mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-23 20:58:55 -05:00
def93a8931
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@5434 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
81 lines
3.2 KiB
Plaintext
81 lines
3.2 KiB
Plaintext
// 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
|
|
audio waveform is computed on-the-fly, with 16-bit integer samples
|
|
sent to the audio output device at a 48000 Hz rate. Generated JT65
|
|
and JT9 signals have continuous phase and constant amplitude, and
|
|
there are no key clicks. The transmitter's power amplifier need not
|
|
be highly linear.
|
|
|
|
.Receiving
|
|
|
|
_WSJT-X_ acquires 16-bit integer samples from the audio input device
|
|
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 for the decoder at intervals of 3456/12000 = 0.288
|
|
s, half the JT9 symbol length.
|
|
|
|
.Decoding
|
|
|
|
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 over the full
|
|
displayed frequency range. Each decoding pass can be described as a
|
|
sequence of discrete blocks. The functional blocks are different for
|
|
the JT65 and JT9 modes. In dual-mode JT9+JT65 operation on computers
|
|
with more than one CPU, decoding computations for the two modes are
|
|
done in parallel.
|
|
|
|
The basic decoding algorithm for JT65 mode is described in the 2005
|
|
{jt65protocol} paper. The following list summarizes the corresponding
|
|
algorithm for JT9 mode. Blocks are labeled here 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 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 interleaving imposed at the
|
|
transmitter
|
|
|
|
+decode9+:: Retrieve a 72-bit user message using the sequential
|
|
Fano algorithm
|
|
|
|
|
|
+unpackmsg+:: Unpack a human-readable message from the 72-bit
|
|
compressed format
|
|
|
|
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.
|
|
|
|
With marginal or unrecognizable signals the sequential Fano algorithm
|
|
can take exponentially long times. If the +sync9+ step in the above
|
|
sequence finds many seemingly worthy candidate signals and 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 for a given signal if
|
|
it takes too long. The choices *Fast | Normal | Deepest* on the
|
|
*Decode* menu provide the user with a three-step adjustment of the
|
|
timeout limit.
|