Time Frequency Resolution In BESA

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Module information
Modules BESA Research Complete (or Basic+Coherence)
Version 5.2 or higher

FAQ

The time-frequency resolution of the complex demodulation used in BESA depends on the user-settings for “Frequency and Time Sampling” in the “Coherence” tab of the ERP dialog box. The following table lists the 50% power and amplitude drops, respectively, as a function of the time-frequency sampling setting. Frequencies are given relative to the center frequency of a sharp frequency oscillation, latencies are given to the latency of a sharp time event. For example, with a time-frequency setting of 1.0 Hz, 50 ms, a sine wave with sharp frequency f will be smoothed to a time-frequency signal that has a full power width at half maximum of 2×1.42 Hz. In other words, at frequencies f ± 1.42 Hz, the displayed power in the time-frequency plot is half of the displayed power at frequency f. A sharp time-event, (e.g. a sharp spike in the time-domain) will be smoothed to a time-frequency signal of 2×78.8 ms full power width at half maximum.

Time-Frequency Sampling 50% Power Drop 50 % Amplitude Drop
0.20 Hz / 250 ms ±0.283 Hz, ±394 ms ±0.397 Hz, ±554 ms
0.25 Hz / 200 ms ±0.354 Hz, ±315 ms ±0.496 Hz, ±443 ms
0.5 Hz / 100 ms ±0.708 Hz, ±158 ms ±0.993 Hz, ±222 ms
1.0 Hz / 50 ms ±1.42 Hz, ±78.8 ms ±1.99 Hz, ±111 ms
2.0 Hz / 25 ms ±2.83 Hz, ±39.4 ms ±3.97 Hz, ±55.4 ms
2.5 Hz / 20 ms ±3.54 Hz, ±31.5 ms ±4.96 Hz, ±44.3 ms
5.0 Hz / 10 ms ±7.08 Hz, ±15.8 ms ±9.93 Hz, ±22.2 ms
10.0 Hz / 5 ms ±14.2 Hz, ±7.88 ms ±19.9 Hz, ±11.1 ms

The following two plots show the frequency blurring (left) and the temporal blurring (right) for a time-frequency sampling of 2.0 Hz / 25 ms. When applying a denser frequency sampling, the frequency resolution will increase, and the time resolution will decrease proportionally (i.e. the x-axis scaling will change proportionally).

Time Frequency Resolution.png