Patent Application
20120207307
The present document relates to audio signal processing, in particular to an apparatus and a corresponding method for improving an audio signal of an FM stereo radio receiver.
In an analog FM (frequency modulation) stereo radio system, the left channel (L) and right channel (R) of the audio signal are conveyed in a midside (M/S) representation, i.e. as mid channel (M) and side channel (S). The mid channel M corresponds to a sum signal of L and R, e.g. M=(L+R)/2, and the side channel S corresponds to a difference signal of L and R, e.g. S=(L−R)/2. For transmission, the side channel S is modulated onto a 38 kHz suppressed carrier and added to the baseband mid signal M to form a backwards-compatible stereo multiplex signal. This multiplex signal is then used to modulate the HF (high frequency) carrier of the FM transmitter, typically operating in the range between 87.5 to 108 MHz.
When reception quality decreases (i.e. the signal-to-noise ratio over the radio channel decreases), the S channel typically suffers more than the M channel. In many FM receiver implementations, the S channel is muted when the reception conditions gets too noisy. This means that the receiver falls back from stereo to mono in case of a poor HF radio signal.
Parametric Stereo (PS) coding is a technique from the field of very low bitrate audio coding. PS allows encoding a 2-channel stereo audio signal as a mono downmix signal in combination with additional PS side information, i.e. the PS parameters. The mono downmix signal is obtained as a combination of both channels of the stereo signal. The PS parameters enable the PS decoder to reconstruct a stereo signal from the mono downmix signal and the PS side information. Typically, the PS parameters are time- and frequency-variant, and the PS processing in the PS decoder is typically carried out in a hybrid filter-bank domain incorporating a QMF bank. The document “Low Complexity Parametric Stereo Coding in MPEG-4”, Heiko Purnhagen, Proc. Digital Audio Effects Workshop (DAFx), pp. 163-168, Naples, IT, October 2004 describes an exemplary PS coding system for MPEG-4. Its discussion of parametric stereo is hereby incorporated by reference. Parametric stereo is supported e.g. by MPEG-4 Audio. Parametric stereo is discussed in section 8.6.4 and Annexes 8.A and 8.C of the MPEG-4 standardization document ISO/IEC 14496-3:2005 (MPEG-4 Audio, 3rd edition). These parts of the standardization document are hereby incorporated by reference for all purposes. Parametric stereo is also used in the MPEG Surround standard (see document ISO/IEC 23003-1:2007, MPEG Surround). Also, this document is hereby incorporated by reference for all purposes. Further examples of parametric stereo coding systems are discussed in the document “Binaural Cue Coding—Part I: Psychoacoustic Fundamentals and Design Principles,” Frank Baumgarte and Christof Faller, IEEE Transactions on Speech and Audio Processing, vol 11, no 6, pages 509-519, November 2003, and in the document “Binaural Cue Coding—Part II: Schemes and Applications,” Christof Faller and Frank Baumgarte, IEEE Transactions on Speech and Audio Processing, vol 11, no 6, pages 520-531, November 2003. In the latter two documents the term “binaural cue coding” is used, which is an example of parametric stereo coding.
Even in case the mid signal M is of acceptable quality, the side signal S may be noisy and thus can severely degrade the overall audio quality when being mixed in the left and right channels of the output signal (which are derived e.g. according to L=M+S and R=M−S). When a side signal S has only poor to intermediate quality, there are two options: either the receiver chooses accepting the noise associated with the side signal S and outputs real stereo, or the receiver drops the side signal S and falls back to mono.
A first aspect of the invention relates to an apparatus for improving an audio signal of an FM stereo radio receiver. The apparatus generates a stereo audio signal. The audio signal to be improved may be an audio signal in L/R representation, i.e. an L/R audio signal, or in an alternative embodiment an audio signal in M/S representation, i.e. an M/S audio signal. Typically, the audio signal to be improved is an audio signal in L/R representation since conventional FM radio receivers use an L/R output.
As an exemplary embodiment of the present invention, the apparatus is for an FM stereo radio receiver configured to receive an FM radio signal comprising a mid signal and side signal.
