Slow CW vs. BPSK. & Computer modes]
Fri, 02 Jun 2000 08:36:18 -0400
Talbot Andrew wrote:
> >This is all-important. Extremely weak stations require considerable
> >additional work by the 'computer between the ears' to decipher
> >what is signal and what is noise on the screen - just like aural
> >Morse, but visual. This accounts for several extra dB of gain, and
> >however slow you send the Morse, this advantage is still available.
> >It would take a very sophisticated computer to be as good.
> NO ! With these modes the computer is not doing anything to aid
> extraction of the signal that could not be done with conventional
> hardware. The 'several extra dB gain' is just there because more
> noise has been filtered out. The brain is just looking at amplitude
> changes in this narrow bandwidth and integrating over the signalling
> period (looking at brightness over the length of a dot).
> All Spect.... is, is a bank of narrow filters. There is nothing in
> theory to stop you making a crystal filter, or a whole bank of them this
> narrow. For a display mechanism, a chart recorder - multiple pen ones
> are available. All this technology was available in the 1950s as I'm
> sure those who were around in that era will remember.
> All the computer does is make this much simpler and cheaper and
> available to everyone, it is only a filter and display mechanism so
> please can we stop referring to Spect... as computer modes. It can
> be done in other ways.
> Coherent etc and PSK31 are computer modes, SPECT..... is not, it is
> only a filter bank and display.
> Real computer / DSP modes mean making use of coherent detection and
> error correction - and have nothing to do with the speed of the
> There seems to be no appreciation on this newsgroup of the real value of
> coherent detection - that is, having phase information available when
> the signal is decoded and coherently locking to the signal carrier and
> bit timing. Simple theory, as I covered in a previous email, shows
> mathematically the considerable theoretical advantage of coherent
> detection over non coherent, there is no dispute about this.
> SO unless your ears / eyes can respond to signal phase, is doesn't
> matter how much personal pride says that the brain is a marvelous
> computer, is will never compete against PSK :
> Given the same TOTAL overall signalling speed and comparing like with
> like on the end to end link.
> Repeating CW characters time and time again has to be compared with true
> error correction and low data rate signalling.
> Computer error correction does the same job as the eye interpreting
> between the dots on Spect..., and with well chosen codes can do an
> awful lot better. These optimum codes are only just beginning to appear
> thanks primarily to the huge research effort funded by the mobile phone
> industry, but hopefully Coherent etc will soon make use of them - the
> equations are public information. The maths is similar, just the
> implementation different.
> Coherent etc and PSK31 are computer modes, Spect... is not.
> Andy G4JNT
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