a9y United States
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`US 20150371643A1
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`12y Patent Application Publication o) Pub. No.: US 2015/0371643 A1
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`Ramo et al. 43) Pub. Date: Dec. 24, 2015
`(54) STEREO AUDIO SIGNAL ENCODER Publication Classification
`(75) Inventors: Anssi Ramo, Tampere (FI); Adriana (51) Int.CL
`Vasilache, Tampere (FI); Lasse GI0L 19/008 (2006.01)
`Laaksonen, Nokia (FI); Miikka Ho4S 1/00 (2006.01)
`Vilermo, Siuro (FT); Mikko Tammi, (52) US.CL
`Tampere (FI) CPC ....ccvue GI0L 19/008 (2013.01); H04S 1/007
`(2013.01); HO4S 2420/03 (2013.01)
`(73) Assignee: Nokia Corporation, Espoo (FI) (57) ABSTRACT
`An apparatus comprising a channel analyser configured to
`pp prising Y 2
`(21) Appl. No.: 14/394,211 analyse an audio signal comprising at least two audio chan-
`. nels to determine at least one parameter associated with a
`(22) PCT Filed: Apr. 18, 2012 difference between the at least two audio channels; an encod-
`(86) PCT No.: PCT/IB2012/051943 ing mode determiner configured to select a multichannel
`
`§371 (),
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`(2), (4) Date: Dec. 28,2014
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`STEREO AUDIO SIGNAL ENCODER
`
`FIELD
`
`[0001] The present application relates to a stereo audio
`signal encoder, and in particular, but not exclusively to a
`stereo audio signal encoder for use in portable apparatus.
`
`BACKGROUND
`
`[0002] Audio signals, like speech or music, are encoded for
`example to enable efficient transmission or storage of the
`audio signals.
`
`[0003] Audio encoders and decoders (also known as
`codecs) are used to represent audio based signals, such as
`music and ambient sounds (which in speech coding terms can
`be called background noise). These types of coders typically
`do not utilise a speech model for the coding process, rather
`they use processes for representing all types of audio signals,
`including speech. Speech encoders and decoders (codecs)
`can be considered to be audio codecs which are optimised for
`speech signals, and can operate at either a fixed or variable bit
`rate.
`
`[0004] An audio codec can also be configured to operate
`with varying bit rates. At lower bit rates, such an audio codec
`may be optimized to work with speech signals at a coding rate
`equivalent to a pure speech codec. At higher bit rates, the
`audio codec may code any signal including music, back-
`ground noise and speech, with higher quality and perfor-
`mance. A variable-rate audio codec can also implement an
`embedded scalable coding structure and bitstream, where
`additional bits (a specific amount of bits is often referred to as
`a layer) improve the coding upon lower rates, and where the
`bitstream of a higher rate may be truncated to obtain the
`bitstream of a lower rate coding. Such an audio codec may
`utilize a codec designed purely for speech signals as the core
`layer or lowest bit rate coding.
`
`[0005] An audio codec is designed to maintain a high (per-
`ceptual) quality while improving the compression ratio. Thus
`instead of waveform matching coding it is common to employ
`various parametric schemes to lower the bit rate. For multi-
`channel audio, such as stereo signals, it is common to use a
`larger amount of the available bit rate on a mono channel
`representation and encode the stereo or multichannel infor-
`mation exploiting a parametric approach which uses rela-
`tively fewer bits.
`
`[0006] Real life multichannel signal types which can be
`used include binaural stereo and near-far stereo representa-
`tion. Binaural stereo refers to a stereo signal typically
`obtained through recording sound with two microphones
`arranged with the intent to create a natural three dimensional
`stereo or spatial sound sensation for the listener. Such micro-
`phone arrangements typically include a dummy head, with
`microphones in the dummy head ears, placing a microphone
`near each ear of a real person, or even placing two micro-
`phones at a typical distance of a person’s ears from each other
`(usually such that direct sound between the two microphones
`is blocked). Near-far stereo on the other hand refers to a stereo
`compatible stereo signal typically obtained through record-
`ing sound with two microphones arranged such that one
`microphone is close to the primary sound source, for example
`aperson’s mouth, and the other microphone is slightly further
`away (for example close to a person’s ear if a regular mobile
`phone form factor is used) and concentrating more on record-
`
`Exhibit 1020
`Page 15 of 26
`
`Dec. 24, 2015
`
`ing the ambient sound. In such circumstances the near chan-
`nel can be directly used as the mono input signal.
