A data acquisition filter (19) for a dual beam CT machine uses two spectral filters (74,76) for alternately receiving the signal (64) from each detector (26) depending on the state of the x-ray beam (24). When one filter is filtering the detector signal, the other filter is in a "hold" state where its output and internal values are frozen. Each filter effectively filters only the signal occurring during one beam state without being effected by the signal occurring during the other beam state or by the passage of time during the other beam state The output of the filter in the holding state is constant and may be sampled at any time during this period.