// Status=review
.Transmitting

Immediately before the start of a transmission _WSJT-X_ encodes a
user's message and computes the sequence of tones to be sent.  The
transmitted audio waveform is then computed on-the-fly, using 16-bit
integer samples at a 48000 Hz rate.  Digital samples are converted to
an analog waveform in the sound card or equivalent D/A interface.

.Receiving and Decoding

_WSJT-X_ acquires 16-bit integer samples from the sound card at a 48000
Hz rate and immediately downsamples the stream to 12000 Hz.  Spectra
from overlapping segments are computed for the waterfall display and
saved at intervals of 0.188 s, half the JT9 symbol length.  As shown
in screen shots earlier in this guide, a JT9 signal appears in the
*Cumulative* spectrum as a nearly rectangular shape about 16 Hz wide.
Although there is no clearly visible ``sync tone'' like the one at the
low-frequency edge of a JT65 signal, by convention the nominal
frequency of a JT9 signal is taken to be that of the lowest tone, at
the left edge of the spectrum.

  insert annotated waterfall picture here?

At the end of a reception sequence, about 50 seconds into the UTC
minute, received data samples are forwarded to the decoder.  For
operator convenience the decoder goes through its full procedure
twice: first at the selected Rx frequency, and then in the full
displayed frequency range (or in JT9+JT65 mode, the displayed range
above the blue *JT65 nnnn JT9* marker).  Decoding of clean JT9 signals
in a white-noise background starts to fail below signal-to-noise
ratio -25 dB and reaches 50% copy at -26 dB.

Each decoding pass can be described as a sequence of discrete blocks.
In the following list, blocks are labeled with the names of functional
procedures in the code.

[horizontal]
+sync9+::    Use sync symbols to find candidate JT9 signals 
            in the specified frequency range

Then, at the frequency of each plausible candidate:

[horizontal]
+downsam9+::  Mix, filter and downsample to 16 complex 
            samples/symbol

+peakdt9+::   Using sync symbols, time-align to start of JT9 symbol 
            sequence

+afc9+::    Measure frequency offset and any possible drift

+twkfreq+::   Remove frequency offset and drift

+symspec2+::  Compute 8-bin spectra for 69 information-carrying
            symbols, using the time- and frequency-aligned data;
            transform to yield 206 single-bit soft symbols

+interleave9+:: Remove single-bit symbol interleaving imposed at the
	    transmitter

+decode9+::   Retrieve a 72-bit user message using the sequential
            Fano algorithm for convolutional codes


+unpackmsg+:: Unpack a human-readable message from the 72-bit 
            compressed format

With marginal or unrecognizable signals the sequential decoding
algorithm can take exponentially long times.  If the +sync9+ step in
the above sequence finds many seemingly worthy candidate signals, and
if many of them turn out to be undecodable the decoding loop can take
an inconveniently long time.  For this reason the step labeled
+decode9+ is programmed to ``time out'' and report failure if it is
taking too long.  The choices *Fast | Normal | Deepest* on the Decode
menu provide the user with a three-step adjustment of this timeout
limit.
