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WO2010097748A1 - Codage et décodage stéréo paramétriques - Google Patents

Codage et décodage stéréo paramétriques Download PDF

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Publication number
WO2010097748A1
WO2010097748A1 PCT/IB2010/050763 IB2010050763W WO2010097748A1 WO 2010097748 A1 WO2010097748 A1 WO 2010097748A1 IB 2010050763 W IB2010050763 W IB 2010050763W WO 2010097748 A1 WO2010097748 A1 WO 2010097748A1
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WO
WIPO (PCT)
Prior art keywords
downmix
phase
signal
parts
correction parameter
Prior art date
Application number
PCT/IB2010/050763
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English (en)
Inventor
Arnoldus W. J. Oomen
Erik G. P. Schuijers
Dirk J. Breebaart
Original Assignee
Koninklijke Philips Electronics N.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2010097748A1 publication Critical patent/WO2010097748A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S1/00—Two-channel systems
    • H04S1/002—Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/008—Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S2420/00—Techniques used stereophonic systems covered by H04S but not provided for in its groups
    • H04S2420/03—Application of parametric coding in stereophonic audio systems

Definitions

  • the invention relates to parametric stereo encoding and decoding.
  • Digital encoding of various source signals has become increasingly important over the last decades as digital signal representation and communication increasingly has replaced analogue representation and communication.
  • mobile telephone systems such as the Global System for Mobile communication
  • digital speech encoding are based on digital speech encoding.
  • distribution of media content is increasingly based on digital content encoding.
  • PS Parametric Stereo
  • Fig. 1 illustrates an example of a PS encoding/ decoding scheme.
  • the scheme is based on the generation of an appropriate mono down-mix. Along with the calculation of the mono down-mix, parameters are calculated that enable a PS decoder to regenerate the stereo signal.
  • PS schemes generally rely on a time-frequency representation, which can be e.g. based on a Discrete Fourier Transform (DFT) for parameter analysis and synthesis or a Quadrature Mirror Filterbank (QMF) for a lower-complexity alternative.
  • DFT Discrete Fourier Transform
  • QMF Quadrature Mirror Filterbank
  • HD the Inter-channel Intensity Difference (typically given in dB).
  • IPD the Inter-channel Phase Difference (typically given in radians or degrees).
  • OPD the Overall Phase Difference (typically given in radians or degrees).
  • the ICC the Inter-channel Coherence (typically calculated to be independent of inter-channel phase differences).
  • the encoder typically estimates such parameters for each sub-band in each time frame based on the mono downmix and the original stereo signals.
  • Other systems use both IID and IC to add a certain sense of ambience (captured by IC) and sound source positions (IID). It has also been proposed to use phase differences (IPD and OPD) as these contain important sound source localization properties.
  • PS can be used to provide signals of different qualities and has the advantage of being scalable.
  • PS has been standardized within the MPEG-4 audio standard (ISO/IEC 14496- 3:2005 Part 3: Audio). It has successfully been adopted into the High-Efficiency Advanced Audio Coding - HE-AAC v2 profile and also by the 3 rd Generation Partnership Project 3GPP Release 6 as Enhanced aacPlus for use in cellular communication systems.
  • these latter standards use PS approaches with a profile/level that does not use the phase parameters IPD and OPD as described in ISO/IEC 14496-3:2005 Part 3: Audio. Nevertheless, for higher bitrates it is advantageous to exploit phase parameters as these provide significant improvements in perceived quality.
  • the PS technology has been employed as a building block in constructing a multi-channel audio coding tool, MPEG Surround (ISO/IEC 23003-1 :2007 Part 1 : MPEG Surround).
  • MPEG Surround ISO/IEC 23003-1 :2007 Part 1 : MPEG Surround
  • OTT One-To-Two
  • the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.
  • a decoder for generating a multi-channel signal comprising: means for receiving a parametrically encoded signal comprising a downmix of multiple channels, a set of upmix parameters including a phase parameter, and a phase correction parameter, the phase correction parameter being provided for second parts of the downmix but not for first parts of the downmix; upmixing means for generating the multi-channel signal by upmixing the downmix based on the set of upmix parameters; modifying means for modifying the upmixing in response to the phase correction parameter for the second parts of the downmix.
  • phase information with parametric multichannel (and specifically stereo) encoding may provide improved performance for some parts of a parametrically encoded multichannel signal, it may also provide suboptimal performance for other parts.
  • the inventors have realized that whereas an upmixing considering phase parameters may provide improved performance when the multichannel signal has certain characteristics, it may also provide reduced performance when the multichannel signal has other characteristics.
  • improved operation can be achieved by only including additional phase information, in the form of a phase correction parameter, for some parts of the downmix and not for other parts of the downmix. This may specifically allow reduced data rate while maintaining a suitably high quality level.
  • the inventors have realized that when estimating upmix phase operations, these can be derived appropriately from parameters such as HD and IPD for some multichannel signal characteristics but not for other multichannel signal characteristics. Typically, such estimation is unsuitable for signals of the multiple channels that are sufficiently out of phase with each other. This may lead to significant and noticeable audio quality degradation.
  • the inventors have realized that such disadvantages may be mitigated while maintaining a suitably low data rate by a decoder being adapted to modify the upmix operation based on the presence of a phase correction parameter for only some parts of the signal.
