, with zero distortion (other than an echo cancelation with music only caused by the chapel audio system) and has allowed me to be fully mobile with my setup. For the first time I feel like I've found a long-term solution to the audio problem that doesn't involve gangly cable runs or frustrating audio interference, and I wanted to share it with anyone else who may benefit from this in their own wards and stakes.