[wsjt-devel] FT8 notes

[Joe T. <jo...@pr...>]

Hi all,

Thanks for a busy ~20 hours of many people testing FT8.  I now have 
accumulated a directory with 527 *.wav files, each of which has at least 
one visible FT8 signal.  The files were recorded at K1JT at either 
14.079 or 50.313 MHz.

Running the r7753 stand-alone slow-mode decoder jt9[.exe] on this 
collection of files produces 574 valid decodes and 0 false decodes with 
total execution time 39.8 s.  The average time to process a 15 s Rx 
sequence on this machine (Core i7-6700 @ 3.4 GHz) is thus 39.8/527 = 
0.076 s.  Not bad!

Here's the detailed execution-time breakdown from timer.out:

  Name                 Time  Frac     dTime dFrac    Calls
----------------------------------------------------------
  jt9                39.828  1.00     0.734  0.02        1
   read_wav           0.121  0.00     0.121  0.00    27404
   decft8            38.973  0.98     0.133  0.00      527
    sync8             4.191  0.11     4.191  0.11      527
    ft8b             34.648  0.87     0.051  0.00     2821
     bpd174           1.480  0.04     1.480  0.04     2821
     osd174          33.117  0.83    33.117  0.83     2210
----------------------------------------------------------
                                     39.828  1.00

Note that 83% of the execution time is spent in routine osd174, 11% in 
sync8, and 4% in bpd174 (a contraction for subroutine name bpdecode174).

It turns out that only 14 of the 574 decodes were produced by osd174, 
the "ordered statistics" decoder.  The rest came from bpd174.  With 
osd174 deactivated, timer.out looks like this:

  Name                 Time  Frac     dTime dFrac    Calls
----------------------------------------------------------
  jt9                 6.641  1.00     0.891  0.13        1
   read_wav           0.168  0.03     0.168  0.03    27404
   decft8             5.582  0.84     0.094  0.01      527
    sync8             3.887  0.59     3.887  0.59      527
    ft8b              1.602  0.24     0.109  0.02     2821
     bpd174           1.492  0.22     1.492  0.22     2821
     osd174           0.000  0.00     0.000  0.00     2210
----------------------------------------------------------
                                      6.641  1.00

Now the average execution time for a 15 s Rx sequence is just 13 ms!

We need to keep decoding time very short -- say, well under 1 s -- so 
that auto-sequencing, if not the human operator, can select proper 
responses to received messages.  Fortunately, we already have good 
baseline performance -- and we have a number of "knobs" to play with.

	-- Joe, K1JT