Excitation signal bandwidth extension

A method of generating a high band extension of a low band excitation signal (eLB) that has been obtained by source-filter model based encoding of an audio signal, including the steps of

• upsampling (S1) the low band excitation signal (eLB) to a predetermined sampling frequency (fS);
• determining (S2) a modulation frequency (Ω) from an estimated measure representing a fundamental frequency (F0) of the audio signal;
• modulating (S3) the upsampled low band excitation signal (eLB↑) with the determined modulation frequency to form a frequency shifted excitation signal;
• high-pass filtering (S4) the frequency shifted excitation signal;
• estimating (S5) a compression factor (λ);
• attenuating (S6) the high-pass filtered frequency shifted excitation signal based on the estimated compression factor.

The method of any of the preceding claims, wherein the upsampled low band excitation signal (eLB↑) is modulated by

• A·cos(l·Ω)
• where
• A is a predetermined constant,
• l is a sample index, and
• Ω is the modulation frequency.

An apparatus for generating a high band extension of a low band excitation signal (eLB) defined by parameters representing a CELP encoded audio signal, said apparatus including

• upsamplers (20) configured to upsample a low band fixed codebook vector (uFCB) and a low band adaptive codebook vector (uACB) to a predetermined sampling frequency (fS);
• a frequency shift estimator (22) configured to determine a modulation frequency (Ω) from an estimated measure representing a fundamental frequency (F0) of the audio signal;
• a modulator (24) configured to modulate the upsampled low band adaptive codebook vector (uACB↑) with the determined modulation frequency to form a frequency shifted adaptive codebook vector;
• a compression factor estimator (28) configured to estimate a compression factor (λ);
• a compressor (34) configured to attenuate the frequency shifted adaptive codebook vector and the upsampled fixed codebook vector (uFCB↑) based on the estimated compression factor;
• a combiner (40) configured to form a high-pass filtered sum (eHB) of the attenuated frequency shifted adaptive codebook vector and the attenuated upsampled fixed codebook vector.

An apparatus for generating a high band extension of a low band excitation signal (eLB) that has been obtained by source-filter model based encoding of an audio signal, said apparatus including

• an upsampler (20) configured to upsample the low band excitation signal (eLB) to a predetermined sampling frequency (fS);
• a frequency shift estimator (22) configured to determine a modulation frequency (Ω) from an estimated measure representing a fundamental frequency (F0) of the audio signal;
• a modulator (24) configured to modulate the upsampled low band excitation signal (eLB↑) with the determined modulation frequency to form a frequency shifted excitation signal;
• a high-pass filter (26) configured to high-pass filter the frequency shifted excitation signal;
• a compression factor estimator (28) configured to estimate a compression factor (λ);
• a compressor (34) configured to attenuate the high-pass filtered frequency shifted excitation signal based on the estimated compression factor.