Hi Igor,
Thank you for your careful scrutiny of internal behavior of the JT65
decoder, and for pointing out what you imagine could be a problem.
I believe there is nothing wrong.
Under certain circumstances the values p1=0 and p2=0 may occur.
Variable psum is prevented from having a zero value by line 39 in
demod64a.f90, not because of "Fortran accuracy". Later calculations of
p1/p2 do not cause problems because the actual calculation is
effectively p1/(p2+0.01) -- see line 130 in ftrsd2/c. Symbols affected
by these calculations are effectively "erased", so they do not cause
false decodes.
-- 73, Joe, K1JT
On 5/19/2016 5:29 AM, Игорь Ч wrote:
>
> Hi Joe and All,
> .
> Observing broken symbols in demod64a.f90, using this diagnostics in line 54
> to catch it in r6643:
> if(p1.lt.0.0001) print *,j,p1
> .
> Sometimes, likely after the signal subtraction, first two information message
> symbols
> getting 0.0(0) values through the whole signal spectrum(all 64 frequencies).
> In this case next 3 information symbols
> are also broken with very high p1 value due to the low psum value. Probably
> some first SYNC symbols
> are also affected with this issue.
> .
> Fortunately, due to the Fortran accuracy, psum value for 0.0 spectrum is
> equal 10E-6 and there is no
> overflow occuring for p1/p2 calculations.
> .
> I tried to trace this issue down to the roots and saw in decode65a that cx()
> array samples from
> twkfreq65 coming with 0.0(0) values, have not got enough experience to trace
> it down further.
> .
> Probably some false decodes are caused by this defect, and both decoders BM
> and FTRSD should have
> sensitivity degradation for some weak signals.
> .
> Audio file is there: https://cloud.mail.ru/public/FDWa/PUMc3B8yD
> .
> There is the diagnostics output:
> .
> 1540 -1 0.8 279 # UA3QUP G7OAE R-15
> 1540 -12 0.5 492 # KK7X DJ0QO JN39
> 1540 -1 2.3 809 # CQ DH6FAJ JO41
> 1540 -2 0.6 1019 # N1FF IU2EWY -19
> 1540 -3 -0.8 1223 # W4IMD DL8DBN JO41
> 1 0.0000000000000000
> 2 0.0000000000000000
> 1540 -19 2.1 1417 # SV1CIF F4FSY -06
> 1540 -4 0.3 1665 # 4S6NCH 4Z5ML KM72
> 1540 -1 0.4 1823 # UR6ISV OM2BK -10
> 1540 -4 0.4 2074 # YV4DHS DF9QV JO32
> 1540 -1 0.4 2308 # CQ 5P1KZX JO57
> 1540 -19 0.4 507 # CQ EW6BN DX
> 1 0.0000000000000000
> 2 0.0000000000000000
> 1540 -1 0.4 1040 # CQ IK3FUS JN55
> 1540 -5 1.2 1221 # W4IMD DM8PV JO60
> 1 0.0000000000000000
> 2 0.0000000000000000
> 1540 -4 0.1 1640 # CQ RW0BT NP39
> 1540 -15 0.5 1665 # 4S6NCH DL6GD JN37
> 1540 -6 0.7 1901 # CQ GM4ZET IO86
> 1 0.0000000000000000
> 2 0.0000000000000000
> 1540 -12 1.2 2095 # YV4DHS IW2CXB JN45
> .
> .
> 73
> Igor UA3DJY
>
>
>
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