  • the upmixing is specifically in response to the phase parameter which e.g. may be an IPD parameter.
  • the upmixing accordingly reflects a phase characteristic of the multiple channels.
  • Each of the parameters may be provided as a set of parameter values for frequency and time intervals (e.g. time frequency tiles or blocks).
  • a set of parameter values may be provided for each of a set of frequency and time blocks.
  • the second parts may correspond to a subset of frequency time blocks.
  • the set of parameters may specifically include HD, IC and/or IPD parameters.
  • the phase correction parameter may be an absolute phase correction parameter and may specifically be a replacement phase parameter.
  • the phase correction parameter may provide relative correction values, such as a relative offset or correction to a parameter value that is calculated by or employed in the decoder.
  • the decoder further comprises: first determining means arranged to determine an overall phase offset in response to the set of upmix parameters for the first parts of the downmix; second determining means arranged to determine the overall phase offset in response to the phase correction parameter for the second parts of the downmix; and wherein the upmixing means is arranged to upmix the downmix based on the overall phase offset.
  • the invention may provide an improved audio quality to data rate ratio in many scenarios.
  • the approach may allow the upmixed signal to reflect overall phase offsets without artifacts and degradations often associated with such approaches.
  • the invention may allow the upmix to take the overall phase offset into account with substantially improved performance for out of phase signals.
  • the overall phase offset can be calculated from other PS parameters.
  • the overall phase offset may specifically be an OPD parameter.
  • the overall phase offset is indicative of a phase difference between the downmix and at least one of the multiple channels.
  • the second determining means is arranged to further determine the overall phase offset in response to the set of upmix parameters.
  • the downmix is a phase compensated downmix for the second parts relative to the first parts, and the phase correction parameter is indicative of the phase compensation.
  • the system may allow a downmix to be generated wherein the contributions from two stereo channels do not cancel each other even if the input signals are out of phase. This may allow improved performance and an improved use of phase information in the upmix. In addition, it may allow the energy variation of the downmix signal to be reduced.
  • the phase compensation may ensure that the downmix and set of parameters is such that the decoder can calculate suitable phase parameters to use in the upmix from the set of parameters. Furthermore, as the compensation is different for the first and second parts, the compensation can be optimized for different signal characteristics in the two parts. This may e.g. allow a reduced data rate.
  • phase compensation for the second parts may for example be represented by:
  • S is the phase compensated downmix
  • L and R are the left and right signal values of a stereo input signal (typically time frequency block values) respectively
  • ⁇ and ⁇ are design parameters (often set to one or chosen such that the power of the downmix signal corresponds to the sum of the powers of the left and right signals)
  • phase compensation for the first parts may for example be represented by:
  • ⁇ and ⁇ are design parameters (typically set to one or chosen such that the power of the downmix signal corresponds to the sum of the powers of the left and right signals).
  • the angle ⁇ represents a constant phase compensation.
  • a phase of the downmix is constant in the first parts and the phase compensation varies dynamically during the second parts. This may provide improved performance in many embodiments. Specifically, it may in many embodiments allow the data rate to be reduced as the phase compensation for the first parts may be known and therefore need not be dynamically modified.
  • the upmixing can be based on an assumption of a known constant phase compensation being performed at the encoder (including the encoder using a phase compensation with zero values, i.e. with no phase compensation).
  • the phase of the downmix may be constant in the sense that the generation of the downmix applies a constant phase shift to the left and right signals.
  • the phase shift may specifically be zero (such as e.g. for a simple summation of the right and left signals.
  • the decoder further comprises: first determining means arranged to determine an overall phase offset for the downmix as a predetermined function of the set of stereo upmix parameters; and wherein the upmixing means is arranged to upmix the downmix based on the overall phase offset.
  • the predetermined function is arranged to estimate the overall phase offset of at least one of the multiple channels relative to a constant phase of the downmix during the first parts.
  • the upmixing means is arranged to estimate a difference signal for a difference between channels of the multichannel signal based on the downmix scaled with a prediction coefficient derived from the set of upmix parameters, and to generate the multichannel signal based on a sum and a difference of the downmix and said difference signal; and wherein the modifying means is arranged to modify the prediction coefficient for the second parts in response to the phase correction parameter.
  • This may provide improved performance, and in particular improved perceived audio quality, in many scenarios. In many embodiments, it may provide a reduced complexity, reduced data rate and/or improved audio quality.
  • the upmixing means is arranged to enhance the difference signal by adding a scaled decorrelated mono downmix.
  • the phase correction parameter is indicative of a correction for an interchannel phase difference of the set of upmix parameters.
  • the second parts correspond to parts of the parametric signal associated with signals of the multiple channels meeting an out of phase criterion.
  • phase information in upmixing is particularly sensitive to out of phase signals and that this can be addressed while maintaining a relatively low data rate by using phase correction parameters for (only) the parts of the signal associated with out of phase characteristics.
  • the parts of the parametric signal that are associated with the channels meeting the out of phase criterion may correspond to a time interval and/or frequency interval around a time- frequency for which the phase criterion is met.
  • the phase correction parameter may gradually change around such a detection to ensure a smooth transition from the first parts (i.e. from parts of the downmix for which no phase correction parameter is provided).
  • the out of phase criterion comprising a requirement that a phase difference between signals of the multiple channels fall within the intervals of [ ⁇ -a; ⁇ ] and [- ⁇ ;- ⁇ +b] where a and b are each less or equal to ⁇ /8. This may provide a particularly advantageous criterion for many scenarios and embodiments.
