No empty room recording

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Hi,
I'm working with meg data set but I do not have empty room recording.
How can I compute noise co-variance without empty room recording?
Can I use identity matrix?

External Email - Use Caution

You can use mne.make_ad_hoc_cov, which creates a diagonal matrix, or use a
covariance computed from a baseline interval before your events of interest
(if it's an event-related paradigm).

Eric