Linear phase filters delay all frequencies by equal amounts, and this is often a desirable property in audio and other applications. A filter phase response is linear in whenever its impulse response is symmetric, i.e.,
in which case the frequency response can be expressed as
Substituting this into the QMF perfect reconstruction constraint (11.34) gives
When is even, the right hand side of the above equation is forced to zero at . Therefore, we will only consider odd , for which the perfect reconstruction constraint reduces to
We see that perfect reconstruction is obtained in the linear-phase case whenever the analysis filters are power complementary. See  for further details.
Conjugate Quadrature Filters (CQF)
Quadrature Mirror Filters (QMF)