Return to KLUBNL.PL main page
| To: | "LF Group \(E-mail\)" <[email protected]> |
|---|---|
| Subject: | LF: Signalling margins and Shannon |
| From: | "Talbot Andrew" <[email protected]> |
| Date: | Mon, 19 Feb 2001 16:59:03 -0000 |
| Reply-to: | [email protected] |
| Sender: | <[email protected]> |
|
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
|
| <Prev in Thread] | Current Thread | [Next in Thread> |
|---|---|---|
| ||
| Previous by Date: | LF: Re: Some signals, LAWRENCE MAYHEAD |
|---|---|
| Next by Date: | Re: LF: Slower and slower, WarmSpgs |
| Previous by Thread: | LF: Transatlantic, Mitch Powell |
| Next by Thread: | LF: Re: Signalling margins and Shannon, Alberto di Bene |
| Indexes: | [Date] [Thread] [Top] [All Lists] |