USING.md

Using sdrx

sdrx is run from the command line in a terminal window. If you have not already build sdrx, look at the build page for instructions.

sdrx takes an aeronautical channel to listen to as the argument. Besides the channel, options are available and can be listed with --help (his page does not always cover all available options so use this page together with --help to get the full picture):

$ cd sdrx/build
$ ./sdrx --help
$ ./sdrx --gain 30 122.455

To stop the program, just press Crtl-C in the terminal and wait. This will stop sdr cleanly as Ctrl-C is handled properly. If you have multiple devices connected you can easily run multiple instances of sdrx in different terminal windows.

The defaults for volume and squelsh level should be good as is. RF gain can be adjusted according to the local signal environment.

The R820T(2)/R860 tuner chip has three gain stages, LNA, Mixer and VGA (sometimes referred to as IF). Each stage can be set to a value between 0 and 15 and each increment of the value represents a change in gain (typically an increase of about +3dB). The gain you give to sdrx as an argument will be translated into one value between 0 and 15 for each stage according to an sdrx internal mapping table. This mapping table is the same as is used in the official librtlsdr library (look in the source code for the details). As an alternative, it is also possible to set the three gain stages directly with stage values like this (LNA = 5, MIX = 8 and VGA = 10):

$ ./sdrx --gain 5:8:10 122.455

This gives very good control over how the total gain is distributed in the R820 tuner and is the preferred way of setting gain when you run a external LNA in front of your device.

sdrx use quite narrow filters so if your RTL dongle does not have a TCXO, take your time to find out the proper frequency correction and supply that with the --fq-corr option. For Airspy devices the correction concept is not used at all and any --fq-corr given is silently ignored.

If you have multiple devices connected, use --list to list what devices that sdrx recognize on your system and what sample rates they support:

$ ./sdrx --list

To use a specific device, it's serial is used and you must ensure that all devices have unique serials. Use rtl_eeprom -s MYSERIAL from the standard librtlsdr package to set unique serials for your RTL devices. Airspy devices normaly have unique serials and you do not have to worry about them.

Note 1: Unlike many other programs that support RTL and/or Airspy dongles, sdrx does not use the "device id" concept at all. An "id" (typically a low number like 0 or 1) is not a stable way to reference a dongle since the id may change over time as devices are plugged in and removed from the USB bus. The serial number concept is, on the other hand, a stable and predictive way to reference a specific dongle as long as every dongle on the system have been given a unique name.

Note 2: The term "serial" is a bit misleading since it actually is a text string based on a USB descriptor. It is prefectly fine to set a serial on a RTL device containing text.

Note 3: Airspy R2 devices are described as "AirSpy NOS" when listing available devices. This is what they call themselves when queried over the USB bus and is nothing sdrx can do anything about.

Support for multiple channels is available as well. Just specify the channels as arguments. The channels must fit inside 80% of the sampling frequency used (see below for explanation):

$ ./sdrx --gain 40 118.105 118.280 118.405 118.505

The more channels you specify, the more loaded the channelization thread will be. Please monitor your system load when running sdrx with many channels to get an understaning of how much you can load your specific system. Especially Airspy devices combined with many channels consume quite some processing power at the moment.

If the connection to a device is lost while sdrx is running, i.e. the device is being unplugged from the USB bus, sdrx will auto reconnect when the device is plugged in again. There is no need to restart the program just because a device disappears for some reason. Some RTL based dongles have rather flimsy USB connectors and a device easily disconnects by just moving it sligthly.

Sample rate defaults to 1.44MS/s for RTL devices and 6MS/s for Airspy devices if not set explicitly. Change to your liking with the --sample-rate option:

$ ./sdrx --sample-rate 2.56 118.280 118.405 118.505

As stated earlier, the sample rate dictates the RF bandwidth that can be used. If, for example, a sample rate of 2.56 MS/s is used, the available RF bandwidth will be 2.56 * 0.8 = 2.048 MHz. For a rate of 1.44 MS/s it will be 1.44 * 0.8 = 1.152 MHz. And so on.

Available rates for each device is shown in the output from the --list option.

Note 4: When specifying a sample rate, use the exact text of the rate as shown when using --list, i.e. if the list say 2.56, you enter 2.56. If the list say 0.96, you enter 0.96. If the list say 10, you enter 10. And so on. Do not include "MS/s" after the rate value. Do not use comma (,) as decimal separator. Do not set anything else other than what is shown with --list.

Output in single channel mode

Besides playing audio when the squelch is open, sdrx write signal power measurements to the console while running at approximately three times a second.

