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Method and apparatus for reducing correlated errors in subband coding systems with quantizers
| Details |
Inventors: Kovacevic, Jelena;
Assignee: AT&T IPM Corp. (Coral Gables, FL)
Primary Examiner: MacDonald; Allen R.
Assistant Examiner: Dorvil; Richemond
Attorney, Agent or Firm: Brown; Kenneth M.
A method and apparatus for reducing correlated errors in subband coding systems with quantizers is disclosed. A subband coding system comprises a plurality of subband analysis filters to divide the frequency spectrum of the input signal into subbands, individual subband quantizers for coding each subband by a preselected number of quantization levels, corresponding subband decoders and subband synthesis filters. The transfer function of each of the subband synthesis filters is advantageously determined based on the transfer functions of the subband analysis filters as well as on the characteristics of the quantizer used to code the corresponding subband. Specifically, the synthesis filter transfer functions may be based on a perfect reconstruction filter bank or a quadrature mirror filter bank, as well as on the gain factors of a gain plus additive noise linear model for the Lloyd-Max quantizers used to code the corresponding subbands. That portion of the error between the input signal and the replica signal as reconstructed by the system which is correlated to the input signal may be advantageously reduced or eliminated, irrespective of that portion of the error which is uncorrelated to the input signal. Thus, the total error in a final signal may be advantageously reduced by the subsequent application of prior art techniques for the reduction of random, uncorrelated noise. |
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DETAILED DESCRIPTION The limitations of the prior art techniques are overcome and a technical advance is made in accordance with the present invention based, in part, on a recognition that it is advantageous in subband coders with quantizers to reduce that part of the reconstruction error which is correlated with the input signal, even at the expense of increasing uncorrelated (random) error.
It has been found that if most or all of the correlated error is eliminated it becomes possible to ultimately achieve a reconstructed signal with less total error than the aforementioned prior approach.
This is so because well-known random noise removal techniques may be advantageously applied to the output of the subband coding system to reduce uncorrelated error.
Thus, even though a subband coding system in accordance with one aspect of the present invention may produce a resultant reconstructed signal with more total error than a prior system, it will nonetheless be preferred because its error is exclusively or primarily uncorrelated with the input signal.
Typical error from optimal quantization methods (such as Lloyd-Max quantization)includes both correlated and uncorrelated error.
Therefore, an illustrative embodiment of the present invention advantageously incorporates characteristics of the quantizers in the design of synthesis filters.
Thus, in accordance with an illustrative embodiment of the present invention, a technique is provided for reducing correlated errors in subband coding systems with quantizers.
In accordance with one aspect of this embodiment, a subband synthesis filter for processing signals in one subband has a transfer function based on both the transfer function of the subband analysis filter bank and on the characteristics of the corresponding quantizer used to code that subband.
In accordance with another aspect of an illustrative embodiment of the present invention, a plurality of subband synthesis filters each has a transfer function based on both the transfer function of the subband analysis filter bank and on the characteristics of the quantizer used to code the corresponding subband
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Image Analysis 
