bandsaunter/README.md

# bandsaunter

A signal scanner, recorder and identifier for RTL-SDR receivers.

Give it any number of frequency ranges — typed in by hand or picked from a
built-in US band plan — and it sweeps them, stops on anything above the noise
floor, records it, and works out what kind of signal it was. CW/Morse is
decoded to text.

```
╭──────────────────────────────── receiver ────────────────────────────────╮
│ Rafael Micro R820T/R820T2   2.048 MS/s   gain auto   +0 ppm              │
╰──────────────────────────────────────────────────────────────────────────╯
╭───────────────────────────────── sweep ──────────────────────────────────╮
│ ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━  step 4/6   146 MHz - 146.666667 MHz  │
│               ▆                      █       ▄▄▄        peak -18.0 dBFS  │
│ recording   cycle 2   hits 3   dropped 0   detections 7   up 0:04        │
╰──────────────────────────────────────────────────────────────────────────╯
╭──────────────────────────────────────────────────────────────────────────╮
│ REC 146.52 MHz  [nfm]  ███████████░░░░░░░░░  11.4/30s  SIGNAL  SNR 27 dB │
╰──────────────────────────────────────────────────────────────────────────╯
╭───────────────────────────── recorded signals ───────────────────────────╮
│ 19:38:43      460.025 MHz   8.2s   27.0  P25 Phase 1 C4FM digital voice  │
│ 19:38:43        144.1 MHz   8.2s   27.0  CW / Morse at 18 WPM  "VVV DE…" │
│ 19:38:43       146.52 MHz   8.2s   27.0  Narrowband FM voice (CTCSS 100) │
╰──────────────────────────────────────────────────────────────────────────╯
```

## Install

### From a package (Debian, Ubuntu, Mint)

```bash
./packaging/build-deb.sh              # writes dist/bandsaunter_<version>_all.deb
sudo apt install ./dist/bandsaunter_*.deb
```

apt pulls in every dependency itself, and the package blacklists the DVB-T
driver that would otherwise claim the receiver. Nothing else to do.

### From source

```bash
sudo apt install rtl-sdr librtlsdr0 espeak-ng    # Debian, Ubuntu, Mint
pip install -e .
```

## Dependencies

Everything required is packaged in Debian, Fedora and Arch, so nothing has to
be built.

| | Package | Debian/Ubuntu | Fedora | Arch | Needed for |
|---|---|---|---|---|---|
| **required** | librtlsdr | `librtlsdr0` | `rtl-sdr` | `rtl-sdr` | talking to the receiver at all |
| **required** | NumPy | `python3-numpy` | `python3-numpy` | `python-numpy` | all signal processing |
| **required** | SciPy | `python3-scipy` | `python3-scipy` | `python-scipy` | filters, resampling, spectra |
| **required** | Rich | `python3-rich` | `python3-rich` | `python-rich` | menus and the live display |
| **required** | PyYAML | `python3-yaml` | `python3-pyyaml` | `python-yaml` | settings file and profiles |
| *recommended* | eSpeak NG | `espeak-ng` | `espeak-ng` | `espeak-ng` | clearer spoken timestamps |
| *optional* | rtl-sdr tools | `rtl-sdr` | `rtl-sdr` | `rtl-sdr` | `rtl_test` and friends for diagnosis |
| *optional* | a speech recogniser | **pip only** | **pip only** | AUR | transcribing speech to text |
| *optional* | Matplotlib | `python3-matplotlib` | `python3-matplotlib` | `python-matplotlib` | nothing yet; reserved for plots |

Two notes on the optional ones:

**eSpeak NG is a recommendation, not a requirement.** Without it the spoken
timestamps come from a built-in formant synthesiser, so that feature works on
a machine with nothing else installed. With it they are clearer and render
about three times faster.

