Configuring rawfile outputs

RTLSDR-Airband can record raw I/Q data of any configured narrowband channel. Recorded data can be demodulated or analyzed with external digital signal processing software, like GNURadio or csdr.

Note: This output type cannot be attached to mixers.

Syntax

outputs is a list of outputs where the audio stream of a particular channel is to be routed. An output is a group of settings enclosed in braces { }. The number of outputs per channel is unlimited.

outputs: (
  {
    type = "rawfile";
    directory = "/home/pi";
    filename_template = "TOWER";
#   split_on_transmission = false;
#   include_freq = false;
#   continuous = false;
#   append = true;
#   dated_subdirectories = true;
  }
);

Want more outputs? Just repeat the braced section several times and separate individual sections with a comma, like this:

outputs: (
  {
    # ... settings for output 1
  },
  {
    # ... settings for output 2
  }
# , ... more outputs here
);

Remember, do not put a comma after the closing brace of the last output. This is a syntax error.

Explanation of keywords

• type (string, required) - type of this output. Put "rawfile" here.
• directory (string, required) - the directory path where output files will be stored. It must be created beforehand.
• filename_template (string, required) - the file name prefix. RTLSDR-Airband appends the current date and UTC time to it and creates a new file on top of every hour. A .cf32 suffix is added automatically.
• continuous (boolean, optional) - whether RTLSDR-Airband shall record the data continuously (true) or skip silence periods when squelch is closed (false). The default is false.
• split_on_transmission (boolean, optional) - if enabled, it caused a new file to be created for every transmission heard on the channel. The default is false. If enabled, then continuous option must be disabled.
• include_freq (boolean, optional) - if enabled, it causes the current channel frequency to be appended to the filename. This is especially useful when operating in scan mode with split_on_transmission feature enabled, as it makes it easy to find out the frequency of each saved transmission. The default is false.
• append (boolean, optional) - if set to true (the default), RTLSDR-Airband will append the recording to the file, if it already exists. When false, the old file will be overwritten.
• dated_subdirectories (boolean, optional) - if set to true (default is false), output files will be placed in dated subdirectories with the format <directory>/YYYY/MM/DD

Format of recorded data

Recorded I/Q data is saved as complex float 32-bit, little endian (hence the cf32 file name suffix).

Sampling rate of the recording is:

• 8000 samples (I/Q pairs) per second when RTLSDR-Airband has been built with NFM support disabled
• 16000 samples (I/Q pairs) per second when RTLSDR-Airband has been built with NFM support enabled

Postprocessing examples

csdr

Demodulating AM from I/Q file using csdr and playing it with mplayer:

cat filename.cf32 | csdr amdemod_cf | csdr fastdcblock_ff  | csdr agc_ff 200 0.2 0.05 0.000001 | csdr limit_ff | csdr convert_f_s16 | mplayer -cache 1024 -quiet -rawaudio samplesize=2:channels=1:rate=16000 -demuxer rawaudio -

Note: substitute rate=16000 with rate=8000 if you have built RTLSDR-Airband with NFM disabled.

Demodulating NFM from I/Q file:

cat filename.cf32 | csdr fmdemod_quadri_cf | csdr limit_ff | csdr deemphasis_wfm_ff 16000 100e-6 | csdr convert_f_s16 | mplayer -cache 1024 -quiet -rawaudio samplesize=2:channels=1:rate=16000 -demuxer rawaudio -

GNUradio

You can use the recorded file as an input to your GNUradio flowgraph. Just use File Source block, set the correct file path, set the "Output type" property to "Short" and "Vec length" to 1.

Now you may want to read about configuring other output types for your channels:

• Icecast
• PulseAudio
• UDP audio stream
• Audio file

or jump straight to:

• Running