In spite of new DSP technology appearing in HF data modes, traditional
RTTY has maintained it's popularity. There is evidently a special
quality in RTTY that is missing in newer modes. It may be that RTTY is
easier to use, or more "hands-on", or more sociable since you can
listen-in and join-in which an ARQ link cannot do. It may just be
because RTTY is cheaper.
A new mode is emerging over the last year which may bridge the gap
between RTTY and the high-tech modes. Based on an idea by SP9VRC, the
PSK31 mode is based on the use of low-cost DSP starter kits and
public-domain software, using modern DSP techniques to implement a basic
live-QSO transmit-receive keyboard-to-screen mode. The bandwidth of
PSK31 is much lower than any other data mode, which means it can work at
lower signal levels in today's crowded bands.
The basic idea of PSK is that keying is done by phase-shifting the
carrier rather than frequency-shifting it. The bandwidth is equal to the
baudrate rather than to the shift plus the baudrate. With the chosen
baudrate of 31.25, that brings the bandwidth down from the 300-500Hz of
other modes down to 31Hz. By using an alphabet similar to morse with
short codes for common letters, the text speed of PSK31 comes out at
about 50 wpm. By using the narrowest possible filters in the transmitter
and receiver, the performance of PSK31, even without error-correction,
is certainly better than most, and has the added advantage for live QSOs
that the performance degrades very gracefully as the signal drops. For
paths where errors occur in bursts rather than randomly, PSK31 can be
switched to use a convolutional code at the transmitter and a matching
Viterbi decoder at the receiver. QPSK, with 4 phase-shifts instead of
two, is used for this, but the bandwidth and text speed stay the same.
The price paid for this robustness is a tighter frequency stability
requirement and a delay of 640mS in the decoder.