[Fwd: LF: SlowCW and Dwell periods]
Fri, 02 Apr 1999 07:24:08 -0500
Andy Talbot wrote:
> >I don't know what "dwell" really means in this topic but I
> >think it's the time of data collection and folding/averaging
> >in background. What means "smoothing"? In time or frequency
> >domain? If it means "data windowing" the WELCH algorithm
> >outperforms the other (Rectangular, Gaussian, Parzen,
> >Hanning, etc.) since it does not "smear" too much,
> >especially in case of LORAN-C background.
> My understanding of the GRAM software is that the 'dwell' time is just a
> period of wasted time idling between performing an FFT on each set of data
> rather than increasing the FFT length. If this is the case then there is no
> advantage at all in going to longer dot lengths USING THE GRAM SOFTWARE as
> there is no way of further narrowing the receive bandwidth with a narrower
> FFT bin. In which case alll the power being transmitted during each dwell
> period is going to waste. I seem to recall that the author of the
> programme quickly included the dwell option at the request of LF operators,
> suspect that but not being particularly interested in the radio related
> aspects of that software did not try to take it any further. I may be
> wrong, but the dwell option appeared so quickly that a simple delay is the
> most likely route for generating it.
> The lowest bandwidth therefore possible is from 8000 Hz sampling with a
> 16384 Point FFT giving a bin size of 0.49 Hz and therefore a bandwidth of
> close to 1 Hz using a Hamming window. Therefore, with 1 Hz BW the 3
> seconds dots everyone seems to have settled on as being optimum will fill 3
> FFT samples ans show up clearly as a line of three pixels or more depending
> on what overlap has been incorporated in the data sampling. But there will
> NEVER be any advantage in going to longer dots unless the sampling rate is
> reduced (will GRAM allow 5513 Hz sampling ?) or the FFT size can be
> increased above 16384.
> I use the DSP EVM board to mix the audio down to DC and then data reduce /
> filter to arbitrarily low bandwidths to give the full advantage of going to
> longer dot. periods with no wasted dwell time. The current limit is in the
> region of a reciptocal-minute bandwidth, around 10mHz, meaning dot lengths
> of 40 - 60 seconds could be used and it would be a straightforward job to
> increase the decimation further.
> GRAM does not use the mix down and decimate technique, but it just may be
> possible for the author to include a longer FFT routine as an option if
> asked. I have seen million point FFTs processed on Pentium PCs in less than
> one second, so it is certainly possible! It is also possible to ignore
> alternate samples of the input data, giving an effectively reduced input
> sampling rate - that is what decimation is all about - but the audio must
> then be pre-filtered to prevent any alliasing terms being generated as no
> filtering is being done in software.
> However, if a CW receiver with a narrow filter giving an audio output in the
> region of say 500 - 900 Hz is used, there is no reason why the 8kHz sampling
> can't be decimated by four giving an effective sampling of 2kHz, and four
> times the resolution bandwidth with the same 16K FFT. That is a 6dB S/N
> improvement immediately for a software mod as straighforward as including a
> dwell time.
> 6dB of capability is being wasted anyway, using these non-coherent energy
> detection techniques, but that's another story ...............
> Andy G4JNT