Translations:The shaping filter/14/en

Noncausal filters cannot be realized in real time because it is impossible for any present output to depend on future inputs. However, noncausal filters find widespread use in what can be called nominal time, which refers to a time scale on recorded data. The data have been observed and documented during some interval during the real-time past, and the time scale attached to the data refers to this historical time. With reference to this historical (or nominal) time scale, both the past and future of the recorded data are available to us. Thus, we can use noncausal filters to process the recordings. This technique is precisely the one used by a historian positioning herself or himself in the midst of some historical event whose future outcome is known already. It is also a technique we can use to analyze seismic records, because the seismograms were acquired some time ago in the field, and now we have the entire set of field recordings laid out before us.