Active learning using inexpensive sound cards for circuits and communications experiments

The sound card has become standard equipment in most computer ensembles. We discuss how Sound Blaster 16 cards, and their later generation successors, can be used to create a simple and inexpensive (audio frequency) network- or spectrum-analyzer. By use of existing shareware software, students may use the sound card to excite an external circuit with an audio tone, and simultaneously record the response of the circuit. In past work we have used various audio tones to excite the circuit, including linear- and exponentially-(frequency)-chirped tones, and white noise. By coupling the sound card inputs and outputs to an external breadboard containing simple circuits (such as RC low-pass and high-pass filters), we demonstrated the ability to excite these circuits, and through post-processing of the recorded data, extract both the magnitude and phase of the transfer function for the circuit under test. In this paper we focus on methods in which audio files can be passed among different applications. In particular, we illustrate the ability to import and export audio data from and into MatLab and Cool Edit, using Cool Edit's superior sound recording/playback capabilities in conjunction with MatLab's powerful signal and array-processing capabilities. We also present examples of several "communication theory" related experiments, designed to give students practical hands-on experience with basic analog (FM & AM) modulation/demodulation techniques.