At 01:22 PM 12/7/00 +0000, Jim Moritz wrote:
Dear LF Group,
Was interested to see Bill's report of the reception of LEK using
slow CW. I played the .WAV file back using spectrogram - I have to
admit I could not make out the CW by ear (it's rather a short
sample), but the signal is clear enough on the spectrogram trace -
it looks like 3s dots, once the compression factor of 40 is taken
into account.
Something else that is interesting is the noise on the display. If you
look at a night-time spectrogram at my QTH, it is like a crazy tartan
pattern, with vertical streaks every few seconds due to QRN, and
horizontal ones every few Hz due to various signals and Loran. On
VE2IQ's sample, there is nothing in the bandwidth apart from LEK
and what appears to be random noise (although I suppose this
would only correspond to a bandwidth of about 10Hz in real time) -
this is what it sounds like too. Is this an effect of the 'Crunch'
compression process, or does it mean that Bill's QTH is very quiet,
or mine is very noisy?
I have lots of noise here!
The CRUNCH process does have the effect of "spreading out the spectrum"
around the frequency of interest. The basic process I use is to *mix*
the signal down to some very low frequency (in this case 800 Hz was
mixed down to 20 Hz) - then to *multiply* it back up to 800 Hz. The
frequency multiplication process is what makes the desired signal seem
to stand out all by itself. For example, if there was an interfering
carrier 10 Hz away from LEK's signal, after mixing down it would still
be 10 Hz away, but after multiplying back up to 800 Hz it would be 400 Hz
away and then gets taken out by the 300 Hz bandpass filter on the
output. I could have used a much *narrower* bandpass filter on the
output in this particular case because I know Lyle's station is always
right on the published frequency, but in general I keep it wide because
lowfer beacons are often several Hz off frequency and they risk going
outside the passband (after multiplication) if the filter is too narrow.
Bill VE2IQ
|
