Technote 138: Transfer Function Measurements with APx500 Audio Analyzers

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Create DateFebruary 19, 2019
Last UpdateJune 7, 2020

Technote 138 discusses the Transfer Function measurement added to APx500 audio measurement software in release version 5.0. We provide background information on transfer function measurements in general, followed by some practical examples of applying this measurement technique to some difficult audio test problems. The technote has also been updated to reflect the multiple simultaneous input types feature (multi-input) added in the version 6.0 release of APx software.

One of the key attributes of transfer function analysis is that it provides a means of measuring the frequency response of a device using any broadband signal, including speech and music. This makes it an ideal choice for analyzing devices used for speech communication (i.e., smart speakers, smartphones, headset microphones, etc.). Many of these devices incorporate DSP algorithms that require the use of speech signals, and some are designed to block sinusoidal signals altogether. Transfer function analysis greatly simplifies measuring the frequency response of such devices.

Included in TN138 is a description of the prepending of a short-burst MLS (Maximum Length Sequence) signal to the stimulus signal as a method to ensure a good match. An MLS is a special waveform that is much like white noise, but has properties that make it easy to detect by the match algorithm. A series of .wav files containing short MLS signals at different sample rates are available at Transfer Function MLS Prepend .wav Files.

TN138 - Transfer Function Measurements with APx500 Audio Analyzers
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