Compression of multi channel audio at low bit rates using theAMR-WB+ codec
Abstract: The purpose of this thesis work done at Ericsson Research in Luleå was toinvestigate the possibilities of encoding 5.1 surround sound for low bitrates using the AMR-WB+ audio encoding technique.In the first phase of the work, investigations of the inter-channeldependencies were carried out, and the main tool used was Matlab. Severalefforts were made aiming to reduce the total energy of the signal. This wasdone by decorrelating the sound channels using ordinary linear predictors.Decorrelation attempts were performed in the time domain as well as in thefrequency domain. After decorrelating the channels, each channel was encodedin an individual bit rate using the mono mode of AMR-WB+ with bit ratesproportional to the relative energy content. However, listening testscombined with studies of the energy reduction achieved by decorrelationsindicated that, for a given bit rate, there is in general no advantagedecorrelating the signal compared to not doing so.The second phase of the thesis work mainly consisted of modifying theexistent C code of the AMR-WB+ mono coder in order to suit it for 5.1 soundencoding and decoding. The goal was "the simplest possible", an encoderthat encoded each unmodified sound channel individually, distributing the bitrates over the channels aiming to keep the sums of the bit rates over thechannels approximately constant over time. Opposed to the five "normal"sound channels, the encoding of the LFE (Low Frequency Element) was done in aspecific way making use of this, compared to the other channels, somewhatconstrained signal. Furthermore, the method of encoding high frequency soundsof AMR-WB+, the BWE (BandWidth Extension), needed to be modified at some extent.Finally, the third phase consisted mainly of evaluation of the product madeand of making comparisons to competing low bit rate multi channel coders.
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