Voiced, unvoiced or noise modes in a CELP vocoder
AssigneeHughes Electronics Corporation
InventorsKumar Swaminathan, Kalyan Ganesan, Prabhat K. Gupta
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Abstract
A bit rate Codebook Excited Linear Predictor (CELP) communication system which includes a transmitter that organizes a signal containing speech into frames of 40 millisecond duration, and classifies each frame as one of three modes: voiced and stationary, unvoiced or transient, and background noise.
Claims24
A method of processing a signal having a speech component, the signal being organized as a plurality of frames, the method comprising the steps, performed for each frame, of:
- measuring a value for at least one speech characteristic of a frame, wherein the speech characteristic is selected from the group consisting of spectral stationarity, pitch stationarity, high-frequency content, and energy;
- comparing the measured value of the selected speech characteristic with at least two thresholds, including a high threshold representing a high value of the selected speech characteristic and a low threshold representing a low value of the selected speech characteristic;
- and
- setting a first flag if the measured value exceeds the high threshold;
- and
- setting a second flag if the measured energy value is below the low threshold;
- determining whether the frame lacks a substantial speech component based on the determined flags;
- classifying the frame in a noise mode if the frame lacks a substantial speech component, and in a speech mode otherwise;
- and
- generating an encoded frame in accordance with a noise mode coding scheme if the frame is classified in the noise mode, and in accordance with a speech coding scheme if the frame is classified in the speech mode.
A method of encoding a signal having a speech component, the signal being organized as a plurality of frames, comprising the steps of:
- measuring a value for at least one speech characteristic of a frame, wherein the speech characteristic is selected from the group consisting of spectral stationarity, pitch stationarity, high-frequency content, and energy;
- comparing the measured value of the selected speech characteristic with at least two thresholds, including a high threshold representing a high value of the selected speech characteristic and a low threshold representing a low value of the selected speech characteristic;
- setting a first flag if the measured value exceeds the high threshold;
- and
- setting a second flag if the measured value is below the low threshold;
- determining whether the frame lacks a substantial speech component based on the determined flags;
- classifying the frame in a noise mode, depending on whether the frame lacks a substantial speech component, and in a speech mode otherwise;
- and
- generating an encoded frame in accordance with a noise coding scheme when the frame is classified in the noise mode, and in accordance with a speech coding scheme when the frame is classified in the speech mode.
An encoder for encoding a signal having a speech component, the signal being organized as a plurality of frames, comprising:
- means for measuring a value for at least one speech characteristic of a frame from among the plurality of frames, wherein the speech characteristic is selected from the group consisting of spectral stationarity, pitch stationarity, high-frequency content, and energy;
- a speech characteristic value measurer for comparing the measured value of the selected speech characteristic with at least two thresholds, including a high threshold representing a high value of the selected speech characteristic and a low threshold representing a low value of the selected speech characteristic, setting a first flag if the measured value exceeds the high threshold, and setting a second flag if the measured value falls below the low threshold;
- means for determining whether the frame lacks a substantial speech component based on an evaluation of the determined flags;
- a mode classifier for classifying the frame in a noise mode if the frame lacks a substantial speech component, and in a speech mode otherwise;
- and
- a frame encoder for generating an encoded frame in accordance with a noise mode coding scheme when the frame is classified in the noise mode, and in accordance with a speech coding scheme when the frame is classified in the speech mode.
The encoder of claim 24, wherein the spectral stationarity measurer determines a first set of filter coefficients corresponding to the frame and a second set of filter coefficients corresponding to a previous signal frame, and determines a cepstral distortion and a residual energy for the frame based on the determined first and second sets of filter coefficients, wherein the spectral stationarity measurement is based on the cepstral distortion and residual energy determinations.
Dependencies are inferred from the claim text.