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Fix transition bandwidth reported in 'filter too short' error (#11406)#14005

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CedricConday wants to merge 3 commits into
mne-tools:mainfrom
CedricConday:fix/notch-filter-error-message
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Fix transition bandwidth reported in 'filter too short' error (#11406)#14005
CedricConday wants to merge 3 commits into
mne-tools:mainfrom
CedricConday:fix/notch-filter-error-message

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@CedricConday

@CedricConday CedricConday commented Jun 30, 2026

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Fixes #11406

What was wrong

When a requested FIR filter_length is too short, the raised error reports the transition bandwidth as transition * sfreq / 2.0. But transition is computed as (prev_freq - this_freq) / 2.0 where the frequencies are already normalized to the Nyquist rate, so it is already half the Nyquist-normalized width. Multiplying by sfreq / 2.0 therefore displays half the real transition band — e.g. a requested l_trans_bandwidth=1.0 Hz is reported as 0.50 Hz.

this_N (the required length) is computed correctly; only the message is wrong.

Fix

Report transition * sfreq, which is the true transition bandwidth.

Test

Adds test_filter_too_short_error_reports_correct_transition: a too-short filter with l_trans_bandwidth=1.0 now reports 1.00 Hz transition band. Verified it reports 0.50 Hz on main and 1.00 Hz with this change.


AI-assisted, human-reviewed — I'm an AI engineer; I find, fix, and test with AI, then review and verify before opening.

…-tools#11406)

The error reported transition * sfreq / 2.0, but transition is already half
the Nyquist-normalized width, so it displayed half the actual transition
band (e.g. 0.50 Hz for a requested 1.0 Hz). Report transition * sfreq. Adds
a regression test.
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incorrect error message in raw.notch_filter

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