  • the parametrically encoded signal comprises a phase correction parameter presence indication indicative of the second parts.
  • the phase correction parameter presence indication comprises a common indication for all time frequency blocks of each encoding frame of the parametrically encoded signal.
  • the phase correction parameter presence indication comprises individual presence indications for a plurality of sets of time frequency blocks of the down-mix.
  • the individual presence indications may cover all time frequency blocks of the downmix signal.
  • the phase correction parameter comprises individual parametric values for a plurality of sets of time frequency blocks of the second parts.
  • the individual parametric values may cover all time frequency blocks of the downmix signal.
  • an encoder for encoding a multichannel signal comprising: downmix means for generating an encoded downmix of multiple channels of the multichannel signal; parameter means for generating a set of upmix parameters relating the downmix to the multiple channels; means for determining at least one section of the downmix in which a deviation between a value of phase parameter derived from the set of upmix parameters and a target value for the phase parameter meets a criterion; means for generating a phase correction parameter for a part of the encoded downmix associated with the at least one section; and means for generating an encoded signal comprising the encoded downmix, the set of upmix parameters and the phase correction parameter for the part of the encoded downmix.
  • the means for determining at least one section may specifically determine the at least one section as a section for which two signals of the multiple channels meet an out of phase criterion.
  • a method of generating a multi-channel signal comprising: receiving a parametrically encoded signal comprising a downmix of multiple channels, a set of upmix parameters including a phase parameter, and a phase correction parameter, the phase correction parameter being provided for second parts of the downmix but not for first parts of the downmix; generating the multi-channel signal by upmixing the downmix based on the set of upmix parameters; and modifying the upmixing in response to the phase correction parameter for the second parts of the downmix.
  • a method of encoding a multichannel signal comprising: generating an encoded downmix of multiple channels of the multichannel signal; generating a set of upmix parameters relating the downmix to the multiple channels; determining at least one section of the downmix in which a deviation between a value of phase parameter derived from the set of upmix parameters and a target value for the phase parameter meets a criterion; generating a phase correction parameter for a part of the encoded downmix associated with the at least one section; and generating an encoded signal comprising the encoded downmix, the set of upmix parameters and the phase correction parameter for the part of the encoded downmix.
  • Fig. 1 is an illustration of an example of a parametric encoding system in accordance with the prior art
  • Fig. 2 illustrates an example of a transmission system for communication of a stereo audio signal in accordance with some embodiments of the invention
  • Fig. 3 illustrates an example of a relationship between an OPD and IPD phase parameter of a parametric encoding system
  • Fig. 4 illustrates an example of a relationship between an OPD-IPD and IPD phase parameter of a parametric encoding system
  • Fig. 5 illustrates an example of a parametric encoder in accordance with some embodiments of the invention.
  • Fig. 6 illustrates an example of a parametric decoder in accordance with some embodiments of the invention.
  • Fig. 2 illustrates a transmission system for communication of a stereo audio signal in accordance with some embodiments of the invention.
  • the transmission system comprises a transmitter 201 which is coupled to a receiver 203 through a network 205 which specifically may be the Internet.
  • the transmitter 201 is a signal recording device and the receiver 203 is a signal player device but it will be appreciated that in other embodiments a transmitter and receiver may be used in other applications and for other purposes.
  • the transmitter 201 and/or the receiver 203 may be part of a transcoding functionality and may e.g. provide interfacing to other signal sources or destinations.
  • the transmitter 201 comprises a digitizer 207 which receives an analog stereo signal that is converted to a digital PCM (Pulse Code Modulated) stereo signal by sampling and analog-to- digital conversion.
  • a digitizer 207 which receives an analog stereo signal that is converted to a digital PCM (Pulse Code Modulated) stereo signal by sampling and analog-to- digital conversion.
  • the digitizer 207 is coupled to the encoder 209 of Fig. 2 which encodes the PCM signal in accordance with a Parametric Stereo (PS) encoding algorithm.
  • the encoder 209 is coupled to a network transmitter 211 which receives the encoded signal and interfaces to the Internet 205.
  • the network transmitter may transmit the parametrically encoded signal to the receiver 203 through the Internet 205.
  • the receiver 203 comprises a network receiver 213 which interfaces to the Internet 205 and which is arranged to receive the parametrically encoded signal from the transmitter 201.
  • the network receiver 213 is coupled to a decoder 215.
  • the decoder 215 receives the parametrically encoded signal and decodes it in accordance with a PS decoding algorithm.
  • the receiver 203 further comprises a signal player 217 which receives the decoded audio signal from the decoder 215 and presents this to the user.
  • the signal player 217 may comprise a digital-to-analog converter, amplifiers and speakers as required for outputting the decoded audio signal.
  • the parametric stereo coding scheme utilizes phase information for the stereo channels. Specifically, a downmix is generated at the encoder 209 together with parametric stereo parameters that can be used by the decoder to upmix the downmixed mono signal. These parameters include at least one phase parameter which specifically may be an IPD.
  • the decoder 215 recreates the original signal by upmixing the downmixed mono signal to a stereo channel using the set of stereo upmix parameters. Thus, the upmixing specifically considers a phase characteristics for at least one of the stereo channels of the downmix.
