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. A decoder configured to decode a bit stream encoded by an encoder, the decoder comprising:
a demultiplexer for demultiplexing the encoded bitstream for obtaining a lowband core decoder signal and level parameters, a level parameter representing a total power in a frequency band of a signal having two channels;
a lowband core decoder for producing a lowband output signal, the lowband output signal having a lowband mono signal or a lowband stereo signal; and
a high-frequency reconstruction device for generating a synthetic highband using the lowband output signal and the level parameter and for combining the synthetic highband and the lowband output signal.
2. A decoder configured to decode a bit stream encoded by an apparatus for encoding a power spectral envelope, the decoder comprising:
an input interface configured for receiving the bit stream comprising a downmix of a stereo signal or a multichannel signal having two channels, the downmix having a set of frequency bands, balance parameters for each frequency band, and level parameters, a level parameter representing a total power of the two channels for each frequency band, and
a converter for configured for converting the balance parameters and the level parameters into power values of the first channel for each frequency band and power values of the second channel for each frequency band; and
a parametric stereo decoder configured for calculating a decoded stereo signal using the downmix and the power values of the first channel for each frequency band and the power values of the second channel for each frequency band.