Stanford Research Systems www.thinksrs.com-SR1 SweptSine free download

File name www.thinksrs.com-SR1_SweptSine.pdf

Swept Sine Chirps for Measuring Impulse Response Ian H. Chan Design Engineer Stanford Research Systems, Inc. Log-sine chirp and variable speed chirp are two very useful test signals for measuring frequency response and impulse response. When generating pink spectra, these signals posses crest factors more than 6dB better than maximum- length sequence. In addition, log-sine chirp separates distortion products from the linear response, enabling distortion-free impulse response measurements, and variable speed chirp offers flexibility because its frequency content can be customized while still maintaining a low crest factor. 1. Introduction Impulse response and, equivalently, frequency response measurements are fundamental to characterizing any audio device or audio environment. In principle, any stimulus signal that provides energy throughout the frequency range of interest can be used to make these measurements. In practice however, the choice of stimulus signal has important implications for the signal-to-noise ratio (SNR), distortion, and speed of the audio measurements. We describe two signals that are generated synchronously with FFT analyzers (chirp signals) that offer great SNR and distortion properties. They are the log-sine chirp and the variable speed chirp. The log-sine chirp has a naturally useful pink spectrum, and the unusual ability to separate non-linear (distortion) responses from the linear response [1,2]. The utility of variable speed chirp comes from its ability to reproduce an arbitrary target spectrum, all the while maintaining a low crest factor. Because these signals mimic sines that are swept in time, they are known generically as swept sine chirps. 2. Why Swept Sine Chirps? 1.0 Most users are probably familiar with 0.5 measuring frequency response at discrete Amplitude (V) frequencies. A sine signal is generated at one 0.0 frequency, the response is measured at that frequency, and then the signal is changed to another -0.5 frequency. Such measurements have very high signal-to-noise ratios because all the energy of the -1.0 signal at any point in time is concentrated at one 0.100 0.101 0.102 Time (s)