  • the inventors have realized that improved performance can be achieved by identifying scenarios where such an upmix may result in problems and using a phase correction parameter for corresponding parts of the downmix to overcome or mitigate such problems.
  • the inventors have realized that including a phase correction parameter for parts of the downmix associated with a detection of the stereo channels to be encoded being out of phase may address a number of problems.
  • the data rate for other parts of the downmix may be kept unchanged thereby resulting in a reduced average data rate.
  • phase parameters such as IPD and OPD in the upmix may contribute significantly to the perceived quality in PS based audio codecs and may in particular substantially improve sound source localization.
  • the usage of such parameters introduces several challenges and especially for signals that are (nearly) out-of- phase (either in single time/frequency tiles or in more continuous regions of a signal), significant degradation in perceived quality can occur if phase relations are not handled correctly. It is worth noting that such degradations do not result from quantization errors, but tend to be related to phase continuity in downmix and decoder output channels.
  • Stereo microphone techniques will often cause time delays between the recorded signals. For example in classical music recordings, this will occur in many cases especially when recordings are made from a limited set of microphones. The resulting time delays will cause out-of-phase signals in some frequency regions (depending on the inter- channel delay).
  • Advanced panning techniques may employ temporal panning instead of the conventional amplitude panning. This technique will also create signals that are out-of-phase in some frequency bands. Cross-talk cancellation techniques deliberately introduce negative correlations to widen the perceived sound stage.
  • stereo signals that are out of phase may cancel each other out and may furthermore result in phase parameters being calculated which vary very significantly for even small variations in the individual stereo signals. For example, small signal variations resulting in a relative phase change of, say 2°, may result in a calculated IPD value changing from +179° to -179°. Also, the relationship between IPD and OPD values may change very substantially for small changes of signals that are substantially out of phase. This may introduce phase discontinuities (in time or frequency) which may result in very noticeable artifacts.
  • a PS decoder may use the HD, IC and IPD parameters to determine a stereo signal from a downmix signal. This process is typically performed by upmixing the downmix and a decorrelated signal using a mixing matrix that depends on the HD, IC and IPD values.
  • IPD only describes the relative phase modification between the two output signals and not the phase difference between the downmix and the individual stereo channels, it cannot provide any information of how the phase modification should be distributed across the output stereo channels.
  • the OPD parameter is indicative of the phase offset between the downmix and at least one of the stereo channels and it thus reflects how the phase should be distributed between the channels.
  • the OPD may accordingly be included in the encoded signal by an encoder.
  • the mentioned document proposes that an OPD estimation is performed at the decoder side such that the OPD value is not included in the encoded signal but is instead calculated by the decoder from the other parameter values.
  • the output stereo signals L', R' can be constructed from the down-mix S and the decorrelated signal D by:
  • V expO (OPD)Xm n S + Tn 12 D) , ,
  • the OPD may e.g. be calculated from (ref. e.g. Jimmy Lapierre and Roch Lefebvre, "On Improving Parametric Stereo Audio Coding", Presented at the 120th Convention, 2006 May 20-23 Paris, Audio engineering Society, France, Preprint 6804):
  • OPD z(lO ⁇ DI2 ° + IC exp(jIPD))
  • Fig. 3 illustrates the relationship between the OPD and the IPD
  • Fig. 4 illustrates the relationship between OPD-IPD (the phase modification applied to the right output channel) and the IPD parameter.
  • the phase discontinuity amounts to ⁇ radians resulting in the effect that small variations of IPD around ⁇ results in the stereo output signals generated by the upmix completely changing signs.
  • the signs of the output signals (e.g. in each time frequency block for which the IPD is close to ⁇ ) may flip between signs.
  • IPD may occur in both the temporal and frequency dimensions. For example, discontinuities can occur in time (e.g. if IPD changes in time from just below to just above ⁇ ). However, as the processing of segments include time averaging (e.g. as part of FFT or QMF windowing) this may lead to cancellation of the output signal. In particular, such an approach typically results in audible artifacts perceived as 'clicks' or 'warbling' sounds. Similarly, the IPD changes (and thus sign inversions) may occur in the frequency domain between one subband and the next (e.g. if IPD in one band is just below ⁇ and just above ⁇ in a neighboring band). This may similarly result in noticeable artifacts.
  • such the encoder 209 divides the signal into first and second parts where the first parts correspond to time and/or frequency sections that are not associated with an out of phase scenario whereas the second parts correspond to time and/or frequency sections that are associated with an out of phase scenario.
  • the encoder then proceeds to include a phase correction parameter for the second parts but not for the first parts.
  • This phase correction parameter is specifically calculated to compensate for phase discontinuities and is included in the encoded signal.
  • the decoder 215 then proceeds to use the phase correction parameter when upmixing the downmix of the second parts and specifically uses this to compensate for phase discontinuities.
  • the inclusion of the phase correction parameter in the encoded signal may increase the data rate relative to an approach where e.g. OPD values are always calculated at the decoder. However, the approach may provide substantially improved audio quality and support for out of phase signals. Furthermore, as out of phase signals typically only occur for a small proportion of the frequencies and/or time, the increase in the data rate may be kept relatively small.
  • the encoder 209 comprises a downmix unit 501 which receives an input stereo signal.
  • the downmix unit 501 proceeds to generate a mono downmix for the two stereo channels of the input stereo signal.
