Content-transfer-encoding: 8bit

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 Shannon

And 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





-- 

The Information contained in this E-Mail and any subsequent correspondence

is private and is intended solely for the intended recipient(s).

For those other than the recipient any disclosure, copying, distribution, 

or any action taken or omitted to be taken in reliance on such information is

prohibited and may be unlawful.