Method and apparatus for frequency-domain downmixing with block-switch forcing for audio decoding functions

• US 6,931,291 B1
• Filed: 05/08/1997
• Issued: 08/16/2005
• Est. Priority Date: 05/08/1997
• Status: Expired due to Term

• Current Assignee: STMicroelectronics Asia Pacific Pte Limited (STMicroelectronics NV)
• Original Assignee: STMicroelectronics Asia Pacific Pte Limited (STMicroelectronics NV)
• Inventors: Alvarez-Tinoco, Mario Antonio, George, Sapna, Yang, Haiyun
• Primary Examiner(s): Tran, Sinh
• Assistant Examiner(s): Graham, Andrew
• Application Number:
  US09/423,413
• Time in Patent Office: 3,022 Days
• Field of Search: 700/94, 704/220, 704/229, 704/500, 704/503, 375/242, 846/25, 341/67, 381/22
• US Class Current: 700/94
• CPC Class Codes: G10L 19/008 Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
• CPC Class Codes: G10L 19/022 Blocking, i.e. grouping of samples in time; Choice of analysis windows; Overlap factoring
• CPC Class Codes: H04S 1/007 in which the audio signals are in digital form data reduction aspects thereof based on psychoacoustics G10L19/02

Abstract

An audio decoder solution is here provided where a reduction in computing power is required. The proposed method consists of forcing the multiple output channels to only one type of inverse transformation format. A format of long transform length is more suitable for input signals whose spectrum remains stationary or quasi-stationary. This provides a greater frequency resolution, improved coding performance and a reduction of computing power required. Another format of two or more short transform lengths, possessing greater time resolution, is more desirable for rapidly changing signals with time. The computer power required for two or more short transforms should be higher than for only one transformation. The time versus frequency resolution trade-off should be considered when selecting a transform block length. Advantage is taken of human hearing behaviour to reduce the computing power of a processing engine (e.g. DSP) when downmixing from an M-channel input to a P-channel output is required. The encoder provides spectral information concerning the transmitted audio signal frame. This information corresponds to signals which are stationary/quasi-stationary or changing rapidly with time. Some analysis is required to decide which input channels are forced to long or short block conversion prior to frequency-domain downmixing and transformation.

Claims

1. An audio decoder, comprising:

a demultiplexer for receiving a data signal and demultiplexing the data signal into a plurality of M frequency-domain·

input data channels;

means for downmixing said M frequency-domain input channels into P frequency-domain channels, where M>

P and P>

0, M and P both integers;

means for selecting an inverse transformation length and forcing data blocks in the P frequency-domain channels into the selected length and performing an inverse transformation of the P frequency-domain channels according to the selected length, so as to produce P audio sample output channels.

2. The audio decoder of claim 1, wherein the means for selecting and performing an inverse transformation is biased to the selection of a long transform length.

3. The audio decoder of claim 2, further including means for determining a transformation length of each of said M frequency-domain input channels.

4. The audio decoder of claim 3, wherein the inverse transform length is selected according to the transformation lengths of the M frequency-domain input channels.

5. The audio decoder of claim 4, wherein the transformation length of the M frequency-domain input channels comprises one of either a long transform length or a short transform length.

6. The audio decoder of claim 5, wherein when the number of input channels having a long transform length is less than or equal to the integer value of M/2, then the inverse transformation of the P frequency-domain channels is performed using a short selected inverse transformation length.

7. The audio decoder of claim 5, wherein when the number of input channels having a short transform length is less than the integer value of M/2, then the inverse transformation of the P frequency-domain channels is performed using a long selected inverse transformation length.

8. A method of audio data decoding, comprising:

receiving a data signal and demultiplexing the data signal into a plurality of M frequency-domain input data channels;

downmixing said M frequency-domain input channels into P frequency-domain channels, where M>

P and P>

0, M and P both integers;

selecting an inverse transformation length and force block-switching of data blocks in the P frequency-domain channels to the selected length; and

performing an inverse transformation of the P frequency-domain channels according to the selected length, so as to produce P audio sample output channels.

9. The method of audio data decoding of claim 8, further including determining a transformation length of each of said M frequency-domain input channels.

10. The method of audio data decoding of claim 8, wherein the selection of an inverse transformation length is biased to the selection of a long transform length.

