The recent correspondence on Slow-Audio is of interest to me as it would
need techniques very close to those I have been using with the PC/soundcard
combination to receive slow Morse.
I am using FFT/inverse FFT filtering to filter signals in the 1000Hz region
through selectable bandwidths from 30 down to 4 Hz. To speed up a signal 16
times I then transpose the signal down to one sixteenth of the frequency
and then take an average of successive groups of 16 samples which restores
the pitch back up to the original, and then stores this in a .WAV file. It
is thus possible to record 1 wpm CW and play it back at 16 wpm and at the
original pitch. Speed-up factors of X2 to X16 are available.
This program is at an early stage but if anyone would like to have a play
then download:
http://www.lsear.freeserve.co.uk/qrss.zip
The program runs under Windows95 and requires a modest PC with a soundcard.
It is at an early stage but has been tested on 80m between myself and G3SMC,
and also KF4HEY has some ongoing tests using simulation software. It works,
but I cannot guarantee that it will outperform the skillful use of the
waterfall display. However, it may be that under some conditions the ear may
be better than the eye. One slight problem occurs with some selected
frequencies where the audio blocks do not join up seamlessly and give a
clicking on the audio.
As far as slow audio goes, I wonder if it would be possible to record 2 kHz
bandwidth speech into a .WAV file at 44100Hz and then alter the sampling
frequency in the file header to 11025Hz. Playing this would result in the
speech with a duration four times as long but in one quarter the bandwidth.
The frequency band would span 1 to 500Hz, so this would need to be
transposed up to 300-800Hz in order to go through an SSB filter. Reception
could be by similar techniques to those used in QRSS.EXE referred to above
but with a further transposition to restore the original pitch. With the
SB16 card, it would be possible to play a .WAV file at 5513Hz though a 250Hz
bandwidth but not all soundcards allow the choice of non standard
frequencies.
Sorry about the long email, but I think it worth mentioning that I was in
the LF group at the start of things and became interested in the DSP aspect,
indeed it was Andy Talbot's suite of PowerBasic programs that gave me a
start. The DSP bit took me off at a tangent and I have spent the intervening
time developing programs for Hellschrieber and MFSK. One development,
SLOWFELD.EXE, where FFT bins are plotted as the individual pixels in a form
of Hellschreiber running at 2 characters/minute, would I humbly suggest, be
ideal for LF use, since it gives a bandwidth of 2Hz. Take a look at:
http://www.lsear.freeserve.co.uk/slowfeld.zip
Again it uses a PC/soundcard with Windows95 but it allows for the on/off
keying of a simple Tx as well as outputting audio for SSB. Maybe it has been
tried on LF but I have had no confirmation of this
73
Lionel
G3PPT
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