1. A system for predicting the onset of atrial fibrillation (AF) from an electrocardiogram (ECG) signal representing a patient, comprising: a signal processing component that determines parameters representing the activity of the heart of the patient from the ECG data, the signal processing component comprising a plurality of R-wave detectors that detect R-waves with respective channels of the ECG signal and a multichannel comparison component that confirms that each R-wave is detected in a threshold number of channels; a feature extraction component that calculates a plurality of features from the determined parameters, at least one feature being calculated from the detected R-waves; and a classification component that determines an AF index for the patient, representing the likelihood that the patient will experience AF, from the calculated plurality of features.