From VE2IQ
It worked on first try.  I will now optimize the filter coefficients

and

write a single program to go directly from digitized samples on disk at
7200 s/s to a .wav file time-compressed at 8000 s/s.

Anybody else tried this?

It was a bit of fun after my 393km 73kHz contact with G3PLX in 1997
using 50 second dot CW.  Peter recorded my signals in I/Q format after
mixing down to zero IF at a sampling rate of  6.25 Hz, then sent me the
raw data file.   I treated this as if it were sampled at a much higher
rate - trivial in DSP by just pretending it is at a higher rate -
effectively multiplying the signalling by 1000 times.  Then upconverted
to audio by multiplying the I/Q pairs by SIN / COS samples of a tone of
800 Hz.  The resulting samples were then stored in a .WAV file which
could be played back conventionally.

50s dots became 50ms which was about 24 WPM (which 'JNT could copy coz
he knew what was being sent).   A signal comfortably visible on the
screen (10 - 15dB S/N in 0.05 Hz bandwidth) became a conventional CW
signal easy to copy - with the same S/N effectively in 50Hz bandwidth.
Incidently, there was no ringing on this in spite of the narrow filter
due to the ability to perfectly optimise DSP filtering.

When we tried with weaker signals, it was interesting, but probably not
surprising that a signal that became difficult to see also became
difficult to hear.  To my mind, and memory of three years ago, the
results for audio and visual were very similar.

Doing the same rate conversion on an early evening to late morning
3.5MHz Dopplergram recording, (see G3PLX's article in RadCom a couple of
years ago), resulted in the most wonderful 'whale' like whistling and
tones as the ionosphere shifted the signal up and down and introduced
extra components.  Another in the 'natural sounds' series -  wonder if a
recording company would be interested ?

Andy  G4JNT

Quote
I don't know if this has been tried before, but I just tried it and it
worked nicely.

The general idea is to record an ultra-slow CW signal, then to play it
back speeded up so the human operator can copy the CW message by ear
at normal speed.  Ham operators have years of experience trying to dig
weak CW signals out of the noise - by ear - not by looking at a
picture
on a computer monitor.

I used VA3LK's ultra-weak test signal.
Using FFTZZ, I knew the signal was being received at 803 Hz - after
some
minutes of integration, the spectral line came up out of the noise, so
I
knew beforehand what the exact frequency was.  Unfortunately the
signal
was way too weak to be able to decode by any spectral display
technique
available to me, so I hit upon the idea of time-compression.

I recorded some 578 seconds of audio to hard disk at 7200 samples per
sec.
Then I post-processed that file as follows:

1.  Run it through a narrow bandpass filter centered on 803 Hz.

2.  Multiply the resulting data with a sinewave at 825.4 Hz - that
    acts like a mixer producing sum and difference frequencies at
    1628.4 and 22.4 Hz respectively.

3.  Run that waveform through a 32-point FIR lowpass filter to keep
    only the 22.4 Hz component.

4.  Keep only 1 resulting filtered sample out of 32 - essentially
    compressing the total recording time by a factor of 32.

5.  Make it into a .wav file specified as sampled at 8000 s/s and
    lasting 16.25 seconds.

When I played the 16.25 second wav file back I could actually hear the
CW at a reasonable speed (about 35.5 times faster than it was
transmitted)
and at a reasonable tone (about 800 Hz).  The ident was easily
recognized.

It worked on first try.  I will now optimize the filter coefficients
and
write a single program to go directly from digitized samples on disk
at
7200 s/s to a .wav file time-compressed at 8000 s/s.

Anybody else tried this?

Bill VE2IQ

Unquote

I have the .WAV file and will send to anyone who asks.

Larry
VA3LK

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