This study addresses device-specific distortion observed in recorded audio, to identify a built-in system-on-a-chip (SoC) in a portable device. A swept sinusoidal signal is emitted from a loudspeaker and is recorded by the portable device used in this study. The three types of distortion observed by spectral analysis of the recorded signals are the folded components at frequencies symmetrical across 4 kHz and 8 kHz of the signal component, non-harmonic and non-subharmonic distortion components whose frequencies are 4 kHz below and multiples of 4 kHz above the signal frequency, and mixed non-subharmonics and folded components in the low-frequency region. They are also observed using the correlation matrix on temporal amplitude variations among frequencies derived from the recorded speech signals.
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