Method and apparatus for reducing aliasing in cascaded...

Data processing: speech signal processing – linguistics – language – Speech signal processing – For storage or transmission

Reexamination Certificate

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C704S205000, C708S313000, C708S402000

Reexamination Certificate

active

06718300

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to cascaded filter banks, and more particularly, to methods and apparatus for reducing the aliasing in such cascaded filter banks.
BACKGROUND OF THE INVENTION
In signal compression applications, a filter bank with unequal band splitting is often desired to obtain the maximum coding gain. In audio coding applications, for example, band splitting also determines how well the time and frequency shape of the quantization noise after the signal has been decoded is matched to the psycho-acoustic audibility threshold. See, for example, D. Sinha et al., “The Perceptual Audio Coder,” Digital Audio, Section 42, 42-1 to 42-18, (CRC Press, 1998), incorporated by reference herein.
To obtain such unequal band splitting, tree structures are often used. A tree structure may implement, for example, wavelet transforms, such as those described in M. Vetterli & J. Kovacevic, “Wavelets and Subband Coding,” Prentice Hall (1995), where one or more frequency bands
120
,
130
, such as the lower frequency band
120
, is further split into additional subbands
140
-
1
through
140
-
3
, as shown in FIG.
1
. Other approaches take a uniform filter bank and join several subbands into one wider band to increase bandwidth, such as described in H. S. Malvar, “Lapped Biorthogonal Transforms for Transform Coding with Reduced Blocking and Ringing Artifacts,” International Conference on Acoustics, Speech, and Signal Processing, Munich, Germany, 2421-2424, (1997). Another approach joins several pieces of different uniform filter banks using transition filters, such as described in J. Princen & J. D. Johnston, “Audio Coding with Signal Adaptive Filterbanks,” ICASSP, Detroit, Mich., 3071-3074, (1995).
A cascaded filter bank, such as the cascaded filter bank
100
shown in
FIG. 1
, has different filter banks
110
,
160
following each other to obtain different frequency and time resolutions. The problem with a cascaded filter bank
100
is that the filters are not perfect, and hence the downsampling after each subband filter leads to aliasing. Aliasing occurs when signals with energy outside the filter passband are “mirrored” into the passband of the filter.
If this mirrored signal is in the passband of the following filter, the attenuation of the signal is determined only by the first filter of the cascade. This leads to a poor frequency selectivity of the combined filter bank.
FIG. 2
displays the magnitude response
200
of a particular band of a filter bank consisting of a cascade with 128 bands in the first stage and 8 bands in the following stage, for a total of 1024 bands, if all subbands of the first stage are split. The aliasing is manifested in the magnitude response
200
as high peaks other than the main lobe, partly with less than 10 dB attenuation. As a comparison, the desired magnitude response
300
can be seen in
FIG. 3
, which shows a particular band of a 1024 band uniform filter bank.
An approach to reduce this type of aliasing has been incorporated into the MPEG1/Layer 3 audio coder, described in Madisetti & D. B. Williams, eds., “The Digital Signal Processing Handbook,” CRC Press, IEEE Press, Boca Raton, Fla. (1997), incorporated by reference herein. The MPEG-1/Layer 3 audio coder uses a cascaded filter bank, with 32 bands in the first stage and 6 or 18 bands (switchable) in the second stage. For this special type of filter bank, a butterfly like structure is used at the output of the second stage to reduce the aliasing between adjacent bands. See, for example, B. Edler, “Aliasing Reduction in Subbands of Cascaded Filter Banks with Decimation,” Electronics Letters, Vol. 28, No. 12, pp. 1104-1105, June 1992, incorporated by reference herein.
While the alias reduction techniques incorporated in the MPEG-1/Layer 3 audio coder works for reducing the aliasing in neighboring bands, it only reduces the aliasing from one neighbor, and only in specific filter banks, where the aliasing of neighboring bands has a phase shift of 180 degrees. A need exists for a method and apparatus that reduce aliasing on more than one neighboring band, so that a fixed phase relationship is not required.
SUMMARY OF THE INVENTION
Generally, a method and apparatus are disclosed for reducing aliasing between neighboring subbands in cascaded filter banks. According to the present invention, a higher frequency resolution is obtained from a set of subbands. The signals of these subbands are first fed into an alias reduction filter bank to reduce the aliasing. If the first stage filter bank is a modulated uniform filter bank with M
1
bands, and the alias reduction filter bank has M
2
bands, then to obtain alias cancellation, the alias reduction filter bank has to have the same frequency response as the synthesis filter bank for the first stage, but with the frequency scaled by the ratio of the sampling rates, M
1
/M
2
. After the alias reduction filter stage, an analysis filter bank to obtain a higher frequency resolution follows.
According to a further aspect of the invention, a synthesis filter bank for a first stage analysis filter bank can be “thinned out” by keeping only every M
1
/M
2
th
sub-structure. In one disclosed implementation, a two band alias reduction filter is generated from a four band synthesis filter bank with reduced aliasing. The resulting two band filter bank has similar frequency responses (up to the scaling) as the four band filter bank, and hence can be used for the alias reduction.


REFERENCES:
patent: 5436940 (1995-07-01), Nguyen
patent: 5606319 (1997-02-01), Yatim et al.
patent: 5852806 (1998-12-01), Johnston et al.
patent: 6029126 (2000-02-01), Malvar
patent: 6144937 (2000-11-01), Ali
B. Edler, “Aliasing Reduction in Subbands of Cascaded Filter Banks with Decimation,” Electronics Letters, vol. 28, No. 12, 1104-1105 (Jun. 1992).

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