[amsat-bb] Re: Decoding wideband recordings

Phil Karn karn at philkarn.net
Thu Aug 18 20:11:44 PDT 2011


On 8/18/11 8:15 AM, Patrick Strasser OE6PSE wrote:

> For decoding please be aware that I/Q via sound interfaces has a weak
> spot at the centre frequency. The interfaces all have a high pass
> characteristic below like 30Hz, which means the resulting spectrum has a

Yes, I'm familiar with this. I will ask users to put this notch in the
center of the transponder band (145.938 MHz) so that the BPSK-1000
beacon will appear at about -18 kHz. 0 Hz will correspond to the notch,
the transponder will appear between -16 kHz to 0 Hz and the FM signals
(voice, sstv) will appear in the positive frequencies centered at +12 kHz.

It will be easier if Doppler is *not* corrected for so I can do that in
software. This will move the notch +/- 3.3 kHz around the center of the
passband, and that might interfere a little with the top of the
transponder passband between AOS and closest approach.

If for some reason the I/Q channels are swapped or there's a polarity
inversion simply flipping the sign on the frequency should take care of
it. The BPSK-1000 signal is symmetric so it can tolerate sideband
inversion. (USB reception on a conventional SSB receiver is recommended
only because the CW beacon is 1 kHz below the BPSK-1000 beacon. This
puts the CW beacon at an audio frequency of 500 Hz when the BPSK-1000
suppressed carrier is at its nominal 1500 Hz. It could be received just
fine in LSB mode but then the CW beacon would have to be tuned to an
uncomfortably high pitch of 2.5 kHz, and it would be harder to tune it
accurately by ear.)

> What I wonder is how good BPSK1000 survives speech encoders, like MP3.
> Is there any experience already?

Much to my amazement, during prelaunch testing I was able to decode
BPSK-1000 that had been run through lossy codecs like MP3. But I don't
recommend it. If the recording data rate is a problem, reduce the
sampling rate while ensuring that the 2 kHz wide BPSK-1000 signal is
completely captured within the Nyquist bandwidth of your A/D converter.
E.g., 8000 or 9600 Hz should be fine provided the BPSK-1000 signal is
centered at 1500 Hz. Although the demodulator assumes an A/D sampling
rate of 48 kHz, a recording with a lower sampling rate can be converted
back to 48 kHz with a program like 'sox' as long as the information is
still there.

73 Phil




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