OFFgrid-SDR 1.3: Panorama, Scan, an auto notch and a lot more

OFFgrid-SDR has moved on quite a bit since version 1.1, so here's a round-up of everything new in versions 1.2 and 1.3, plus a few of the more interesting problems along the way.

As always, it's still one file (index.html), still runs from a pen drive, and still needs no internet. Grab the latest from GitHub: github.com/diddy-boy/OFFgrid-SDR

==== PANORAMA: UP TO 10 MHZ AT ONCE ====

An RTL-SDR stick can only see about 2 MHz at a time. Panorama gets round that by sweeping: it hops the stick across a wider range, takes a spectrum at each stop, and stitches the slices into one picture, up to 10 MHz wide.

• Around me: centred on where you're tuned, 2, 4, 6, 8 or 10 MHz wide

• Three HF slices: 0.5–10, 10–20 and 20–28.8 MHz

• Click any signal in the panorama and the receiver tunes straight to it, ready to listen

• Click Panorama again and you're back exactly where you were

A 10 MHz sweep takes around half a second on a real stick.

How it works, for the curious:

• While sweeping, the stick switches to 10 ms sample blocks (normally 50 ms), so little time is wasted after each retune

• After each hop, the blocks that were already on their way from the old frequency are thrown away, then one fresh block is captured

• Only the clean middle 80% of each slice is used, avoiding the roll-off at the edges and the DC spike in the centre

• The slices are level-matched, so the joins don't show as steps in the noise floor

• The top HF slice stops at 28.8 MHz on purpose: that's where both the RTL-SDR Blog V4 (its upconverter) and the Nooelec (direct sampling) switch their HF input, and those switch-overs are slow. Staying below it keeps every hop fast on both sticks.

==== SCAN YOUR MEMORIES ====

Press Scan, tap the memories you want scanned (they glow green), and set the squelch. Scan hops through them and stops on any with a signal, showing "SCAN: HOLDING ON A SIGNAL", then carries on 3 seconds after the signal ends. Anything you do by hand ends the scan.

It's ideal for airband, PMR446 or a handful of favourite stations.

==== IDENTIFY: WHAT'S ON THIS FREQUENCY? ====

The Identify button (above the waterfall, when online) now offers three lookups for the frequency you're tuned to:

• What's on this frequency? A web search

• Ask AI: asks Google's AI Mode what's usually there, given the frequency, mode and UTC time

• Shortwave radio schedule: below 30 MHz, opens shortwave.live's schedule database for that exact frequency, showing who's scheduled and when

Only the frequency (and for the AI, the mode and time) is sent, and only when you click. Offline, it's simply greyed out.

==== AUTO NOTCH: GOODBYE WHISTLES ====

The new Notch button finds steady whistles (heterodynes from a nearby carrier, the classic shortwave annoyance) and removes each with a very narrow 20 Hz notch, within about a second.

The first design was the textbook one, an adaptive "line enhancer" that predicts the audio and subtracts what's predictable. It removed the whistles, but it also ate the speech: over a few milliseconds a human voice is quite predictable too.

So it was rebuilt as "detect, then notch". Every tenth of a second it takes a fine spectrum of the audio and looks for narrow peaks that stay on exactly the same frequency for about a second. A whistle does; the harmonics of a voice keep moving as the pitch changes, so they're left alone.

The result: a whistle drops by around 40 dB, while the speech is essentially untouched. It handles up to three whistles at once and follows slowly drifting ones.

==== SSB FILTER WIDTHS AND PASSBAND SHIFT ====

• The filter badge now cycles SSB through 1.8, 2, 2.4 and 2.8 kHz with one click

• A passband shift slides the filter up or down without changing the pitch, to dodge a station just above or below

• Both are remembered per band and saved in memories

==== FASTER BAND AND SAMPLE-RATE CHANGES ====

Changing the sample rate (including going into and out of 2 m and airband, which use 2.4 MS/s) used to fully restart the stick: close the USB connection, reopen it, re-initialise the tuner. It now just pauses the sample stream, sets the new rate on the RTL chip and carries on. The pause dropped from about 0.8 s to about 0.1 s.

Testing it turned up a nice race condition. Starting a new sample stream is asynchronous, and if the next change asked it to stop before it had really started, the stop did nothing and the stream started anyway: an orphan stream that nothing would ever stop. Waiting for the start to complete fixed it. It's the kind of bug that only appears when you change things quickly, which is exactly what Panorama and Scan do.

==== A CLEARER TUNING LINE ====

The tuning line used to be amber, and it vanished over strong signals, which show as yellow and orange on the waterfall. It's now white with a thin dark edge, so it shows up over any colour.

==== SMALLER IMPROVEMENTS AND FIXES ====

• Each band button now sets its own mode and tuning step, and remembers your choices. Before, CW's 10 Hz steps could follow you into a broadcast band, so waterfall clicks missed the 5 kHz channels.

• Coming back from 2 m to a shortwave broadcast band no longer leaves you in NFM

• Clicking the display in general coverage (free tuning, e.g. at 252 kHz) no longer complains about band edges

• The squelch is now a clean on/off gate, like a real radio, with a little hysteresis so it doesn't chatter

• The Blanker and Notch are green toggle buttons, matching the other noise filters

• Shorter mode captions, with the full explanations in tooltips

• New demo station on 5.985 MHz, a talk station with a whistle, to try the Notch

• Demo mode no longer serves a moment of the old frequency's signal after a retune

==== THANKS ====

Thanks to everyone who has tried OFFgrid-SDR, and as ever to Jacobo Tarrío, whose webrtlsdr and signals libraries make it possible.

If OFFgrid-SDR is useful to you, there's a "Buy me a coffee" button in the ? help panel.

73!