  • the downmix may e.g. be generated as a simple summation of the signals of the two stereo channels, i.e. as:
  • L and R are the signal values of left and right input stereo channels.
  • the downmix unit 501 then proceeds to encode the downmix signal to generate an encoded downmix. It will be appreciated that any suitable encoding may be used. Typically, the encoding will include the generation of a number of time frequency tiles representing the downmix signal in the frequency domain for each of a plurality of time segments as will be well known to the skilled person.
  • the downmix unit 501 is coupled to a parameter generation unit 503 which proceeds to generate parametric stereo parameters based on the downmix and the original input signals.
  • the parameter unit 503 may generate a set of stereo upmix parameters comprising HD and IPD values, as well as in some embodiments IC values. It will be appreciated that any suitable approach for generating the stereo upmix parameters may be used without detracting from the invention.
  • the parameter unit 503 generates a set of IID and IPD values for each of a plurality of time frequency blocks.
  • each time frequency block corresponds to one time segment or (encoding) frame and a given frequency band (such as an ERB band) which is wider than the frequency span of the time frequency tiles used for the encoding of the downmix signal.
  • the parameter unit 503 is fed to an out-of-phase detection unit 505 which is arranged to detect at least one section of the downmix in which the stereo channels meet an out of phase criterion.
  • Each section for which an out-of-phase condition is detected may correspond to one or more time frequency blocks.
  • the criterion may be a requirement that the IPD value falls within one of the intervals [ ⁇ -a; ⁇ ] and [- ⁇ ;- ⁇ +b] where a and b are suitable design parameter (which typically may be identical). Particularly advantageous performance trade-offs have been found when a and b are each less or equal to ⁇ /8.
  • the out-of-phase detection unit 505 may identify all time frequency blocks for which the IPD value falls within these intervals and thus all the time frequency blocks for which the input stereo channels are considered to be (sufficiently) out of phase.
  • other out of phase criteria may be used, such as e.g. a criterion including a requirement that an interchannel correlation is below a given value.
  • the out-of-phase detection unit 505 is coupled to a correction unit 507 which is further coupled to the parameter unit 503.
  • the correction unit 507 receives an indication of all the sections (e.g. all the time frequency blocks) that are considered to be out of phase. It then proceeds to select which parts of the downmix signal a phase correction parameter should be included.
  • the parts are selected to include the time frequency blocks identified by the out-of-phase detection unit 505 but are typically not restricted to these. Indeed, typically surrounding time frequency blocks are also included in order to ensure a smooth and gradual transition from the parts for which there is no phase correction parameter to the time frequency blocks which are detected as being sufficiently out of phase.
  • the correction unit 507 then proceeds to calculate a phase correction parameter.
  • phase correction parameter values for all the time frequency blocks identified by the correction unit 507 are calculated.
  • the phase correction parameter may be a value which can offset a phase parameter value calculated by the decoder such that phase discontinuities are avoided.
  • the phase correction value may specify that e.g. ⁇ should be added or subtracted to the phase parameter value calculated at the decoder.
  • the phase correction parameter may be a relative parameter which is relative to a phase parameter calculated by the decoder or to parameters or phase values for other time frequency blocks.
  • the phase correction value may be a replacement value that directly provides the parameter value to be used by the decoder 215.
  • the downmix unit 501 and the correction unit 507 are coupled to a multiplexer 509 which combines the encoded downmix, the set of upmix stereo parameters and the phase correction parameter into a single encoded signal.
  • the encoder 209 generates a parametrically encoded stereo signal comprising a mono downmix as well as parametric stereo upmix parameters that can be used to upmix this downmix signal.
  • These stereo parameters include a phase parameter and particularly include an IPD parameter.
  • the encoded signal comprises a phase correction parameter, but only for parts of the encoded signal for which the input stereo signal are considered to be associated with input stereo channels being sufficiently out of phase.
  • the signal is divided into first parts wherein no phase correction parameter is provided and second parts in which a phase correction parameter is provided.
  • the second parts represent parts of the signal for which the encoder estimates that the phase based upmixing is likely to introduce mistakes or artifacts unless the phase correction parameter is used.
  • Fig. 6 illustrates elements of the decoder 215 in more detail.
  • the decoder 215 comprises a demultiplexer 601 which receives the parametrically encoded signal from the encoder. It furthermore comprises an upmix unit 603 which is arranged to generate a stereo signal from the downmix.
  • the upmixing may specifically use the downmix signal as well as a second signal derived therefrom (e.g. a decorrelated signal derived from the downmix). For example, a matrix multiplication may be applied to each time frequency tile of the downmix and the decorrelated signal.
  • the decoder 215 further comprises a parameter derivation unit 605 which is coupled to the demultiplexer 601 and the upmix unit 603.
  • the parameter derivation unit 605 generates suitable upmix values based on the set of stereo upmix parameters. Specifically, for the first parts of the downmix (for which no phase correction parameter is provided), the parameter derivation unit 605 calculates suitable upmix values based on a nominal or default algorithm. This default algorithm may e.g. use a predetermined function for calculating phase values based on the provided set of parameters.
  • the decoder 215 also comprises a correction unit 607 which is coupled to the demultiplexer 601 and the parameter derivation unit 605 and which is arranged to modify the upmixing operation based on the phase correction parameter.