The apparatus comprises a parametric stereo (PS) parameter estimation stage. The parameter estimation stage is configured to determine one or more PS parameters based on the L/R or M/S audio signal in a frequency-variant or frequency-invariant manner. The one or more parameters may include a parameter indicating inter-channel intensity differences (HD or also called CLD—channel level differences) and/or a parameter indicating an inter-channel cross-correlation (ICC). Preferably, these PS parameters are time- and frequency-variant.
Moreover, the apparatus comprises an upmix stage. The upmix stage is configured to generate the stereo signal based on a first audio signal and the one or more PS parameters.
The first audio signal is obtained from the L/R or M/S audio signal, e.g. by a downmix operation in a downmix stage. The first audio signal may be obtained from the audio signal in case of an L/R representation by a downmix operation according to the following formula: DM=(L+R)/a, with DM corresponding to the first audio signal. For example, the parameter a is selected to be 2. In case of DM=(L+R)/a, the first audio signal essentially corresponds to the received mid signal M. In more advanced adaptive downmix schemes, the two parameters a1, a2 for combining the two channels according to the formula DM=L/a1+R/a2 may be different and/or may depend on the PS parameters and/or other signal properties.
In case of an M/S representation at the output of the FM stereo radio receiver, the first audio signal may simply correspond to the M signal of the M/S audio signal at the output.
The PS parameter estimation stage can be part of a PS encoder. The upmix stage can be part of a PS decoder.
The apparatus is based on the idea that due to its noise the received side signal may be not good enough for reconstructing the stereo signal by simply combining the received mid and side signals; nevertheless, in this case the side signal or the side signal's component in the L/R signal may be still good enough for stereo parameter analysis in the PS parameter estimation stage. These PS parameters may be then used for reconstructing the stereo signal.
Thus, the apparatus enables improved stereo reception under conditions of intermediate or even large noise in the side signal. It should be noted that the term “noise” is usually used in this specification to refer to the noise introduced from the limitations of the radio transmission channel (as opposed to the noise-like signal component originating in the actual audio signal being broadcast).
Instead of using a received noisy side signal to create the stereo audio signal, an improved side signal generated at receiver may be used. The improved side signal may be generated with help of techniques from PS coding. These include e.g. the generation of components of the improved side signal by means of a decorrelator operating on the first audio signal as input. Data about reception conditions and/or an analysis of the received stereo signal can be used to adaptively control the generation of the improved side signal and also the generation of the audio output signals.
According to another embodiment, the apparatus further comprises a decorrelator configured to generate a decorrelated signal based on the first audio signal. The upmix stage may generate the stereo signal based on the first audio signal, the one or more PS parameters and the decorrelated signal or at least frequency band of the decorrelated signal.
Instead of using the decorrelated signal, the upmix stage may use the received side signal for the upmix, e.g. in case of good reception conditions when the noise of the received side signal is low. Therefore, according to an embodiment, for the upmix selectively the received side signal or the decorrelated signal is used. More preferably, the selection is frequency-variant. For example, the upmix stage may use the received side signal for lower frequencies and may use the decorrelated signal as a pseudo side signal for higher frequencies since the higher the frequency, the larger is the noise density. This is a typical property of the FM demodulation in case of additive (white) noise on the radio channel. This will be explained in detail later in the specification.
The received side signal or at least one or more frequency components thereof may be used for upmix if the first signal corresponds to the mid signal. In case of a different downmix scheme (which is different from (L+R)/a for generating the first audio signal), a residual signal may be used for upmix instead of using the received side signal. Such a residual signal indicates the error associated with representing original channels by their downmix and PS parameters and is often used in PS encoding schemes. The above remarks to the use of the received side signal also apply to a residual signal.
The selection between the received side signal and the decorrelated signal for upmix may be signal-dependent or in other words signal-adaptive.
According to yet another embodiment, the selection depends on the reception conditions indicated by a radio reception indicator, such as the signal strength and/or on an indicator indicative of the quality of the received side signal. In case of good reception conditions (i.e. high strength), the received side signal can be preferably used for upmix (in some cases, not for the highest frequencies), whereas in case of intermediate reception conditions (i.e. lower strength), the decorrelated signal can be used for upmix.
