[RTTY] Baud racing

[Kok Chen]

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