  • the correction unit 607 receives the phase correction parameter and controls the parameter derivation unit 605 to modify the generated upmix values to reflect the correction indicated by the phase correction parameter.
  • the decoder 215 generates a stereo signal based on a default phase based upmixing during (most) parts of the received signal while at the same time modifying these approaches for parts of the signal wherein the stereo channels are considered sufficiently out of phase to be likely to result in artifacts and degradation if the default upmixing is used.
  • the decoder 215 may be arranged to receive IPD, HD and ICC parameters in the encoded signal. It may further be arranged to calculate an OPD parameter from these parameters using a predetermined function, such as:
  • OPD z(lO IID/2 ° + ICexp(jIPD))
  • Sd is a decorrelated signal generated from the downmix s e.g. by allpass filtering (as will be known to the skilled person).
  • the upmixing may use a conventional upmix operation based on the phase parameters IPD and OPD.
  • the decoder rather than using communicated OPD values, the decoder itself estimates/calculates suitable OPD values from the received PS parameters.
  • the encoder may be arranged to perform the same calculation thereby deriving the OPD value that is calculated by the decoder. However, for the second parts typically associated with sections of the signal for which the stereo signals are sufficiently out of phase to possibly result in phase discontinuities, the encoder proceeds to generate an offset phase correction. For example, for a time frequency block where the OPD varies by close to ⁇ compared to a neighbor time frequency block (where the neighbor may be in time and/or frequency), the encoder may include a phase correction parameter that indicates that a value of ⁇ should be added (or subtracted) from the calculated OPD value. Alternatively, the phase correction parameter may directly provide an OPD value that should be used by the decoder instead of the decoder calculated value. Thus, the encoder includes overall phase offset correction parameter values that may remove any discontinuities which will result from applying the predetermined function to calculate OPD.
  • the decoder 215 operates in two different modes depending on whether a phase correction parameter is provided or not.
  • an overall phase offset in the form of an OPD is generated based on a predetermined function, and for the second parts the overall phase offset is determined from the phase correction parameter.
  • the overall phase offset is then used as the OPD value of the upmix operation.
  • the overall phase offset is indicative of a phase difference between the downmix and at least one of the stereo channels and thus provides information of how the downmix phase should be distributed across the different output signal. Accordingly, an improved perceived audio quality and especially sound source position perception is achieved.
  • the OPD used by the upmixing may for the second parts be determined based on the overall phase correction value provided in the encoded signal, e.g. when this comprises a replacement value for the decoder generated value.
  • the OPD may in addition to the overall phase correction parameter also be determined based on the set of stereo upmix parameters.
  • the HD, IC and IPD values may be used to generate an OPD estimate using the default function and the resulting value may be offset by the value given by the overall phase offset parameter.
  • the encoder 209 detects whether or not the OPD that can be estimated by the decoder 215 provides a reliable estimate which does not invoke any phase modifications. This information is then signaled in the PS bit-stream e.g. by including a phase correction parameter value for all time frequency blocks that are considered to not be estimated reliably.
  • a correction value may be provided for each segment or for groups of time frequency blocks.
  • the correction values can be transmitted as absolute values, but also differentially with respect to the decoder estimate, differentially over frequency- and/or time (e.g. the correction value is transmitted as an offset to a (time and/or frequency) neighboring time frequency block).
  • the OPD information is estimated from the other PS parameters or is decoded from the bit-stream.
  • the latter case may still employ the estimated data, depending on the coding scheme as outlined above, e.g. by transmitting the difference between the estimated OPD and the OPD derived in the decoder.
  • the parameters for the upmixing are changed for the second parts based on the phase correction parameter, the actual operations and functions may be the same as in traditional parametric stereo upmixing which uses IPD and OPD values.
  • the encoder 209 is arranged to introduce a phase compensation when generating the downmix for the second parts relative to any phase compensation that is applied for the first parts.
  • a phase compensation may specifically ensure that the component from the two stereo channels in the downmix are not out of phase with each other, and thus may prevent or reduce the two stereo signals canceling each other.
  • the encoder may then generate the phase correction parameter to be indicative of the phase compensation that is performed for the second parts.
  • the decoder does not require any phase compensation information for these parts.
  • Such an approach may provide advantageous operation in many embodiments and may in particular provide improved audio quality without substantially increasing the data rate.
  • a preferred property of the down-mix signal is that it is energy preserving on a frequency scale that roughly corresponds to the human auditory system, such as the ERB scale.
  • the energy of the downmix should preferably be equal to the sum of the energies of the individual input signals. This is advantageous to minimize perceptual influences (e.g. coloring) of the down-mix process.
  • using a simple downmix such as simply summing the two stereo signals (and e.g. applying a simple scaling), provides a typically sufficient degree of energy preservation.
  • a constant (such as 0°) phase compensation can be used thereby obviating the need for substantially increasing the data rate.
  • the signals fully or partially cancel each other out resulting in a very high variation in the energy.
  • this cannot typically be compensated by simply scaling the downmix signal since the post-scale factor that would need to be employed would be extremely large, resulting in audible quantization artifacts.
  • the signal will be (almost) zero.
  • the encoder may accordingly apply phase compensation for these second parts.
  • the downmix unit may calculate the downmix as:
  • S is the phase compensated downmix
  • L and R are the left and right signal values (typically time frequency block values) respectively
  • ⁇ and ⁇ are design parameters (often set to one or such that the power of the downmix corresponds to the sum of the powers of the left and right signals) and (pi and Cp 2 are compensating phase values.