In very bad reception conditions with high levels of noise on the side signal, the FM receiver may switch to a mono output mode to decrease the noise of the audio signal. In case of an L/R stereo audio signal at the output of the FM receiver, both channels at the output have the same signal in mono playback. In case of an M/S stereo signal at the output of the FM receiver, the S channel at the output is muted. In the mono output mode the stereo information is missing in the audio signal of the FM receiver. Thus, the PS parameter estimation stage cannot determine PS parameters suitable for creating a real stereo signal in the upmix stage. Even if the FM receiver does not switch to mono output mode in very bad reception conditions, the audio signal at the output of the FM receiver may be too bad for estimation of meaningful PS parameters.
The apparatus can be configured to detect whether the FM receiver has selected mono output of the stereo radio signal and/or can be configured to notice such poor reception conditions (which are too poor for estimation of meaningful PS parameters). In case of detecting mono output or in case of detecting such poor reception conditions, the upmix stage may generate a pseudo stereo signal. The upmix stage use one or more upmix parameters for blind upmix instead of the estimated parameters as discussed above. This mode is referred to as pseudo stereo operation or blind upmix operation.
Blind upmix operation specifies, in this case, that after detecting poor reception conditions or detecting mono output and thus initiating the blind upmix operation, spatial acoustic information—if at all present—in the output signal of the FM receiver is not used for determining the upmix parameters and thus is not considered for the upmix (if there is already a mono output at the output of the FM receiver no spatial acoustic information is present and thus cannot be considered at all). In contrast to the PS operation mode discussed above where the PS parameters are determined for reconstructing the side signal in the output signal of the upmix stage, in blind upmix operation the apparatus does not aim for reconstructing the side signal at the output signal of the upmix stage.
However, blind upmix does not mean that the apparatus is “blind” in that the upmix parameters are necessarily independent of the output signal of the FM receiver. E.g. the output signal of the FM receiver may be monitored whether it is music or speech, and dependent thereon appropriate upmix parameters may be selected.
One embodiment for blind upmix is to use preset upmix parameters. The preset upmix parameters may be default or stored upmix parameters.
Nevertheless, the used upmix parameters may be signal dependent, e.g. upmix parameters for speech and upmix parameters for music. In this case, the apparatus further has a speech detector (e.g. a speech/music discriminator) which detects whether the audio signal is predominantly speech or music. For example, in case of pure music the upmix parameters may be selected such that the downmix signal and the decorrelated version thereof are mixed, whereas in case of pure speech the upmix parameters may be selected such that the decorrelated version of the downmix signal is not used and only the downmix signal is used for upmix to a “mono” left/right signal. In case of an audio signal being a mixture of speech and music, blind upmix parameters may be used which are in between the upmix parameters for pure speech and the upmix parameters for pure music. One can further use interpolated upmix parameters for all states in between.
Advanced blind upmix schemes to pseudo stereo can be envisioned, where an even more advanced analysis of the mono signal is performed and this is used as the basis to derive “artificially generated” or “synthetic” PS parameters.
For a side signal with practically only noise, the apparatus preferably switches to pseudo stereo mode as discussed above. As noted above, the term “noise” here refers to the noise introduced by the bad radio reception (i.e. low signal-to-noise ratio on the radio channel), not to noise contained in the original signal sent to the FM broadcast transmitter.
However, for a side signal with almost no noise, i.e. almost no noise originating from the FM radio transmission, the apparatus preferably switches to normal stereo mode instead of parametric stereo mode. In normal stereo mode, the apparatus' signal improvement functionality is essentially deactivated. For deactivation, the left/right audio signal at the input of apparatus may be essentially fedthrough to the output of the apparatus.
Alternatively, for deactivation only the received side signal (and not the decorrelated signal) is mixed with the first audio signal in the upmix stage. When appropriately selecting the upmix parameters in the upmix stage, the output signal of the upmix stage corresponds to the output signal of the FM transmitter: e.g. when mixing of the first audio signal DM and the received side signal S0 according to
L′=DM+S
0 and R′=DM−S0, in case DM=(L+R)/2 and S0=(L−R)/2.
More preferably in some instances, the normal stereo mode or the parametric stereo mode may be selected in a frequency-variant manner, i.e. the selection may be different for the different frequency bands. This is useful since the signal-to-noise ratio for the received side signal characteristically gets worse for higher frequencies. As discussed above, this is a typical property of the FM demodulation.
Further embodiments of the apparatus are discussed in the dependent claims.