11. The method of audio data decoding of claim 9, wherein the inverse transform length is selected according to the transformation lengths of the M frequency-domain input channels.

12. The method of audio data decoding of claim 11, wherein the transformation length of the M frequency-domain input channels comprises one of either a long transform length or a short transform length.

13. The method of audio data decoding of claim 12, wherein when the number of input channels having a long transform length is less than or equal to the integer value of M/2, then the inverse transformation of the P frequency-domain channels is performed using a short selected inverse transformation length.

14. The method of audio data decoding of claim 12, wherein when the number of input channels having a short transform length is less than the integer value of M/2, then the inverse transformation of me P frequency-domain channels is performed using a long selected inverse transformation length.

15. An audio decoder, comprising:

a downmixing circuit configured to receive M frequency-domain input channels and to downmix the M frequency-domain input channels into P frequency-domain channels, where M>

P and P>

0, M and P both integers;

a P number of transformation circuits, each transformation circuit coupled to a respective frequency-domain channel, each transformation circuit configured to select an inverse transformation length and performing an inverse transformation of the respective frequency-domain channel according to the selected length so as to produce time domain signals, wherein each transformation circuit is configured to select a transformation length of each of the M frequency-domain input channels from one of either a long transform length when the number of input channels having a long transform length is greater than the integer value of M/2, and otherwise selecting a short inverse transformation length;

a P number of overlap-and-add circuits coupled to respective transformation circuits and configured to apply an overlap-and-add operation to the respective time-domain signal; and

a P number of windowing circuits coupled to respective overlap-and-add circuits and configured to implement a windowing function so as to produce P audio sample output signals.

16. A decoding method, comprising:

receiving a plurality of M frequency-domain input data signals and downmixing the M frequency-domain input data signals into P frequency-domain channels, where M>

P and P>

0, M and P both integers;

selecting an inverse transformation length for each P frequency-domain channel and performing an inverse transformation of each P frequency-domain channel according to the selected length to produce P output signals, wherein selecting the inverse transformation length comprises selecting a short inverse transformation length when the number of frequency-domain input signals having a long transformation length is less than or equal to the integer value of M/2, and otherwise selecting a long inverse transformation length;

performing an overlap-and-add function on the P output signals; and

subsequently performing a windowing function to produce audio output signals.

17. An audio decoder, comprising:

a demultiplexer for receiving a data signal and demultiplexing the data signal into a plurality of M frequency-domain·

input data channels each having a respective block length;

a downmixing circuit adapted to downmix the M frequency-domain input channels into P frequency-domain channels, where M>

P and P>

0, M and P are both integers;

a circuit for selecting an inverse transformation length, forcing the P frequency-domain channels to the selected inverse transformation length, and performing an inverse transformation of the P frequency-domain channels according to the selected length to produce P audio sample output channels.

18. An audio decoder, comprising:

a demultiplexer for receiving a data signal and demultiplexing the data signal into a plurality of M frequency-domain input data channels;

a circuit for downmixing the M frequency-domain input channels into P frequency-domain channels, where M>

P and P>

0, M and P are both integers, the circuit configured to de-interleave and zero pad selected frequency-domain channels in accordance with a selected inverse transformation length and to force the P frequency-domain channels to the same inverse transformation length; and

an inverse transformation circuit for performing an inverse transformation of the P frequency-domain channels according to the selected length so as to produced P audio sample output channels.

19. An audio decoding method, comprising:

receiving a data signal an demultiplexing the data signal into a plurality of M frequency-domain input data channels;

downmixing the M frequency-domain input data channels into P frequency-domain channels, where M>

P and P>

0, M and P are both integers; and

selecting an inverse transformation length, forcing the P frequency-domain channels to the selected inverse transformation length and performing an inverse transformation of the P frequency-domain channels according to the selected length so as to produce P audio sample output channels.

20. A method of audio data decoding, comprising:

receiving a data signal and demultiplexing the data signal into a plurality of M frequency-domain input data channels;

downmixing the M frequency-domain input channels into P frequency-domain channels, where M>

P and P>

0, M and P are both integers, and de-interleaving and zero padding the channels so that all P frequency-domain channels have a selected inverse transformation length; and

performing an inverse transformation of the P frequency-domain channels according to the selected length so as to produce P audio sample output channels.