A filter apparatus (110; 300) for use in the control of a respirator comprising: a signal input (111; 301; 311) at which an input signal is present, which input signal contains a useful component and a noise component, wherein the input signal is a physiological signal from a patient; a fast signal path (112; 302; 312) and a slow signal path (113; 303; 313) arranged in parallel to the fast signal path; wherein the signal input (111; 301; 311) is coupled with the inputs of the fast signal path and of the slow signal path, such that the physiological signal passes through the fast signal path and through the slow signal path; wherein the fast signal path contains a filter to bring about fast filtering of the input signal; wherein the slow signal path contains a filter to bring about slow filtering of the input signal; wherein the signal travel time in the fast signal path is shorter by a factor of at least 2 than the signal travel time in the slow path; wherein an output of the slow signal path is coupled with the fast signal path via a signal line (116; 306; 316) to affect the characteristic of the fast signal path; a signal output (114; 304; 314) coupled with an output of the fast signal path and at which an output signal, which contains essentially useful components of the input signal, is present; and another signal line (117; 317) via which a signal is returned from the fast signal path to the slow signal path to affect the functional characteristic of the slow filter path; or a signal line via which an output of the fast signal path is coupled with an input of the slow signal path to affect the functional characteristic of the slow filter path; wherein the physiological signal, which is present at the signal input (111; 301; 311) and which is present at the signal output after its filtering by the filter apparatus, is used to drive a respirator (309; 319); and wherein the fast signal path has a filter with nonlinear properties and the slow signal path has a filter with linear properties or an adaptive filter. a signal input (111; 301; 311) at which an input signal is present, which input signal contains a useful component and a noise component, wherein the input signal is a physiological signal from a patient; a fast signal path (112; 302; 312) and a slow signal path (113; 303; 313) arranged in parallel to the fast signal path; wherein the signal input (111; 301; 311) is coupled with the inputs of the fast signal path and of the slow signal path, such that the physiological signal passes through the fast signal path and through the slow signal path; wherein the fast signal path contains a filter to bring about fast filtering of the input signal; wherein the slow signal path contains a filter to bring about slow filtering of the input signal; wherein the signal travel time in the fast signal path is shorter by a factor of at least 2 than the signal travel time in the slow path; wherein an output of the slow signal path is coupled with the fast signal path via a signal line (116; 306; 316) to affect the characteristic of the fast signal path; a signal output (114; 304; 314) coupled with an output of the fast signal path and at which an output signal, which contains essentially useful components of the input signal, is present; and another signal line (117; 317) via which a signal is returned from the fast signal path to the slow signal path to affect the functional characteristic of the slow filter path; or a signal line via which an output of the fast signal path is coupled with an input of the slow signal path to affect the functional characteristic of the slow filter path; wherein the physiological signal, which is present at the signal input (111; 301; 311) and which is present at the signal output after its filtering by the filter apparatus, is used to drive a respirator (309; 319); and wherein the fast signal path has a filter with nonlinear properties and the slow signal path has a filter with linear properties or an adaptive filter.