A second aspect of the invention relates to an apparatus for generating a stereo signal based on left/right or mid/side audio signal of an FM stereo radio receiver. The apparatus is configured for noticing that the FM stereo receiver has selected mono output of the stereo radio signal or the apparatus is configured for noticing poor radio reception. The apparatus comprises a stereo upmix stage. The upmix stage is configured to generate the stereo signal based on a first audio signal and one or more upmix parameters for blind upmix in case the apparatus notices that the FM stereo receiver has selected mono output of the stereo radio signal or the apparatus notices poor reception. The first audio signal is obtained from the left/right or mid/side audio signal.
The upmix parameters for blind upmix may be preset parameters, such as default or stored parameters.
The apparatus allows generation of a pseudo stereo signal having a low level noise in case of very bad reception conditions with high levels of noise on the side signal. In such reception conditions, the FM receiver may switch to mono mode to decrease the noise of the audio signal or the L/R or M/S audio signal may be too bad for estimation of meaningful PS parameters. This is detected and then upmix parameters blind upmix are used for generating a pseudo stereo signal. This was already discussed in connection with the first aspect of the invention.
As also discussed in connection with the first aspect of the invention, the apparatus may comprise a detection stage for detecting whether the FM stereo receiver has selected mono output of the stereo radio signal.
According to an exemplary embodiment, the apparatus further comprises an audio type detector, such as a speech detector indicating whether the audio signal at the output of the FM transmitter is predominantly speech or not. In this case, the upmix parameters are dependent on the indication of the speech detector. E.g. the apparatus uses upmix parameters in case of speech and different upmix parameters in case of music as discussed in detail in connection with the first aspect of the invention.
The apparatus according to the second aspect of the invention may further include the features of the apparatus according to the first aspect of the invention and vice versa.
A third aspect of the invention relates to an FM stereo radio receiver configured to receive an FM radio signal comprising a mid signal and a side signal. The FM stereo radio receiver includes an apparatus for improving the audio signal according to the first and second aspects of the invention.
A fourth aspect of the invention relates to a mobile communication device, such as a cellular telephone. The mobile communication device comprises an FM stereo receiver configured to receive an FM radio signal. Moreover, the mobile communication device comprises an apparatus for improving the audio signal according to the first and second aspects of the invention.
A fifth aspect of the invention relates a method for improving a left/right or mid/side audio signal of an FM stereo radio receiver. The features of the method according to the fifth aspect correspond to the features of the apparatus according to the first aspect. One or more PS parameters are determined based on the left/right or mid/side audio signal in a frequency-variant or frequency-invariant manner. The stereo signal is generated based on said first audio signal and the one or more PS parameters by an upmix operation.
The remarks to the first aspect of the invention also apply to the fifth aspect of the invention.
A sixth aspect of the invention relates to a method for generating a stereo signal based on left/right or mid/side audio signal of an FM stereo radio receiver. The features of the method according to the sixth aspect correspond to the features of the apparatus according to the second aspect. It is noticed that the FM stereo receiver has selected mono output of the stereo radio signal or in an alternative embodiment poor radio reception is noticed. In case the FM stereo receiver has selected mono output of the stereo radio signal or in case of poor radio reception, the stereo signal is generated based on a first audio signal and one or more upmix parameters for blind upmix, such as preset upmix parameters.
The remarks to the second aspect of the invention also apply to the sixth aspect of the invention.
The invention is explained below by way of illustrative examples with reference to the accompanying drawings, wherein
Instead of using a left/right representation at the output of the FM receiver 1 and the input of the apparatus 2, a mid/side representation may be used at the interface between the FM receiver 1 and the apparatus 2 (see M, S in
Optionally, a signal strength signal 6 indicating the radio reception condition may be used for adapting the audio processing in the audio processing apparatus 2. This will be explained later in this specification.
The combination of the FM radio receiver 1 and the audio processing apparatus 2 corresponds to an FM radio receiver having an integrated noise reduction system.
An audio signal DM is obtained from the input signal. In case the input audio signal uses already a mid/side representation, the audio signal DM may directly correspond to the mid signal. In case the input audio signal has a left/right representation, the audio signal is generated by downmixing the audio signal. Preferably, the resulting signal DM after downmix corresponds to the mid signal M and may be generated by the following equation:
DM=(L+R)/a, e.g. with a=2,
i.e. the downmix signal DM may correspond to the average of the L and R signals. For different values of a, the average of the L and R signals is amplified or attenuated.
