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