Methods for improving high frequency reconstruction
The present invention proposes a new method and a new apparatus for enhancement of audio source coding systems utilizing high frequency reconstruction (HFR). It utilizes a detection mechanism on the encoder side to assess what parts of the spectrum will not be correctly reproduced by the HFR method in the decoder. Information on this is efficiently coded and sent to the decoder, where it is combined with the output of the HFR unit.
1. Decoder for decoding an encoded signal, the encoded signal including an encoded input signal representing a frequency content of an original audio signal below a predetermined frequency, and an additional information, the decoder comprising:
a coding algorithm for decoding the encoded input signal to produce a decoded input signal;
a reconstructor for reconstructing differences between the original audio signal and a regenerated signal based on the additional information;
a high frequency generator for performing a high frequency regeneration technique to obtain the regenerated signal; and
a producer for producing a high frequency regenerated audio signal based on the decoded input signal, the reconstructed differences and the regenerated signal, wherein the coding algorithm, the reconstructor, the high frequency generator or the producer comprises a hardware implementation.
2. Decoder in accordance with claim 1 ,
in which the additional information specifies a scale factor band, in which a spectral line is to be reconstructed,
in which the encoded signal further comprises spectral envelope data for describing a spectral portion of the audio signal above the predetermined frequency,
in which the producer is arranged for generating a spectral line in the scale factor band, and
in which the producer is further arranged for adjusting spectral lines in the scale factor band so that a given energy for the scale factor band including the generated spectral line is maintained.
3. Decoder in accordance with claim 1 ,
in which the high frequency regenerator includes a synthesis filter bank having synthesis filter bank channels, wherein a scale factor band includes more than one filter bank channels,
in which the encoded signal further includes a spectral envelope vector and a noise-floor level vector, and
wherein the reconstructor is arranged for calculating a level of the reconstructed spectral line based on the spectral envelope vector.
4. Decoder in accordance with claim 3 , wherein the producer is arranged for determining band pass signals for filter bank channels, into which no sine is to be inserted, in a scale factor band in accordance with the following equation
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hfr
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wherein l is a filter bank channel number, wherein l l is the lowest filter bank channel number for the scale factor band, wherein l u is the highest filter bank channel for the scale factor band, wherein x re is the real part of a band pass signal sample output by the HFR block, wherein x im is an imaginary part of the band pass signal sample output by the HFR block, wherein y re and y im are the real part and the imaginary part of an adjusted band pass signal for a filter bank channel, and wherein g hfr is a gain adjustment factor derived from the noise-floor level vector.
5. Decoder in accordance with claim 3 , wherein the reconstructor is arranged for determining a certain scale factor band I s into which a synthetic sine is to be inserted, and
wherein a level of a synthetic sine to be inserted is defined as follows:
g sine ( n )=√{square root over ( e ( n ))}
wherein n is a number of the given scale factor band, and e is the spectral envelope vector, and
wherein the producer is arranged for determining a band pass signal for the channel in which the synthetic sine is to be placed in accordance with the following equation:
y re ( l s )= x re ( l s )· g hfr ( l s )+ g sin ( l s )· φ re ( k )
y im ( l s )= x im ( l s )· g hfr ( l s )+ g sin ( l s )·(−1) 1 s · φ im ( k )
wherein l s is a filter bank channel number, into which a sine is to be inserted, wherein l l is the lowest filter bank channel number for the scale factor band, wherein l u is the highest filter bank channel for the scale factor band, wherein x re is the real part of a band pass signal sample output by the HFR block, wherein x im is an imaginary part of the band pass signal sample output by the HFR block, and wherein y re and y im are the real part and the imaginary part of an adjusted band pass signal for a filter bank channel, and wherein g hfr is a gain adjustment factor derived from the noise-floor level vector,
wherein φ re and φ im form a complex modulation vector for placing a sine into a band pass signal and wherein k is a modulation vector index ranging between 0 and 4.
6. Method for decoding an encoded signal, the encoded signal including an encoded input signal representing a frequency content of an original audio signal below a predetermined frequency, the encoding being performed using a coding algorithm, and additional information describing differences between a regenerated signal and the original audio signal, the regenerated signal being generated by a high frequency regenerating technique from the input signal or a coded and decoded version thereof, the method comprising:
obtaining a decoded input signal, which is produced by decoding the encoded input signal in accordance with the coding algorithm;
reconstructing differences based on the additional information;
performing a high frequency regeneration technique similar to the high frequency regeneration technique for obtaining the differences to obtain the regenerated signal;
producing a high frequency regenerated audio signal based on the decoded input signal, the reconstructed differences and the regenerated signal.
7. A non-transitory computer-readable medium encoded with a computer program for performing a method for decoding an encoded signal, the encoded signal including an encoded input signal representing a frequency content of an original audio signal below a predetermined frequency, the encoding being performed using a coding algorithm, and additional information describing differences between a regenerated signal and the original audio signal, the regenerated signal being generated by a high frequency regenerating technique from the input signal or a coded and decoded version thereof, the method comprising:
obtaining a decoded input signal, which is produced by decoding the encoded input signal in accordance with the coding algorithm;
reconstructing differences based on the additional information;
performing a high frequency regeneration technique similar to the high frequency regeneration technique for obtaining the differences to obtain the regenerated signal;
producing a high frequency regenerated audio signal based on the decoded input signal, the reconstructed differences and the regenerated signal, when the computer program runs on a computer.