A frequency scanner for Gqrx Software Defined Radio receiver

gqrx-scanner is a frequency scanner written in C that uses gqrx remote protocol to perform a fast scan of the band. It can be used in conjunction with the gqrx bookmarks (--mode bookmark) to look for the already stored frequencies or, in a free sweep scan mode (--mode sweep), to explore the band within a specified frequency range (--min, --max options).

The sweep scan uses an adaptive algorithm to remember the active frequencies encountered during the sweep, that prioritizes active stations without stopping to look for new ones.

Gqrx Squelch level is used as the threshold, when the signal is strong enough it stops the scanner on the frequency found. After the signal is lost the scanner waits a configurable ammount of time and restart the loop (--delay option).

In sweep mode the scan of the band is performed fast (well, as fast as it can), self asjusting the scanning speed. On signal detection a fine tuning is performed to pinpoint the nearest carrier frequency (subdivision of upper and lower limits with increasing precision) and, for the already seen carriers, a previous value is used to avoid fine tuning at every hit. This value is also averaged out with the new one in order to converge eventually to the exact frequency in less time (after 4 hits of the same frequency).

• Support for Gqrx bookmarks file
• Fast sweep scan with adaptive monitor of the most active stations
• Frequency range constrained scan (also for bookmarks)
• Multiple Tag based search in bookmark scan mode ("Tag1|Tag2|TagX").
• Automatic Frequency Locking in sweep scan mode
• Interactive monitor to skip, ban or pause a frequency manually
• Automatic recording of detected signals
• Voice Activity Detection (VOX) for squelch-bypassing listen timeout (Linux only)

Gqrx Remote Protocol must be enabled: Tools->Remote Control. See this. PipeWire and pw-cat are required for VOX (Linux only). See "Voice Activity Detection (VOX)" section below.

It is advisable to disable AGC during the scan: adjust the fixed gain lowering the noise floor to at least -60/-70 dBFS and set the squelch level to -50/-40 dBFS, depending on the band activities and noise levels.

The best results are obtained in relative quiet frequencies with sporadic transmissions. If the band is cluttered with harmonics and other types of persistent noise, avoid the sweep scan and use the bookmarks mode with a higher squelch level (or use the 'b' key to ban a frequency from the scan).

In sweep mode use a limited bandwidth of about 2 MHz in order to avoid VFO and noise floor levels chainging during the sweep. If you don't provide the --min, --max option, set the demodulator frequency to the middle of the screen and start from there in order to avoid the panadapter to move during the scan.

You may also consider to adjust FFT options: FFT size and rate (on FFT Settings) to improve performances (and cpu usage). I have found better result with high fft size (64536) and 17 fps refresh rate, but this depends on your hardware.

Usage:
gqrx-scanner
		[-h|--host <host>] [-p|--port <port>] [-m|--mode <sweep|bookmark>]
		[-f <central frequency>] [-b|--min <from freq>] [-e|--max <to freq>]
		[-d|--delay <lingering time in milliseconds>]
		[-l|--max-listen <[probe_time:]hangup_time>]
		[-t|--tags <"tag1|tag2|...">]
		[-v|--verbose]
		[-r|--record]
		[--vox]

-h, --host <host>            Name of the host to connect. Default: localhost
-p, --port <port>            The number of the port to connect. Default: 7356
-m, --mode <mode>            Scan mode to be used. Default: sweep
                               Possible values for <mode>: sweep, bookmark
-f, --freq <freq>            Frequency to scan with a range of +- 1MHz.
                               Default: the current frequency tuned in Gqrx Incompatible with -b, -e
-b, --min <freq>             Frequency range begins with this <freq> in Hz. Incompatible with -f
-e, --max <freq>             Frequency range ends with this <freq> in Hz. Incompatible with -f
-s, --step <freq>            Frequency step <freq> in Hz. Default: 10000
-d, --delay <time>           Lingering time in milliseconds before the scanner reactivates. Default 2000
-l, --max-listen <time>      Maximum time to listen to an active frequency. Default 0 (no limit).
                               With --vox, use -l [probe_time:]hangup_time
                               probe_time (ms): Time to wait for voice on a new carrier.
                               hangup_time (ms): Time to wait after voice drops.
                               Example: --vox -l 1000:5000
-x, --speed <time>           Time in milliseconds for bookmark scan speed. Default 250 milliseconds.
                               If scan lands on wrong bookmark during search, use -x 500 (ms) to slow down speed
-y  --date                   Date Format, default is 0.
                               0 = mm-dd-yy
                               1 = dd-mm-yy
-t, --tags <"tags">          Filter signals. Match only on frequencies marked with a tag found in "tags"
                               "tags" is a quoted string with a '|' list separator: Ex: "Tag1|Tag2"
                               tags are case insensitive and match also for partial string contained in a tag
                               Works only with -m bookmark scan mode
-r, --record                 Enable recording of detected signals
-v, --verbose                Output more information during scan (used for debug). Default: false
--vox                        Enable voice activity detection. Requires PipeWire + pw-cat
                               Uses -l [probe_time:]hangup_time — see -l help.
--help                       This help message.

These keyboard shortcuts are available during scan:

[space] OR [enter]  :   Skips a locked frequency (listening to the next).
'b'                 :   Bans a locked frequency, the bandwidth banned is about 10 Khz from the locked freq.
'c'                 :   Clears all banned frequencies.
'p'                 :   Pauses scan on locked frequency, 'p' again to unpause.

