A monitoring apparatus comprising a multichannel pressure sensing sensor for measuring a ballistocardiographic signal of a human body is provided. The monitoring apparatus comprises a manner for selecting a time window for heart inter beat interval including two consecutive heart beats to be estimated, defining a spectrum for the signal averaging between at least two measurement channels of the sensor, a cepstrum from the logarithm of the spectrum, and a heart inter beat interval. A method for defining a heart inter beat interval is also provided, where a ballistocardiographic signal of a body is measured with a multichannel pressure sensing sensor, a time window for heart inter beat interval including two consecutive heart beats to be estimated is selected, a spectrum for the signal averaging between at least two measurement channels of the sensor, a cepstrum from the logarithm of the spectrum, and a heart inter beat interval are defined.
