Part 11 of this series provided a general description of digital signal processing (DSP) as used in software-defined radios (SDRs) and included an overview of a full-featured radio that uses a PC to perform all DSP and control functions. Part 2 described Visual Basic source code that implements a full-duplex quadrature interface to a PC sound card. As previously described, in-phase(I) and quadrature (Q) signals give the ability to modulate or demodulate virtually any type of signal.
The Tayloe Detector, described in Part 1, is a simple method of converting a modulated RF signal to baseband in quadrature, so that it can be presented to the left and right inputs of a stereo PC sound card for signal processing. The full-duplex DirectX8 interface, described in Part 2, accomplishes the input and output of the sampled quadrature signals. The sound-card interface provides an input buffer array, inBuffer(), and an output buffer array, outBuffer(), through which the DSP code receives the captured signal and then outputs the processed signal data.
More at http://www.arrl.org/tis/info/pdf/021112qex027.pdf
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