Spread spectrum based detection using a sound card

Lock-in amplifiers (LIA) are currently used for signal recovery in the presence of high noise [1]. They are based on low-pass filtering the product of a sinusoidal signal by a reference sinusoid with the same frequency, effectively calculating the zero-time correlation between sinusoid and signal (Figure 1). To ensure that the frequencies of signal and reference are the same, in a LIA signal and reference are phase-locked. We note that LIAs can be sensitive to narrowband noise with a frequency closer to the reference frequency than the bandwidth of the low-pass filter. In this respect, generating a signal synchronous with a pseudorandom reference, and detecting it by correlation with the sequence itself, offers promise of a better noise immunity [2]. In this work we investigate a computer sound card as an acquisition system for the extraction of signal by correlation with a pseudorandom sequence.