Sascha Disch

A Dedicated Decorrelator for Parametric Spatial Coding of Applause-like Audio Signals

Parametric spatial coding of multichannel content has acquired widespread use in today’s audio codecs to enable efficient operation in the medium bitrate range. However, applause signals are still challenging to code with good perceptual quality using these techniques. To obtain a sufficiently low side information bitrate, the spatial parameter update rate is usually chosen in the range of several tens of milliseconds which is too low to restore a convincing spatial listener envelopment of applause-like signals. Moreover, state of the art decorrelators that are mandatory in parametric multichannel decoders inevitably deteriorate the signal quality of transient sound events, like handclaps, by dispersion. Therefore, we propose a novel decorrelation and parametrization technique for efficient coding of applause sounds: transient events are separated from the core decoder output and a dedicated decorrelator algorithm distributes the transients in the spatial image according to parametric guiding information transmitted in the bitstream. Listening tests show a substantial improvement in subjective quality compared to standard methods.

MPEG Unified Speech and Audio Coding – Bridging the Gap

Speech and audio coding schemes originate from different worlds. Speech coding schemes typically assume a source model i.e. the human vocal tract. General audio coding schemes primarily rely on a sinkmodel i.e. the human auditory system. While speech coding schemes work well for the signal class they were designed for at very low rates, they are known to fail for general audio signals even at higher rates. In contrast, general audio coders work well for any content at higher rates, but typically have limited performance especially for speech signals at very low rates. Recently the ISO/MPEG group started a standardization activity to develop a new Unified Speech and Audio Coding scheme. A state of the art AAC based general audio coder, featuring transform coding, parametric bandwidth extension and parametric stereo coding,was extended by source model coding tools. All codec modules were further improved and revised for enhanced performance in particular at very low bitrates. The new unified coding scheme outperforms dedicated speech and general audio coding schemes and bridges the gap between both worlds. This paper describes the new codec in detail and shows how the goal of consistent high quality for all signal types is reached.