Ulrich Krämer

Frequency warping in low delay audio coding

The goal of the presented schemes is to obtain an ultra low delay audio coder with a good performance even at low bit rates (around 64 kb/s). The problem to be solved is to gain sufficient frequency resolution at low frequencies for precise low frequency psycho-acoustics and quantization noise shaping, because the ear has a higher frequency resolution at lower frequencies. Our approach is to use warped linear noise shaping pre- and post-filters, and a short DFT for the psycho-acoustic model (length 256), but with frequency warping. We compare four different psychoacoustic versions: DFT with no warping; DFF without warping using warped pre- and post-filters; warping with the so-called NDFT (WDFT); DFT with all-pass delay chain pre-processing. Listening tests show that the best performance is obtained using WDFT.