Decode data signals, starting with on-off keying

Much of what a scanner finds is not speech.  Doorbells, tyre-pressure
sensors, weather stations, remote controls, paging and packet radio all
carry something a receiver can read, and until now the answer was "OOK /
ASK data burst" and a WAV file.  Now the bits come out.

The observation the whole thing is built on is that whatever the
modulation, a data signal is the same shape once it has been sliced: a
train of alternating runs whose lengths carry the information.  On-off
keying gives that directly -- the carrier is up or it is down -- and
two-level FSK gives exactly the same thing from the discriminator, one
tone or the other.  So both reduce to a run-length train and everything
after that is shared.

What the runs mean is the line code, and it is worked out from the runs
alone rather than configured, because each code makes a different
prediction about which of the two histograms is the bimodal one: PWM
(EV1527, PT2262, and nearly every 433 MHz remote), PPM, Manchester, and
plain NRZ.  Four-level FSK is recognised as such and read as symbols
rather than sliced down the middle, which produces bits that mean
nothing; where a frame sync word appears the system is named outright.

Two protocols carry their own framing and checksums and so are read in
full.  POCSAG paging: all three rates tried because nothing in the signal
says which it is, every codeword checked and single-bit errors corrected
against the BCH code, and the address, function letter and message text
reported.  AX.25 as APRS uses it: the frame check has to come out right
before a frame is reported at all, and the sender's callsign goes onto
the map with everyone else's.

The hard half is refusing what is not data.  Noise sliced at a threshold
produces runs and runs produce bits, so three things guard against it:
the runs have to quantise to the line code's own grid; most of the bursts
in a capture have to decode the same way, because one lucky window in
eight is a coincidence and that is exactly what SSB voice produced; and,
much the strongest, the packet has to repeat, because bits that come back
identical six times did not come from noise.  A reading with none of that
behind it is reported as nothing at all rather than as a bit string with
a low number beside it that somebody will read anyway.  Across 27
recordings of speech, music, static, a bare carrier, Morse and PSK it
returns nothing 27 times.

A firm decode also outranks the content check, which is statistical: a
burst of keying demodulated as FM audio is a buzz and the speech detector
likes a buzz, but a frame whose own checksum came out right is not a
statistic.  Such a capture is kept and filed as data, not as voice.

What comes out is written to a _data.txt beside the recording, shown on
the live display and in the line-per-hit output, and takes the place of
the transcript at the top of saunterbrowse -- where it is searchable, so
"which page mentioned engine 4" is a question that can be asked.
`bandsaunter analyze` decodes a file you already have.

The simulator gained two honest transmitters to test against: a
pulse-width remote that repeats a real payload, and a pager that sends
real POCSAG batches with real BCH check bits.  Random keying exercises
the classifier but leaves a decoder nothing to get right.  The POCSAG
encoder lives next to the decoder rather than in the test helpers, so a
bug shared by both cannot hide.

Fixed along the way:

- Rich reads a square bracket as markup, and a decoded page is arbitrary
  text off the air.  "[/x]" in a message ended the live display with a
  MarkupError; so did typing "[/" at saunterbrowse's search prompt.
  Everything that did not come from this program is escaped now.

- Otsu returned the first bin of a plateau.  Two populations with nothing
  between them -- silence and full carrier, which is what on-off keying
  is -- make every threshold in the gap equally good, and taking the
  first put it hard against the lower population with the hysteresis band
  outside the data entirely, so nothing sliced at all.

- Estimating the symbol clock by counting along a cumulative grid is a
  fixed point: a unit two per cent small produces two per cent more
  symbols and reproduces itself exactly.  Rounding each run on its own
  converges instead, because every run votes independently.  The grid is
  then the right way to extract the bits, where rounding runs one at a
  time drifts.

- A clipped first repeat used to truncate every other repeat to its
  length.  The consensus is taken over the commonest length now.

761 -> 869 tests.

bandsaunter/tui.py

@@ -559,7 +559,26 @@ It is recognised by its symbol rate and its refusal to pause, named on screen,
 and skipped within a second or two.  Turn 'Skip trunk control channels' off
 only if you are collecting them for a decoder.  If digital voice calls are
 being skipped by mistake, raise 'Control channel patience'."""),
-    "9": ("Callsigns and the map", """
+    "9": ("Reading data signals", """
+Much of what a scanner finds is not speech: doorbells, tyre-pressure sensors,
+weather stations, remote controls, paging, packet radio.  All of it is read.
+
+Whatever the modulation, a data signal comes down to the same shape once it has
+been sliced — a train of runs whose lengths carry the information — and which
+line code it is (PWM, PPM, Manchester, NRZ) is worked out from the runs alone
+rather than configured.  Four-level FSK, as P25 and DMR use it, is recognised
+as such and read as symbols; where a frame sync word appears the system is
+named.
+
+POCSAG paging and APRS packet carry their own checksums, so those are read in
+full: the address and message text of a page, the callsign and position of an
+APRS beacon.  A decoded callsign goes onto the map with the rest.
+
+What keeps it honest is repetition.  Noise sliced at a threshold produces runs,
+and runs produce bits, so a reading with nothing behind it is reported as
+nothing at all.  These transmitters send the same packet three to ten times
+over, and bits that come back identical every time did not come from noise."""),
+    "10": ("Callsigns and the map", """
 Anyone who identifies themselves in a transcript is picked out and looked up in
 the FCC's published licence register: the name, the town, the class of licence.
 The callsign is the only thing sent, and each one is asked about once and then
@@ -575,7 +594,7 @@ can reach.
 Turn 'Look callsigns up' off to keep the scan entirely offline; callsigns are
 still found, and named by country from their prefix.  Clear 'Map file' to stop
 writing the map.  US licence records are public, and include addresses."""),
-    "10": ("Keys during a scan", """
+    "11": ("Keys during a scan", """
 q   stop the scan
 p   pause and resume
 s   skip the signal being recorded and carry on sweeping