On 03/01/2019 15:33, Frank Kirschner wrote:
Many years ago, I had a home-brew RTTY TU. It had two 88 mH torroids
in the tuned circuits and a differential amplifier as a detector.
Occasionally, the received signal would fade below the noise, but the
Kleinschmidt teleprinter would keep printing the decoded text. I have
tried several current software packages for decoding RTTY signals, and
all of them require quite strong signals to decode at all, and very
strong signals to provide 100% print.
I was wondering if the decoding method used in WSJT-X for FT8 could be
ported to a RTTY decoder. I realize that FT8 has a lower symbol rate,
which provides more time to integrate, and incorporates redundancy,
which improves accuracy. But the theoretical lower limit for decoding
RTTY is something like -5 dB S/N, and the current programs are nowhere
near that.
If RTTY facilities could be integrated with WSJT-X, not only would the
decoding be better, but the log scanning functions of WSJT-X and
JTAlert would be better, significantly improving contest operation.
I confess I haven't looked at the decoding techniques used by RTTY
software, but I suspect, based on performance, that it could be improved.
73,
Frank
KF6E
Hi Frank,
this is well covered ground, some existing Baudot decoders have very
good performance with built in weak and fading signal models to optimize
them.
The techniques used to decode FT8 are based around a block message
format that includes parity bits and checksums for advanced forward
error detection and correction, Baudot is a character based stream code
with no forward error correction capability. RTTY was designed for an
era where automation was mechanical and quite limited, just having a
code that could be used to modulate a transmitter and be printed by a
receiver was about the sum of the technical design. RTTY's *only*
similarity with FT8 is that it is a frequency shift keyed constant
amplitude signal. Even the simplest addition of a single parity bit to
detect, but not correct, single bit errors would need a fundamental
non-backwards compatible change to the Baudot code.
73
Bill
G4WJS.
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