A physiological signal is recorded over a set interval against time and the sequence is divided into individual sections. Successive subsets of the sequence are mapped as an allocated series of symbols in accordance with a criterion. This addresses whether a signal magnitude rises or falls relative to the previous signal. Frequencies of the various signal series are determined. A test parameter is determined, to characterize behavior of series in terms of frequency in differing correlation steps, especially for correlation and anti-correlation within a section. Relative differences of the test parameters are determined for two adjacent sections. This is compared with a given threshold, as a criterion for time-constancy of the recorded signal. An Independent claim is included for the corresponding implanted apparatus, especially a defibrillator or pacemaker with an operational control program carrying out the evaluation as described.
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