The apparatus further comprises an upmix stage 4 also called stereo mixing module or stereo upmixer. The upmix stage 4 is configured to generate a stereo signal L′, R′ based on the audio signal DM and the PS parameters 5. Preferably, the upmix stage 4 does not only use the DM signal but also uses a side signal or some kind of pseudo side signal (not shown). This will be explained later in the specification in connection with more extended embodiments in
The apparatus 2 is based on the idea that due to its noise the received side signal may too noisy for reconstructing the stereo signal by simply combining the received mid and side signals; nevertheless, in this case the side signal or side signal's component in the L/R signal may be still good is enough for stereo parameter analysis in the PS parameter estimation stage 3. The resulting PS parameters 5 can be then used for generating a stereo signal L′, R′ having a reduced level of noise in comparison to the audio signal directly at the output of the FM receiver 1.
Thus, a bad FM radio signal can be “cleaned-up” by using the parametric stereo concept. The major part of the distortion and noise in an FM radio signal is located in the side channel which may be not used in the PS downmix. Nevertheless, the side channel is even in case of bad reception often of sufficient quality for PS parameter extraction.
In all the following drawings, the input signal to the audio processing apparatus 2 is a left/right stereo signal. With minor modifications to some modules within the audio processing apparatus 2, the audio processing apparatus 2 can also process an input signal in mid/side representation. Therefore, the concepts discussed herein can be used in connection with an input signal in mid/side representation.
The PS encoder 7 generates—based on the stereo audio input signal L, R—the audio signal DM and the PS parameters 5. Optionally, the PS encoder 7 further uses a signal strength signal 6. The audio signal DM is a mono downmix and preferably corresponds to the received mid signal. When summing the L/R channels to form the DM signal, the information of the received side channel may be completely excluded in the DM signal. Thus, in this case only the mid information is contained in the mono downmix DM. Hence, any noise from the side channel may be excluded in the DM signal. However, the side channel is part of the stereo parameter analysis in the encoder 7 as the encoder 7 typically takes L=M+S and R=M−S as input (consequently, DM=(L−FR)/2=M).
Experimental results indicate that a received side signal that contains intermediate levels of noise may not be good enough for reconstructing stereo itself but can be good enough for stereo parameter analysis in a PS encoder 7.
The mono signal DM and the PS parameters 5 are used subsequently in the PS decoder 8 to reconstruct the stereo signal L′, R′.
The use of a residual signal in an PS encoder/decoder is e.g. described in the MPEG Surround standard (see document ISO/IEC 23003-1:2007, MPEG Surround) and in the paper “MPEG Surround—The ISO/MPEG Standard for Efficient and Compatible Multi-Channel Audio Coding”, J. Herre et al., Audio Engineering Convention Paper 7084, 122nd Convention, May 5-8, 2007.
The PS parameter estimation stage 3 may estimate as PS parameters 5 the correlation and the level difference between the L and R inputs. Optionally, the parameter estimation stage receives the signal strength 6 which may be the signal power at the FM receiver. This information can be used to decide about the reliability, e.g. in case of a low signal strength 6, of the PS parameters 5. In case of a low reliability the PS parameters 5 may be set such that the output signal L′, R′ is a mono output signal or a pseudo stereo output signal. In case of a mono output signal, the output signal L′ is equal to the output signal R′. In case of a pseudo stereo output signal, default PS parameters may be used to generate a pseudo or default stereo output signal L′, R′.
The PS decoder module 8 comprises a stereo mixing matrix 4a and a decorrelator 10. The decorrelator receives the mono downmix DM and generates a decorrelated signal S′ which is used as a pseudo side signal. The decorrelator 10 may be realized by an appropriate all-pass filter as discussed in section 4 of the cited document “Low Complexity Parametric Stereo Coding in MPEG-4”. The stereo mixing matrix 4a is a 2×2 upmix matrix in this embodiment.
Dependent upon the estimated parameters 5, the matrix 4a mixes the DM signal with the received side signal S0 or the decorrelated signal S′ to create the stereo output signals L′ and R′. The selection between the signal S0 and the signal S′ may depend on a radio reception indicator indicative of the reception conditions, such as the signal strength 6. One may instead or in addition use a quality indicator indicative of the quality of the received side signal. One example of such a quality indicator may be an estimated noise (power) of the received side signal. In case of a side signal comprising a high degree of noise, the decorrelated signal S′ may be used to create the stereo output signal L′ and R′, whereas in low noise situations, the side signal S0 may be used. Various embodiments for estimating the noise of the received side signal are discussed later in this specification.