Linux only — requires a PipeWire based distro (pipewire-bin package) and a compatible audio setup. MacOSX is not supported due to the lack of a portable audio interception method.

VOX uses PipeWire to capture the receiver's audio produced by an arbitrary demodulator program and detect speech, decoupling the listen timer from the carrier state. The -l option accepts two values separated by a colon:

• probe_time: how long to wait for voice on a new carrier before skipping (short, e.g. 1000ms)
• hangup_time: how long to wait after voice stops before moving on (long, e.g. 5000ms)

A single value sets both to the same time (backward compatible).

On digital modes like DMR the carrier is always present even when no voice channel is active, so the traditional squelch-based approach will either stop the scan or timeout when using the -l option. VOX solves this by analyzing the demodulated audio for actual speech.

Without VOX, the scanner listens to any signal that opens the squelch (noise, hum, interference) for the full -l duration. With VOX, the pitch-based voice detector extends the listening window indefinitely as long as speech is detected. When speech stops, the hangup_time counts down before the scanner moves on. Frequencies with a carrier but no voice (digital modes, noise) skip after probe_time expires, allowing fast scanning without cutting off real conversations.

By default, pw-cat --record captures from the default audio monitor (all audio played through the speakers). No setup script is needed:

./gqrx-scanner --vox -l 1000:5000

If you run other audio sources (music, notifications, etc.) that could falsely trigger VOX, the gqrx-scan-setup-audio.sh script isolates just your demodulator's audio. It creates a virtual null sink that intercepts the app's audio, duplicating it so the scanner can analyze it for voice without muting the speakers:

1. Creates a virtual "null sink" (gqrx-scanner-intercept)
2. Redirects your demodulator (DSD, GQRX, etc.) to this null sink
3. Sets up a loopback from the null sink back to your speakers

Audio flow: app → null sink → (speakers + scanner capture)

1. Find your demodulator's name:

   ./gqrx-scan-setup-audio.sh list-processes
   

2. Attach the interceptor:

   ./gqrx-scan-setup-audio.sh attach dsd
   

   ("gqrx-scanner-intercept" appears in pw-cli or pavucontrol Recording)

3. Run the scanner with VOX:

   ./gqrx-scanner --vox -l 1000:5000
   

   (probe for voice 1s on new carriers, hang 5s after voice stops)

   A single value sets both probe and hangup:

   ./gqrx-scanner --vox -l 5000
   

4. Clean up when done:

   ./gqrx-scan-setup-audio.sh detach
   ./gqrx-scan-setup-audio.sh cleanup
   

Performs a sweep scan with a range of +-1Mhz from the demodulator frequency in Gqrx:

./gqrx-scanner

Search all bookmarks for frequencies in the range +- 1 Mhz from the demodulator frequency in Gqrx:

./gqrx-scanner -m bookmark

Performs a sweep scan from the central frequency 144.000 MHz using the range 143.000-145.000 MHz:

./gqrx-scanner -f 144000000

Performs a scan using Gqrx bookmarks, monitoring only the frequencies tagged with "DMR" or "Radio Links" in the range 430MHz-431MHz:

./gqrx-scanner -m bookmark --min 430000000 --max 431000000 --tags "DMR|Radio Links"

Performs a sweep scan from frequency 430MHz to 431MHz, using a delay of 3 secs as idle time after a signal is lost, restarting the sweep loop when this time expires:

./gqrx-scanner --min 430000000 --max 431000000 -d 3000

VOX bookmark scan with 1s probe and 5s hangup, searching only DMR frequencies (see "Voice Activity Detection (VOX)" section for setup):

./gqrx-scanner -m bookmark --vox -l 1000:5000 --tags "DMR"

$ ./gqrx-scanner -m bookmark -f 430000000
Frequency range set from 429.000 MHz to 431.000 MHz.
[04-08-17 19:51:54] Freq: 430.037 MHz active [ Beigua                   ], Level: -40.20/-50.70  [elapsed time 03 sec]
[04-08-17 19:51:57] Freq: 430.288 MHz active [ ponte dmr                ], Level: -39.00/-50.70  [elapsed time 43 sec]
[04-08-17 19:52:40] Freq: 430.887 MHz active [ DMR                      ], Level: -30.50/-50.70  [elapsed time 11 sec]
[04-08-17 19:53:23] Freq: 430.900 MHz active [ Genova DMR               ], Level: -32.20/-50.70  [elapsed time 14 sec]

cmake .
make
sudo make install

The test suite requires the cmocka testing framework. Install it first:

Ubuntu/Debian:

sudo apt-get install libcmocka-dev

Fedora/RHEL:

sudo dnf install libcmocka-devel

macOS (via Homebrew):

brew install cmocka

Once cmocka is installed, build and run tests:

cmake .
make
ctest

For verbose test output:

ctest --verbose

Note: If cmocka is not installed, the build will proceed normally but tests will be disabled. You'll see a message: "cmocka not found - tests disabled."

run ccmake, toggle, and set CMAKE_C_FLAGS=-DOSX

ccmake .
cmake .
make
sudo make install

sudo make uninstall

• set modulation in bookmark search
• parsable output in csv?