Interpolation in a speech decoder of a transmission system on the basis of transformed received prediction parameters

• US 6,157,907 A
• Filed: 02/05/1998
• Issued: 12/05/2000
• Est. Priority Date: 02/10/1997
• Status: Expired due to Fees

• Current Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Taori, Rakesh, Gerrits, Andreas J.
• Primary Examiner(s): Hudspeth, David R.
• Assistant Examiner(s): Storm, Donald L.
• Application Number:
  US09/018,980
• Time in Patent Office: 1,034 Days
• Field of Search: 704/221, 704/222, 704/265, 704/223
• US Class Current: 704/221
• CPC Class Codes: G10L 19/07 Line spectrum pair [LSP] vocoders
• CPC Class Codes: G10L 19/12 the excitation function being a code excitation, e.g. in code excited linear prediction [CELP] vocoders
• CPC Class Codes: G10L 2019/001 Interpolation of codebook vectors

Abstract

A transmission system wherein a speech signal is represented by a plurality of prediction parameters updated once per frame. Each frame comprises a plurality of sub-frames in which an excitation signal generated by a fixed codebook and an adaptive codebook is updated. In order to enhance the reconstructed speech quality the prediction coefficients are interpolated at the decoder by an LPC coefficient interpolator to obtain interpolated prediction coefficients for each sub-frame. According to the present invention the interpolation of the prediction coefficients is not based on the prediction coefficients used for transmission, such as reflection coefficients or Log Area Ratios, but on Line Spectral Frequencies. This reduces degradation of speech quality due to interpolation.

Claims

1. Transmission system comprising a transmitter with a speech encoder, said speech encoder comprising means for deriving from an input signal a symbol sequence including a representation of a plurality of prediction coefficients and a representation of an excitation signal, said representation of prediction coefficients being an untransformed representation, said transmitter being coupled via a transmission medium to a receiver with a speech decoder, said transmitter being arranged for transmitting said symbol sequence, the speech decoder comprising transformation means for deriving a transformed representation of said plurality of prediction coefficients from said untransformed representation of prediction coefficients, the speech decoder comprising interpolation means for deriving interpolated prediction coefficients from the transformed representation of the prediction coefficients, and the speech decoder being arranged for reconstructing a speech signal on basis of the interpolated prediction coefficients, said transformed representation of prediction coefficients being based on line spectral frequencies, and being more suitable for interpolation than said untransformed representation of prediction coefficients.

2. Transmission system according to claim 1, wherein the interpolation means are arranged for deriving in dependence of a control signal, the interpolated prediction coefficients from the representation of the prediction coefficients or for deriving the interpolated prediction coefficients from the transformed representation of the prediction coefficients.

3. Transmission system comprising a transmitter with a speech encoder, said speech encoder comprising means for deriving from an input signal a symbol sequence including a representation of a plurality of prediction coefficients and a representation of an excitation signal, said transmitter being coupled via a transmission medium to a receiver with a speech decoder, said transmitter being arranged for transmitting said symbol sequence, the speech decoder comprising transformation means for deriving a transformed representation of said plurality of prediction coefficients from said representation of prediction coefficients, the speech decoder comprising interpolation means for deriving interpolated prediction coefficients from the transformed representation of the prediction coefficients, and the speech decoder being arranged for reconstructing a speech signal on basis of the interpolated prediction coefficients, said transformed representation of prediction coefficients being based on line spectral frequencies, and being more suitable for interpolation than said representation of prediction coefficients, and said transformation means being arranged for determining reflection coefficients from said representation of prediction coefficients, for determining a-parameters from said reflection coefficients, and for determining said line-spectral frequencies from said a-parameters.

4. Transmission system according to claim 3, wherein said interpolation means are arranged for determining interpolated line spectral frequencies from said line-spectral frequencies, and for converting said interpolated line spectral frequencies to a-parameters.

5. Receiver for receiving a symbol sequence representing a speech signal, said symbol sequence including a representation of a plurality of prediction coefficients and a representation of an excitation signal, said representation of prediction coefficients being an untransformed representation, said receiver comprising a speech decoder for deriving a reconstructed speech signal from said symbol sequence, the speech decoder comprising transformation means for deriving a transformed representation of said plurality of prediction coefficients from said untransformed representation of prediction coefficients, the speech decoder comprising interpolation means for deriving interpolated prediction coefficients from the transformed representation of the prediction coefficients, and the speech decoder being arranged for deriving said reconstructed speech signal on basis of said interpolated prediction coefficients, said transformed representation of prediction coefficients being based on line spectral frequencies, and being more suitable for interpolation than said untransformed representation of prediction coefficients.

6. Receiver according to claim 5, wherein the interpolation means are arranged for deriving in dependence of a control signal, the interpolated prediction coefficients from the representation of the prediction coefficients or for deriving the interpolated prediction coefficients from the transformed representation of the prediction coefficients.

