All this stuff about faster baud rate RTTY brings up a subject that more people
should be made aware of. That not all digital modes require the same bandwidth
to transmit the same number of bits per second.
I had already alluded to the need of lots of bandwidth to pass more data per
second with RTTY.
The key to not being so wasteful of bandwidth is to use a lower baud rate even
if you have higher bit rates. MFSK16, Olivia and DominoEX all transmit 4 bits
per symbol, instead of 1 bit per symbol as in RTTY. Because of that, for a
given data rate (bit rate), the keying sidebands outside and away from the
tones used by these Multi-bit FSK signals are much narrower than the keying
sidebands from an RTTY signal outside of its own Mark and Space tones.
The spectrum of an MFSK signal is like a filter with a flat top and sharp
skirts, while RTTY looks like a filter with very wide skirts.
DominoEX, for example is no harder to tune than RTTY (it is actually easier to
tune, since what ZL1BPU calls "incremental frequency shift keying" does not
require you to fine tune a signal once a single tone appears in the receiver's
passband -- the decoder will lock into the signal automatically). And when
conditions are harsh, simply turn on forward error correction. The higher
speed DominoEX 22 will do just about 150 words per minute, which should be
faster anyone can type.
You can see the bandwidth versus data rate in average English words per minute
(for comparison, 45.45 baud Baudot RTTY is equivalent to 60 WPM) in the first
plot on this web page:
http://homepage.mac.com/chen/w7ay/cocoaModem/UsersManual/mfskManual/mfskManual/dominoex.html
60 WPM RTTY (45.45 baud) with 170 Hz shift occupies a bandwidth around 300 Hz,
100 WPM RTTY (75 baud) requires a bandwidth of around 400 Hz.
When compared to the plot referred to above, the DominoEX family will push
around 100 WPM in the bandwidth of a 45.45 baud (60 WPM) RTTY signal. MFSK16's
bandwidth is narrower than DominoEX by yet another 12% or so (MFSK16 uses 16
tones, DominoEX uses 18 tones) but is a bear to tune, however.
Anyway, food for thought.
73
Chen, W7AY
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