Dear Geri, Lionel, and LF group,
playing with slow voice, I found that the main problem with simply
time-stretching ("transposing") the audio spectrum is the slope of the
transmitter's SSB filter. It is not steep enough to pass a lower band edge of
about 75 Hz. One solution is to digitally shift the whole passband to the
middle of the normal speech spectrum, eg. a 1/4-rate bandwidth of 1450 ...
2000 Hz above 1378 Hz zero beat. Such a function is not normally present in
sound editing software, but by a combination of undersampling-aliasing and
bandpass-filtering, an SSB mixer can be emulated. In the appended .txt file
there is a description of how to (ab)use the GoldWave shareware sound-editor
for this purpose.
For receiving, you can reverse the procedure. An easier way for analog-minded
people like me is to record the slow SSB signal from the RX (eg. Windows
audio-recorder at 22050 samples per s), couple the PC's audio output to the
antenna input of your VLF converter (tuned to 5.5 kHz USB, plus 4x any offset
from the LF link), and replay the recording 4 times fast.
Locally, the scheme worked amazingly well, now I'd like to try it out on the
air. My preferred freq would be 136.88 kHz shifted zerobeat, 136.95 to 137.5
USB. With 16 mW radiated PEP this experiment will be rather restricted, so
who of my neighbours would like to try?
73s de Markus, DF6NM
How to produce time-expanded, frequency-compressed and passband-shifted SSB
using the GoldWave sound editor
by DF6NM, 26.12.2000
Basic principle:
- start with 5512 Hz sampling,
- equalize and filter,
- resample to 22050 Hz causing aliasing around n*5512 Hz,
- filter out USB spectrum above 5512 Hz,
- reduce speed to 1/4.
Working steps in GoldWave:
Record voice using 11025 Hz sampling rate
1. 11025 sps 8bit -> 16bit
2. Lowpass 400 Hz (steepness 1, 6dB/oct to equalize sinc response of alias)
3. Bandpass 300...2500 Hz
3. (Volume *2)
4. Resample 5512 Hz
5. Resample 22050 Hz (causes the desired aliasing above 5512 Hz)
6. Bandpass 5800..8000 Hz (steepn. 20)
7. Volume *4 *4 *4 *4
8. Playback rate 11025 sps
9. Speed *0.5
10. Bandpass 1400...2100 Hz (steepn. 20)
11. (Volume *1.2 or nonlinear dynamics + Bandpass)
12. Save (8bit)
Resulting SSB spectrum 1453...2003 Hz, zero beat 1378.12 Hz.
Using Windows audio-recorder "sndrec32.exe" does not interrupt spectrogram
while playing.
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