**No speech recogniser is packaged for Debian.** `faster-whisper`, `vosk` and
the pocketsphinx Python bindings are all absent from the archive, so
transcription can only be installed with pip:

```bash
pip install faster-whisper     # best on radio audio, ~120 MB
pip install vosk               # ~10 MB plus a 40 MB model, weaker on noise
bandsaunter transcribe --engines
```

That is why transcription is `Suggests:` rather than `Depends:` in the
package. Debian Policy forbids anything in the archive from requiring
software outside it, and a `postinst` that reaches out to PyPI would break
offline and reproducible installs — so a package simply cannot pull these in.
Transcription is therefore off by default and reports plainly when no
recogniser is present, rather than the install failing or the feature
appearing broken.

Mixing the two is nonetheless fine here. Modern Debian marks the system
Python as externally managed (PEP 668), so a pip install lands in your user
site directory:

```bash
pip install --user faster-whisper      # ~/.local/lib/python3.x/site-packages
```

which is on `sys.path` for the system interpreter. A bandsaunter installed
from the `.deb` into `/usr/lib/python3/dist-packages` picks it up with no
further configuration — verified, not assumed. A virtual environment works
too, as long as bandsaunter runs inside it.

If you would rather keep everything under apt, the only route is packaging a
recogniser for Debian yourself. That is a real undertaking for whisper: it
would mean packaging ctranslate2, tokenizers, onnxruntime and their
dependencies, several of which are large C++ or Rust projects. It is why
none of them are there.

### Other distributions

```bash
sudo dnf install rtl-sdr python3-numpy python3-scipy python3-rich \
                 python3-pyyaml espeak-ng                    # Fedora
sudo pacman -S rtl-sdr python-numpy python-scipy python-rich \
               python-yaml espeak-ng                          # Arch
brew install librtlsdr espeak-ng && pip install -e .          # macOS
```

### Letting your user reach the receiver

The DVB-T television driver claims RTL dongles on sight and has to be kept
away from them. The `.deb` does this for you; from source:

```bash
echo 'blacklist dvb_usb_rtl28xxu' | sudo tee /etc/modprobe.d/blacklist-rtlsdr.conf
sudo rmmod dvb_usb_rtl28xxu        # or just unplug and replug the receiver
```

If the device is found but cannot be opened, your user needs permission for
it. Most distributions ship a udev rule with `rtl-sdr`; failing that:

```bash
echo 'SUBSYSTEM=="usb", ATTRS{idVendor}=="0bda", ATTRS{idProduct}=="2838", MODE="0666"' \
  | sudo tee /etc/udev/rules.d/20-rtlsdr.rules
sudo udevadm control --reload-rules && sudo udevadm trigger
```

### Check it worked

```bash
bandsaunter devices --test      # opens the receiver and captures a test block
bandsaunter scan -b 2m --simulate   # exercises everything without hardware
```

## Versioning

Releases are named for the day they were made and a revision within that day:

```
2026-08-21_01      first build on the 21st
2026-08-21_02      second build the same day
2026-09-01_01
```

The revision is padded to two digits so that versions sort correctly as text —
without it, revision 10 would sort before revision 2.

Packaging tools cannot use that form directly, so it is converted at the edge
rather than kept as a second version string that could drift:

| Where | Form | Why |
|---|---|---|
| the program, `--version` | `2026-08-21_01` | what you asked for |
| pip, `pyproject.toml` | `2026.8.21.1` | PEP 440 forbids dashes and underscores in a release |
| dpkg, the `.deb` | `2026.08.21.01-1` | Debian versions may not contain underscores |

`bandsaunter/__init__.py` holds the date and revision; the other two forms are
derived from it, and the test suite checks that all three describe the same
release and that both pip and `dpkg --compare-versions` order them correctly.

## Quick start

```bash
bandsaunter                                     # the menus: set up and scan
bandsaunter scan -b 2m -b marine-vhf            # band-plan presets
bandsaunter scan -r 144M-148M -r 420M-450M      # your own ranges
bandsaunter scan -b 2m --simulate               # try it without hardware
```

## Two ways to drive it

Everything is available both ways. Run `bandsaunter` with no arguments for the
menus, or pass flags for scripting — the two are generated from the same
table of settings, so neither can offer something the other cannot.

```
  1  Frequency ranges       3 configured
  2  Band plan              107 US presets
  3  Settings               record no limit, hang 6s, squelch +12 dB, keep voice, cw
  4  Saved settings and profiles
  h  Help
  s  Start scanning
  q  Quit
```

