Audio coding method and apparatus using backward adaptive prediction

• US 6,012,025 A
• Filed: 01/28/1998
• Issued: 01/04/2000
• Est. Priority Date: 01/28/1998
• Status: Expired due to Term

• Current Assignee: Nokia Technologies Oy (Nokia Corporation)
• Original Assignee: Nokia Mobile Phones UK Limited (Nokia Corporation)
• Inventors: Yin, Lin
• Primary Examiner(s): Hudspeth, David R.
• Assistant Examiner(s): Chawan, Vijay B
• Application Number:
  US09/014,712
• Time in Patent Office: 706 Days
• Field of Search: 704/206, 704/204, 704/219, 704/229, 704/220, 704/205, 704/211, 704/222, 704/224, 704/230
• US Class Current: 704/219
• CPC Class Codes: G10L 19/04 using predictive techniques

Abstract

A method of coding an audio electrical signal using backward adaptive prediction. A first time frame of the audio electrical signal to be coded is received and transformed into the frequency domain using a modified discrete cosine transform (MDCT). The resulting frequency spectrum has 1024 spectral components. Subsequent time frames of the audio electrical signal are then received and the MDCT is applied to each in turn so as to generate a stream of spectral data values for each spectral component. For each stream, a set of prediction coefficients is calculated for each spectral value using a predetermined number of previously received consecutive spectral values of the stream. Using the set of linear prediction coefficients, a predicted spectral value is generated and the error between the predicted spectral value and the corresponding actual spectral value calculated. The calculated errors provide a coded representation of the spectral value stream.

Claims

1. A method of coding an audio electrical signal using backward adaptive prediction, the method comprising the steps of:

(a) receiving a first time frame of an audio electrical signal to be coded;

(b) transforming the time frame into the frequency domain to generate a frequency spectrum having 512 or more spectral components;

(c) receiving subsequent time frames of said audio electrical signal and repeating step (b) for these frames in sequence to generate a stream of spectral data values for each spectral component;

(d) for each said stream,

calculating a set of prediction coefficients for each spectral data value using the covariances of a predetermined number of previously determined reconstructed spectral values of the stream,

using said set of prediction coefficients to generate a predicted spectral value, and

calculating the error between the predicted spectral value and the corresponding actual spectral data value, and

(e) constructing the calculated errors wherein the calculated errors provide a coded representation of a spectral data value stream and said errors can be recombined with predicted spectral values to obtain reconstructed spectral values for producing a coded audio signal.

2. A method according to claim 1, wherein the prediction order is two.

3. A method according to claim 1 and comprising recalculating the prediction coefficients only after receipt of multiple spectral values and using the same coefficients for several consecutive spectral values.

4. A method according to claim 3, wherein said multiple is two.

5. A method according to claim 3 and comprising switching between a low coefficient update rate and a high update rate immediately upon detection of a transient in the audio signal to be coded.

6. A method according to claim 1, wherein said predetermined number of spectral values is four or more.

7. A method according to claim 1, wherein said predetermined number of spectral values is ten or less.

8. A method according to claim 1, wherein a least squares method is used for evaluating the prediction coefficients.

9. A method according to claim 1, wherein said covariances are determined as:

##EQU6##10.

10. A method according to claim 9, wherein the prediction coefficients are determined according to:

11. A method of decoding a coded audio electrical signal, the decoding method comprising the steps of:

receiving as an input signal a sequence of error values corresponding to the coded audio signal and separating these error values into spectral component streams;

for each component stream, determining a corresponding predicted spectral component value for each error value using a set of prediction coefficients, the prediction coefficients being calculated using covariances of a predetermined number of previously determined consecutive predicted spectral component values for that stream, and combining the error value and the predicted spectral value to provide a reconstructed spectral value; and

substantially reconstructing said audio signal by combining and frequency-to-time transforming the reconstructed spectral values of all of the component streams.

12. Apparatus for coding an audio electrical signal using backward adaptive prediction, the apparatus comprising:

an input for receiving an audio electrical signal to be coded;

a time-to-frequency domain transformer for transforming sequentially received time frames of the received audio signal from the time domain to the frequency domain to provide frequency spectra having 512 or more spectral components;

signal processing means associated with each spectral component for receiving as a stream the associated spectral values, for calculating for each spectral value a set of prediction coefficients using covariances of a predetermined number of previously reconstructed spectral values, for using said set of prediction coefficients to generate a predicted spectral value, and for calculating the error between the predicted value and the corresponding actual spectral value, the calculated errors providing a coded representation of the received spectral value stream and wherein said error can be recombined with predicted spectral values to obtain reconstructed spectral values for producing a coded audio signal.

13. Apparatus for decoding a coded audio electrical signal, the apparatus comprising:

an input for receiving a sequence of error values corresponding to the coded audio signal; and

signal processing means for separating said sequence of error values into separate spectral component streams and for determining for each error value a corresponding predicted spectral value using a set of prediction coefficients, the signal processing means being arranged to calculate the prediction coefficients, using covariances of a predetermined number of previously determined consecutive reconstructed spectral values, the signal processing means being further arranged to combine each error value with the corresponding predicted spectral value to provide a reconstructed spectral value and to substantially reconstruct said audio signal by combining and frequency-to-time transforming the reconstructed spectral values of all of the streams.

14. A mobile communications device comprising:

coding apparatus for coding an audio electrical signal using backward adaptive prediction, comprising;

an input for receiving an audio electrical signal to be coded;

a time-to-frequency domain transformer for transforming sequentially received time frames of the received audio signal from the time domain to the frequency domain to provide frequency spectra having 512 or more spectral components;

signal processing means associated with each spectral component for receiving as a stream the associated spectral values, for calculating for each spectral value a set of prediction coefficients using covariances of a predetermined number of previously reconstructed spectral values, for using said set of prediction coefficients to generate a predicted spectral value, and for calculating the error between the predicted value and the corresponding actual spectral value, the calculated errors providing a coded representation of the received spectral value stream and wherein said errors can be recombined with predicted spectral values to obtain reconstructed spectral values; and

decoding apparatus for decoding a coded audio electrical signal, comprising;

an input for receiving a sequence of error values corresponding to the coded audio signal; and

signal processing means for separating said sequence of values into separate spectral component streams and for determining for each error value a corresponding predicted spectral value using a set of prediction coefficients, the signal processing means being arranged to calculate the prediction coefficients, using covariances of a predetermined number of previously determined consecutive reconstructed spectral values, the signal processing means being further arranged to combine each error value with the corresponding predicted spectral value to provide a reconstructed spectral value and to substantially reconstruct said audio signal by combining and frequency-to-time transforming the reconstructed spectral values of all of the streams.