Listen the way the tools that work on this band listen

Still nothing, with rtl-433 receiving the same sensors on the same aerial from
a different receiver.  That settles where the fault is not: not the aerial,
not the sensors, not the band.  So the sensible thing is to stop differing
from the configuration known to work on that aerial, and this differed from it
in three ways, every one of them mine.

It tuned a quarter of a megahertz to one side of 433.92 and shifted the signal
back in software, to keep the receiver's own spike off a signal that works by
being switched off.  That is a real effect and avoiding it this way is a bad
trade: at the sample rate this ran at, the shift is followed by a filter, and
a filter narrow enough to reject the spike is narrow enough to lose a
transmitter that has drifted -- or to lose the signal outright if a dongle
presents its samples the other way round, which is not a thing to depend on.
The spike is a steady addition to the envelope and a burst rises clear of it.
Tuning straight at the sensors now, which is what the established tools do.

It sampled at a megasample a second where a quarter of one is plenty: the
shortest pulse these send is two hundred microseconds, which is fifty samples
at the lowest rate a dongle will do.  The extra rate bought nothing but the
room for that filter to exist in.  At 250 kS/s nothing after the mixer is
narrower than the band, so the offset now does nothing at all whatever it is
set to, and says so.

And it turned on the RTL2832's digital gain control along with the tuner's.
The two pump: the gain winds up through the silence between one burst and the
next, lifting the noise towards the signal and squeezing the very difference
the burst detector works on.  It matters here in a way it does not for
aircraft, where a frame is found by correlating a preamble over microseconds
rather than by comparing a burst with the quiet around it.

Three more faults found while going over the rest of it, all the same mistake
in different clothes -- treating the middle of a distribution as though it
were the quiet part of one.

The check that skips an empty block measured the peak against the median.  A
recording that is mostly burst measures its own burst against its own burst,
finds no difference and is discarded as silence, which is what happened to
every short capture.  The gate's scatter had the same trouble one level down
and could come out above the peak, which is the one setting that cannot be
right, so it is now capped below it.

And the rule deciding where one message ends keyed on the middle gap in a
burst.  Where a one is drawn as a gap three times a zero and most of the bits
are zeroes, the middle gap is the short one, twice it still falls inside the
message, and every one-bit ended a burst -- the message coming apart into
pieces of three pulses.  It keys on the widest gap now, which is a fact about
the message rather than about the data it happened to carry.

The pieces of a message are also put back together after being sliced rather
than before.  Grouping has to be tight, because the group is what fixes the
threshold and a group holding two sensors of unequal strength fixes it on the
louder; but the gaps inside one message run from two hundred microseconds on
the newer sensors to four thousand on the oldest, and a grouping tight enough
for the first tears the second into a bit at a time.  So each piece is
measured at its own amplitude and joined to its neighbours afterwards, and
anything that comes out longer than the longest message there is gets cut at
its largest gaps.

Measured rather than argued: across four hundred and eighty combinations of
pulse and gap timing, 464 now read where 417 did; across twenty-one gap-keyed
combinations, 18 where 12 did; and eighty seconds of receiver noise still
yields nothing at all.

--from-iq FILE reads a saved capture instead of the receiver, so a recording
made where the aerial is can be worked on anywhere, as many times as it takes.
Everything downstream of the dongle is the real thing, which is what tells a
receiver problem and a decoder problem apart.  --save-iq writes the settings
beside the samples, a file of raw samples with no record of its rate being
unreadable by anything.  The invented garden now waits like a dongle instead
of running as fast as the machine allows, which it should have done from the
start: --seconds meant nothing against it and a capture came out fifty times
too large.