Settings are grouped, show their current value against the built-in default,
and carry their own help:

```
   #  setting                 value        what it does
   1  Record for *            no limit     longest one signal may hold the receiver
   2  Wait for quiet *        6 s          quiet time before the sweep resumes
   3  Absolute limit          900 s        ceiling on one capture, even when 'Record for' is 0
* differs from the built-in default

Number to change it, ?N for help on one, d to reset the group, blank to go back.
```

`?2` explains a setting in full, including the command-line flag that does the
same thing. Typing a search term instead of a number finds settings by any
word in their name or description — `voice score` finds the speech threshold.

Values may be typed with their units: `5 s`, `2.048 MHz`, `12 dB`, `48k`, or
`no limit` for the settings that accept 0.

## Settings that persist

Settings are saved to `~/.config/bandsaunter/config.yaml` and picked up by every
later run. Save them from the menus (**4 → s**) or from the command line:

```bash
bandsaunter config                          # open the settings menu
bandsaunter config hang_seconds=6 record_seconds=0   # set and save directly
bandsaunter config --show                   # every setting, with defaults
bandsaunter config --describe hang_seconds  # explain one in full
bandsaunter config --path                   # where the file lives
bandsaunter config --reset                  # back to defaults
```

Three layers apply in order, each overriding the last:

1. the saved settings file
2. a named profile, if `--profile` is given
3. any flags on the command line

So a saved squelch of 12 dB stays in force while `--hang 1.5` overrides just
the hang for one run. `--no-config` ignores the file entirely; `--save` stores
the resulting settings as the new default.

Named profiles live beside it in the same directory:

```bash
bandsaunter scan -b 2m -b 70cm --record 0 --hang 6 --save-profile local
bandsaunter scan -p local
bandsaunter profiles
```

## Entering frequencies

**By hand** — repeat `-r` as many times as you like; there is no limit on the
number of start/end pairs.

```bash
bandsaunter scan -r 144M-148M -r 462.5M-467.8M -r 929M-932M
```

A range is `start-end`, with optional `/step` and `@mode`:

| Form | Meaning |
|---|---|
| `144M-148M` | explicit start and end |
| `144-148M` | the unit carries over to the left end |
| `146.52M` | a single frequency |
| `144M-148M/25k` | with a channel step |
| `144M-148M/25k@nfm` | and a forced demodulator |

Units may be written `144M`, `144 MHz`, `144000k`, or plain Hz. A bare number
below 10000 is read as MHz, so `-r 162.4-162.55` does what you expect.

**From the US band plan** — 107 presets across 18 categories:

```bash
bandsaunter bands --categories          # list categories
bandsaunter bands --category Aviation   # everything in one category
bandsaunter bands pager                 # search
bandsaunter scan -b gmrs -b railroad -b noaa-weather
```

Each preset carries its own channel spacing, demodulator and bandwidth, so
`-b marine-vhf` scans 25 kHz channels in NFM while `-b fm-broadcast` uses
200 kHz WFM, without being told.

Each amateur band also has a **complete** entry that covers the whole band and
picks the demodulator per segment, because a band is not one mode:

```bash
bandsaunter scan -b 2m-complete
```

scans 144-148 MHz as CW below 144.1, SSB to 144.3 and FM above it, so a CW
beacon at the bottom is decoded to text while a repeater at the top is
demodulated as FM — in one sweep. There is one for every band from 160 m to
33 cm: `160m-complete`, `80m-complete`, ... `70cm-complete`, `33cm-complete`.

Where an amateur band overlaps another service the amateur reading wins inside
a complete-band sweep — 433 MHz is treated as 70 cm rather than as the ISM
band it shares — while scanning `-r 433.9M` on its own still treats it as ISM.

A few presets stand for a *set* of others, so scattered segments can be picked
in one go:

```bash
bandsaunter scan -b all-cw --record 0 --hang 6
```

`all-cw` covers every CW allocation in the plan — 160, 80, 40, 30, 20, 17, 15,
12, 10, 6 and 2 metres — as eleven separate ranges rather than one span from
1.8 to 144 MHz. That is 1.25 MHz of spectrum in total, so a full pass takes
under a second and CW gets decoded to text as it turns up. Direct sampling
switches itself on for the HF segments and off again from 12 m upward; the HF
part needs an HF antenna to be worth anything.

