Constant-phase sine/cosine filter

The sine/cosine filter has several applications in audio equipment thanks to its virtually constant phase shift. As an example, the design shown here is tailored for use in a surround-sound encoder, in which the surround information is shifted 90° and then added to the stereo channels. For a proper effect, the bandwidth has to he limited to 100 Hz to 7 kHz with the aid of a filter. Unfornatunatly, that filter causes further phase shifts. The alternative solution to this problem is phaw correction for high and low-pass filters described elsewhere in this publication.