Full suite 2331 passed; this work checked against sixteen deliberately broken
builds, two of which it survived until the tests were made to catch them, and
one change was removed for being unable to earn a test at all.  Built as
2026-09-07_03.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016PsWPTweCT6pwxKngvVxcg

packaging/bandsaunter.1

@@ -1,5 +1,5 @@
 .\" Generated by packaging/make-man.py -- do not edit by hand.
-.TH BANDSAUNTER 1 "2026-09-07" "bandsaunter 2026-09-07_02" "User Commands"
+.TH BANDSAUNTER 1 "2026-09-07" "bandsaunter 2026-09-07_03" "User Commands"
 .SH NAME
 bandsaunter \- scan, record and identify radio signals with an RTL-SDR
 .SH SYNOPSIS
@@ -2264,18 +2264,24 @@ and one byte of sum is all that is left. Corroboration does not help either,
 the copies of a message being identical. What gives that window away every
 time is that it ends a whole byte before the burst does.
 .SS Getting it off the air
-The receiver is tuned a little to one side of 433.92 MHz, because every
-RTL-SDR puts a spike of its own at whatever it is tuned to, and a spike
-sitting on top of a signal that works by being switched on and off is the one
-thing that stops it being off. The sensors are shifted back to the middle in
-software, which puts the spike out at the edge instead.
-.B \-\-offset 0
-tunes straight at them, which is worth trying once to see what the spike was
-costing.
+The receiver is tuned straight at 433.92 MHz, at 250 kS/s, with the tuner's own
+gain control doing its job and the RTL2832's digital AGC left off, which is
+what the established tools for this band do.
 .PP
-A running average of the complex samples then rejects the spike, and only
-after that is the magnitude taken \[em] filtering before detection rather than
-after is what keeps the neighbours out of the envelope of the sensor.
+The digital AGC is off because it pumps: it winds the gain up through the
+silence between one burst and the next, lifting the noise towards the signal
+and squeezing the difference this depends on. It matters here in a way it does
+not for aircraft, where a frame is found by correlating a preamble over a few
+microseconds rather than by comparing a burst with the quiet around it.
+.PP
+.B \-\-offset
+tunes to one side of the sensors and shifts them back in software, which
+avoids the spike every RTL-SDR puts at whatever it is tuned to. It is off by
+default: the spike is a steady addition to the envelope and the burst rises
+clear of it, while a filter narrow enough to reject the spike is narrow enough
+to lose a transmitter that has drifted. At the default sample rate it does
+nothing whatever it is set to, there being nothing after the mixer narrower
+than the band.
 .PP
 Finding the bursts is done in two passes, and the reason is having more than
 one sensor. The first pass asks only where anything is happening at all, and
@@ -2372,8 +2378,18 @@ on every quiet second.
 .PP
 .BI \-\-save\-iq " FILE"
 writes the raw samples alongside, for anything the diagnosis cannot settle. It
-is 8 MB a second at the default rate, so bound it with
-.BR \-\-seconds .
+is 2 MB a second at the default rate, so bound it with
+.BR \-\-seconds ;
+sixty seconds is plenty, every sensor reporting at least twice in that. The
+settings it was taken at are written beside it, a file of raw samples with no
+record of its sample rate being unreadable by anything.
+.PP
+.BI \-\-from\-iq " FILE"
+reads one back instead of the receiver, so a recording made where the aerial is
+can be worked on anywhere. Everything downstream of the dongle is the real
+thing, which is what tells a receiver problem and a decoder problem apart: a
+capture that yields nothing on replay yields nothing for anybody, and one that
+yields readings on replay and not on the air is a setting.
 .SH WEATHER OPTIONS
 Every option the weather side takes, in the four groups the menu shows them
 in. Each is a flag here and a line in the menu, and both come from one table
@@ -2399,7 +2415,7 @@ Leave it automatic first. If a sensor you know is there never appears, try 40 or
 .B --rate
 Sample rate \[em] how fast to sample; a quarter of a megasample is the minimum (Hz).
 .br
-Setting name \fBrate\fR, default \fB1.024 MHz\fR.
+Setting name \fBrate\fR, default \fB250 kHz\fR.
 .br
 Accepts: at least 250000.
 .TP
@@ -2411,14 +2427,14 @@ Setting name \fBfrequency\fR, default \fB433.92 MHz\fR.
 Accepts: at least 3e+08, at most 1e+09.
 .TP
 .B --offset
-Tuning offset \[em] how far to one side of the sensors to tune the receiver (Hz).
+Tuning offset \[em] how far to one side of the sensors to tune, if at all (Hz).
 .br
-Setting name \fBoffset\fR, default \fB250 kHz\fR.
+Setting name \fBoffset\fR, default \fBstraight at them\fR.
 .br
 Accepts: at least 0.
 .RS
 .PP
-A quarter of the sample rate is right and is the default. Only set it to zero to see what the spike was costing.
+Leave it at zero. It is worth trying a quarter of the sample rate only if the diagnosis shows bursts that will not slice.
 .RE
 .TP
 .B --simulate / --no-simulate