The menus do both: browse the band plan by category, or type in start/end
pairs one after another. Ranges can be listed, removed, toggled on and off,
and have their demodulator changed from the ranges menu.

## The two dwell settings

These are the settings that decide how the scanner behaves when it finds
something:

| Setting | Flag | What it does |
|---|---|---|
| Record for X seconds before continuing | `--record 30` | The longest a single signal may hold the receiver. `0` means stay as long as it keeps transmitting. |
| Wait for X seconds of no signal before continuing | `--hang 3` | How long the channel must stay quiet before the sweep resumes. **Gaps shorter than this are recorded straight through.** |

```bash
bandsaunter scan -b 2m --record 30 --hang 3
```

Whichever comes first wins, and that is worth being clear about: **`--record`
overrides `--hang`.** A transmission still in progress at the record limit is
cut off there, however long the hang time is. If a recording keeps ending at
exactly 30 seconds, that is the default record limit doing it, not the hang —
set `--record 0`. The scan reports it when this happens.

Both are measured in **samples**, not wall-clock time, so a 30 second setting
produces a 30.0 second recording.

Supporting settings:

- `--min-record 0.5` — discard anything shorter, so brief noise spikes leave
  nothing behind on disk.
- `--revisit 8` — ignore a frequency for this long after recording it, so a
  busy repeater does not monopolise the sweep.
- `--max-record 900` — absolute ceiling on one capture, applied even when
  `--record` is 0.
- `--threshold 8` — squelch, in dB above the measured noise floor.

### Capturing both sides of a conversation

`--hang` is what holds a recording open across the natural pauses in two-way
traffic. Set it longer than the gap between overs and the whole exchange lands
in one file:

```bash
bandsaunter scan -b 2m --record 0 --hang 6
```

`--record 0` is the important half. Without it the per-signal cap cuts the
exchange off mid-sentence no matter what the hang time is — the default 30
seconds is a common surprise. `--max-record` (default 900 s) still bounds an
unlimited capture so nothing runs away.

To make it the permanent default:

```bash
bandsaunter config record_seconds=0 hang_seconds=6
```

"Quiet" means *no real signal*, not merely a closed squelch. Silence, static
and interference all count towards the timer, so a burst of noise during a
pause does not reset it and park the receiver on a finished conversation.
Recognising that a signal carries nothing takes a couple of seconds of
evidence, so expect the tail to run a little past `--hang` in that case.

Once a capture has produced real content it is never abandoned as noise, since
a quiet spell between overs would otherwise throw the conversation away.

## Only real signals get recorded

A power threshold cannot tell a transmission from a hump of interference, so
every capture is checked for *content* before it is kept. Recording happens
only for:

| Category | What it means |
|---|---|
| `voice` | speech structure in the demodulated audio: a pitch track in the 70-400 Hz range that drifts the way intonation does, pauses between phrases, syllable-rate envelope modulation, and formants that move |
| `cw` | a keyed carrier whose timing resolves as Morse |
| `digital` | an identified keying scheme: discrete FSK levels, an M-PSK phase line, or on-off keying -- corroborated by a symbol rate |

Everything else -- static, hum, switch-mode power supply harmonics, clock
spurs, bare carriers -- is discarded, and the files it wrote are deleted.

The check runs *while* the capture is still going, so interference is dropped
after a second or two instead of holding the receiver for the whole record
time.

```bash
bandsaunter scan -b 2m                          # voice, CW and digital (default)
bandsaunter scan -b 2m --accept voice           # voice only
bandsaunter scan -b ism-433 --accept digital    # data bursts only
bandsaunter scan -b 2m --keep-carriers          # also keep unmodulated carriers
bandsaunter scan -b 2m --keep-everything        # no content check at all
```

Tuning knobs: `--min-voice-score` (0-1, default 0.45) sets how speech-like
audio must be; `--min-signal-score` sets the confidence needed to keep
anything; `--verify-max` caps how long a contentless capture is given to prove
itself.

