Effective deployment of temporal noise shaping (TNS) filters
In the MPEG2 Advanced Audio Coder (AAC) standard, Temporal Noise Shaping (TNS) is currently implemented by defining one filter for a given frequency band, and then switching to another filter for the adjacent frequency band when the signal structure in the adjacent band is different than the one in the previous band. The AAC standard limits the number of filters used to either one filter for a “short” block or three filters for a “long” block. In cases where the need for additional filters is present but the limit of permissible filters has been reached, the remaining frequency spectra are simply not covered by TNS. This current practice is not an effective way of deploying TNS filters for most audio signals. We propose two solutions to deploy TNS filters in order to get the entire spectrum of the signal into TNS. The first method involves a filter bridging technique and complies with the current AAC standard. The second method involves a filter clustering technique. Although the second method is both more efficient and accurate in capturing the temporal structure of the time signal, it is not AAC standard compliant. Thus, a new syntax for packing filter information derived using the second method for transmission to a receiver is also outlined.
1. A method of deploying filters for use in processing audio signals, comprising:
calculating a filter for each of a plurality of frequency bands;
determining a distance between coefficients of filters in adjacent frequency bands;
merging filters with a shortest distance between coefficients to produce a set of final filters;
determining whether a stronger signal is mixed with one or more weaker signals in frequency bands covered by the final filters; and
recalculating ones of the final filters determined to have a stronger signal mixed with one or more weaker signals in a corresponding frequency band, the ones of the final filters being recalculated for the respective stronger signal.
2. The method of claim 1 , wherein said filters are TNS filters.
3. The method of claim 1 , wherein said coefficients are PARCOR coefficients.
4. The method of claim 1 , wherein said merging involves calculating a new filter for a frequency range comprising said adjacent frequency bands of said filters with said shortest distance.
5. A method of deploying filters for use in processing audio signals, comprising:
calculating a filter for each of a plurality of frequency bands;
comparing coefficients of filters in adjacent frequency bands to identify a pair of filters with a shortest Euclidean distance between coefficients;
merging said pair of filters to produce a set of final filters;
repeating all previously recited acts until a predetermined number of total filters is reached;
determining whether a stronger signal is mixed with one or more weaker signals in frequency bands covered by the final filters; and
recalculating ones of the final filters determined to have a stronger signal mixed with one or more weaker signals in a corresponding frequency band, the ones of the final filters being recalculated for the respective stronger signal.
6. The method of claim 5 , wherein said coefficients are PARCOR coefficients.
7. The method of claim 5 , wherein said merging involves calculating a new filter for a frequency band comprising said adjacent frequency bands of said filters with said shortest Euclidean distance.
8. The method of claim 5 , further comprising:
using at least one of said recalculated ones of the filters for an entire extent of said frequency range.
9. The method of claim 8 , wherein said strongest signal is identified based on energy/bin within said frequency range.