A high-frequency resolution is small sub-bands in the lower frequency region, whereas a lower resolution in the higher frequency region with wide sub-bands should be the basis for an adequate calculation of the masked thresholds in the frequency domain. This would lead to a tree structure of the filterbank. The poly- phase filter network used for the sub-band filtering has a parallel structure, which does not provide sub-bands of different widths. Nevertheless, one major advantage of the filterbank is given by adapting the audio blocks optimally to the requirements of the temporal masking effects and inaudible pre-echoes. A second major advantage is the small delay and complexity. To compensate for the lack of accuracy of the spectrum analysis of the filterbank, a 1024-point FFT for Layer II is used in parallel with the process of filtering the audio signal into 32 sub-bands.The prototype QMF filter is of order 511, optimised in terms of spectral resolution and rejection of sidelobes, which is better than 96 dB. This rejection is necessary for a sufficient cancellation of aliasing distortions. This filterbank provides a reasonable trade-off between temporal behaviour on one side and spectral accuracy on the other side. A time/frequency mapping providing a high number of sub-bands facilitates the bit rate reduction, due to the fact that the human ear perceives the audio information in the spectral domain with a resolution corresponding to the critical bands of the ear, or even lower. These critical bands have a width of about 100 Hz in the low-frequency region, that is below 500 Hz, and a width of about 20% of the centre frequency at higher frequencies.The requirement of having a good spectral resolution is unfortunately contradict- ory to the necessity of keeping the transient impulse response, the so-called pre- and post-echo, within certain limits in terms of temporal position and amplitude com- pared to the attack of a percussive sound. Knowledge of the temporal masking behaviour [Fastl, 1977] gives an indication of the necessary temporal position and amplitude of the pre-echo generated by a time/frequency mapping in such a way that this pre-echo, which normally is much more critical compared to the post-echo, is masked by the original attack. In association with the dual synthesis filterbank located in the decoder, this filter technique provides a global transfer function optimised in terms of perfect impulse response perception.
đang được dịch, vui lòng đợi..