Tagged: Audio

rtlmic: Wireless Microphone Receiver for RTL-SDR

Over on GitHub a new program called rtlmic has recently been uploaded. The program descriptions reads: 

rtlmic is a multichannel FM microphone receiver/demodulator for RTL-SDR cards. It outputs realtime audio to JACK.

Basic usage is simply:

$ rtlmic [channel 1 frequency] [channel 2 frequency]…

This program may be able to be used as a replacement for wireless microphone base stations at events. The software allows you to capture as many channels as your CPU can handle, within the active bandwidth of the RTL-SDR. There are also settings for tweaking the companding ratio and tau, deemphasis tau and FM deviation all of which affect the output audio and can be used to optimize the frequency response of the microphone.

The audio outputs directly to Jack audio which is an audio piping API, which simply routes the audio out to wherever you choose it to go.

A typical wireless microphone base station and microphones.
A typical wireless microphone base station and microphones.

Setting audio levels correctly when decoding with an RTL-SDR

When decoding a digital signal with an RTL-SDR the digital audio is usually piped from receiver software like SDR# via stereo mix or software like Virtual Audio Cable/VBCable into the decoding software. The decoding software expects a clean audio signal with the volume levels set not too loud, but also not too quiet. Usually this can be achieved by trial and error.

However amateur radio hobbyist K3RRR found that setting the audio level correctly was critical for maximizing decodes on his digital HF signals when using his RTL-SDR. To correctly set the audio levels he uses a program called Peak Level Meter and another called Soundcard Oscilloscope. Peak Level Meter is used to ensure that the audio levels are set correctly and Soundcard Oscilloscope is used to ensure that the audio is not being over driven into square waves.

Checking for over driven audio waveforms in Soundcard Oscilloscope.
Checking for over driven audio waveforms in Soundcard Oscilloscope.