  • phase compensation can be used to ensure that the energy of the downmix is maintained sufficiently constant.
  • the phase values may be selected such that phase discontinuities (e.g. across time or frequency) are avoided.
  • the phase compensation may be selected such that the OPD of the downmix relative to the phase compensated signal will always correspond to that calculated at the decoder 215.
  • the decoder 215 in order to generate the original stereo signals instead of the phase compensated stereo signals, the decoder 215 must reverse the operation of the phase compensation.
  • the phase correction parameter may indicate the phase compensation that has been applied thereby allowing the decoder 215 to reverse this phase compensation.
  • the data rate increase is typically relatively low.
  • the encoder may be arranged to use the following downmix for the first parts:
  • the decoder may then for these first parts estimate an OPD value as previously described and perform the described OPD and IPD based upmixing resulting in the desired output stereo signals.
  • the encoder 209 may use the following downmix:
  • the encoder can determine the HD, IC and IPD values that relate the downmix to the phase compensated stereo signals given by:
  • phase values are selected such that the OPD values estimated from the HD, IC and IPD values will always be appropriate and will not have any phase discontinuities (basically the phase values (pi and ⁇ 2 are selected such that the phase compensated stereo signals L' and R' are not close to being out of phase with each other).
  • the decoder 215 can generate the phase compensated stereo signals L' and R' based on reliable parameter values including reliable IPD and OPD values.
  • the encoder may include a phase correction parameter which specifies the phase compensation that has been performed for the second parts.
  • the applied phase values Cp 1 and ⁇ 2 may be included for each frequency time block of the second parts.
  • the decoder 215 may then modify the upmixing to include the operation:
  • the correction unit 607 modifies the upmixing based on the phase correction parameter for the second parts.
  • the matrix operation to reverse the phase compensation will be included in the upmix matrix.
  • the same operations may be performed but with modified values.
  • the stereo signals may be generated without an explicit generation of OPD values.
  • stereo signals can be generated based on the downmix and the generation of a difference signal generated from the downmix.
  • a difference signal comprising a difference between the left stereo signal and the right stereo signal may be predicted based on the mono downmix scaled with a prediction coefficient, where the prediction coefficient is derived from the spatial parameters.
  • the upmixing unit may then generate the left output signal and the right output signal based on a sum and a difference of the mono downmix signal and said difference signal.
  • the left right output signals can be reconstructed as follows:
  • c is a gain normalization constant and is a function of the stereo parameters.
  • Gain normalization ensures that a power of the mono downmix signal is (approximately) equal to a sum of powers of the left signal and the right signal.
  • the encoder sum signal may be calculated as:
  • the spatial parameters are determined in the encoder beforehand and transmitted to the decoder.
  • the spatial parameters are determined on a frame-by- frame basis for each time/frequency tile as:
  • iid is an interchannel intensity difference
  • ice is an interchannel coherence
  • ipd is an interchannel phase difference
  • (/,/) and (r,r) are the left and right signal powers respectively and (/, r) represents the non-normalized complex- valued covariance coefficient between the left and right signals.
  • k ale represents the DFT bins corresponding to a parameter band.
  • other complex domain representation could be used, such as e.g. a complex exponentially modulated QMF bank as described in P. Ekstrand, "Bandwidth extension of audio signals by spectral band replication", in Proc. 1 st IEEE Benelux Workshop on Model based Processing and Coding of Audio (MPCA-2002), Leuven, Belgium, Nov. 2002, pp. 73 - 79.
  • the ice is calculated as:
  • the gain normalization constant c is expressed as:
  • the value of the gain normalization constant c is typically limited as:
  • the prediction coefficient used to calculate the difference signal is based on estimating the difference signal from the mono downmix using waveform matching.
  • Said waveform matching comprises e.g. a least-squares match of the mono downmix signal onto the difference signal, resulting in the difference signal provided as:
  • the least-squares matching a waveform matching using a different norm from L2-norm can be used.
  • the p-norm error - ⁇ • sf could be e.g. perceptually weighted.
  • the least-squares matching is advantageous as it results in relatively simple calculations for deriving the prediction coefficient from the transmitted spatial image parameters.
  • the prediction coefficient my specifically be given as a function of the stereo parameters:
  • the upmixing unit may specifically enhance the difference signal by adding a scaled decorrelated mono downmix signal.
  • the mono downmix is decorrelated, e.g. using all- pass filters to generate a decorrelated mono downmix.
  • a first part of the difference signal is calculated by scaling the mono downmix with the prediction coefficient. Additionally the decorrelated mono downmix is also scaled by a scale factor.
  • a resulting second part of the difference signal is consequently added to the first part of the difference signal resulting in an enhanced difference signal.
  • the mono downmix and the enhanced difference signal are then used to calculate the left signal and the right signal.
  • the decorrelated mono downmix can be obtained by means of filtering the mono downmix. This filtering generates a signal with a similar spectral and temporal envelope as the mono downmix, but with a correlation substantially close to zero such that it corresponds to a synthetic variant of the residual component derived in the encoder. This effect is achieved by means of e.g. allpass filtering, delays, lattice reverberation filters, feedback delay networks or a combination thereof.
