#### Phase in the Stopband

Practical zero-phase filters are zero-phase in their passbands, but
may switch between 0 and in their stopbands (as illustrated in
the upcoming example of Fig.10.2). Thus, typical zero-phase
filters are more precisely described as *piecewise constant-phase
filters*, where the constant phase is 0 in all passbands, and
over various intervals within stopbands. Similarly, practical
``linear phase'' filters are typically truly linear phase across their
passbands, but typically exhibit discontinuities by radians in their
stopband(s). As long as the stopbands are negligible, which is the
goal by definition, the -phase regions can be neglected
completely.

**Next Section:**

Trigonometric Identities, Continued

**Previous Section:**

-Phase Filters