packaging/make-man.py

@@ -1376,18 +1376,24 @@ and one byte of sum is all that is left. Corroboration does not help either,
 the copies of a message being identical. What gives that window away every
 time is that it ends a whole byte before the burst does.
 .SS Getting it off the air
-The receiver is tuned a little to one side of 433.92 MHz, because every
-RTL-SDR puts a spike of its own at whatever it is tuned to, and a spike
-sitting on top of a signal that works by being switched on and off is the one
-thing that stops it being off. The sensors are shifted back to the middle in
-software, which puts the spike out at the edge instead.
-.B \-\-offset 0
-tunes straight at them, which is worth trying once to see what the spike was
-costing.
+The receiver is tuned straight at 433.92 MHz, at 250 kS/s, with the tuner's own
+gain control doing its job and the RTL2832's digital AGC left off, which is
+what the established tools for this band do.
 .PP
-A running average of the complex samples then rejects the spike, and only
-after that is the magnitude taken \[em] filtering before detection rather than
-after is what keeps the neighbours out of the envelope of the sensor.
+The digital AGC is off because it pumps: it winds the gain up through the
+silence between one burst and the next, lifting the noise towards the signal
+and squeezing the difference this depends on. It matters here in a way it does
+not for aircraft, where a frame is found by correlating a preamble over a few
+microseconds rather than by comparing a burst with the quiet around it.
+.PP
+.B \-\-offset
+tunes to one side of the sensors and shifts them back in software, which
+avoids the spike every RTL-SDR puts at whatever it is tuned to. It is off by
+default: the spike is a steady addition to the envelope and the burst rises
+clear of it, while a filter narrow enough to reject the spike is narrow enough
+to lose a transmitter that has drifted. At the default sample rate it does
+nothing whatever it is set to, there being nothing after the mixer narrower
+than the band.
 .PP
 Finding the bursts is done in two passes, and the reason is having more than
 one sensor. The first pass asks only where anything is happening at all, and
@@ -1484,8 +1490,18 @@ on every quiet second.
 .PP
 .BI \-\-save\-iq " FILE"
 writes the raw samples alongside, for anything the diagnosis cannot settle. It
-is 8 MB a second at the default rate, so bound it with
-.BR \-\-seconds .
+is 2 MB a second at the default rate, so bound it with
+.BR \-\-seconds ;
+sixty seconds is plenty, every sensor reporting at least twice in that. The
+settings it was taken at are written beside it, a file of raw samples with no
+record of its sample rate being unreadable by anything.
+.PP
+.BI \-\-from\-iq " FILE"
+reads one back instead of the receiver, so a recording made where the aerial is
+can be worked on anywhere. Everything downstream of the dongle is the real
+thing, which is what tells a receiver problem and a decoder problem apart: a
+capture that yields nothing on replay yields nothing for anybody, and one that
+yields readings on replay and not on the air is a setting.
 .SH WEATHER OPTIONS
 Every option the weather side takes, in the four groups the menu shows them
 in. Each is a flag here and a line in the menu, and both come from one table