Methods and apparatus for coding digital audio signals using a filtered quantizing noise
The invention concerns an encoder for an input audio signal (S(z)) comprising a combination module combining the input audio signal with an intermediate counter-reaction signal forming a modified input signal and a quantification module scalable for the rate ( 91 ) of said modified input signal, delivering a binary raster of quantification indexes of a predetermined rate. This encoder is such that it comprises: a reverse quantification module ( 93 ) of a part of the quantification indexes of the binary raster corresponding to the indexes of a rate lower than the predetermined rate, for predetermining a reconstructed signal (S Mic (z)), a module for determining ( 94 ) a quantification noise (Q Mic (z)) derived from at least quantification modules and reverse quantification modules in series, a determination module ( 111 ) for a filtering function of the quantification noise from said reconstructed signal and a filtering module fit for applying ( 33 ) said filtering function to said quantification noise in order to obtain said intermediary counter-reaction signal. The invention also relates to an encoding method applied by the described encoder and a corresponding decoding method applied by a decoder.
1. A method of coding an input audio signal comprising the steps:
combining said input audio signal with an intermediate feedback signal forming a modified input signal;
quantizing said modified input signal;
delivering a quantizing index binary frame of a predetermined bit rate;
inverse quantizing of some of the quantizing indices of the binary frame corresponding to the indices of a core bit rate lower than the predetermined bit rate, to determine a core bit rate reconstructed signal;
determining quantizing noise generated at least by the succession of the quantizing step and the inverse quantizing step;
determining a quantizing noise filter function from said core bit rate reconstructed signal; and
applying said filter function to said quantizing noise to obtain said intermediate feedback signal.
2. The method as claimed in claim 1 , wherein the quantizing step provides at least core bit rate quantizing indices corresponding to PCM coding type core coding.
3. The method as claimed in claim 1 , wherein said intermediate feedback signal is calculated from a prediction of the quantizing noise and a prediction of the filtered quantizing noise, the filtered quantizing noise being obtained by adding the intermediate signal and the quantizing noise.
4. The method as claimed in claim 1 , wherein the quantizing noise filter function is furthermore calculated from periodicity characteristics of the input signal.
5. The method as claimed in claim 1 , wherein the scalable quantizing step of predetermined bit rate is a PCM scalar quantizing step, the inverse quantizing step of the indices of a core bit rate lower than the predetermined bit rate applies to 8-K bits of the binary frame to determine a core bit rate reconstructed signal, and there is also effected a step of inserting K data bits.
6. A method for scalable decoding of an audio signal, said method comprising:
a first step of inverse quantization decoding quantizing indices of a core bit rate to form a core bit rate reconstructed signal;
a second step of inverse quantization decoding quantizing indices of a second bit rate higher than the core bit rate, executed as a function of the bit rate received on decoding, to form a reconstructed signal of higher bit rate;
a third step of obtaining a difference signal between said core bit rate reconstructed signal and said higher bit rate reconstructed signal;
a fourth step of determining a correction filter function from the core bit rate reconstructed signal;
a filtering step capable of applying said correction filter function to said difference signal to obtain a corrective term;
a fifth step of synthesizing a corrected reconstructed signal at higher bit rate capable of combining said corrective term and one of the reconstructed signals.
7. The method as claimed in claim 6 , wherein said corrective term is applied to said core bit rate reconstructed signal.
8. The method as claimed in claim 6 , wherein said corrective term is applied to said higher bit rate reconstructed signal.
9. A decoding method as claimed in claim 6 , further comprising a step of receiving information regarding insertion of bits on coding and, in the case of insertion information on K bits, a step of masking the K bits inserted is effected before the first step of core bit rate inverse quantization.
10. An input audio signal coder comprising a combination module combining the input audio signal with an intermediate feedback signal to form a modified input signal and a bit rate scalable quantizing module for quantizing said modified input signal to deliver a binary frame of quantizing indices of predetermined bit rate, said input audio signal coder further comprising:
a module for inverse quantization of some of the quantizing indices of the binary frame corresponding to the indices of a core bit rate lower than the predetermined bit rate, to determine a core bit rate reconstructed signal;
a module for determining a quantizing noise generated at least by the succession of the quantizing step and the inverse quantizing step;
a module for determining a quantizing noise filter function from said core bit rate reconstructed signal;
a module for applying said filter function to said quantizing noise to obtain said intermediate feedback signal.
11. An audio signal decoder comprising:
a first module for inverse quantization decoding quantizing indices at core bit rate to form a core bit rate reconstructed signal;
a second module for inverse quantization decoding quantizing indices at a second bit rate higher than the core bit rate, executed as a function of the bit rate received on decoding, to form a reconstructed signal of higher bit rate;
a module for obtaining a difference signal between said core bit rate reconstructed signal and said higher bit rate reconstructed signal;
a module for determining a correction filter function from the core bit rate reconstructed signal;
a module for applying said correction filter function to said difference signal to obtain a corrective term;
a module for synthesizing a corrected reconstructed signal at higher bit rate by combining said corrective term and one of the reconstructed signals.
12. A computer program stored in a memory of a coder or a decoder or on a memory medium adapted to cooperate with a reader of the coder or the decoder, said computer program comprising code instructions for executing the steps of the coding method as claimed in claim 1 when said program is executed by a processor of the coder or the decoder.
13. A computer program stored in a memory of a coder or a decoder or on a memory medium adapted to cooperate with a reader of the coder or the decoder, said computer program comprising code instructions for executing the steps of the decoding method as claimed in claim 6 when said program is executed by a processor of the coder or the decoder.