A typical output look like this:

$ ./sdrx -g 45 118.105
...
10:57:00: Level[X   -37.2] 118.105[ 0.0] [-22.5|-22.6|-23.4] [  0.00] [SNR][low|mid|hig][imbalance]
10:57:01: Level[X   -39.6] 118.105[ 0.0] [-22.1|-22.9|-22.2] [  0.00] [SNR][low|mid|hig][imbalance]
10:57:01: Level[X   -39.5] 118.105[ 0.0] [-21.1|-22.3|-22.8] [  0.00] [SNR][low|mid|hig][imbalance]
10:57:01: Level[XXX -27.5] 118.105[38.0] [-21.4| 16.5|-21.7] [-12.28] [SNR][low|mid|hig][imbalance]
10:57:02: Level[XXX -27.5] 118.105[38.7] [-22.7| 16.6|-21.6] [-30.74] [SNR][low|mid|hig][imbalance]
10:57:02: Level[XXX -27.5] 118.105[39.7] [-21.8| 16.9|-24.2] [-30.66] [SNR][low|mid|hig][imbalance]
...

Open squelch is indicated by the channel name getting a yellow background. Level is a measurement of the signal level before the ADC in dB (dBFS) and can be used to see if the receiver is oversteered at the IF/ADC stage or not.

mid is the power level in dB for the center band of the channel you are listenting to, i.e fc +/- 2.8kHz and low and hig are the power levels for the bands just outside of the channel, i.e. fc - 3.5-4.9kHz and fc + 3.5-4.9kHz.

The noise floor for the channel is estimated as the power just outside of where the modulated audio resides (i.e. the low and hig measurements). SNR is the difference between "signal power" and "noise power" and is checked against the desired squelch level to determine if the squelsh should be open or not.

imbalance is a measure of how "off" the receiver is compared to the frequency that the transmitter is using. If it is negative, you are tuned above the transmitter and if it is positive you are tuned below the transmitter frequency.

The imbalance is determined by calculating the energy balance between "negative" and "positive" frequencies from the FFT used for the squelch. Since AM is a symmetric modulation around the carrier, the imbalance should be 0 if you are tuned to the transmitters frequency.

Output in multi channel mode

In multi channel mode, audio from the different channels are distributed between the left and right speaker in a virtual five speaker audio panorama. This will create the sense of a set of five speakers front of the listener.

Together with the visual presentation in the terminal, this will increase the awareness of what channels that are active. The channels will be placed in the panorma based on their order on the command line.

Output in the terminal looks about the same as for single channel but only the SNR is shown for each individual channel:

$ ./sdrx -g 40 118.105 118.280 118.405
...
10:57:00: Level[XX    -39.6] 118.105[ 0.0] 118.205[ 0.0] 118.280[ 0.0] 118.405[ 0.0]
10:57:01: Level[XX    -39.6] 118.105[ 0.0] 118.205[ 0.0] 118.280[ 0.0] 118.405[ 0.0]
10:57:01: Level[XX    -39.6] 118.105[ 0.0] 118.205[ 1.0] 118.280[ 0.0] 118.405[ 0.0]
10:57:01: Level[XXXX  -27.5] 118.105[ 1.3] 118.205[ 0.0] 118.280[27.0] 118.405[ 0.0]
10:57:02: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[28.5] 118.405[ 0.0]
10:57:02: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[28.9] 118.405[ 0.0]
10:57:02: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[29.0] 118.405[ 0.0]
10:57:03: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[29.4] 118.405[ 0.0]
10:57:03: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[28.2] 118.405[ 0.0]
10:57:03: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[28.4] 118.405[ 0.0]
10:57:04: Level[XXXX  -27.5] 118.105[ 1.1] 118.205[ 0.0] 118.280[29.8] 118.405[ 0.0]
10:57:04: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[28.9] 118.405[ 0.0]
10:57:04: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[28.5] 118.405[ 0.0]
10:57:05: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[27.9] 118.405[ 0.0]
10:57:05: Level[XXXX  -27.5] 118.105[ 0.0] 118.205[ 0.0] 118.280[28.8] 118.405[ 0.0]
10:57:05: Level[XX    -39.6] 118.105[ 1.0] 118.205[ 0.0] 118.280[ 0.0] 118.405[ 0.0]
10:57:06: Level[XX    -39.6] 118.105[ 1.8] 118.205[ 0.0] 118.280[ 1.2] 118.405[ 0.0]
10:57:06: Level[XX    -39.6] 118.105[ 0.0] 118.205[ 0.0] 118.280[ 0.0] 118.405[ 0.0]
...