Brian Lloyd wrote:
On Fri, Mar 12, 2010 at 12:05 AM, [email protected]
<[email protected]> wrote:
"Theoretically", it starts automatically when an APT start sequence is
detected. I do not know how to enhance the process at the moent. Maybe a
better filter. The APT frequency has some margin, but if it is too large, it
starts decoding each time 300 hz is detected in noise. So, the method should
be refined.
Instead of comparing the 300Hz power to a constant threshold, compare
it to the power in nearby bins in order to determine S:N power at
300Hz. That should be self-adjusting.
Quite interesting; it might be used as well for the stop frequency at
450 hz.
At the moment, the frequencies are detected in the function
fax_implementation::decode_apt :
It counts the number of transitions between [0,25] and [ 229,255],
divided by the number of samples. This ratio is multiplied by the
sample_rate 11025 hz, then compared to 300 hz (450 hz for stop), with an
empirical margin of a couple of Hz. I took this algorithm from hamfax
but it does not work very well when the signal is poor.
At the moment, I am concentrating on the detected bugs but later, do you
please know whether there are already available tools in fldigi to have
this done ?
Thanks again.
Cheers
R
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