`
`[0007] On playback using headphones the perception of a
`binaural stereo recording is generally such that the person
`listening feels as if they are in the recording environment
`themselves. The near-far stereo representation on the other
`hand may be played back such that one ear receives the near
`channel while the other ear receives the far channel audio
`information. Thus the experience is similar to a traditional
`monaural phone call hearing the talker in one ear and hearing
`the ambient sound of the recording environment instead of
`their own environmental ambient sounds through the other
`ear. Both real life stereo signal types can therefore be consid-
`ered as representations that provides the listener with a natu-
`ral and enjoyable feeling of the recording environment.
`
`SUMMARY
`
`[0008] There is provided according to a first aspect a
`method comprising: analysing an audio signal comprising at
`least two audio channels to determine at least one parameter
`associated with a difference between the at least two audio
`channels; selecting a multichannel audio signal encoding
`dependent on the at least one parameter; and encoding the
`audio signal with the multichannel audio signal encoding.
`
`[0009] Analysing an audio signal comprising at least two
`audio channels to determine at least one parameter associated
`with a difference between the at least two audio channels may
`comprise: generating a frequency domain representation for
`the at least two audio channels of the audio signal; separating
`the frequency domain representation for the at least two audio
`channels of the audio signal into at least two frequency bands;
`and generating at least one parameter associated with the
`difference between two audio channels for a frequency band.
`
`[0010] The parameter may comprise at least one of: a rela-
`tive energy signal level associated with the at least two audio
`channels; a correlation value associated with the at least two
`audio channels; and a time shift value associated with the at
`least two audio channels.
`
`[0011] Selecting a multichannel audio signal encoding
`dependent on the at least one parameter may comprise: select-
`ing an initial default multichannel audio signal encoding;
`selecting a second audio signal multichannel audio signal
`encoding dependent on a first selection of the at least one
`parameter; and maintaining the second audio signal multi-
`channel audio signal encoding dependent on a second selec-
`tion of the at least one parameter.
`
`[0012] The first selection of the at least one parameter may
`be a combination of a relative energy signal level and a
`correlation value associated with the at least two audio chan-
`nels, and wherein selecting the second audio signal multi-
`channel audio signal encoding dependent on a first selection
`of the at least one parameter may comprise selecting the
`second audio signal multichannel audio signal encoding
`where the combination is greater than a determined threshold
`value.
`
`[0013] The second selection of the at least one parameter
`may be a relative energy signal level associated with the at
`least two audio channels, and wherein maintaining the second
`audio signal multichannel audio signal encoding may com-
`prise maintaining the second audio signal multichannel audio
`signal encoding where the relative energy signal level is less
`than a second determined threshold value.
`
`
`
`
`
`
`
`
`US 2015/0371643 Al
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`[0014] The multichannel audio signal encoding may com-
`prise at least one of: binaural encoding; and near-far stereo
`encoding.
`
`[0015] Encoding the audio signal with the multichannel
`audio signal encoding may comprise: combining the at least
`two audio channels to form a single combined channel audio
`signal; encoding the single combined channel audio signal;
`and generating data associated with the at least two audio
`channels using the multichannel audio signal encoding such
`that the data enables the at least two audio channels to be
`reproduced from the single combined channel audio signal.
`[0016] According to a second aspect there is provided a
`method comprising: receiving an encoded audio signal;
`selecting a multichannel audio signal decoding dependent on
`afirstpart of the encoded audio signal; and decoding a second
`part of the encoded audio signal, the second part of the audio
`signal encoded with a multichannel audio signal encoding,
`such that the decoding the second part of the encoded audio
`signal generates an audio signal comprising at least two audio
`channels.
