SCS PTC-IIex, Manual

Page: 166 / 220

11 PSK31
Chapter 11
11 PSK31
11.1 General
The PSK31 system developed by Peter Martinez, G3PLX, offers the RTTY enthusiast a
modern variation to this by now somewhat old fashioned operating system. It has
markedly reduced bandwidth requirements and a better resistance to interference.
The operating technique is very similar to the normal operation of Baudot-RTTY,
although a more sensitive fine-tuning is required compared to the older system, as a
PSK31 signal requires a tuning accuracy of a few Hertz.
Technically, the system is a differentially phase modulated, single carrier, with a Baud
rate of 31.25 steps per second. In practice, DBPSK and DQPSK are employed, but with a
heavy leaning towards DBPSK. DQPSK, despite the use of convolutional Coding (k = 5),
has shown itself to be more prone to errors at very low SNR's compared to uncoded
DBPSK. The waveform pulse of PSK31 is the "Raised Cosine in time" so that the
DBPSK signal switches between two-tone (when only phase changes are taking place)
and a plain single tone carrier. The bandwidth between the first null points of the
spectrum is 62.5 Hz. A specialty of PSK31 is that it allows the entire ASCII character set,
even the back space" character is included.
11.2 Activation and use of the PSK31 terminal
The command to activate the PSK31 mode is
PSKTerm
in the
cmd:-
menu. Command
parameters are not required, and will be ignored if given. After turning on the PSK31
terminal, the PTC-IIex is in the receive condition. Various PSK31 relevant settings are at
their default or previously set values. The
Quit
command closes the PSK31 terminal,
and returns to the normal STBY condition. Text input may be carried out whilst in the
receive condition, and is placed in the type-ahead buffer. (This can be deleted at any time
with <Ctrl-X>.
At the frontpanel PSK31 mode is indicated by lighting the RTTY LED together with the
BDPSK or DQPSK LED. The tuning indicator takes on a different look. It has been
changed to a small
spectroscope , so that the small band PSK31 signals can be tuned
precisely to the correct frequency. Each LED represents the power in a 9.375 Hz wide
frequency window. The
spectroscope encompasses a frequency range of approx. 140 Hz.
Lower frequencies are to the right, higher to the left. This is, at first sight, intuitively
wrong. It does however make it easier to tune when using the usually clockwise turning
VFO control of the modern transceiver. The brighter the LED glows, the higher the
energy content within that appropriate window. (refer to chapter
11.7 , page 155 on how
this display may be used to tune in PSK31 signals)
11.3 Carrier Frequencies
The audio carrier frequency for receive and transmit are calculated using the settings for
MARK-frSPACE-frequency/2. If one has MARK=1400 Hz and SPACE=1200
Hz, then the carrier frequency is 1300 Hz. A special command for the PSK31 AF-carrier
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