As an example, in case of good reception conditions (i.e. the signal strength is high), the signal S0 is used for upmixing, whereas in case of bad conditions the upmixing is based on the decorrelated signal S′. Preferably, the decision whether the stereo mixing module 4 uses the received side signal S0 or S′ is frequency dependent, e.g. for lower frequencies the received side signal S0 is used and for higher frequencies the decorrelated signal S′ is used. This will be discussed more in detail in connection with
The frequency-variant or frequency-invariant selection between the signal S0 and the signal S′ may be done in the upmix stage 4 (e.g. by selector means in the upmix stage 6 which are controlled e.g. in dependency of the signal strength 6). Alternatively, the frequency-variant or frequency-invariant selection between the signal S0 and the signal S′ may be performed in the parameter estimation stage 3 (e.g. in dependency of the signal strength 6), and the parameter estimation stage 3 then sends upmix parameters to the upmix stage 6 that cause that the respectively selected signal (either S0 or S′) is used for the upmix, e.g. the upmix parameters relating to the signal S0 are set to zero and the parameters relating to S′ are not set to zero in case of selecting S′. Alternatively, a selection signal (not shown) may be send to the upmix stage 6.
The upmix operation is preferably carried out according to the following matrix equation:
Here, the weighting factors α, β, γ, δ determine the weighting of the signals DM and S. The mono downmix DM preferably corresponds to the received mid signal. The signal S in the formula corresponds either to the decorrelated signal S′ or to the received side signal S0. The upmix matrix elements, i.e. the weighting factors α, β, γ, δ, may be derived e.g. as shown the cited paper “Low Complexity Parametric Stereo Coding in MPEG-4” (see section 2.2), as shown in the cited MPEG-4 standardization document ISO/IEC 14496-3:2005 (see section 8.6.4.6.2) or as shown in MPEG Surround specification document ISO/IEC 23003-1 (see section 6.5.3.2). These sections of the documents (and also sections referred to in these sections) are hereby incorporated by reference for all purposes.
Preferably, the selection between S′ and S0 is frequency dependent. This is shown in
If the received side signal S0 corresponds to S0=(L−R)/2 and L′=M+S0 and R′=M−S0, the mono downmix DM should preferably correspond to (L+R)/2; this allows perfect reconstruction, i.e. L′=L and R′=R.
Instead of using a PS upmixer using the received side signal S0, a generalized PS upmixer using a residual signal may be used. The resulting signals L′, R′ are function of the PS parameters, the residual signal and the mono downmix.
The mono downmix signal DM may be generated by adding the L, R channels with same weighting factors (e.g. using weighting factors of 1 or using weighting factors of ½). The signal DM then corresponds to the received mid signal. When using weighting factors of ½, the amplitude of the signal DM is half of the amplitude of the signal DM in case when using weighting factors of 1.
Optionally, some form of noise reduction may be also applied to the signal L/R or the signal DM (and/or the S0 signal if used). E.g. some noise reduction may be applied to the signal DM (see the optional noise reduction stage 11 in
In certain reception conditions, the FM receiver 1 only provides a mono signal, with the conveyed side signal being muted. This will typically happen when the reception conditions are very bad and the side signal is very noisy. In case the FM stereo receiver 1 has switched to mono playback of the stereo radio signal, the upmix stage preferably uses upmix parameters for blind upmix, such as preset upmix parameters, and generates a pseudo stereo signal, i.e. the upmix stage generates a stereo signal using the upmix parameters for blind upmix.
There are also embodiments of the FM stereo receiver 1 which switch at too poor reception conditions to mono playback. If the reception conditions are too poor for estimation of reliable PS parameters 5, the upmix stage preferably uses upmix parameters for blind upmix and generates a pseudo stereo signal based thereon.
