kopia lustrzana https://github.com/jamescoxon/dl-fldigi
259 wiersze
7.9 KiB
Plaintext
259 wiersze
7.9 KiB
Plaintext
/**
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\page rtty_page RTTY
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\tableofcontents
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The RTTY modulator and demodulator have been extensively changed with
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version 3.21.67. The new design was a cooperative effort of
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Stefan, DO2SMF, and Dave, W1HKJ with extensive testing performed by Ed,
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W3NR, and Dick, AA5VU. Chen, W7AY, was a silent contributor to
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the design by virtue of his excellent technical papers on RTTY
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modulation and demodulation, which he so generously placed in the
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public domain.
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<br>
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fldigi can operate on a wide range of RTTY symbol rates and bandwidths. The
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selection of symbol rate and bandwidth is made on
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the \ref rtty_fsk_configuration_page "RTTY configuration tab". The three most
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common in amateur radio use can be selected from the mode menu. These are
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<br>
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<center>
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Mode | Symbol Rate | Typing Speed | Bandwidth
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:------:|:-----------:|:------------------:|:----------:
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RTTY 45 | 45.45 baud | 6.0 cps (60 wpm) | 270 Hz
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RTTY 50 | 50.0 baud | 6.6 cps (66 wpm) | 280 Hz
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RTTY 75 | 75.0 baud | 10.0 cps (100 wpm) | 370 Hz
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</center>
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<br>
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These modes were a result of mechanical and electrical
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designs of the early TTY machines. The 45.45 baud and 75 baud
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machines were for the US / Canadian market and used 60 Hz
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synchronous motors. The 50 baud machines were for the European
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market and used 50 Hz synchronous motors.
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<br>
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<br>
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fldigi can encode and decode many other symbol rates and bandwidths. "Custom"
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combinations are set up on the RTTY configuration tab. You
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probably will never have to do that unless you like experimenting with
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unusual RTTY modes.
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<br>
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\section rtty_modulator RTTY modulator
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All of the modem signals that fldigi produces are audio signals. That
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includes the RTTY signal. fldigi can encode and decode an RTTY signal
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that is anywhere within the passband of the sideband transceiver. It is
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not limited to the traditional tone pairs around 2100 Hz. Each of
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the generated Mark / Space signals are on-off-keyed (OOK), bandwidth
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limited signals. The resultant waveform is not an FM type signal
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of constant amplitude. Therefore the <b>transmit audio and RF
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amplifiers must be linear</b>, just like the requirement for PSK signals.
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There are performance gains using this approach. The principal being a
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reduction in inter symbol interference which gives much improved
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performance by the receiver. The waterfall, time domain, and
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spectrum signatures of the transmitted signal look like this:
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<br>
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\image html w1aw-rtty-80.png "W1AW on air signal"
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\image latex w1aw-rtty-80.png "W1AW on air signal" width=3.5in
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<br>
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\image html rtty-transmit.png "AFSK signal "
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\image latex rtty-transmit.png "AFSK signal " width=3.5in
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<br>
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\image html rtty-spectrum.png "Spectrum"
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\image latex rtty-spectrum.png "Spectrum" width=1.5in
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<br>
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You must operate your transceiver in the USB
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mode for the fldigi RTTY signal to be the correct polarity. If your
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transceiver is set to LSB then use the fldigi "Rev" button to reverse
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the sense of the mark and space signals.
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<br>
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You must maintain transmitter LINEARITY in the AUDIO AMPLIFIERs.
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Do not think that you can improve performance by over driving the audio
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input. A good operating procedure for most transceivers is the
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set the audio level to the level for which there is just barely a hint
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of ALC. Then reduce the input to 80% of that power level.
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Over driving an AFSK signal is as disastrous as over driving a PSK
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signal. This is an actual on air signal that was being over
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driven (but not on purpose):
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<br>
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\image html rtty_overdrive.png "Overdriven RTTY Signal"
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\image latex rtty_overdrive.png "Overdriven RTTY Signal" width=6.0in
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<br>
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Joe also performed a series of tests on an Icom 706 mkIIg transceiver. The
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results of those tests are very enlightening.
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<br>
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"Two views - the 2 KHz span and a 10 KHz span. The 10 KHz span
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shows the one failing of the IC-706mkIIg and other rigs with analog
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modulation - opposite sideband and carrier leakage. This one
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isn't too bad but one can see carrier at -50 dBc and opposite sideband
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at -55 dBc +/-. I do use a high audio frequency to minimize
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harmonic issues.
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<br>
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For fun I've attached versions at 70 W in 10K, 5K, and 2K spans. The narrow
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spans clearly show the benefits of reducing the audio until output power
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drops 1.5 dB.
