This example is an AM radio receiver, implemented with an envelope detector. It can be used to listen to broadcast stations on the MF (AM Broadcast) and HF (Shortwave Broadcast) bands, as well as aviation communication on the VHF airband. It uses the RTL-SDR as an SDR source, plays audio with PulseAudio, and shows two real-time plots: the RF spectrum and the demodulated audio spectrum.

This example requires an RF upconverter to listen to stations on the HF and MF bands with the RTL-SDR.

This AM envelope demodulator composition is available in LuaRadio as the AMEnvelopeDemodulator block.

Flow Graph

local radio = require('radio')

if #arg < 1 then
    io.stderr:write("Usage: " .. arg[0] .. " <frequency>\n")

local frequency = tonumber(arg[1])
local tune_offset = -100e3
local bandwidth = 5e3

-- Blocks
local source = radio.RtlSdrSource(frequency + tune_offset, 1102500)
local tuner = radio.TunerBlock(tune_offset, 2*bandwidth, 50)
local am_demod = radio.ComplexMagnitudeBlock()
local dcr_filter = radio.SinglepoleHighpassFilterBlock(100)
local af_filter = radio.LowpassFilterBlock(128, bandwidth)
local af_gain = radio.AGCBlock('slow')
local sink = os.getenv('DISPLAY') and radio.PulseAudioSink(1) or radio.WAVFileSink('am_envelope.wav', 1)

-- Plotting sinks
local plot1 = radio.GnuplotSpectrumSink(2048, 'RF Spectrum', {yrange = {-120, -40}})
local plot2 = radio.GnuplotSpectrumSink(2048, 'AF Spectrum', {yrange = {-120, -40},
                                                              xrange = {0, bandwidth},
                                                              update_time = 0.05})

-- Connections
local top = radio.CompositeBlock()
top:connect(source, tuner, am_demod, dcr_filter, af_filter, af_gain, sink)
if os.getenv('DISPLAY') then
    top:connect(tuner, plot1)
    top:connect(af_gain, plot2)

Usage: examples/rtlsdr_am_envelope.lua <frequency>

Running this example in a headless environment will inhibit plotting and record audio to the WAV file am_envelope.wav.

Usage Example

Listen to WWV at 5 MHz (with a 125 MHz upconverter):

$ ./luaradio examples/rtlsdr_am_envelope.lua 130e6

Listen to an AM radio station at 560 kHz (with a 125 MHz upconverter):

$ ./luaradio examples/rtlsdr_am_envelope.lua 125.560e6