### What makes this hold up against interference

Static is good at imitating most of the things that look like structure, so
each test is built so that noise cannot pass it:

- **A pitch track that moves gates the voice score**, rather than contributing
  a share of it. Dynamics, syllable-rate modulation and energy in the voice
  band are all things hiss does too; weighted alongside voicing they were
  enough to carry noise over the line on their own. Speech is the only thing
  here that produces a pitch period that drifts, so nothing is called voice
  without one. A steady tone or mains hum has a perfectly stable "pitch" and
  is rejected for exactly that reason. Measuring drift needs several voiced
  frames, so the requirement eases for a short over that cannot supply them;
  the steady-tone guard still applies.
- **A symbol rate has to hold still across the capture.** The estimator always
  returns its best peak, so on noise it reports one rate for the first half of
  a capture and a different one for the second. Real data keeps one.
- **Keying has to land on a grid.** On/off contrast alone is not evidence: a
  signal fading across the squelch produces plenty of it, with run lengths
  that fit no symbol period at all.
- **A phase line has to be created by the exponentiation.** An unmodulated
  carrier -- including the gaps between phrases on an FM channel -- already
  has a line at every power, and would otherwise look like textbook PSK.
- **The demodulator is chosen from the signal**, not from the band plan, by
  probing briefly before recording. An AM signal inside a
  band listed as FM would otherwise be recorded through the wrong detector,
  giving audio that is useless to listen to and impossible to judge. Three
  ratios settle it — how much the envelope varies, how far the tone swings,
  and how much power sits in a carrier — because those hold steady over a
  fraction of a second whatever is being said. Running the full classifier on
  so short a probe was tried and is not reliable: speech makes any modulation
  look bursty over half a second, and AM came back as on-off keying while FM
  came back as AM. The probe is played into the recording rather than
  discarded, so a short over does not lose its opening.
- **Content is judged only on the audio that was actually recorded**, and only
  on the stretches the squelch called signal. Demodulating two ways and
  keeping whichever scored higher is cherry-picking, and on noise one of the
  two always flatters it.

The gate needs roughly two seconds of audio to judge speech reliably, so pair
it with `--record 5` or more rather than very short capture limits.

### Keeping up with the radio

An RTL-SDR only delivers samples while the host is actively reading. Anything
that arrives while the program is busy demodulating is discarded by the
driver, and a recording then holds *less* than really went by -- which plays
back too fast.

Two things keep that from happening:

- **Captures stream asynchronously.** A ring of USB transfers stays queued in
  a background thread, so the dongle is never waiting for the host. Sweeping
  still uses plain reads, because each dwell is an independent snapshot and a
  gap between them costs nothing.
- **The signal path is fast enough to keep up.** Decimation computes only the
  samples that survive, rather than filtering at the input rate and throwing
  away seven of every eight outputs; the quarter-rate local oscillator is the
  four-step cycle 1, -j, -1, +j and needs no trigonometry; and pitch tracking
  runs through the FFT instead of a direct autocorrelation per frame. Together
  those took the capture loop from 65% of the real-time budget to under 10%.

If the host does fall behind anyway, the scan reports how many samples were
lost rather than silently producing a fast recording.

### Why the threshold is what it is

The sweep uses peak-hold, which keeps the largest value each FFT bin reached
during the dwell. That finds bursty traffic that averaging would bury -- but
it also means noise alone rides several dB above the measured floor. On this
hardware, empty spectrum reaches 5-9 dB above a percentile floor with nothing
transmitting.

So `--threshold` is a margin over *noise*, not over the floor: the offset that
noise alone clears is computed from the detector (segment count and bin count)
and added automatically. A threshold of 8 means 8 dB of real headroom. It is
deliberately not measured from the spectrum -- a spread estimated from the
data reads five times higher across the packed broadcast FM band than on empty
spectrum, which would suppress exactly the stations you are looking for.