7. Receiver for receiving a symbol sequence representing a speech signal, said symbol sequence including a representation of a plurality of prediction coefficients and a representation of an excitation signal, said receiver comprising a speech decoder for deriving a reconstructed speech signal from said symbol sequence, the speech decoder comprising transformation means for deriving a transformed representation of said plurality of prediction from said representation of prediction coefficients, the speech decoder comprising interpolation means for deriving interpolated prediction coefficients from the transformed representation of the prediction coefficients, and the speech decoder being arranged for deriving said reconstructed speech signal on basis of said interpolated prediction coefficients, said transformed representation of prediction coefficients being based on line spectral frequencies, and being more suitable for interpolation than said representation of prediction coefficients, and said transformation means being arranged for determining reflection coefficients from said representation of prediction coefficients, for determining a-parameters from said reflection coefficients, and for determining said line-spectral frequencies from said a-parameters.

8. Receiver according to claim 7, wherein said interpolation means are arranged for determining interpolated line spectral frequencies from said line-spectral frequencies, and for converting said interpolated line spectral frequencies to a-parameters.

9. Speech decoder for deriving a reconstructed speech signal from a symbol sequence, said symbol sequence including a representation of a plurality of prediction coefficients and a representation of an excitation signal, said representation of prediction coefficients being an untransformed representation, and said symbol sequence being received from a speech encoder, the speech decoder comprising transformation means for deriving a transformed representation of said plurality of prediction coefficients from said untransformed representation of prediction coefficients, the speech decoder comprising interpolation means for deriving interpolated prediction coefficients from the transformed representation of the prediction coefficients, and the speech decoder being arranged for reconstructing a speech signal on basis of the interpolated prediction coefficients, said transformed representation of prediction coefficients being based on line spectral frequencies, and being more suitable for interpolation than said untransformed representation of prediction coefficients.

10. Speech decoder according to claim 9, wherein the interpolation means are arranged for deriving in dependence of a control signal, the interpolated prediction coefficients from the representation of the prediction coefficients or for deriving the interpolated prediction coefficients from the transformed representation of the prediction coefficients.

11. Speech decoder for deriving a reconstructed speech signal from a symbol sequence, said symbol sequence including a representation of a plurality of prediction coefficients and a representation of an excitation signal, and said symbol sequence being received from a speech encoder, the speech decoder comprising transformation means for deriving a transformed representation of said plurality of prediction coefficients from said representation of prediction coefficients, the speech decoder comprising interpolation means for deriving interpolated prediction coefficients from the transformed representation of the prediction coefficients, and the speech decoder being arranged for reconstructing a speech signal on basis of the interpolated prediction coefficients, said transformed representation of prediction coefficients being based on line spectral frequencies, and being more suitable for interpolation than said representation of prediction coefficients, and said transformation means being arranged for determining reflection coefficients from said representation of prediction coefficients, for determining a-parameters from said reflection coefficients, and for determining said line-spectral frequencies from said a-parameters.

12. Speech decoder according to claim 11, wherein said interpolation means are arranged for determining interpolated line spectral frequencies from said line-spectral frequencies, and for converting said interpolated line spectral frequencies to a-parameters.

13. A speech decoding method for deriving a reconstructed speech signal from a symbol sequence, said symbol sequence including a representation of a plurality of prediction coefficients and a representation of an excitation signal, said representation of prediction coefficients being an untransformed representation, before deriving the reconstructed speech signal receiving the symbol sequence from a speech encoder, the method comprising deriving a transformed representation of said plurality of prediction coefficients from said untransformed representation of prediction coefficients, the method comprising deriving interpolated prediction coefficients from the transformed representation of the prediction coefficients, and the method comprising reconstructing a speech signal on basis of the interpolated prediction coefficients, said transformed representation of prediction coefficients being based on line spectral frequencies, and being more suitable for interpolation than said untransformed representation of prediction coefficients.

14. A speech decoding method for deriving a reconstructed speech signal from a symbol sequence, said symbol sequence including a representation of a plurality of prediction coefficients and a representation of an excitation signal, before deriving the reconstructed speech signal receiving the symbol sequence from a speech encoder, the method comprising deriving a transformed representation of said plurality of prediction coefficients from said representation of prediction coefficients, the method comprising deriving interpolated prediction coefficients from the transformed representation of the prediction coefficients, the method comprising reconstructing a speech signal on basis of the interpolated prediction coefficients, said transformed representation of prediction coefficients being based on line spectral frequencies, and being more suitable for interpolation than said representation of prediction coefficients, and the method determining reflection coefficients from said representation of prediction coefficients, determining a-parameters from said reflection coefficients, determining said line-spectral frequencies from said a-parameters.

15. A speech decoding method according to claim 14, in said method determining interpolated line spectral frequencies from said line-spectral frequencies, and converting said interpolated line spectral frequencies to a-parameters.