Subband splitting of a signal is often performed with a two-channel uniform filterbank. The filterbank is used several times to achieve finer bandsplitting, in a filter tree:
This way, the coder can split the input data into uniform bands, in each of which a quantizer, adopted to the masking threshold (see section 3), is applied.
The filterbank filterbank consists of four filters, , , and . The F's are used to split the input data in a high and a low frequency band. Both bands are then subsampled with a factor of 2. To reconstruct, both bands are upsampled again, and filtered with the G's. Thus, the output can be written as
To obtain aliasing cancellation, the following is required:
so that perfect reconstruction is achieved. This is, though, often hard to achieve. The filters used in most coders are Quadrature Mirror Filters (QMF), which achieve aliasing cancellation by choosing as the mirror image of (around ):
To get good reconstruction without relying on the aliasing cancellation, which does not work perfectly in the presence of quantization noise, the QMF filters have to have a steep pass-to-stop-band transition.