A Quadrature Mirror Filter (QMF) is a type of digital filter that is commonly used in signal processing, particularly in applications like subband coding, audio compression, and wavelet transforms. The primary purpose of a QMF is to split a signal into two frequency bands, typically low and high frequencies, in such a way that the original signal can be perfectly reconstructed when these bands are combined.
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For an orthonormal wavelet refinement filter, is equivalent to the displayed identity. A high-pass filter can be chosen with coefficients . In frequency, its symbol is , up to a constant phase. The two channels split an approximation space into a coarser approximation and its orthogonal complement.