Re: [wsjt-devel] FST4 Doppler Spread Overshoot Calculation

[Joe Taylor via wsjt-devel]

Hi Ryan,

Many thanks for pointing out the error in calculating the value of 
Doppler Spread for FST4 decodes!  You are (almost) 100% correct, and the 
bug will be fixed in the next release of WSJT-X.  In Fortran subroutine 
lib/fst4_decode.f90 the line


  xi1=i - 1 + (sum2-0.25*sum1)/(sum2-sum2z)

will be replaced by

  xi1=i - 1 + (0.25*sum1-sum2)/(sum2-sum2z)

I say 'almost' only because there was a stray right-parenthesis after 
'sum1' in your rendering of the corrected line of code.


Thanks again for catching this error!

-- 73, Joe, K1JT

On 11/21/2023 12:53 PM, Tolboom, Ryan via wsjt-devel wrote:

Good Evening,

I'm looking at lib/fst4_decode.f90, specifically the code where it 
calculates the frequency in the channel gain function, /g/, at which it 
reaches a certain percentage of the signal power. This can be found on 
line 999 of lib/fst4_decode.f90 
.


To prevent zero Doppler Spreads it uses a linear interpolation to get a 
floating point "index" between bins of the FFT. It does this by setting 
xi1 (and xi2, xi3 in a similar fashion) to the current index minus one 
plus the difference between the current power (sum2) and the power we're 
looking for (sum2 * 0.25, for xi1) divided by the difference between the 
current power (sum2) and the previous power (sum2z):


xi1=i-1+(sum2-0.25*sum1)/(sum2-sum2z)

If I'm interpreting this correctly this will not yield a correct result.

Let me give you an example:

For simplicity, assume we have four bins with powers 1, 2, 9, 2, and 1 
respectively. Assume the bins are enumerated from zero.


image.png

The total power would be 15 and 25% of that would be 3.75. As we look 
for that 25% mark, we'd find it in the do loop when i hits 2, sum2 is 
12, and sum2z is 3. xi1 would then be set to:


xi1 = 2 - 1 + (12 - 3.75)/(12 - 3) = 1 + 8.25/9 = 1.92

In other words, 92% of the way to bin 1 from bin 2. Shouldn't this be 
the other way around? Shouldn't it be 92% of the way working backwards 
from bin 2 to bin 1 or 8% of the way from bin 1 to bin 2? It seems like 
3.25 is only slightly larger than 3 and significantly less than 12.


i believe the overshoot correction should be the difference between the 
power we're looking for and the previous power divided by the difference 
between the current power and the previous power:


xi1 =  i - 1 + (0.25 * sum1) - sum2z)/(sum2 - sum2z)

Which in this case would yield:

xi1 = 2 - 1 + (3.75 - 3)/(12 - 3) = 1 + 0.75/9 = 1.08

Please let me know if I'm missing something and thanks for your time,

-Ryan Tolboom N2BP


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[wsjt-devel] FST4 Doppler Spread Overshoot Calculation

[Tolboom, Ryan via wsjt-devel]

Good Evening,

I'm looking at lib/fst4_decode.f90, specifically the code where it
calculates the frequency in the channel gain function, *g*, at which it
reaches a certain percentage of the signal power. This can be found on line
999 of lib/fst4_decode.f90

.

To prevent zero Doppler Spreads it uses a linear interpolation to get a
floating point "index" between bins of the FFT. It does this by setting xi1
(and xi2, xi3 in a similar fashion) to the current index minus one plus the
difference between the current power (sum2) and the power we're looking for
(sum2 * 0.25, for xi1) divided by the difference between the current power
(sum2) and the previous power (sum2z):

xi1=i - 1 + (sum2-0.25*sum1)/(sum2-sum2z)

If I'm interpreting this correctly this will not yield a correct result.

Let me give you an example:

For simplicity, assume we have four bins with powers 1, 2, 9, 2, and 1
respectively. Assume the bins are enumerated from zero.

[image: image.png]

The total power would be 15 and 25% of that would be 3.75. As we look for
that 25% mark, we'd find it in the do loop when i hits 2, sum2 is 12, and
sum2z is 3. xi1 would then be set to:

xi1 = 2 - 1 + (12 - 3.75)/(12 - 3) = 1 + 8.25/9 = 1.92

In other words, 92% of the way to bin 1 from bin 2. Shouldn't this be the
other way around? Shouldn't it be 92% of the way working backwards from bin
2 to bin 1 or 8% of the way from bin 1 to bin 2? It seems like 3.25 is only
slightly larger than 3 and significantly less than 12.

i believe the overshoot correction should be the difference between the
power we're looking for and the previous power divided by the difference
between the current power and the previous power:

xi1 =  i - 1 + (0.25 * sum1) - sum2z)/(sum2 - sum2z)

Which in this case would yield:

xi1 = 2 - 1 + (3.75 - 3)/(12 - 3) = 1 + 0.75/9 = 1.08

Please let me know if I'm missing something and thanks for your time,

-Ryan Tolboom N2BP
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