`
`[0017] Decoding a second part of the encoded audio signal
`may comprise: generating a first channel audio signal from a
`first section of the second part of the encoded audio signal;
`and generating at least one further channel audio signal from
`a second section of the second part of the encoded audio
`signal dependent on the multichannel audio signal decoding
`indicated by the first part of the encoded audio signal.
`[0018] The first channel may be a left channel audio signal
`and the at least one further channel audio signal may be a right
`channel audio signal.
`
`[0019] The first channel may be a combined channel audio
`signal and the at least one further channel audio signal may
`comprise a left channel signal and a right channel audio
`signal.
`
`[0020] According to a third aspect there is provided a
`method comprising: determining at least one channel pair
`distance value for an audio signal comprising at least a pair of
`audio channels; encoding the audio signal with a multichan-
`nel audio signal encoding to generate at least an encoded
`signal and difference signal; and generating an equivalent
`difference signal dependent on the difference signal, the at
`least one channel pair distance value and an encoded channel
`distance value.
`
`[0021] The method may further comprise receiving the
`encoded channel distance value.
`
`[0022] Receiving the encoded channel distance value may
`comprise at least one of: determining an encoded channel
`distance value from a user input; and receiving an encoded
`channel distance value from a decoder.
`
`[0023] The method may comprise receiving the audio sig-
`nal from a pair of microphones, wherein a first audio channel
`may be from a first microphone and a second audio channel
`may be from a second microphone, wherein determining the
`at least one channel pair distance value may comprise deter-
`mining the distance between the first microphone and the
`second microphone.
`
`[0024] According to a fourth aspect there is provided a
`method comprising: receiving an encoded signal and an
`equivalent difference signal; reproducing a pair of audio
`channels with a determined channel distance dependent on
`the encoded signal and the equivalent difference signal.
`[0025] The method may further comprise: determining an
`encoded channel distance value; and generating a pair of
`audio channels with a desired channel distance dependent on
`
`Exhibit 1020
`Page 16 of 26
`
`Dec. 24, 2015
`
`the encoded signal, the equivalent difference signal, the
`encoded channel distance value and the desired channel dis-
`tance.
`
`[0026] According to a fifth aspect there is provided an
`apparatus comprising at least one processor and at least one
`memory including computer program code for one or more
`programs, the at least one memory and the computer program
`code configured to, with the at least one processor, cause the
`apparatus at least to perform: analysing an audio signal com-
`prising at least two audio channels to determine at least one
`parameter associated with a difference between the at least
`two audio channels; selecting a multichannel audio signal
`encoding dependent on the at least one parameter; and encod-
`ing the audio signal with the multichannel audio signal
`encoding.
`
`[0027] Analysing an audio signal comprising at least two
`audio channels to determine at least one parameter associated
`with a difference between the at least two audio channels may
`cause the apparatus to perform: generating a frequency
`domain representation for the at least two audio channels of
`the audio signal; separating the frequency domain represen-
`tation for the at least two audio channels of the audio signal
`into at least two frequency bands; and generating at least one
`parameter associated with the difference between two audio
`channels for a frequency band.
`
`[0028] The parameter may comprise at least one of: a rela-
`tive energy signal level associated with the at least two audio
`channels; a correlation value associated with the at least two
`audio channels; and a time shift value associated with the at
`least two audio channels.
`
`[0029] Selecting a multichannel audio signal encoding
`dependent on the at least one parameter may cause the appa-
`ratus to perform: selecting an initial default multichannel
`audio signal encoding; selecting a second audio signal mul-
`tichannel audio signal encoding dependent on a first selection
`of the at least one parameter; and maintaining the second
`audio signal multichannel audio signal encoding dependent
`on a second selection of the at least one parameter.
`
`[0030] The first selection of the at least one parameter may
`be a combination of a relative energy signal level and a
`correlation value associated with the at least two audio chan-
`nels, and wherein selecting the second audio signal multi-
`channel audio signal encoding dependent on a first selection
`of the at least one parameter may cause the apparatus to
`perform selecting the second audio signal multichannel audio
`signal encoding where the combination is greater than a deter-
`mined threshold value.