## Signal identification

Every recording is classified from its own IQ. The classifier measures
occupied bandwidth, envelope statistics, discriminator levels, phase
behaviour, spectral flatness and symbol rate, then combines those with the
frequency to name the signal:

| Family | Recognised as |
|---|---|
| Analogue voice | Narrowband FM (with CTCSS tone or DCS), wideband FM (stereo pilot detected), AM, SSB (USB/LSB) |
| CW | Keyed carrier, **decoded to text** with the speed in WPM |
| Digital voice | P25 C4FM, DMR (TDMA burst structure), NXDN, D-STAR |
| Data | POCSAG and FLEX paging, ACARS, AIS, APRS/AFSK1200, 2-FSK and 4-FSK, BPSK/QPSK/8-PSK |
| Other | Unmodulated carriers, OOK/ISM devices, ADS-B and UAT, DME/TACAN pulses, wideband OFDM/cellular |

Each result carries a confidence and the reasoning behind it:

```
146.520038 MHz   3.0s  SNR 27.6 dB  Narrowband FM voice (CTCSS 100.0 Hz) (88%)
                       4.9 kHz wide, 0.9 kHz rms deviation
```

Low SNR reduces confidence rather than producing a confident wrong answer.

### CW / Morse

Keyed carriers are decoded to text. The speed is measured from the signal, so
nothing has to be configured, and anything from about 8 to 40 WPM reads
reliably:

```
144.1 MHz  12.0s  SNR 50.8 dB  CW / Morse at 18 WPM  CW "VVV DE W1AW FN31"
```

The decoder runs its own CW detector over the captured IQ, so Morse is found
even when the recording itself was made in FM or SSB.

## Output

Everything lands in one directory, named
`frequency--yyyy-mm-dd_hour_minute_second-modulation.wav`:

```
0014.058000MHz--2026-08-21_20_35_41-cw.wav
0098.299255MHz--2026-08-21_20_24_01-wfm.wav          demodulated audio
0098.299255MHz--2026-08-21_20_24_01-wfm.json         identification, features, timing
0098.299255MHz--2026-08-21_20_24_01-wfm.cf32         raw IQ         (with --iq)
0098.299255MHz--2026-08-21_20_24_01-wfm.sigmf-meta   SigMF sidecar  (with --iq)
0098.299255MHz--2026-08-21_20_24_01-wfm_transcription.txt  (with --transcribe)
0098.361991MHz--2026-08-21_20_24_18-wfm.wav
0146.520000MHz--2026-08-21_20_31_02-nfm.wav
1090.000000MHz--2026-08-21_20_38_12-raw.wav
scan_log.jsonl                                       one line per hit
scan_log.csv                                         the same, as a spreadsheet
```

Frequency leads and is padded to four digits, so a plain directory listing
sorts by frequency across the whole tuning range — unpadded, 1090 MHz would
sort before 146 MHz. Each channel's captures group together with the
timestamp ordering them. Every artefact of one capture shares a stem, and the
modulation suffix is what the signal was *identified* as, so the file is
renamed once the capture has been analysed.

### Where files go

The directory is asked for the first time bandsaunter is run and remembered
afterwards:

```
Recordings, transcripts and the scan log are all written to one directory.
Where would you like them?

  recordings directory (~/bandsaunter):
```

It is an ordinary setting, so it can be changed at any time:

```bash
bandsaunter config output_dir=~/somewhere-else
bandsaunter scan -b 2m -o /tmp/just-this-once
```

The question is only asked when there is someone to answer it: a scan run from
a script or with output redirected uses the default rather than blocking, and
`--no-config` skips it entirely.

### One file per frequency

With `--combine`, each frequency gets a single file that every later reception
is appended to, so a whole watch on a channel plays back as one recording:

```bash
bandsaunter scan -b 2m --combine --record 0 --hang 6
```

```
recordings/
  146.520000MHz.wav      every transmission heard on 146.520, in order
  147.100000MHz.wav
  146.520692MHz--2026-08-21_13_21_20-nfm.json   what each capture was
  scan_log.csv
```

Each transmission is preceded by **its date and time read aloud**, so the file
says when everything was heard without needing the log open. Receptions within
`--combine-tolerance` (6.25 kHz by default) of each other count as the same
channel, which covers the few hundred hertz a detection wanders by.