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<br>
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Audio was connected to the IC-706mkIIg via the "DATA" jack rather
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than the mic connector or "Mod In" pin of the ACC jack. Using this
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input avoids several potential problems - including the constant
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swapping between mic and digital connections and remembering to turn off the
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compressor when switching to digital operation. The "Data" input is
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also 6 dB less sensitive than "Mod in" making it that much less likely
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that one will significantly over drive the the transceiver and create
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distortion in the audio stages ahead of the modulator".
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<br>
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The green area is 400 Hz wide.
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<center>
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\image html Icom_FSK.png "Image A"
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\image latex Icom_FSK.png "Image A" width=4.0in
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</center>
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Transceiver operated in FSK mode<br>
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Power: 100 W<br>
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<center>
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\image html fldigi_Icom_AFSK.png "Image B"
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\image latex fldigi_Icom_AFSK.png "Image B" width=4.0in
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</center>
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Transceiver in USB, fldigi AFSK audio drive<br>
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Space at 2125, Mark at 2295 Hz<br>
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Power: 100 Watts, ALC just extinguished<br>
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<center>
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\image html fldigi_Icom_AFSK-1dB.png "Image C"
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\image latex fldigi_Icom_AFSK-1dB.png "Image C" width=4.0in
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</center>
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Transceiver in USB, fldigi AFSK audio drive<br>
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Space at 2125, Mark at 2295 Hz<br>
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Power: reduced to 80 Watts (-1 dB)<br>
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<center>
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\image html fldigi_Icom_AFSK-1r5dB.png "Image D - 2 K span"
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\image latex fldigi_Icom_AFSK-1r5dB.png "Image D - 2 K span" width=4.0in
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</center>
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Transceiver in USB, fldigi AFSK audio drive<br>
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Space at 2125, Mark at 2295 Hz<br>
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Power: reduced to 70 Watts (-1.5 dB)</td>
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<center>
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\image html fldigi_70W_1000Hz.png "Image F - 10 K span"
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\image latex fldigi_70W_1000Hz.png "Image F - 10 K span" width=4.0in
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</center>
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Transceiver in USB, fldigi AFSK audio drive<br>
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Space at 830 Hz, Mark at 1000 Hz<br>
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Power: 70 Watts. The LSB leakage is clearly <br>
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seen at approximately -55 dB<br>
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<center>
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\image html fldigi_70W_1000Hz_5K.png "Image G - 5 K span"
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\image latex fldigi_70W_1000Hz_5K.png "Image G - 5 K span" width=4.0in
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</center>
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Transceiver in USB, fldigi AFSK audio drive<br>
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Space at 830 Hz, Mark at 1000 Hz<br>
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Power: 70 Watts. The LSB leakage is clearly <br>
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seen at approximately -55 dB.<br>
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<center>
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\image html fldigi_70W_2stop.png "Image H - 2 K span"
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\image latex fldigi_70W_2stop.png "Image H - 2 K span" width=4.0in
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</center>
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Transceiver in USB, fldigi AFSK audio drive<br>
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Space at 830 Hz, Mark at 1000 Hz<br>
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Stop Bits set to 2 vice 1.5<br>
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Power: 70 Watts. Compare this to image D<br>
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<br>
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\section rtty_demodulator RTTY demodulator
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Fldigi's demodulator uses a design based on theoretical work published by
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Kok Chen, W7AY,
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<a href="http://www.w7ay.net/site/Technical/ATC/">
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http://www.w7ay.net/site/Technical/ATC/.</a>
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<br>
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\image html RTTY-demodulator.png "Demodulator"
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\image latex RTTY-demodulator.png "Demodulator" width=5.0in
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<br>
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The mark and space signals are
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converted to base band and then filtered in a low pass filter.
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Each filter is a variant of Chen's enhanced Nyquist filter. It is
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implemented using a Fast Overlap-and-Add Fourier Transform.
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<br>
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Each time the baud rate is selected the program must "rebuild" the
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digital RTTY filter. The filter parameters are optimized for the
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baud rate.
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<br>
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The detector is an optimized Automatic Threshold Correcting (ATC) type
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described in Chen's paper.
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<br>
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To start decoding a signal simply left click on the signal and the AFC should
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lock on to the signal.
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<br>
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The digiscope display will extinguish when the Rx signal level falls below
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the squelch setting.
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<br>
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It is possible to use fldigi to generate the keying waveform for use with an
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FSK type of transmitter. See \ref pseudo_fsk_page "Pseudo FSK for a
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description of how this can be accomplished.
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<br>
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See \ref rtty_fsk_configuration_page "RTTY Configuration Page"
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<br>
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\ref rtty_page "Return to Top of Page"
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<br>
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\ref main_page "Return to Main Page"
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*/
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