`
`[0031] The second selection of the at least one parameter
`may be a relative energy signal level associated with the at
`least two audio channels, and wherein maintaining the second
`audio signal multichannel audio signal encoding may cause
`the apparatus to perform maintaining the second audio signal
`multichannel audio signal encoding where the relative energy
`signal level is less than a second determined threshold value.
`
`[0032] The multichannel audio signal encoding may com-
`prise at least one of: binaural encoding; and near-far stereo
`encoding.
`
`[0033] Encoding the audio signal with the multichannel
`audio signal encoding may cause the apparatus to perform:
`combining the at least two audio channels to form a single
`combined channel audio signal; encoding the single com-
`bined channel audio signal; and generating data associated
`with the at least two audio channels using the multichannel
`
`
`
`
`
`
`
`
`US 2015/0371643 Al
`
`audio signal encoding such that the data enables the at least
`two audio channels to be reproduced from the single com-
`bined channel audio signal.
`
`[0034] According to a sixth aspect there is provided an
`apparatus comprising at least one processor and at least one
`memory including computer program code for one or more
`programs, the at least one memory and the computer program
`code configured to, with the at least one processor, cause the
`apparatus at least to perform: receiving an encoded audio
`signal; selecting a multichannel audio signal decoding depen-
`dent on a first part of the encoded audio signal; and decoding
`a second part of the encoded audio signal, the second part of
`the audio signal encoded with a multichannel audio signal
`encoding, such that the decoding the second part of the
`encoded audio signal generates an audio signal comprising at
`least two audio channels.
`
`[0035] Decoding a second part of the encoded audio signal
`may cause the apparatus to perform: generating a first channel
`audio signal from a first section of the second part of the
`encoded audio signal; and generating at least one further
`channel audio signal from a second section of the second part
`of the encoded audio signal dependent on the multichannel
`audio signal decoding indicated by the first part of the
`encoded audio signal.
`
`[0036] The first channel may be a left channel audio signal
`and the at least one further channel audio signal may be a right
`channel audio signal.
`
`[0037] The first channel may be a combined channel audio
`signal and the at least one further channel audio signal may
`comprise a left channel signal and a right channel audio
`signal.
`
`[0038] According to a seventh aspect there is provided an
`apparatus comprising at least one processor and at least one
`memory including computer program code for one or more
`programs, the at least one memory and the computer program
`code configured to, with the at least one processor, cause the
`apparatus at least to perform: determining at least one channel
`pair distance value for an audio signal comprising at least a
`pair of audio channels; encoding the audio signal with a
`multichannel audio signal encoding to generate at least an
`encoded signal and difference signal; and generating an
`equivalent difference signal dependent on the difference sig-
`nal, the at least one channel pair distance value and an
`encoded channel distance value.
`
`[0039] The apparatus may further be caused to perform
`receiving the encoded channel distance value.
`
`[0040] Receiving the encoded channel distance value may
`cause the apparatus to perform at least one of: determining an
`encoded channel distance value from a user input; and receiv-
`ing an encoded channel distance value from a decoder.
`[0041] The apparatus may be caused to perform receiving
`the audio signal from a pair of microphones, wherein a first
`audio channel may be from a first microphone and a second
`audio channel may be from a second microphone, wherein
`determining the at least one channel pair distance value may
`cause the apparatus to perform determining the distance
`between the first microphone and the second microphone.
`[0042] According to an eighth aspect there is provided an
`apparatus comprising at least one processor and at least one
`memory including computer program code for one or more
`programs, the at least one memory and the computer program
`code configured to, with the at least one processor, cause the
`apparatus at least to perform: receiving an encoded signal and
`an equivalent difference signal; and reproducing a pair of
`
`Exhibit 1020
`Page 17 of 26
`
`Dec. 24, 2015
`
`audio channels with a determined channel distance dependent
`on the encoded signal and the equivalent difference signal.
`[0043] The apparatus may be caused to perform: determin-
`ing an encoded channel distance value; and generating a pair
`of audio channels with a desired channel distance dependent
`on the encoded signal, the equivalent difference signal, the
`encoded channel distance value and the desired channel dis-
`tance.
