Channels and decoding

A channel listens on one frequency. That frequency belongs to the channel, not to the radio feeding it: retuning the radio leaves every channel where it was. A channel whose frequency falls outside what the radio is sampling stays set up and goes quiet, and its face offers to tune the radio over it.

Add a channel

Add a channel from + Node, then wire the Device’s IQ output into the channel’s IQ input. Connect the outputs you need:

Channel catalog

The node palette lists every channel type under Decoders, whether it produces audio or events. The server reports the exact catalog for the running build; this is the current list.

Coverage varies by protocol. The catalog lists implemented signal paths, but optional services, trunking variants, and vendor extensions may be unsupported. Check the mode-specific limits below.

What the maturity labels mean

Generated fixtures catch decoding errors, but do not establish tolerance to transmitter drift, keying transients, adjacent-channel interference, or multipath. Most decoders have only this coverage.

Committed recordings add regression coverage for DMR, ADS-B, FreeDV 1600, and a busy FT8 slot. Their origins and expected output are listed in the fixture library. A recording test does not necessarily verify the whole live receive path, and a maturity label applies only to the services tested.

Some decoders also use worked examples from their standards, including ADS-B frames, APRS compressed positions, CCIR 476 characters, and radio-clock minutes. Iridium tests use an off-air bit sequence with a synthetic waveform, which tests real framing but not real RF conditions.

VDL Mode 2, HFDL, Inmarsat Classic Aero, Inmarsat STD-C, and Digital Selective Calling use decoders from xng. Their labels describe the sdr– integration, separately from upstream testing.

ISM sensors

A Sub-GHz channel decodes known sensor payloads and displays raw frames for other signals. Supported devices are grouped by pulse coding:

Readings can include temperature, humidity, soil moisture, wind speed and direction, rainfall, tyre pressure, and power. Renault TPMS adds Manchester coding after framing; Toyota TPMS adds differential Manchester.

The decoder checks pulse timings and the device’s checksum, digest, or parity before reporting a reading. Unrecognised bursts remain available in the raw timing view. FSK sensors use bit periods of 55–58 µs; the default minimum pulse width admits these signals.

Pulse slicing, payload layouts, validation rules, and CRC/LFSR digest routines follow rtl_433, licensed GPL-2.0-or-later.

Experimental mode limits

To add on-air coverage, contribute a short IQ capture restricted to the relevant band, with its expected decoded output. See Build and test and the contribution guide.

Pager text

POCSAG uses seven-bit characters. Some German networks use DIN 66003, which replaces ASCII brackets and related punctuation with umlauts and ß.

sdr– applies this mapping when the affected character appears inside a word beside a lowercase letter: M}nchen becomes München, and Stra~e becomes Straße. Otherwise it keeps ASCII, so [ALARM] retains its brackets. Entirely uppercase pages remain ASCII. There is no manual setting.

Sample rate and passband

A channel’s occupied band must fit inside its source device’s current passband. If it does not, move the channel closer to center, raise the device sample rate, or retune the device.

Most channels resample device IQ to their processing rate, provided their occupied band fits in the device passband. The following channels process samples at the device rate and require:

The channel face says so when the current rate cannot work, and offers a compatible one.

Otherwise use the lowest rate that covers the signals you need. Higher rates increase USB traffic, FFT work and CPU load without improving a narrow channel.

Tuning and squelch

Tune a channel by editing its frequency, dragging its marker on a connected Scope, or using the keyboard while the channel is selected. The field takes megahertz; the −25k, −5k, +5k and +25k buttons step it.

The keyboard button beside the field takes a typed frequency. A bare number is read as megahertz; a kHz, MHz or GHz suffix is honoured. The span the radio currently hears is shown below the field — a frequency outside it is accepted, and the channel waits there silently until the radio covers it.

The lock beside the dial holds the channel on its frequency. While it is held, the dial, typed entry, and Scope marker refuse to move the channel; the radio feeding it can still be retuned. The Device node carries the same lock for the radio itself.

A channel node that is not wired to anything yet can still be given a frequency; it is held against the node and applied the moment a radio carries it. A radio nobody has tuned by hand opens over the channels wired into it.

Every channel can gate what it decodes with squelch. Off passes everything through. Manual opens above a level you set; a lower threshold opens more easily.

Auto opens a chosen number of decibels above the channel’s measured noise floor, with no fixed level needed. The level meter under the dial marks where the gate opens.

The channel learns the noise floor during quiet periods. A continuous signal may be mistaken for the floor, requiring a stronger signal to open the gate. Once the gate opens, the floor cannot rise and suppress a long transmission.

Switching from Auto back to Manual restores the level you last set by hand.

NFM adds tone squelch:

• Detect reports any recognized CTCSS tone or DCS code without gating audio.
• CTCSS opens only for a selected standard tone.
• DCS opens only for a selected standard code.

