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Chapter 9: Audio Coding > 9.8 Spectral Band Replication

9.8 Spectral Band Replication

To further reduce the bit rate an extension of MPEG-1 Layer III with the name MP3pro was introduced [Die02, Zie02]. The underlying method, called spectral band replication (SBR), performs a low-pass and high-pass decomposition of the audio signal, where the low-pass filtered part is coded by a standard coding method (e.g. MPEG-1 Layer III) and the high-pass part is represented by a spectral envelope and a difference signal [Eks02, Zie03]. Figure 9.37 shows the functional units of an SBR coder. For the analysis of the difference signal the high-pass part (HP Generator) is reconstructed from the low-pass part and compared to the actual high-pass part. The difference is coded and transmitted. For decoding (see Fig. 9.38) the decoded low-pass part of a standard decoder is used by the HP generator to reconstruct the high-pass part. The additional coded difference signal is added at the decoder. An equalizer provides the spectral envelope shaping for the high-pass part. The spectral envelope of the high-pass signal can be achieved by a filter bank and computing the RMS values of each band-pass signal [Eks02, Zie03]. The reconstruction of the high-pass part (HP Generator) can also be achieved by a filter bank and substituting the band-pass signals by using the low-pass parts [Schu96, Her98]. To code the difference signal of the high-pass part additive sinusoidal models can be applied such as the parametric methods of the MPEG-4 coding approach.


  

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