Described here are methods, devices and systems for characterizing a physiological signal such as an electrocardiogram (ECG) signal. Generally, the method includes receiving the ECG signal generated by an ECG device coupled to a patient. ECG signal quality for a plurality of consecutive beats may be determined based on a dispersion coefficient and a deviation from an average of each of a plurality of ECG parameters. Indexing information may be generated for set of sequences from the plurality of consecutive cardiac beats. Each of the sequences may comprise a first predetermined number of consecutive cardiac beats. The set of the sequences may be ordered based upon the ECG signal quality.