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Time-Varying Filter Banks with Low Delay for Audio Coding

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A design method and filter structure for fast modulated low delay filter banks is presented. The system delay can be pre-specified independent of the filter length. Perfect reconstruction is structurally guaranteed. It is shown how to make this structure time-varying, which includes switching the number of bands, while maintaining perfect reconstruction. Low delay filter banks, besides having the advantage of a low system delay, can reduce pre-echoes in coding applications. Design examples are shown, including a low delay filter bank which can be switched between 1024 and 128 bands. Results of listening tests with the MPEG-AAC coder show that they indeed can be used to reduce pre-echoes in non-stationary signals like castanets.

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