Short-Time Fourier Transform (STFT)
Definition
A signal processing technique that applies a Fourier transform to successive short windows of a signal, producing a time-frequency representation. Used in ENF extraction to track how the dominant frequency near 50 Hz or 60 Hz changes over the course of a recording.
- Input
- A time-domain signal split into successive short windows
- Output
- Time-frequency representation (spectrogram)
- Forensic application
- ENF extraction, tracking the mains frequency near 50 or 60 Hz over a recording
- Key parameter
- Window length, which trades time resolution against frequency resolution
Common questions
Why is a plain Fourier transform not enough for ENF analysis?+
A single Fourier transform over the whole recording gives only the average frequency content, losing when each frequency occurred. ENF analysis needs to see how the mains frequency drifts second by second across the recording so it can be matched against a reference database, which requires the time-localised STFT approach instead.
Does the STFT window length affect how reliable an ENF match is?+
Yes. A window too short gives poor frequency resolution, making small ENF fluctuations hard to measure precisely, while a window too long smears out genuine short-term frequency changes; examiners tune window length against the known behaviour of the local power grid to balance the two.
Related terms
- Cross-Correlation
- A statistical measure of the similarity between two time series as a function of a time lag. In ENF matching, the extracted...
- Electric Network Frequency (ENF)
- The instantaneous frequency of the alternating-current mains supply, nominally 50 Hz in most of the world and 60 Hz in North America....
- ENF Discontinuity
- A break in the continuity of the extracted ENF trace that is inconsistent with the reference database. Can indicate a deletion (missing...
- ENF Reference Database
- A continuously maintained archive of grid frequency measurements recorded at high temporal resolution, used as the ground truth against which a recording's...
- Transmission System Operator (TSO)
- The entity responsible for operating a high-voltage electricity transmission network in a given country or region. TSOs log grid frequency in real...