The file is rewritten to stay valid after every append, so it can be opened
and played while the scan is still running, and a scan stopped part-way still
leaves a working recording. A later run continues the same file rather than
starting a new one.

By default the per-transmission WAVs are removed once they have been added, so
each recording exists in one place; `--keep-individual` keeps both. The `.json`
describing each capture is written either way.

`--no-announce` leaves the timestamps out, and `--announce-frequency` reads the
frequency out as well.

### Where the speech comes from

The announcements use an installed text-to-speech program if there is one
(`espeak-ng`, `espeak`, `pico2wave`, `flite`, `say`) and fall back to a
built-in formant synthesiser otherwise, so the feature works with nothing else
installed. `--announce-engine builtin` forces the built-in one; the scan plan
reports which is in use.

The two are given different wording. An installed engine gets ordinary text,
where punctuation is what produces the phrasing — and the obvious spellings
are traps: espeak-ng reads `14:38:05` as "fourteen thirty, eight zero five",
and an ISO date as "two thousand and twenty six dash zero eight dash twenty
one". It is given `August 21, twenty twenty six, at 14 38 and 05 seconds`
instead. The built-in synthesiser gets the word list it has pronunciations
for. Both are level-matched, so switching between them does not change how
loud the announcements sit against the recordings.

It only has to say numbers, month names and a handful of words, which makes
direct synthesis practical: a glottal source through a cascade of three
formant resonators, with the formant tracks interpolated between phonemes.
The test suite checks by LPC analysis that all ten vowels come out within
130 Hz of their intended first formant and 250 Hz of their second — summing
the resonators in parallel rather than cascading them loses the first formant
entirely and makes every vowel sound the same.

### Transcribing speech to text

With `--transcribe`, anything the content check identified as **voice** is
passed to a speech recogniser and the words are written beside the recording:

```bash
bandsaunter scan -b 2m --transcribe
```

```
146.520000MHz--2026-08-21_12_18_38-nfm.wav
146.520000MHz--2026-08-21_12_18_38-nfm_transcription.txt
146.520000MHz--2026-08-21_12_18_38-nfm.json
```

Only voice is transcribed — running a recogniser over Morse or a data burst
costs seconds and produces nothing. CW is decoded separately and appears in
the metadata as text already.

**A capture with nothing recognisable in it produces no file.** Music, a
carrier with an open mic, a fragment too short to make out: nothing is
written, rather than a directory of placeholders. The transcript is also
copied into the capture's `.json`, which names it only once it exists — so the
metadata never points at a file that was never created. The scan reports the
tally at the end:

```
  2 transcript(s) written, 1 with no recognisable speech
```

Recognition takes seconds per capture, far longer than a capture itself, so it
runs on its own thread and the scan never waits for it; anything still queued
is finished when the scan stops. When recordings are being combined by
frequency there is one transcript per frequency too, each line stamped with
the time:

```
[2026-08-21 12:18:38] this is what the first transmission said
[2026-08-21 12:24:02] and this is the second
```

### Getting a recogniser

Unlike the spoken announcements, this needs an installed engine — recognition
depends on a trained model, so there is no built-in fallback.

```bash
bandsaunter transcribe --engines      # what is installed
pip install faster-whisper        # the recommended one
```

| Engine | Notes |
|---|---|
| `faster-whisper` | best on radio audio; ~120 MB of dependencies, model downloads on first use |
| `whisper` | the original; heavier |
| `whisper-cli` | whisper.cpp, no Python dependencies |
| `vosk` | ~10 MB plus a 40 MB model, fully offline, but weaker on noisy audio |
| `pocketsphinx` | tiny; poor on radio audio |

The difference is easy to measure. Both engines on the same 26-second
off-air recording:

```
faster-whisper   3.4s   "August 21, 2026, at 13.42 and 28 seconds,
                          96.108 megahertz, 13.42 and 47 seconds, ..."
vosk            11.2s   "august twenty one twenty twenty six at thirteen
                          forty two i'm twenty eight seconds ... forty
                          family factories ..."
```

Whisper is both more accurate and three times faster, and it punctuates.
Vosk's advantage is size and that it needs nothing after its model is
downloaded once.

