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New Digital Contest Mode: FT4
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Mike WB2FKO
Quite an interesting development!
The whitepaper shows that Gaussian
Frequency Shift Keying dramatically reduces the modulation
bandwidth compared to FSK. This is essential to allow multiple
stations to occupy the receiver passband, as is the case with
FT8. The channel bandwidth is about twice that of FT8, but their
testing shows up to 50 stations can still reliably use the channel
(proposed 50.318 MHz for FT4 on 6m). It would be interesting to
learn how much sensitivity is lost (how many dB?) by using GFSK
compared to FSK. Note that GFSK is an established modulation
scheme:
They tested the possibility of running
FT4 asynchronously, but it seems this leads to excessive channel
congestion. I suspect the Costas Arrays must still be in there to
get the FEC to work. If so, my hunch is that shorter sequences
will make Internet synchronization of computers more demanding
(tighter) than FT8. I will have a look at the source code when it
is made available next Monday (29 April).
WSJT-X is an open source project, but
it's pretty clear that protocol development was done by private
communication among a select few. Given the large amount of noise
on the WSJT mailing lists, I would have done the same for the sake
of minimizing needless distraction.
FT8 and FT4 are clearly incompatible,
which is why there will be separate frequencies on each band.
Will FT4 be used by 6m operators instead of SSB when the band
really opens up, even in non-contest conditions? Will it displace
FT8 just as FT8 displaced JT65?
-WB2FKO
On 4/22/19 4:01 PM, Alden Oyer wrote:
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Keith Morehouse
..and the WSJT musical chairs performance moves on. At least THIS "experimental" mode is going to have a timer in it's alpha release that will force it to stop working right before the June VHF contest... -W9RM Keith Morehouse via MotoG On Mon, Apr 22, 2019, 4:01 PM Alden Oyer <atoyer@...> wrote:
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Alden Oyer
"FT4 is an experimental digital mode designed specifically for radio contesting. Like FT8, it uses fixed-length transmissions, structured messages with formats optimized for minimal QSOs, and strong forward error correction. T/R sequences are 6 seconds long, so FT4 is 2.5 × faster than FT8 and about the same speed as RTTY for radio contesting. FT4 can work with signals 10 dB weaker than needed for RTTY, while using much less bandwidth." CHeck out: http://physics.princeton.edu/pulsar/k1jt/FT4_Protocol.pdf Enjoy, Alden Oyer, AG5S |
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