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