15 Basics
188
Modulation Scheme
Total bit rates (Bit/s)
DBPSK 200
DQPSK 400
8-DPSK 600
16-DPSK 800
Table 15.1: Total Bit Rate
The complex PT-2 modulation scheme is totally different to the simple FSK. Therefore
it is IMPOSSIBLE to use the FSK modulators found in some transceivers to generate
the signal. The PT-2 signal must always go via the indirect route, by using SSB to
generate the HF signal.
This has actually no disadvantages, providing the transceiver is not overdriven (see
below).
A further very essential difference between the
older
FSK modulation and the multi-tone
DPSK modulation has to be mentioned. With FSK modulation the output power of the
transmitter remains constant during the entire transmission because alternating square
wave pulses of each tone are transmitted, and mathematically the total amplitude adds up
to a constant function. This could be called a
constant envelope
.
As the amplitude remains the same, non-linear amplifiers, or even class C power
amplifiers, can be used without problems. Speaking about a complex modulation method,
e.g. used in PT-2, a more or less
variable envelope
must be considered. This means, in
practice, the following two points have to be observed:
In all modulation methods using a changing amplitude HF signal (e.g. PACTOR-2,
SSB-speech modulation, AM etc)., it is NEVER allowed to overdrive the transmitter
because intermodulation products will be generated broaden the signal.
How to adjust the maximum transmitter power will be described in the
PSKAmpl
command (refer to chapter
on page
). It always has to be taken in consideration
that, with a variable amplitude modulation system, the effective average power is lower
than the peak power. With PACTOR-2 this ratio between peak power and average power
is almost exactly 2. (For insiders: the square root of this ratio is called the crest factor, and
with PT-2 has a value of around 1.45). This value is considerably lower than with other
multi-tone systems, and has shown itself to be very well matched to the usual SSB
transmitter. If one sets a peak power of 100 watts, then the PT-2 signal produces an
average output of about 50 watts. The full PEP output of an SSB power amplifier can be
thus used without great fear of overload, conditions being similar to those existing during
normal SSB speech transmission.
15.3.3
Error control coding
The basic idea behind error correcting codes is that extra checking information is
transmitted along with the required data, so that the
redundancy
of the signal is increased.
The greater the efficiency of the redundancy employed the better the code, and the greater
its error correcting abilities. The ratio of useful information to total information (=useful