Gerard Hotho

A Backward-Compatible Multichannel Audio Codec

We propose in this paper a backward-compatible multichannel audio codec. This codec represents a multichannel audio input signal by a down mix and parametric data. In order to enable backward compatibility, it is necessary to have the possibility of exerting control over the down-mixing procedure. At the same time, in order to achieve a high coding efficiency, both signal and perceptual redundancies should be exploited. In this paper, we describe a codec that unifies the above-mentioned conditions: backward compatibility and exploitation of both signal and perceptual redundancies. The codec combines a high audio quality and a low parameter bit rate. Moreover, its design is flexible, examples of which are the scalability of the audio quality to (in principle) transparency and the possibility to preserve the correlation structure of the original input signals by using synthetic signals. A stereo backward compatible version of the proposed codec is used as a component of the recently standardized MPEG Surround multichannel audio codec.

Audio coding based on frequency variations of sinusoidal components

Coding of an audio signal is provided where an indicator of the frequency variation of sinusoidal components of the signal is used in the tracking algorithm of a sinusoidal coder where sinusoidal parameters from appropriate sinusoids from consecutive segments are linked. By applying an indicator such as a warp factor or polynomial fitting, more accurate tracks are obtained. As a result, the sinusoids can be encoded more efficiently. Furthermore, a better audio quality can be obtained by improved phase continuation.

Multichannel coding of applause signals

We develop a parametric multichannel audio codec dedicated to coding signals consisting of a dense series of transient-type events. These signals of which applause is a typical example are known to be problematic for such audio codecs. The codec design is based on preservation of both timbre and transient-type event density. It combines a very low complexity and a low parameter bit rate (0.2 kbps). In a formal listening test, we compared the proposed codec to the recently standardised MPEG Surround multichannel codec, with an associated parameter bit rate of 9 kbps. We found the new codec to have a significantly higher audio quality than the MPEG Surround codec for the two multichannel applause signals under test. Though this seems promising, the technique presented is not fully mature, for example, because issues related to integration of the proposed codec in the MPEG Surround codec were not addressed.