On Sun, 2010-02-28 at 20:42 +0000, Victor Lazzarini wrote: > Pardon the ignorance, but are the RF signals brought down to AF before > AD conversion? Otherwise, it sounds like a silly SR will be needed. Is > this the flowchart? > > antenna -> RF demodulation -> ADC -> your software -> DAC -> audio
Not quite... > So what exactly does your plugin do? The incoming RF is downconverted to the audio range, but not demodulated. Effectively you've mixed a chunk of RF (7MHz band, in the case of my sample file) down to an audio range but not yet demodulated. At this point you may have a signal at 7066kHz which has been downmixed to 10kHz, but that's outside the receiver passband - you can't hear it. By mixing a further 10kHz signal into the audio you bring that down to 0kHz, and if it's an LSB speech signal it now occupies a range of frequencies up to about 3kHz with an "image" at 20kHz. Now we've got it in the speech range and we can demodulate it by summing the imaginary and real components so that the image cancels out. Think of the SDR hardware as being a pair of direct-conversion receivers with the local oscillators 90 degrees out of phase so the output is a vector rather than a scalar. Gordon MM0YEQ _______________________________________________ Linux-audio-dev mailing list [email protected] http://lists.linuxaudio.org/listinfo/linux-audio-dev
