Efficient and scalable parametric stereo coding for low bitrate audio coding applications
The present invention provides improvements to prior art audio codecs that generate a stereo-illusion through post-processing of a received mono signal. These improvements are accomplished by extraction of stereo-image describing parameters at the encoder side, which are transmitted and subsequently used for control of a stereo generator at the decoder side. Furthermore, the invention bridges the gap between simple pseudo-stereo methods, and current methods of true stereo-coding, by using a new form of parametric stereo coding. A stereo-balance parameter is introduced, which enables more advanced stereo modes, and in addition forms the basis of a new method of stereo-coding of spectral envelopes, of particular use in systems where guided HFR (High Frequency Reconstruction) is employed. As a special case, the application of this stereo-coding scheme in scalable HFR-based codecs is described.
1. Parametric decoder for generating a stereo or multi-channel output signal from a decoder input signal, the decoder input signal comprising;
stereo-width parameters representing degrees of similarity between a first channel and a second channel of a stereo signal or of a multi-channel signal, the parametric decoder comprising:
a calculator for calculating a smoothed stereo-width gain value by smoothing stereo-width gain values over a time segment having several stereo-width parameters, a stereo-width gain value being a function of a corresponding stereo-width parameter; and
a generator for generating the stereo or multi-channel output signal using the smoothed stereo width gain value to control a stereo-width of the stereo or multi-channel output signal.
2. The parametric decoder in accordance with claim 1 , in which the calculator comprises a smoothing filter, the smoothing filter having a first attack time constant to get a first rise-time and having a second release time constant to get a second fall-time, wherein the first attack time constant is shorter than the second release time constant, and wherein the first rise-time is shorter than the second fall-time.
3. The parametric decoder in accordance with claim 2 , further comprising:
a receiver for receiving a signalling of a sudden speech entry and bypassing or resetting the smoothing filter when a sudden speech entry is signalled.
4. The parametric decoder in accordance with claim 2 , further comprising:
a receiver for receiving a signalling of attack time constants, release time constants or other filter characteristics of the smoothing filter, the signalling being generated by an encoder.
5. Method of parametric decoding for generating a stereo or multi-channel output signal from a decoder input signal, the decoder input signal comprising stereo-width parameters representing a degree of similarity between a first channel and a second channel of a stereo signal or of a multi-channel signal, the method comprising:
calculating, by a calculator, a smoothed stereo width gain value by smoothing stereo-width gain values over a time segment having several stereo-width parameters, a stereo-width gain value being a function of a corresponding stereo-width parameter; and
generating, by a generator, the stereo or multi-channel output signal using the smoothed stereo width gain value to control a stereo-width of the stereo or multi-channel output signal.
6. A pseudo stereo or multi-channel generator for generating a stereo or multi-channel output signal, comprising:
a calculator for calculating a smoothed stereo-width gain by smoothing stereo-width gain values over a time segment having several stereo-width parameters, a stereo-width gain value being a function of a corresponding stereo-width parameter, the stereo-width parameters representing degrees of similarity between a first channel and a second channel of a stereo signal or of a multi-channel signal;
a gain device for applying the smoothed stereo width gain to a mono signal derived from a first channel and a second channel of a stereo signal or a multi-channel signal; and
an attenuation device for applying an attenuation to the mono signal, the attenuation being such that an overall level value when changing a stereo width is not significantly altered.
7. Method of pseudo stereo or multi-channel generating a stereo or multi-channel output signal, comprising:
calculating, by a calculator, a smoothed stereo-width gain by smoothing stereo-width gain values over a time segment having several stereo-width parameters, a stereo-width gain value being a function of a corresponding stereo-width parameter, the stereo-width parameters representing degrees of similarity between a first channel and a second channel of a stereo signal or of a multi-channel signal;
applying, by a gain device, the smoothed stereo width gain to a mono signal derived from a first channel and a second channel of a stereo signal or a multi-channel signal; and
applying, by an attenuation device, an attenuation to the mono signal, the attenuation being such that an overall level value when changing a stereo width is not significantly altered.