[Lf] [Fwd: LF: Signalling margins and Shannon]

[Andre' Kesteloot]

Talbot Andrew wrote:

>  Has anyone looked at what Signal to Noise is realistically viewable
> on a spectrogram display ?   Assuming bandwidth equal to FFT bin size,
> my few tests suggest around 10dB but it would be nice to have a more
> authoritative  figure to calculate with.   Obviously depends on colour
> palette and number of colours on the display so after a couple of
> years of experience with Spectrogram and now Argo, what has anyone
> found ?A back of envelope calculation on signalling capabilities using
> DFCW :(Switch off now those not interested in communications theory
> and working out what Could be done
> <:-)-------------------------------------------------For DFCW, signal
> bandwidth needs to be at least twice that of a dot interval in order
> to be able to distinctly see the on-off transitions.   Doubled again,
> since two frequencies are involved, but some overlap of sidebands is
> allowed as we can sttill se which of the frequencies is intended, so
> take a total signalling bandwidth of 3 times the dot interval as being
> the minimum needed.   This gives a signalling rate of   1 / 3 Bit /
> second per Hz   (0.33 B/s/Hz) Shannon's law relates signalling rate to
> Signal to Noise by :R = LOG(1 + S/N)     (LOG to the base 2 and S/N in
> numerical units, not dB)so for R = 0.33, signalling should be possible
> (at an 'arbitrarily low' error rate)  in a S/N of 0.258  = -5.8dB
> (yes, negative S/N)If 10dB S/N is needed for viewing then we are
> almost 16dB down on ShannonAnd this does not even allow for further
> signal degradation due to multipath / fading,The only scheme I have
> ever come across that reckons to get within less than 1 dB of Shannon
> makes use of the very latest Turbo coding schemes now possible with
> high speed DSP, coupled with continuous phase modulation (partially
> related to MSK)    It is / was a contender for third generation mobile
> phones to increase data rate there in the congested bandwidth
> available.   PSK and QAM modulation sits somewhere between.Coding
> :DFCW codes the alphabet into between 1 and 5 bit intervals per
> character plus a gap for inter letter spacing, which, with the two
> frequency level coding, equates to between 4 to 12 bits per
> character.   The letter frequency of plain language text coupled with
> the one bit for E, two for A,N and 4 for Q, Z etc means there is
> probably an average of around 5 - 6 bits per letter, which is not bad
> coding efficiency - simple Baudot manages 7.5 bits / character and
> PSK31 about 5.5 - 6.  The very best dictionary based coding schemes,
> transmitting codes to represent words or even whole phrases, can claim
> 1 bit or less per letter equivalence.However, the coding efficiency
> only affects the time to send the overall message since it dictates
> the Total number of bits needed, not the S/N needed in which to send
> it;  that only dictates the rate.Andy  G4JNT
>
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