Variable rate vocoder

• US 5,778,338 A
• Filed: 01/23/1997
• Issued: 07/07/1998
• Est. Priority Date: 06/11/1991
• Status: Expired due to Term

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Lee, Chong U., Jacobs, Paul E., Gilhousen, Klein S., Tsai, Ming-Chang, Lam, S. Katherine, Gardner, William R.
• Primary Examiner(s): Hudspeth, David R.
• Assistant Examiner(s): Chawan, Vijay B.
• Application Number:
  US08/788,728
• Time in Patent Office: 530 Days
• Field of Search: 395/2.38, 395/2.28, 395/2.31, 395/2.32
• US Class Current: 704/223
• CPC Class Codes: G10L 19/005 Correction of errors induced by the transmission channel, if related to the coding algorithm
• CPC Class Codes: G10L 19/012 Comfort noise or silence coding
• 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 19/22 Mode decision, i.e. based on audio signal content versus external parameters
• CPC Class Codes: G10L 19/24 Variable rate codecs, e.g. for generating different qualities using a scalable representation such as hierarchical encoding or layered encoding
• CPC Class Codes: G10L 2025/786 Adaptive threshold
• CPC Class Codes: G10L 25/78 Detection of presence or absence of voice signals switching of direction of transmission by voice frequency in two-way loud-speaking telephone systems H04M9/10
• CPC Class Codes: H04B 1/66 for reducing bandwidth of signals; for improving efficiency of transmission H04B1/68 takes precedence
• CPC Class Codes: H04J 3/1688 the demands of the users being taken into account after redundancy removal, e.g. by predictive coding, by variable sampling reducing bandwidth of signals in general H04B1/66; in PCM-systems H04B14/046; removal of redundancy in telegraph communication H03M7/30
• CPC Class Codes: H04L 1/0014 by adapting the source coding
• CPC Class Codes: H04L 1/0017 where the mode-switching is based on Quality of Service requirement
• CPC Class Codes: H04L 1/0041 Arrangements at the transmitter end
• CPC Class Codes: H04L 1/0046 Code rate detection or code type detection H04L1/0038 takes precedence; detection of the data rate H04L25/0262; for packet format H04L1/0091
• CPC Class Codes: H04L 1/0057 Block codes H04L1/0061, H04L1/0064 take precedence

Abstract

An apparatus and method for performing speech signal compression, by variable rate coding of frames of digitized speech samples. The level of speech activity for each frame of digitized speech samples is determined and an output data packet rate is selected from a set of rates based upon the determined level of frame speech activity. A lowest rate of the set of rates corresponds to a detected minimum level of speech activity, such as background noise or pauses in speech, while a highest rate corresponds to a detected maximum level of speech activity, such as active vocalization. Each frame is then coded according to a predetermined coding format for the selected rate wherein each rate has a corresponding number of bits representative of the coded frame. A data packet is provided for each coded frame with each output data packet of a bit rate corresponding to the selected rate.

At the decoder, if a frame is lost due to a channel error, the error is masked by maintaining a fraction of the previous frame'"'"'s energy and smoothly transitioning to background noise.

Claims

1. An apparatus for masking frame errors comprising:

memory means for storing at least one previous frame of data and for providing said at least one previous frame of data in response to a frame error signal; and

masking means for receiving said frame error signal and for generating a masking signal in accordance with said at least one previous frame of data and a predetermined error masking format.

2. The apparatus of claim 1 wherein said at least one previous frame of data comprises a last frame of data capable of being decoded.

3. The apparatus of claim 2 wherein said error masking format comprises attenuating the gain of said last frame of data capable of being decoded.

4. The apparatus of claim 1 wherein said error masking format comprises attenuating the gain of said at least one previous frame of data.

5. A method for masking frame errors comprising:

storing at least one previous frame of data;

providing said at least one previous frame of data in response to a frame error signal; and

generating a masking signal in accordance with said at least one previous frame of data and a predetermined error masking format.

6. The apparatus of claim 5 wherein said at least one previous frame of data comprises a last frame of data capable of being decoded.

7. The apparatus of claim 6 wherein said error masking format comprises attenuating the gain of said last frame of data capable of being decoded.

8. The apparatus of claim 5 wherein said error masking format comprises attenuating the gain of said at least one previous frame of data.