`
`[0044] According to a ninth aspect there is provided an
`apparatus comprising: means for analysing an audio signal
`comprising at least two audio channels to determine at least
`one parameter associated with a difference between the at
`least two audio channels; means for selecting a multichannel
`audio signal encoding dependent on the at least one param-
`eter; and means for encoding the audio signal with the mul-
`tichannel audio signal encoding.
`
`[0045] The means for analysing an audio signal comprising
`at least two audio channels to determine at least one param-
`eter associated with a difference between the at least two
`audio channels may comprise: means for generating a fre-
`quency domain representation for the at least two audio chan-
`nels of the audio signal; means for separating the frequency
`domain representation for the at least two audio channels of
`the audio signal into at least two frequency bands; and means
`for generating at least one parameter associated with the
`difference between two audio channels for a frequency band.
`[0046] The parameter may comprise at least one of: a rela-
`tive energy signal level associated with the at least two audio
`channels; a correlation value associated with the at least two
`audio channels; and a time shift value associated with the at
`least two audio channels.
`
`[0047] The means for selecting a multichannel audio signal
`encoding dependent on the at least one parameter may com-
`prise: means for selecting an initial default multichannel
`audio signal encoding; means for selecting a second audio
`signal multichannel audio signal encoding dependent on a
`first selection of the at least one parameter; and means for
`maintaining the second audio signal multichannel audio sig-
`nal encoding dependent on a second selection of the at least
`one parameter.
`
`[0048] The first selection of the at least one parameter may
`be a combination of a relative energy signal level and a
`correlation value associated with the at least two audio chan-
`nels, and wherein the means for selecting the second audio
`signal multichannel audio signal encoding dependent on a
`first selection of the at least one parameter may comprise
`means for selecting the second audio signal multichannel
`audio signal encoding where the combination is greater than
`a determined threshold value.
`
`[0049] The second selection of the at least one parameter
`may be a relative energy signal level associated with the at
`least two audio channels, and wherein the means for main-
`taining the second audio signal multichannel audio signal
`encoding may comprise means for maintaining the second
`audio signal multichannel audio signal encoding where the
`relative energy signal level is less than a second determined
`threshold value.
`
`[0050] The multichannel audio signal encoding may com-
`prise at least one of: binaural encoding; and near-far stereo
`encoding.
`
`[0051] The means for encoding the audio signal with the
`multichannel audio signal encoding may comprise: means for
`combining the at least two audio channels to form a single
`combined channel audio signal; means for encoding the
`
`
`
`
`
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`
`
`US 2015/0371643 Al
`
`single combined channel audio signal; and means for gener-
`ating data associated with the at least two audio channels
`using the multichannel audio signal encoding such that the
`data enables the at least two audio channels to be reproduced
`from the single combined channel audio signal.
`
`[0052] According to a tenth aspect there is provided an
`apparatus comprising: means for receiving an encoded audio
`signal; means for selecting a multichannel audio signal
`decoding dependent on a first part of the encoded audio sig-
`nal; and means for decoding a second part of the encoded
`audio signal, the second part of the audio signal encoded with
`a multichannel audio signal encoding, such that the decoding
`the second part of the encoded audio signal generates an audio
`signal comprising at least two audio channels.
`
`[0053] The means for decoding a second part of the
`encoded audio signal may comprise: means for generating a
`first channel audio signal from a first section of the second
`part of the encoded audio signal; and means for generating at
`least one further channel audio signal from a second section
`of the second part of the encoded audio signal dependent on
`the multichannel audio signal decoding indicated by the first
`part of the encoded audio signal.
`
`[0054] The first channel may be a left channel audio signal
`and the at least one further channel audio signal may be a right
`channel audio signal.
`
`[0055] The first channel may be a combined channel audio
`signal and the at least one further channel audio signal may
`comprise a left channel signal and a right channel audio
`signal.
`
`[0056] According to an eleventh aspect there is provided an
`apparatus comprising: means for determining at least one
`channel pair distance value for an audio signal comprising at
`least a pair of audio channels; means for encoding the audio
`signal with a multichannel audio signal encoding to generate
`at least an encoded signal and difference signal; and means
`for generating an equivalent difference signal dependent on
`the difference signal, the at least one channel pair distance
`value and an encoded channel distance value.
