On 4/27/19 3:37 PM, Bill Somerville wrote:
The approach we propose is to restrict the amount of smoothing applied
to make a GFSK modulated signal to a relatively low amount and to
perhaps apply an even smaller amount of smoothing to facsimile signals
used for subtraction, so as to minimize the overall loss of sensitivity
in a mixed FSK/GFSK environment. Obviously the intent is for all
stations to move to GFSK modulation eventually as it has clear benefits
despite the extra complexity of implementation, so any interim measures
to limit sensitivity loss when decoding FSK modulated signals will be
biased towards best performance for GFSK decoding. If we take that
approach I assume the intention will be to move towards decoding purely
optimized for GFSK modulation over time as users migrate. This in itself
is not a compatibility issue, rather a quality of implementation issue.
Fortunately the Gaussian function convoluted with a raw FSK signal to
make GFSK is continuously variable so we can pick one that exactly suits
our needs and vary it over releases as needed. The final choice of
Gaussian smoothig functions for transmission and for signal subtraction
has been made by doing empirical tests using multiple simulated FT8
signals and using sample data gathered on air. For example measuring how
many potential decodes are lost decoding real-World FSK modulated
signals using a decoder optimized for GFSK modulated signals.
Fortunately for the levels of smoothing we propose this loss of
sensitivity is relatively small as other effects like noise mixed with
signals tend to dominate.
Hi Bill and all,
Some years ago, I have designed some RTTY TU's using tubes, transistors,
IC's and OP amps. I have observed that the place where the optimization
process has given the most remarkable improvement was often the post
detection low-pass filter. On many popular designs, this filter has
simply been neglected.
Starting with the rather classical but conservative Bessel and gaussian
based polynomials, I have observed that the limits of the possible
improvement was quickly reached. But I have found that it's often better
to tolerate a little bit of overshot, rather than to demand zero
overshot. This tolerance of overshot has nearly no drawback but it offer
the ability to obtain a more steep raising and falling edge which is an
advantage for the following stage which produce finally a digital
signal. This result is simply obtained by using a different polynomial,
which is a little bit heuristic.
Such a post-detection low-pass filter is different from a pre-modulation
filter, about which one we are speaking here. But some similarities
remain. I can imagine that a polynomial of the same type could help to
improve the performances in a similar manner as it does in the case of a
RTTY TU.
Just an idea !
Best wishes,
Claude (DJ0OT)
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