A 45-year-old man with severe traumatic brain injury is admitted to the ICU. Clinical examination shows an irreversible coma, absent brainstem reflexes, and a positive apnea test (PaCO₂ 68 mmHg). An EEG is performed as a confirmatory ancillary test. The recording shown displays the pattern marked **A** — electrocerebral inactivity (ECI). Which of the following technical requirements is MANDATORY for this EEG recording to be valid as a confirmatory test for brain death according to ACNS standards?