Method for selecting noise codebook vectors in a variable rate speech coder and decoder

• US 5,875,423 A
• Filed: 10/17/1997
• Issued: 02/23/1999
• Est. Priority Date: 03/04/1997
• Status: Expired due to Fees

• Current Assignee: Mitsubishi Denki Kabushiki Kaisha (Mitsubishi Electric Corporation)
• Original Assignee: Mitsubishi Denki Kabushiki Kaisha (Mitsubishi Electric Corporation)
• Inventors: Matsuoka, Bunkei
• Primary Examiner(s): Hudspeth, David R.
• Assistant Examiner(s): Opsasnick, Michael N.
• Application Number:
  US08/953,437
• Time in Patent Office: 494 Days
• Field of Search: 704/220, 704/221, 704/223, 704/226
• US Class Current: 704/220
• CPC Class Codes: G10L 19/18 Vocoders using multiple modes

Abstract

In a variable rate speech coding method for a CELP speech coding system, an adaptive sound source vector and a first noise source vector are selected from a sound source code book and a noise source code book so that a first synthesized speech signal is obtained which has a minimum distortion relative to an input speech signal. A virtual reference speech signal is generated using a sound source signal which is produced using the adaptive sound source vector. A second noise source vector corresponding to the adaptive sound source vector is selected so that a second synthesized speech signal is obtained which has a minimum distortion relative to the virtual reference speech signal. The sending of a noise source code book index corresponding to the first noise source vector is suspended according to the quality of the second synthesized speech signal.

Claims

1. A variable rate speech coding method for CELP speech coding system including an adaptive sound source code book for storing an adaptive sound source vector repeating sound source signals of previous frames at intervals of a pitch period and a noise source code book for storing noise source vectors, said method comprising the steps of:

selecting and outputting said adaptive sound source vector and a first noise source vector from said adaptive sound source code book and said noise source code book so that a first synthesized speech signal with a minimum distortion relative to an input speech signal is obtained;

synthesizing a virtual reference speech signal by using a sound source signal generated from said adaptive sound source vector;

selecting a second noise source vector corresponding to said adaptive sound source vector so that a second synthesized speech signal with a minimum distortion relative to said virtual reference signal is obtained; and

suspending sending of a noise source code book index corresponding to said first noise source vector according to quality of said second synthesized speech signal.

2. The method according to claim 1, wherein the step of suspending the sending of said noise source code book index comprises the steps of:

converting speech quality of each of said first and second synthesized speech signals and said virtual reference speech signal into a numerical representation relative to the input speech signal;

calculating a threshold value for comparison through utilization of said speech quality of said first synthesized speech signal and said computed virtual reference speech signal;

comparing said second synthesized speech signal with said threshold value; and

deciding whether or not to send a noise source code book index corresponding to said first noise source vector according to a result of comparison.

3. A variable rate speech decoding method for CELP speech decoding system including an adaptive sound source code book for storing an adaptive sound source vector repeating sound source signals of previous frames at intervals of a pitch period and a noise source code book for storing noise source vectors, said method comprising the steps of:

generating a first synthesized speech signal from a sound source generated using both of an adaptive sound source vector and a noise source vector corresponding to an adaptive sound source code book index and a noise source code book index when they are contained in a received signal sequence;

synthesizing a virtual reference speech signal from a sound source generated using said adaptive sound source vector corresponding to said adaptive sound source code book index when said noise source code book index is not contained in said received signal sequence; and

selecting a noise source vector corresponding to an adaptive sound source vector indicated by said received adaptive sound source code book index so that a synthesized speech signal with a minimum distortion relative to said virtual reference speech signal is obtained, and outputting a second synthesized speech signal produced based on a result of said selection.