Optionally, a speech detector 14 may be added to indicate if the received signal is predominantly speech or music. Such speech detector 14 allows for signal dependent blind upmix. E.g. such a speech detector 14 may allow for signal dependent upmix parameters. Preferably, one or more upmix parameters may be used for speech and different one or more upmix parameters may be used for music. Such a speech detector 14 may be realized by a Voice Activity Detector (VAD). Strictly speaking, the upmix stage 4 in
The same approach of using upmix parameters based on the previously estimated PS parameters can be also applied if the FM receiver 1 provides a noisy stereo signal during a short period of time, with the noisy stereo signal being too bad to estimate reliable PS parameters based thereon.
In the following, an advanced PS parameter estimation stage 3′ providing error compensation is discussed with reference to
When assuming that the noise in the side signal is independent of the mid signal:
For compensation of the error in the PS parameters the apparatus 2 preferably has a noise estimate stage which is configured to determine a noise parameter characteristic for the power of the noise of the received side signal that was caused by the (bad) radio transmission. The noise parameter is considered when estimating the PS parameters. This may be implemented as shown in
According to
The actual noisy stereo input signal values lw/ noise and rw/ noise, which are input to the inner PS parameter estimation stage 3′ shown in
l
w/ noise
=m+(s+n)=lw/o noise+n
r
w/ noise
=m−(s+n)=rw/o noise−n
It should be noted that here the received side signal is modeled as s+n, where “s” is the original (undistorted) side signal, and “n” is the noise (distortion signal) caused by the radio transmission channel. Furthermore, it is assumed here that the signal m is not distorted by noise from the radio is transmission channel.
Thus, the corresponding input powers Lw/ noise2, Rw/ noise2 and the cross correlation Lw/ noiseRw/ noise can be written as:
L
w/ noise
2
=E(lw/ noise2)=E((m+s)2)+E(n2)=Lw/o noise2+N2
R
w/ noise
2
=E(rw/ noise2)=E((m−s)2)+E(n2)=Rw/o noise2+N2
L
w/ noise
R
w/ noise
=E(lw/ noise·rw/ noise)=E((lw/o noise+n)·(rw/o noise−n))=Lw/o noiseRw/o noise−N2
with the side signal noise power estimate N2, with N2=E (n2), where “E( )” is the expectation operator.
By rearranging the above equations, the corresponding compensated powers and cross-correlation without noise can be determined to be:
L
w/o noise
2
=L
w/ noise
2
−N
2
R
w/o noise
2
=R
w/ noise
2
−N
2
L
w/o noise
R
w/o noise
=L
w/ noise
R
w/ noise
+N
2
An error-compensated PS parameter extraction based on the compensated powers and cross correlation may be carried out as given by the formulas below:
CLD=10·log10(Lw/o noise2/Rw/o noise2)
ICC=(Lw/o noiseRw/o noise)/(Lw/o noise2+Rw/o noise2)
Such a parameter extraction compensates for the estimated N2 term in the calculation of the PS parameters.
In
A variety of methods can be used for determining the side signal noise power N2, e.g.:
In the following further preferred embodiments of the audio processing apparatus 2 are discussed.
Preferably, the apparatus 2 is configured in such a way that for received side signals with practically only noise, the apparatus 2 smoothly switches to pseudo stereo (blind upmix) operation, as illustrated in
For side signals with almost no noise, the apparatus 2 preferably switches smoothly to normal stereo operation instead of parametric stereo operation. In normal stereo operation, the signal improvement functionality of the apparatus 2 is essentially deactivated. For deactivation, the audio signal at the input of apparatus may be essentially fedthrough to the output of the apparatus 2.
Alternatively, the normal stereo operation may be accomplished by using the received side signal S0, as illustrated in
L′=DM+S
0
,R′=DM−S
0,
in case DM=M=(L+R)/2 and S0=(L−R)/2.
More preferably, the normal stereo mode or the parametric stereo mode may be selected in a frequency-variant manner, i.e. the selection may be different for the different frequency bands. This is useful since the signal-to-noise ratio for the received side signal gets worse for higher frequencies.
The smooth switching between different operation modes may be adapted dynamically to the current reception conditions, in order to provide always the best possible stereo signal at the output of the apparatus 2. In case of a high signal-to-noise ratio normal FM stereo operation (without noise reduction based on PS processing) is preferred, whereas in case of a low signal-to-noise ratio PS processing greatly improves the stereo signal.