  • the scaling factor applied to the decorrelated mono downmix may be compensated for a prediction energy loss.
  • the scaling factor applied to the decorrelated mono downmix ensures that the overall signal power of the output left and right signals matches the signal power of the left and right signal power at the encoder side, respectively.
  • the scaling factor henceforth referred to as ⁇ is interpreted as a prediction energy loss compensation factor.
  • the difference signal d is then expressed as:
  • the scaling factor ⁇ applied to the decorrelated mono downmix can be derived as a function of the spatial parameters:
  • the left and right output signals may then be expressed as:
  • the difference signal d a ⁇ s + ⁇ • s d is respectively added to the downmix s for the left channel and subtracted from the downmix s for the right channel.
  • the left and right output signals can be expressed as:
  • the downmix includes a phase compensation and the downmix may accordingly be given as
  • the phase correction parameter provides the information of the phase compensation for these parts.
  • the left and right output signals may then be calculated as:
  • the phase correction parameter may provide the phase compensation difference Cp 1 -Cp 2 for the time frequency blocks of the second parts of the downmix.
  • the phase correction parameter in this example reflects an interchannel phase difference correction value.
  • the modifying unit 607 thus changes the generation of the upmixing coefficients to be based on the equations which include the correction value Cp 1 -Cp 1 for the time frequency blocks of the second parts but not for the first parts.
  • the parametrically encoded signal may also comprise a phase correction parameter presence indication which is indicative of the second parts.
  • this presence indication which may be distinct from the phase correction parameter itself, may provide an efficient and low data rate indication of which parts of the encoded signal comprises phase correction parameter values that should be included when decoding the encoded stereo signal.
  • the presence indication may specifically be single bit values indicating whether a phase correction parameter should purely be estimated by the encoder or should be replaced (or compensated) by a phase correction parameter included in the encoded stereo signal.
  • a common presence indication may be provided for each segment of the parametrically encoded signal.
  • the encoded signal may be segmented in typically the time domain when being encoded.
  • the presence indication may simply indicate whether there are any phase correction parameter values for the current segment.
  • the presence indication can be a single bit denoting that for the current frame all time frequency blocks can be estimated reliably by the decoder. This may provide a very low data rate overhead (possibly a single bit per segment) and may reduce the complexity and/or resource usage of the decoder.
  • the presence indication may comprise individual presentation indications for a plurality of sets of time frequency blocks of the down-mix. Specifically, each set may correspond to one time frequency block for which individual PS parameters are provided. Further the sets may cover all time frequency blocks of the signal. Thus, specifically a single presence indication bit may be included for each parameter time frequency block indicating whether e.g. the OPD for the block can be purely estimated by the decoder 215 or whether it must take into account a phase correction parameter provided for the block. It will be appreciated that in many embodiments, the phase correction parameter may indeed be provided for each time frequency block that belongs to the second parts of the downmix.
  • the out-of-phase detection unit 505 may be replaced by other detection units arranged to detect one or more sections of the downmix for which the deviation between a value of phase parameter derived from the upmix parameters and a target value for the phase parameter meets a criterion.
  • the target value may specifically be a value for the phase parameter which is calculated directly from the input signals or which has been compensated to reduce or remove undesired characteristics.
  • the target value for an OPD parameter may be calculated from the original input signals and compensated such that any phase discontinuities are removed.
  • the value calculated from the set of upmix parameters may be calculated using a predetermined function which is also used by the decoder 215.
  • the detection unit 505 may be arranged to compare the value that will be calculated by the decoder 215 to the value it should preferable be. Any sections where the deviation (e.g. calculated on the basis of a psychoacoustic model) between these exceed a threshold may thus be identified and corrected by the inclusion of the phase correction parameter.
  • improved audio quality is generated for the specific parts of the signal where the decoder 215 derived parameter value is not sufficiently accurate without affecting the data rate for other parts of the encoded signal.
  • a multichannel system may use a plurality of downmix and upmix blocks (often referred to as One-To-Two, Two-To-Three, Two-To-One and Three-To-Two modules) to encode a multichannel signal.
  • the encoder described above may in such a system be used as a Two-To-One block and the decoder may be used as a One-To-Two block.
  • the principles may e.g. be used in a coding system wherein three channels are encoded as a mono or stereo downmix with parameters allowing the upmix back to three channels.
  • the invention can be implemented in any suitable form including hardware, software, firmware or any combination of these.
  • the invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors.
  • the elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

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Abstract

L'invention concerne un décodeur comportant un récepteur (601) qui reçoit un signal ayant fait l'objet d'un codage paramétrique et comprenant un mélange réduit de canaux multiples, un ensemble de paramètres de mélange élévateur comprenant un paramètre de phase, et un paramètre de correction de phase. Le paramètre de correction de phase est indiqué pour des deuxièmes parties du mélange réduit mais pas pour des premières parties du mélange réduit. Un mélangeur élévateur (603, 605) génère le signal multicanaux par un mélange élévateur du mélange réduit sur la base de l'ensemble de paramètres de mélange élévateur et une unité (607) de modification modifie le mélange élévateur en réponse au paramètre de correction de phase relatif aux deuxièmes parties du mélange réduit. Les deuxièmes parties peuvent être associées à des scénarios où au moins deux des signaux multicanaux codés sont déphasés. L'invention est capable d'assurer une qualité audio améliorée à un débit de données réduit.
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