[RTTY] High speed RTTY

[Kok Chen]

On Jan 27, 2010, at 8:32 PM, Bill, W6WRT wrote:

> Do you think it might be DominoEX? Are there any
> drawbacks to using it in a contest? If it is slower that is not
> necessarily a gad thing if throughput is significantly better.

I don't know if DominoEX will gather steam or fizzle away like the other digital mode-du-jour.  It certainly has many unique features to earn a position in amateur history.  I personally think ZL1BPU did a fantastic job with it (Murray was also the one who came up with the MFSK16 protocol).  

ZL1BPU came up with the IFK scheme in DominoEX to counter the usual criticism of how slowly MFSK16 tunes (and created non-FEC modes to further improve the decoding latency).  It uses a nibble mode Varicode, which makes much more sense than binary Varicode for 16 tone FSK (DominoEX actually uses 18 tones to make sure that successive ones are separated by at least 2 tone slots to avoid intersymbol overlaps that are caused by doppler spreading).  Instead of an idle varicode or a diddle character, Murray used the idle periods to transmit a secondary code set which can be used as a beacon (you can send your callsign repeatedly in the beacon channel, for example, or hey, even a row of unending dots to irritate Phil, GU0SUP :-) :-)

Skip KH6TY is pushing its use as an FM weak signal mode on VHF (i.e., use the audio tones to modulate an FM transmitter instead of an SSB transmitter).

Speed wise, it has many submodes, from DominoEX 22 down to DominoEX 4.  When you add FEC, it becomes about a factor of two slower than non-FEC versions in terms of thruput.  To get a bird's eye view of the non-FEC speed, take a look at the first chart here

http://homepage.mac.com/chen/w7ay/cocoaModem/UsersManual/mfskManual/mfskManual/dominoex.html

The second chart on that page gives a 10,000 ft view of how non-FEC DominoEX performs against BPSK31 (for the same speed, it is perhaps 2 dB more sensitive than BPSK31 under quiet band conditions).

Details of FEC performance under poor propagation can be found here

http://homepage.mac.com/chen/Technical/DominoEX/ITU/index.html

If you compare the DominoEX 8 (with default FEC flutter -- red curve) chart here

http://homepage.mac.com/chen/Technical/DominoEX/ITU/ITU/flutter.html

with the VE3NEA's RTTY flutter chart here,

http://www.dxatlas.com/RttyCompare/

you can see that DominoEX 8 with FEC a whopping 11 to 12 dB advantage compared to RTTY.   That is a power difference between an over 1 kilowatt transmitter and a 100 watt transmitter.

VE3NEA's RTTY and my DominoEX charts are pretty much directly comparable since we both use a reference of 3 kHz noise bandwidth. Both were measured using real world modems, using the same Watterson HF channel simulator models and CCIR parameters (the simulator implementations were different: Alex used PathSim and I used cocoaPath).  

With FEC turned on, DominoEX 8 does run only at about half the output character rate of RTTY.  But that 12 dB is hard to argue against when conditions become bad.  When conditions are good, turn the FEC off and get that factor of two back.  

The slower DominoEX modes are even better, but they are probably too slow for people who are used to playing RTTY.   But if you are interested in beating RTTY by only 6 dB, you can use the DominoEX modes that are faster than 45.45 baud RTTY :-).

I don't know if KH6TY will succeed in giving DominoEX better visibility, and even if he does, if its success in the VHF/FM weak signal world will translate into success in the HF bands.   DominoEX has been around for a while now and never took off. 

If I remember correctly, PSK31 was not very popular either in the earlier years, until the same KH6TY introduced waterfall tuning to PSK31 with his DigiPan program.  Once tuning became easy, PSK31's use shot up like a hockey stick. 

73
Chen, W7AY