1. Field of the Invention
The present invention relates to the processing of audio signals and, more particularly, the coding of multi-channel audio signals.
2. Description of the Related Art
Parametric multi-channel audio coders generally transmit only one full-bandwidth audio channel combined with a set of parameters that describe the spatial properties of an input signal. For example, FIG. 1 shows the steps performed in an encoder 10 described in International Application No. WO2003/90208, filed Apr. 22, 2003.
In an initial step S1, input signals L and R are split into subbands 101, for example, by time-windowing followed by a transform operation. Subsequently, in step S2, the level difference (ILD) of corresponding subband signals is determined; in step S3, the time difference (ITD or IPD) of corresponding subband signals is determined; and in step S4, the amount of similarity or dissimilarity of the waveforms which cannot be accounted for by ILDs or ITDs, is described. In the subsequent steps S5, S6, and S7, the determined parameters are quantized.
In step S8, a monaural signal S is generated from the incoming audio signals, and finally, in step S9, a coded signal 102 is generated from the monaural signal and the determined spatial parameters.
FIG. 2 shows a schematic block diagram of a coding system comprising the encoder 10 and a corresponding decoder 202. The coded signal 102, comprising the sum signal S and spatial parameters P, is communicated to a decoder 202. The signal 102 may be communicated via any suitable communications channel 204. Alternatively, or additionally, the signal may be stored on a removable storage medium 214, which may be transferred from the encoder to the decoder.
Synthesis (in the decoder 202) is performed by applying the spatial parameters to the sum signal to generate left and right output signals. Hence, the decoder 202 comprises a decoding module 210 which performs the inverse operation of step S9 and extracts the sum signal S and the parameters P from the coded signal 102. The decoder further comprises a synthesis module 211 which recovers the stereo components L and R from the sum (or dominant) signal and the spatial parameters.
One of the challenges is to generate the monaural signal S, step S8, in such a way that, on decoding into the output channels, the perceived sound timbre is exactly the same as for the input channels.
Several methods of generating this sum signal have been suggested previously. In general, these methods compose a mono signal as a linear combination of the input signals. Particular techniques include:
1. Simple summation of the input signals. See, for example, ‘Efficient representation of spatial audio using perceptual parametrization’, by C. Faller and F. Baumgarte, WASPAA′01, Workshop on applications of signal processing on audio and acoustics, New Paltz, New York, 2001.2. Weighted summation of the input signals using principle component analysis (PCA). See, for example, International Patent Application No. WO2003/85645, filed Mar. 20, 2003 and International Patent Application No. WO2003/85643 filed Mar. 20, 2003. In this scheme, the squared weights of the summation sum up to one and the actual values depend on the relative energies in the input signals.3. Weighted summation with weights depending on the time-domain correlation between the input signals. See for example ‘Joint stereo coding of audio signals’, by D. Sinha, European Patent Application No. EP 1 107 232 A2. In this method, the weights sum to +1, while the actual values depend on the cross-correlation of the input channels.4. U.S. Pat. No. 5,701,346 to Herre et al. discloses weighted summation with energy-preservation scaling for downmixing left, right, and center channels of wideband signals. However, this is not performed as a function of frequency.
These methods can be applied to the full-bandwidth signal or can be applied on band-filtered signals which all have their own weights for each frequency band. However, all of the methods described have one drawback. If the cross-correlation is frequency-dependent, which is very often the case for stereo recordings, coloration (i.e., a change of the perceived timbre) of the sound of the decoder occurs.
This can be explained as follows: For a frequency band that has a cross-correlation of +1, linear summation of two input signals results in a linear addition of the signal amplitudes and squaring the additive signal to determine the resultant energy. (For two in-phase signals of equal amplitude, this results in a doubling of amplitude with a quadrupling of energy.) If the cross-correlation is 0, linear summation results in less than a doubling of the amplitude and a quadrupling of the energy. Furthermore, if the cross-correlation for a certain frequency band amounts −1, the signal components of that frequency band cancel out and no signal remains. Hence, for simple summation, the frequency bands of the sum signal can have an energy (power) between 0 and four times the power of the two input signals, depending on the relative levels and the cross-correlation of the input signals.