`
`[0057] The apparatus may further comprise means for
`receiving the encoded channel distance value.
`
`[0058] The means for receiving the encoded channel dis-
`tance value may comprise at least one of: means for deter-
`mining an encoded channel distance value from a user input;
`and means for receiving an encoded channel distance value
`from a decoder.
`
`[0059] The apparatus may comprise means for receiving
`the audio signal from a pair of microphones, wherein a first
`audio channel may be from a first microphone and a second
`audio channel may be from a second microphone, wherein the
`means for determining the at least one channel pair distance
`value may comprise means for determining the distance
`between the first microphone and the second microphone.
`[0060] According to a twelfth aspect there is provided an
`apparatus comprising: means for receiving an encoded signal
`and an equivalent difference signal; and means for reproduc-
`ing a pair of audio channels with a determined channel dis-
`tance dependent on the encoded signal and the equivalent
`difference signal.
`
`[0061] The apparatus may comprise: means for determin-
`ing an encoded channel distance value; and generating a pair
`of'audio channels with a desired channel distance dependent
`on the encoded signal, the equivalent difference signal, the
`encoded channel distance value and the desired channel dis-
`tance.
`
`Exhibit 1020
`Page 18 of 26
`
`Dec. 24, 2015
`
`[0062] According to a thirteenth aspect there is provided an
`apparatus comprising: a channel analyser configured to anal-
`yse an audio signal comprising at least two audio channels to
`determine at least one parameter associated with a difference
`between the at least two audio channels; an encoding mode
`determiner configured to select a multichannel audio signal
`encoding dependent on the at least one parameter; and a
`channel encoder configured to encode the audio signal with
`the multichannel audio signal encoding.
`
`[0063] The channel analyser may comprise: a time to fre-
`quency domain converter configured to generate a frequency
`domain representation for the at least two audio channels of
`the audio signal; a filter configured to separate the frequency
`domain representation for the at least two audio channels of
`the audio signal into at least two frequency bands; and a
`parameter determiner configured to generate at least one
`parameter associated with the difference between two audio
`channels for a frequency band.
`
`[0064] The parameter determiner may comprise at least one
`of: a relative energy signal level determiner configured to
`determine a relative energy signal level associated with the at
`least two audio channels; a correlation determiner configured
`to determine a correlation value associated with the at least
`two audio channels; and a shift determiner configured to
`determine a time shift value associated with the at least two
`audio channels.
`
`[0065] The encoding mode determiner may be configured
`to: select an initial default multichannel audio signal encod-
`ing; select a second audio signal multichannel audio signal
`encoding dependent on a first selection of the at least one
`parameter; and maintain the second audio signal multichan-
`nel audio signal encoding dependent on a second selection of
`the at least one parameter.
`
`[0066] The first selection of the at least one parameter may
`be a combination of a relative energy signal level and a
`correlation value associated with the at least two audio chan-
`nels, and wherein the encoding mode determiner may be
`configured to select the second audio signal multichannel
`audio signal encoding where the combination is greater than
`a determined threshold value.
`
`[0067] The second selection of the at least one parameter
`may be a relative energy signal level associated with the at
`least two audio channels, and wherein the encoding mode
`determiner may be configured to maintain the second audio
`signal multichannel audio signal encoding where the relative
`energy signal level is less than a second determined threshold
`value.
`
`[0068] The multichannel audio signal encoding may com-
`prise at least one of: binaural encoding; and near-far stereo
`encoding.
`
`[0069] The channel encoder may comprise: a mono chan-
`nel generator configured to combine the at least two audio
`channels to form a single combined channel audio signal; a
`mono channel encoder configured to encode the single com-
`bined channel audio signal; and a further channel encoder
`configured to generate data associated with the at least two
`audio channels using the multichannel audio signal encoding
`such that the data enables the at least two audio channels to be
`reproduced from the single combined channel audio signal.
`[0070] According to a fourteenth aspect there is provided
`an apparatus comprising: an input configured to receive an
`encoded audio signal; a multichannel decoding determiner
`configured t