Guide Time machine

Rewind (time machine)

The "Rewind" button in the diagnostics row keeps the last seconds of the radio's raw samples in memory, so the signal from just before something happened can be saved as well as what comes after it.

• Memory only. The ring is 8-bit I/Q (2 bytes per sample) in RAM. "Reaches back" and "Memory limit" set its size; the memory limit wins, and the window shortens to fit (the window is shown). 10 Msps at 512 MB reaches back about 25 s.
• Samples kept. 1/2 ... 1/16 averages every 2 ... 16 samples before storing: the window is that many times longer, but only the middle part of the band (1/N of it) survives, so use it for a channel at the centre.
• A save writes onair_rewind-<mode>_<MHz>_<rate>_<time>.cs8 (plus a .sigmf-meta and .sigmf-data link when SigMF is on) into the recordings folder: "Save the last (s)" before the trigger and "Then keep recording (s)" after it. The slider's bar shows how much history there is and how much of it a save keeps. A file is never bigger than 4 GB, and a save is refused if the disk has less room than the file needs.
• Triggers. The "Save now" button; the General receiver's squelch opening (tick the box); an automation rule with the action Save the rewind (seconds to keep after the event; any event can be the rule's trigger: a decode, a pass, an aircraft), or save_rewind(seconds) in a script.
• No loss on the live path. The analysis thread only converts a block and copies it into the pre-allocated ring (no lock held while copying, no allocation, no waiting for the disk); a separate thread copies the window out and writes the file. A retune empties the history (it would be at another centre frequency). A file source is not recorded.
• Applies to every mode: it taps the radio's samples before any mode-specific processing.

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See also: Tutorials and guides, FAQ, Troubleshooting.