Method for detecting and masking bad frames in coded speech signals

• US 5,255,343 A
• Filed: 06/26/1992
• Issued: 10/19/1993
• Est. Priority Date: 06/26/1992
• Status: Expired due to Fees

• Current Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Original Assignee: Northern Telecom Limited (Nortel Networks Corporation)
• Inventors: Su, Huan-Yu
• Primary Examiner(s): Fleming, Michael R.
• Assistant Examiner(s): HAFIZ, TARIQ R
• Application Number:
  US07/904,638
• Time in Patent Office: 480 Days
• Field of Search: 381/36, 381/43, 381/31, 381/38, 381/51, 381/46, 395/2, 371/3, 371/43, 371/5.5
• US Class Current: 704/242
• CPC Class Codes: H04L 1/0083 Formatting with frames or packets; Protocol or part of protocol for error control

Abstract

The process for detection and masking of bad frames in a coded speech signal resulting from channel transmission errors has been improved. The coded speech signal has a first group of bits comprising the most perceptually significant bits of the coded speech signal, a second group of bits comprising the second most perceptually significant bits and a third group of bits comprising the least perceptually significant bits. The coded speech signal is de-interleaved to obtain a first series of bits comprising the first and second group and a second series of bits comprising the third group of bits. The first series of bits are convolutionally decoded for recovering the first and second group of bits, with the first group of bits containing error protected bits. A CRC check is performed by sending the recovered bits of the first group to cyclic redundancy decoding means. The first group of bits is forwarded to speech decoder means if the CRC check is successful. The frame energy and LPC parameters are replicated for the first and third group of bits and the frame energy, LPC parameters, lag value and sub-frame gain are replicated for the second group of bits, and subsequently the replicated bits are forwarded to the speech coder means, if the CRC check is not successful.

Claims

1. A method of improving the detection and masking of bad frames in a coded speech signal resulting from channel transmission errors, said coded speech signal having a first group of bits comprising the most significant bits of said coded speech signal, error detection bits and error correction bits, a second group of bits comprising the second most significant bits and error correction bits and a third group of bits comprising the least significant bits, said method comprising the steps of:

de-interleaving said coded signal to obtain a first series of bits comprising said first and second group of bits and a second series of bits comprising said third group of bits;

convolutionally decoding said first series of bits for recovering said first and second group of bits;

performing a Cyclic Redundancy Code (CRC) check on recovered bits of said first group;

if said CRC check is successful, utilizing said third group of bits and said recovered bits of said first group for speech decoding and performing a maximum likelihood check on said second group of bits;

if said maximum likelihood check is successful, utilizing said recovered bits of said second group for speech decoding; and

if said maximum likelihood check is unsuccessful, replicating the pitch lag value and sub-frame gain for each sub-frame of said bad frame, from the last sub-frame of the previously accepted frame.

2. A method as defined in claim 1, further comprising the steps of:

if said CRC check is not successful, replicating the frame energy and LPC parameters for said first and third group of bits;

replicating the frame energy, LPC parameters, lag value and sub-frame gain for said second group of bits; and

utilizing the replicated bits of said first, second and third groups for speech decoding.

3. A method as defined in claim 1, wherein said convolutional decoding comprises a Viterbi maximum likelihood convolution decoding.

4. A method as defined in claim 3, wherein said maximum likelihood check is successful if a metric assigned to the final surviving path by said Viterbi decoding is less than a predetermined threshold.

5. A method as defined in claim 4, wherein said metric is defined a the Hamming distance.