Updates for rtl_fm overhaul
Source: Archived version from November 23, 2023
Rtl_fm is a little utility I wrote for the rtl-sdr project. The program was made to fill a gap in software defined radio: all the computers weaker than a Pentium 4. Basically, an Atom processor does not have enough oomph to demodulate something as simple as narrow band FM using the standard tools. (Recently a high performance FM demodulator was released, Simple FM, but it works only passably on newer Atoms.) So rtl_fm was written with one goal, efficiency, in mind.
The very first version ran with plenty of cycles to spare. As an unexpected bonus, it ended up being efficient enough to easily run on small ARM boards such as the Raspberry Pi. GnuRadio is a really great program easily worth a thousand bucks. But it was designed to run on $500 computers with $500 SDR hardware. Whereas this is made for $20 SDRs plugged into $20 computers. More powerful Windows and OSX machines can easily handle several instances of rtl_fm at once.
Rtl_fm is a general purpose analog demodulator. It can handle FM, AM, and SSB. It can scan more than a hundred frequencies a second. For digital modes, piping the audio into multimon-ng works very well. On a Raspberry Pi, rtl_fm + multimon uses just over half of the CPU. If you are interested in ADS-B decoding, check out the rtl_adsb utility. It uses only 6% CPU on a Raspberry Pi and can run on targets as small and adorable as a 16MB TL-R703N. If you are using Arch Linux, I've already packaged these tools for you, and you can install them with pacman -S rtl-sdr. If you need something more specific, feel free to send me an email.
Throughout this guide, I'll be using Sox's play command to play the audio. This is because Sox works on Linux, OSX, Windows, and pretty much anything else you can run a compiler on. (If you are using Windows, rename the sox binary to play first.) Sox will automatically resample the audio to keep your soundcard happy and can apply denoising filters to keep your ears happy. If you want to run closer to the metal, feel free to use the utilities provided by Alsa, OSS, or Pulse. But I won’t be talking about them in this guide.
Before starting, make sure your dongle works. rtl_test should return a list of supported gain values and not produce error messages. Make sure you are using a USB 2.0 port as well, otherwise you get really weird errors.
It is also very important to always make sure rtl_fm and Sox are both set to use the same data rate. This is probably the most common problem people run into.
rtl_fm -M wbfm -f 89.1M | play -r 32k -t raw -e s -b 16 -c 1 -V1 -This will tune to a broadcast FM station.
-fgives the frequency to tune in.-M wbfmsays to use wideband FM mode, but this is really a shortcut for a tweaked narrowband FM mode. It expands fully into:
rtl_fm -f 89.1M -M fm -s 170k -A fast -r 32k -l 0 -E deemp | play -r 32k ...-f ...indicates the frequency to tune to-M fmmeans narrowband FM-s 170kmeans to sample the radio at 170k/sec-A fastuses a fast polynomial approximation of arctangent-r 32kmeans to lowpass/resample at 32kHz-l 0disables squelch-E deempapplies a deemphasis filter
Needless to say, this is a lot just to listen to the local radio. So use -M wbfm instead. If conditions are poor, you might need to manually add some gain with -g 20 or so.
Operating police scanners is illegal in some countries, notably Germany.
rtl_fm -M fm -f 154.42M -f 154.75M -f 154.82M -f 154.89M -s 12k -g 50 -l 70 | play -r 12k ...This one will take a lot more fiddling to work properly. First, you of course need channels used by your municipality. And this will only work if your local services have not upgraded to P25 or some other trunked system.
-M fmnarrowband FM-f ...frequency to tune, use multiple times for scanning-s 12ksample rate, about as narrow as possible for FM voice-g 50set gain to maximum (use a value appropriate to your dongle)-l 70set squelch to 70. The exact value varies a lot. Changing the gain or sample rate will require a change in the squelch to compensate.