Compander applies 2:1 audio expansion to receive signals transmitted with matching compression. Enable it only for a companded NFM link; ordinary NFM speech can become too quiet with expansion. The corresponding transmit setting applies compression. Expansion stops 20 dB below the reference level, and sub-audible tones are excluded from level tracking.

Audio processing

Audio channels share an Audio block. Processing is off by default except for AGC on AM and SSB. The stages run in the order shown below.

The blanker runs on IQ; the remaining stages run on audio. Removing impulses before filtering reduces the ringing they would otherwise cause.

Slow-scan television

An SSTV picture takes 36 seconds to four and a half minutes to receive, depending on mode. Tune to the SSB carrier; the channel processes the 1000–2600 Hz video subcarrier above it.

A transmission names its own mode in the VIS header that precedes it. Follow VIS, the default, reads that header and recognizes Robot 36 and 72, Martin M1 and M2, Scottie S1, S2 and DX, PD50, PD90, PD120 and PD180, and Wraase SC2-180. Pick a mode by hand when the header was missed or corrupted; the decoder then starts on any header it sees and scans it as the mode you chose.

Slant correction tracks each line’s sync pulse instead of free-running from the header, which keeps the picture upright when your sample clock and the transmitter’s disagree. Leave it on unless you are diagnosing the sync itself.

Keep unfinished pictures decides what happens when a transmission fades or is cut short. On, the lines that did arrive are kept; off, only a picture that scanned to its last line is.

Wire the channel’s video output into a Video node to watch a picture build up line by line. Every finished picture, and every kept partial, is also stored on the server as a PNG and listed in the channel’s own panel, so a picture that arrived while no browser was connected is still there. The store holds 24 hours of pictures, capped at 512 of them.

Surveying a DECT network

The dect channel surveys base stations on one DECT carrier. It reads the 64-bit A-field in each burst for identity, configuration, and authentication or ciphering signalling. It does not decode the B-field containing call audio and user data.

A DECT carrier is 1.728 MHz wide and the channel runs at 2.304 MHz, so the receiver needs at least that much bandwidth and must reach the band: 1880–1900 MHz in Europe, 1920–1930 MHz in the US. An RTL-SDR tops out below the band and cannot be used; a HackRF or an SDRplay can.

Set Band so carrier numbers resolve to frequencies, and set Side to Base if you only want the fixed part, Handset for portables, or Both. Carrier 0 is the highest frequency in the European band (1897.344 MHz) and they count downwards in 1.728 MHz steps to carrier 9 at 1881.792 MHz; the US band counts upwards from 1921.536 MHz.

Each base station transmits a dummy bearer once per 10 ms frame in a fixed slot, cycling through the identity and system-information messages. The decoder groups bursts by their slot timing, so several base stations sharing one carrier stay apart, and folds each one into a single record:

• RFPI — the 40-bit Radio Fixed Part Identity, broadcast on the Nt channel. It splits into the access rights class (A residential, B private multi-cell, C public, D GSM/UMTS, E direct), the manufacturer, installer or operator code, the fixed part number and sub-number, and the radio fixed part number that separates cells within one system. Class C and D encode single-cell versus multi-cell in the low bit of the RPN.
• System information — the carrier the base is on and its frequency, which slot pair it uses, how many transceivers it has, which of the ten carriers it says are available, and its primary scan carrier number.
• Capabilities — the fixed part capabilities broadcast, decoded bit by bit: slot types, frequency control, handover, the connectionless services, and the higher-layer services.
• Security — whether the base advertises standard authentication (DSAA) and standard ciphering (DSC), and, separately, whether encryption was actually negotiated on the air. MAC encryption-control messages are followed through request, confirm and grant, so a bearer shows as encrypted only once the grant is seen. A cipher key index is reported when the base uses the keyed variant.
• Handsets — the PMIDs seen in encryption handshakes, plus the FMID of the fixed part.

Each A-field must pass its R-CRC check. Burst and error counts appear beside each station.

Advertised ciphering support does not establish whether a call uses encryption. The reported encryption state follows observed request, confirm, and grant messages; missing signalling is not proof that a call is unencrypted.

Following a DMR trunk system

Add a DMR trunk system node, connect a Device’s iq output, and enter the control-channel frequency in MHz. Select a system type or use auto-detect. The node manages its own DMR decoders.

Following runs on the server even when no browser is connected. Traffic channels must fit in the source radio’s passband. If a grant falls outside it, the node reports the failure. Increase the sample rate or retune to include the required frequencies.

Enable Record calls to buffer completed calls and their audio in memory. Encrypted calls retain metadata only. Disable it to follow traffic without buffering audio.

Where decoder output goes

Decoder events are typed on the server and timestamped with source and frequency information. Choose the destination that matches the job:

Decoder log history is bounded, so a busy unattended receiver cannot grow the database forever.

Pictures are not decoder-log rows. A completed SSTV picture writes one line to the log recording what arrived, while the pixels go to the picture store and are served from GET /api/images.