Method and apparatus for encoding/decoding speech signal

An apparatus for decoding a speech signal, the apparatus comprising:

• a demultiplexing unit to receive and to demultiplex a variable bit rate bitstream, and to extract an immittance spectral frequencies (ISF) index, a gain vector quantization (VQ) index, a code index, and a pitch index from the variable rate bitstream with at least two indexes of the ISF index, the gain VQ index, the code index, and the pitch index having been encoded at a variable bit rate;
• a linear predictive coefficient decoding unit to decode a linear predictive coefficient using quantizer information included in the ISF index;
• a gain decoding unit to decode an adaptive codebook gain and a fixed codebook gain using the quantizer information included in the gain VQ index;
• a fixed codebook decoding unit to decode a fixed codebook vector using fixed codebook information used in the code index;
• an adaptive codebook decoding unit to decode an adaptive codebook vector using pitch allocation bit information included in the pitch index;
• an excitation signal configuration unit to configure an excitation signal using the decoded adaptive codebook gain and fixed codebook gain;
• and
• a synthesis filter unit to synthesize the excitation signal with the ISF index,
• wherein the quantizer information included in the ISF index indicates a linear predictive coefficient quantizer selected for control of the variable bit rate of the ISF index, based on a comparison between reserved bits and a reference value.

A method for encoding a speech signal, the method comprising:

• determining, using at least one processing device, an immittance spectral frequencies (ISF) index;
• determining a pitch index;
• determining a code index from a fixed codebook;
• determining a gain vector quantization (VQ) index;
• and
• generating a variable rate bitstream including the ISF index, the pitch index, the code index, and the gain VQ index, the variable rate bitstream with at least two indexes of the ISF index, the gain VQ index, the code index, and the pitch index encoded at a variable bit rate,
• wherein compares reserved bits with a reference value, and selects a linear predictive coefficient quantizer for control of the variable bit rate of the ISF index, based on a result of the comparison.