Tutorial Signal workbench

Tutorial: Signal workbench (take a recording apart)

What it is. The Signal workbench analyses a recording without a radio. You open an audio file or an IQ file, see a waveform and a spectrogram, select a time range and a band, measure the signal (peaks, bandwidth, carrier offset, SNR, symbol rate, a modulation hint, bursts), demodulate it with AM, FM, SSB, CW or a digital demodulator, look at the eye and constellation, and read the recovered bit stream. It is the tool for signals you do not yet know how to decode.

What you need

• A recording. No radio is needed. Supported inputs: audio (WAV, MP3, FLAC, OGG, M4A: a real signal, or a stereo file read as I/Q) and IQ (.cs8, .cu8, .cf32, .ci16, .cu16, SigMF, IQ WAV). You can make one with Record IQ in the Source panel, see Record IQ then the Signal workbench and Signal samples for ready-made files.
• The Demo signal button loads a built-in test signal if you have no file.
• No frequency list: the centre frequency only labels the plots.

Before you start

• For a raw IQ file you must tell the workbench the Format, the Sample rate and optionally the Centre Hz. SigMF and WAV carry them. A wrong sample rate makes every frequency and symbol-rate figure wrong by the same factor.
• A stereo audio file is read as I (left) and Q (right) only if stereo is I/Q is ticked; otherwise it is mixed to a real signal.

Step by step

1. In the mode list open General & tools and choose Signal workbench (second line Files: audio and IQ). The tab is called Workbench; it has five views: Signal, Measure, Demodulate, Bits and Export.
2. In Signal: type a path in File or press Open..., set Format, Sample rate and Centre Hz for raw IQ, and press Load (or Demo signal).
3. Look at the waveform and the spectrogram. Set FFT size, Window and Overlap; Colours picks the map, with auto or manual min/max levels. Full, To selection, - and + control the zoom.
4. Drag in the spectrogram to select a time range and a band (Selection time ... to ..., band ... to ... show the numbers). Clear selection removes it.
5. Go to Measure and press Measure selection. Choose the FFT, how many peaks (top), and whether to estimate modulation, symbol rate and bursts.
6. Go to Demodulate, choose the Demodulator and press Demodulate selection. Press Listen to hear audio demodulators; Stop ends it.
7. Digital demodulators fill the Bits view. Export the results in Export.

What you should see

• Signal: the waveform on top and the spectrogram below, with cursors and the selection.
• Measure: a peaks table (frequency, level), the -3 dB bandwidth, the -20 dB bandwidth and the 99 % bandwidth, the carrier offset, the SNR, the symbol rate estimate (with use as baud to copy it to the demodulator), modulation hints, and power over time with the bursts found. Click a burst to select it and zoom the Signal view to it.
• Demodulate: audio level for analogue demodulators; for digital ones also the eye diagram, the constellation (for PSK) and a histogram, and the bit stream length.

Reading the screen

• Demodulators: AM, FM, USB, LSB, CW give audio. ASK/OOK, 2-FSK, 4-FSK, AFSK, BPSK, QPSK, 8PSK, MSK and GMSK also recover the bit stream.
• Baud and Auto-detect: the symbol rate; the auto-detect fills it from the measurement. band from selection takes the filter from your band selection; carrier recovery and differential options apply to PSK; roll-off sets the filter; swap which side is 1 flips the bit sense.
• Tones for AFSK include presets such as Bell 202 (1200 / 2200 Hz at 1200 baud, as used by packet radio and APRS); you can also set the mark and space tones, FM first and the CW pitch. Clock recovery options and a loop bandwidth slider tune the symbol timing.
• Bits: invert, differential decode, NRZI, a Manchester setting, reverse bits in each byte, Byte alignment and LSB first in bytes; a view of bits, hex or text; bytes per line; Find a bit pattern (also inverted, with a tolerance for mismatches), then jump to hits; pages with < and >.
• Export: the selection as a recording (WAV two-channel I/Q or mono for a real recording, cf32, or SigMF with the centre frequency; a selected band is moved to baseband and written at a lower rate), the demodulated audio (.wav), the bits (.txt or .bin) and the peaks (.csv).

Options and settings

• Colours and the dB range, FFT size and Overlap change what you can see; a long FFT shows narrow carriers, a short one shows short bursts.

Troubleshooting

• Nothing in the spectrogram. Wrong format or sample rate. Try cs8 versus cu8, or set the rate from the file's name.
• Bits look random. The wrong baud rate, polarity or modulation. Use Auto-detect, then try invert, differential decode or NRZI.
• Eye diagram closed. The symbol rate is not right or the selection includes noise; tighten the selection.
• Audio is silent. Select a time and a band that contains the signal before demodulating.
• Constellation spins. Tick carrier recovery.

Tips

• Make a wide, short recording, then select exactly the burst you care about.
• Export the selection as SigMF so you can open it again or share it with the centre frequency kept.
• Use Find with a known preamble to prove that the bit order is right.

Legal and privacy notes

Analysing a recording is passive. What you decode from signals may be protected by law; only keep or publish recordings you may.

Under the hood

The spectrogram is a short-time FFT; the measurements use a Welch-averaged spectrum, an edge detector against the noise floor and a symbol-rate estimator. Demodulators: envelope and threshold (ASK/OOK), FM discriminator (FSK, MSK, GMSK), two-tone correlators (AFSK), and coherent mixing with carrier and clock recovery (PSK).

Related

Mode page: Signal workbench | Record IQ then the Signal workbench | Signal samples | Spectrum explorer | Settings and files