`--transcribe-model` selects the size (`tiny.en`, `base.en`, `small.en`,
`medium.en`) and `--transcribe-language` fixes the language — worth setting,
since on a short noisy clip automatic detection often guesses wrong and
returns nonsense in another language.

Existing recordings can be transcribed after the fact:

```bash
bandsaunter transcribe recordings/                 # every WAV in a directory
bandsaunter transcribe one.wav --stdout
```

Re-examine anything later:

```bash
bandsaunter analyze recordings/2026-08-19/.../iq.cf32     # identify
bandsaunter analyze recordings/2026-08-19/.../audio.wav   # decode CW
```

## Live controls

The display sizes itself to the terminal, giving up the spectrum row, then the
hit list, then the key hints as space runs short. A frame taller than the
terminal cannot be redrawn in place, so an oversized one would leave a copy of
itself behind on every refresh.

For the same reason the driver's own messages are suppressed while a scan
runs: librtlsdr writes them straight to file descriptor 2 from C — including
`Allocating 15 zero-copy buffers` on *every* capture — and they draw over the
display and break its cursor tracking. `bandsaunter devices` still shows them,
since that is the command to run when something is wrong, and
`BANDSAUNTER_DRIVER_MESSAGES=1` restores them everywhere.


| Key | Action |
|---|---|
| `q` | stop |
| `p` | pause / resume |
| `s` | skip this signal, resume sweeping |
| `l` | lock out this frequency for the rest of the run |
| `+` / `-` | adjust the squelch threshold |

## Built-in help

Press `h` in the menus for topics covering setup, how the sweep works, why
nothing (or too much) is being recorded, capturing conversations, where files
go, HF reception and the keys available during a scan. Typing a setting name
there explains that setting instead.

From the command line, `bandsaunter config --describe <setting>` does the same,
and `bandsaunter scan --help` lists every flag grouped the same way as the menus.

## HF

Frequencies below 24 MHz need direct sampling, which most RTL-SDR dongles
support on the Q branch. It is selected automatically:

```bash
bandsaunter scan -b 40m-cw --record 60      # 40 m CW, decoded to text
bandsaunter scan -b am-broadcast
```

You will need an HF antenna; the tuner is bypassed in this mode, so there is
no front-end filtering or gain.

## How the sweep works

- The band is covered in steps of `sample_rate x usable_fraction / 2`. The
  local oscillator is parked *below* the span each step covers, so the
  RTL2832's DC spike never lands inside the frequencies being searched.
- The noise floor is measured per FFT bin as a sliding low percentile, which
  follows the receiver's passband shape and steps over signals. There is no
  warm-up period, and a station that transmits constantly does not learn
  itself into the floor.
- The sweep uses peak-hold rather than averaging across each dwell, so bursty
  traffic — CW, packet, a short over — is not averaged into the noise.
- On a hit, the receiver retunes with a quarter-rate LO offset (moving the DC
  spike off the signal), probes once at ~60 Hz resolution to measure the real
  occupied bandwidth, and picks the demodulator from that plus the band plan.

## Without hardware

`--simulate` swaps in a synthetic receiver carrying one of each interesting
signal type, which is also what the test suite runs against:

```bash
bandsaunter scan -r 144M-148M --simulate
```

## Testing

```bash
python -m pytest
```

Covers DSP invariants, frequency parsing, the classifier against synthetic
signals at several SNRs and random seeds, Morse decoding from 8 to 40 WPM, the
voice detector against synthetic speech and against noise, tones and hum, and
full scan runs through the simulator checking that `--record` and `--hang` are
obeyed, that static and bare carriers are never written to disk, and that
audio, IQ and metadata are correct.

The simulator's voice transmitters carry synthesised speech -- glottal pulses
through moving formants, compressed the way a real transmitter compresses,
then gated into syllables and phrases -- because sine tones would not exercise
the speech detector at all.

## Legal note

Receiving is not the same as being allowed to use or divulge what you hear. In
the US, the ECPA prohibits intercepting cellular and other private
communications, and rebroadcasting or acting on what you receive is separately
restricted. Check your local rules.