Preferably, the generation of the mono downmix DM in the PS encoder 7 should be done such that as little as possible noise from the side signal leaks into the mono downmix DM. This can require different downmix techniques than those typically used in a PS encoder (such as an MPEG-4 PS encoder for MPEG-4) which is normally employed in the context of a very low bitrate coding system. This can be as simple as a fixed (non-adaptive) downmix DM=M=(L+R)/2, where the downmix simply correspond to the mid signal. Furthermore, the upmix in the PS decoder 8 is typically adapted to the actual downmix technique used in the PS encoder 7.
It should be noted that although in several drawings the PS encoder 7 and the PS decoder 8 are shown as separate modules, it is of course advantageous in the context of an efficient implementation to merge PS encoder 7 and the PS decoder 8 as much as possible.
The concepts discussed herein can be implemented in connection with any encoder using PS techniques, e.g. an HE-AAC v2 (High-Efficiency Advanced Audio Coding version 2) encoder as defined in the standard ISO/IEC 14496-3 (MPEG-4 Audio), an encoder based on MPEG Surround or an encoder based on MPEG USAC (Unified Speech and Audio coder) as well as encoders which are not covered by MPEG standards.
In the following, by way of example, a HE-AAC v2 encoder is assumed; nevertheless, the concepts may be used in connection with any audio encoder using PS techniques.
HE-AAC is a lossy audio compression scheme. HE-AAC v1 (HE-AAC version 1) makes use of spectral band replication (SBR) to increase the compression efficiency. HE-AAC v2 further includes parametric stereo to enhance the compression efficiency of stereo signals at very low bitrates. An HE-AAC v2 encoder inherently includes a PS encoder to allow operation at very low bitrates. The PS encoder of such an HE-AAC v2 encoder can be used as the PS encoder 7 of the audio processing apparatus 2. In particular, the PS parameter estimating stage within a PS encoder of an HE-AAC v2 encoder can be used as the PS parameter estimating stage 3 of the audio processing apparatus 2. Also the downmix stage within a PS encoder of an HE-MC v2 encoder can be used as the downmix stage 9 of the apparatus 2.
Hence, the concept discussed in this specification can be efficiently combined with an HE-AAC v2 encoder to realize an improved FM stereo radio receiver. Such an improved FM stereo radio receiver may have an HE-MC v2 to recording feature since the HE-AAC v2 encoder outputs an HE-AAC v2 bitstream which can stored for recording purposes. This is shown in
Optionally, the PS encoder 7 may be modified for the purpose of FM radio noise reduction to support a fixed downmix scheme, such as a downmix scheme according to DM=(L+R)/a.
The mono downmix DM and the PS parameters 8 may be fed to the PS decoder 8 to generate the stereo signal L′, R′ as discussed above. The mono downmix DM is fed to an HE-AAC v1 encoder for perceptual encoding of the mono downmix DM. The resulting perceptual encoded audio signal and the PS information are multiplexed into an HE-MC v2 bitstream 18. For recording purposes, the HE-AAC v2 bitstream 18 can be stored in a memory such as a flash-memory or a hard-disk.
The HE-MC v1 encoder 17 comprises an SBR encoder and an MC encoder (not shown). The SBR encoder typically performs signal processing in the QMF (quadrature mirror filterbank) domain and thus needs QMF samples. In contrast, the MC encoder typically needs time domain samples (typically downsampled by a factor 2).
The PS encoder 7 within the HE-AAC v2 encoder 16 typically provides the downmix signal DM already in the QMF domain.
Since the PS encoder 7 may already send the QMF domain signal DM to the HE-AAC v1 encoder, the QMF analysis transform in the HE-AAC v1 encoder for the SBR analysis can be made obsolete. Thus, the QMF analysis that is normally part of the HE-AAC v1 encoder can be avoided by providing the downmix signal DM as QMF samples. This reduces the computing effort and allows for complexity saving.
The time domain samples for the MC encoder may be derived from the input of the apparatus 2, e.g. by performing the simple operation DM=(L+R)/2 in the time domain and by downsampling the time domain signal DM. This approach is probably the cheapest approach. Alternatively, the apparatus 2 may perform a half-rate QMF synthesis of the QMF domain DM samples.
It should be noted that the PS encoder and PS decoder can be partly merged if both are implemented in the same module.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/005481 | 9/7/2010 | WO | 00 | 4/24/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61241113 | Sep 2009 | US |
