/**
\page psk_page PSK - BPSK, BPSKR, QPSK, and Multi-Channel Modems

\tableofcontents

\section bpsk_modem  BPSK and QPSK modems

PSK are narrow band low symbol rate modes using either single or
multiple carrier differential Binary Phase Shift Kying, BPSK, or Quadrature
Phase Shift Keying, QPSK.
<br>

PSK63FEC and the PSKxxxR modes are forward error correcting modes. PSK63FEC
is compatible with the MultiPsk mode of the same name. The PSKxxxR, or
robust, modes use both forward error correction and interleaving to achieve
about 4 dB s/n improvement over standard PSK. These modes are use primarily
by the PskMail user community. They are the invention of John Douyere,
VK2ETA, a member of the fldigi development team.
<br>

In addition to the binary phase shift keying the signal is 100%
raised-cosine amplitude modulated at the symbol rate. This reduces the power
to zero at the phase change. Because of this amplitude modulation, the
signal bandwidth is relatively narrow. Synchronization at the receiver
is straight forward because it can be recovered from the amplitude
information. Differential PSK is used to provide continuous phase changes
when idle (to maintain sync), and by allowing the receiver to measure phase
difference from symbol to symbol, to reduce the effects of ionospheric
Doppler phase changes which modulate the signal. The slower modes are
more affected by Doppler, and the QPSK modes are particularly affected.
<br>

With no interleaver and limited coding length, the QPSK mode Forward Error
Correction coding gain is limited, and under burst noise conditions on
HF the performance is usually worse than the BPSK option at the same
baud rate. In general the narrow-band BPSK modes work well on a quiet
single-hop path, but give poor performance in most other conditions.
<br>

\image html psk-signal-oscope.png "PSK63 signal transmitting text data - oscilloscope view"
\image latex psk-signal-oscope.png "PSK63 signal transmitting text data - oscilloscope view" width=5.0in
<br>

\image html psk-signal-waterfall.png "PSK63 signal transmitting text data - waterfall view"
\image latex psk-signal-waterfall.png "PSK63 signal transmitting text data - waterfall view" width=1.0in
<br>

\image html qpsk-signal-oscope.png "QPSK63 signal transmitting text data - oscilloscope - waterfall view"
\image latex qpsk-signal-oscope.png "QPSK63 signal transmitting text data - oscilloscope - waterfall view" width=5.0in
<br>

\image html qpsk-signal-waterfall.png "QPSK63 signal transmitting text data - waterfall view"
\image latex qpsk-signal-waterfall.png "QPSK63 signal transmitting text data - waterfall view" width=1.0in

<br>

The two oscilloscope views above clearly show the combined phase and
amplitude modulation of these modes.<br>

<br>

With these modes, a very linear transmitter is required. Over-driven
operation results in excessive bandwidth, poorer reception and
difficult tuning. Overdrive usually occurs by having the audio
signal much too large. These are very sensitive modes and usually very little
power is required. QRP operation of 80, 40, 30 and 20 meters can provide
nearly 100% copy over multi-hop paths. In many instances PSK can provide
better decoding than CW.
<br>

Setting up for a good clean on air signal that will receive the accolades
of your QSO partners is easy. Follow the instructions on using the
\ref tune_page "Tune" button</a> and you will have a clean on signal.
<br>

Good reception of PSK signals requires that the demodulator be phase locked
to the incoming signal. Fldigi has both a fast acquire / slow tracking AFC
system. Place the red bandwidth bar (see above) so that it overlies
the desired signal and then press the left mouse button. The signal
should quickly lock on a decoding should commence immediately. It is
almost impossible to visually tell whether a BPSK or QPSK signal is
being received. Under very high s/n you might be able to hear the
difference, but that is even difficult for most operators. If you are not
able to decode a signal that looks like a BPSK and the bandwidth of the
signal matches the baud rate then it might be a QPSK signal. Just change
mode a try reacquiring the signal.
<br>

\section multi_bpsk_modem  Multi-Channel PSK modems

\image html MultiChannelBPSKWaterfall.png "PSK63R20C signal transmitting text data - waterfall view"
\image latex MultiChannelBPSKWaterfall.png "PSK63R20C signal transmitting text data - waterfall view" width=4.0in

<br>
\ref psk_page "Return to Top of Page"
<br>
\ref main_page "Return to Main Page"


*/