The airband has a lot of channels to scan. Explicitly listing them all is not practical, so use a channel range.
rtl_fm -M am -f 118M:137M:25k -s 12k -g 50 -l 280 | play -r 12k ...-M amAM demodulation since Aircraft radio 108-136 MHz is AM.-f start:stop:intervala range of frequencies to scan-s 12ksame as above-g 50same as above-l 280squelch level, exact value varies a lot
Pagers are considered private communications in some areas (notably the US) and may be illegal to read.
rtl_fm -M fm -f 929.77M -s 22050 -g 10 -l 250 | multimon -t raw /dev/stdinExact frequency and gain depend on your area. Most of these options should be familiar by now. The only new thing is -s 22050 (the only sample rate multimon supports) and the rather peculiar incantation multimon needs to use standard input. Frequency scanning also works in conjunction with multimon.
Maybe you have a poor quality wifi link and don’t want to send a full 1MS/s data stream around. Here is how to make a quick and dirty mp3 stream that will average 4KB-8KB/sec for voice channels. Other codecs may be swapped in as desired.
On the dongle side:
rtl_fm ..... | sox -traw -r24k -es -b16 -c1 -V1 - -tmp3 - | socat -u - TCP-LISTEN:8080On the loudspeaker side:
netcat don.gle.ip.adr 8080 | play -t mp3 -Comment: Q: start/stop rtl_fm automatically based on time A: cron and ti...
Q: Rtl_fm says it tunes to some other frequency instead of mine.
This is normal behavior, to dodge the DC spike present in E4000 tuners. The software tunes slightly off center and then corrects for the offset during demodulation.
Q: How to do skip channels when providing a range?
You don’t. Use multiple ranges instead.
Q: How do I filter noise?
That is really hard. But Sox provides filters for it, and you can use these filters in your play command.
If you want to stream directly to a file, provide the file name as the final argument.
-M fm | wbfm | am | usb | lsb | raw
Selects a demodulator.
-M fmnarrow band FM-M wbfmwide band FM-M amamplitude modulation-M lsblower sideband-M usbupper sideband-M rawraw output, 2x16 bit IQ pairs. Purely debugging and no sane person has any use for this.
-f <freq> | <start:stop:step>
Tune to a frequency. Value can be specified as an integer (89100000), a float (89.1e6), or as a metric suffix (89.1M). Multiple values or ranges will be scanned.
Scanning requires a non-zero squelch level.
-s <sample_rate>
Bandwidth of signal.
-d <index>
When using multiple dongles, this indicates which. You can also identify dongles by the text in the serial number field of the EEPROM.
-g <gain>
A floating point gain value. The dongle will use the closest gain setting available.
-p <error>
Correct for the parts-per-million error in the crystal.
-F 0 | 9
Enable a higher quality downsampling FIR than the default boxcar filter.
-F 0is okay to use while-F 9is still a work in progress.
-A std | fast | lut
Choose how arctan is computed. Options select between the standard (floating) lib, a fast polynomial integer approximation, and a precomputed look-up-table.
-E dc | deemp | direct | offset
-E dcdc blocking filter-E deempde-emphasis filter for WBFM-E directdirect sampling mode-E offsetoffset tuning mode (E4000 only)
Use multiple -E option to enable multiple features.
These kind of stink and are complicated and hard to explain.
Squelch is required for scanning multiple frequencies, to indicate when a frequency should be skipped. When used with a single frequency, the output will be muted instead. Default value is 0/off.
Squelch values are extremely sensitive to external conditions (noise and antenna) as well as the other options used. For example, squelch should be halved when the sample rate is doubled. When in doubt, start at zero and work upwards.
The number of squelch hits to count up to. Default value is 20. So the squelch must be triggered 20 times consecutively before rtl_fm moves on to the next channel.
Negative delay values cause rtl_fm to exit instead of hopping to another channel or muting the output. This is useful for scripts or trunked systems.
In some odd circumstances, you might want to apply an extra low pass downsampling/resampling stage. This is for that, but most of the time it is rarely needed. It will also change the output rate. It is also a very poor quality resampler. Use sox for resampling.
Note: This Markdown file is based on the archived version of the page from November 23, 2023. It includes all the main content, code examples, and comments from the original page, formatted for readability.