Apparatus and Method for Improving the Perceived Quality of Sound Reproduction
by Combining Active Noise Cancellation and a Perceptual Noise Compensation
Description
The present invention relates to audio signal processing and, in particular, to an apparatus
and method for improving the perceived quality of sound reproduction by combining
Active Noise Cancellation and Perceptual Noise Compensation, e.g., by improving the
perceived quality of reproduction of sound over headphones.
Audio signal processing becomes more and more important. In many listening scenarios,
e.g., in a cabin of a vehicle, the audio signals are presented in a noisy environment and
thereby, their sound quality and intelligibility is affected. One approach to reduce the
impact of environmental noise on the listening experience is Active Noise Cancellation
(Active Noise Control) see, e.g., [1], [2]. ANC (ANC = Active Noise Cancellation) reduces
the interfering noise at the receiver side to varying degree. In general, low-frequency noise
components can be canceled more successfully than high-frequency components, and
stationary noise can be canceled better than non-stationary, and pure tone better than
random noise.
Active Noise Cancellation is a technique to suppress acoustic noise based on the principle
of acoustic interference. The basic idea of canceling the interfering noise by using a phaseinverted
copy of it has first been described in Paul Lueg's patent in 1936, see [7].
The principles of ANC are summarized in [1] and [2]. The sound field emitted by the noise
source (primary source) is measured using a transducer. This reference signal is used to
generate a secondary signal which is fed into a secondary loudspeaker. If the acoustic wave
emitted by the secondary source (the so-called "anti-noise") is exactly out of phase with
the acoustic wave of the noise, the noise is canceled due to destructive interference in the
region behind the loudspeaker and opposite the noise source, the "zone of quiet". Ideally,
plane wave transducers are used for both, microphone and loudspeaker.
Although the anti-noise can be generated by delaying and scaling the measurement of the
primary noise, the anti-noise is often computed adaptively to cope with possible variations
in the acoustic path between noise and anti-sound transducer. Such implementations are
based on adaptive filters whose filter coefficients are computed by minimizing an error
signal using the Least-Mean Square (LMS), filtered-X LMS algorithm (FXLMS), leaky
FXLMS or other optimization algorithms.
ANC can be implemented as either feedforward control or feedback control.
Fig. 3 illustrates a block diagram of an ANC implementation with feedforward structure. A
noise source 310 emits primary noise 320. The primary noise 320 is recorded by a
reference microphone 330 as an environmental audio signal d(t). The environmental audio
signal is fed into an adaptive filter 340. The adaptive filter is configured to filter the
environmental audio signal d(t) to obtain a filtered signal. The filtered signal is employed
to steer a loudspeaker 350.
As already stated, the structure illustrated by Fig. 3 is a feedforward structure. In a
feedforward structure, the referenced microphone may, e.g., be placed such that the
primary noise is picked up before it reaches the secondary source, as shown in Fig. 3.
Often, a second microphone is mounted after the secondary source to measure the residual
noise signal. In such a structure, the second microphone represents a residual noise
microphone or an error microphone. Such a structure is shown in Fig. 4.
Fig. 4 illustrates a block diagram of an ANC implementation with feedforward structure
with an additional error microphone 460. An adaptive algorithm computes the filter
coefficients for generating the anti-noise using the referenced microphone signal such that
the residual noise is minimized.
Fig. 5 illustrates a block diagram of an ANC implementation with feedback structure.
Implementations in feedback structures, as shown in Fig. 5 use only one microphone for
measuring the error and generating the secondary signal. A feedback ANC system for
headphone application is described in [8].
The effect of the cancellation depends on the accuracy of the superposition of the sound
fields of the noise source and the secondary source. In practice, the interfering noise signal
is not removed completely. ANC is especially suitable for low-frequency noise signal
components and stationary signals, but fails to remove high-frequency and non-stationary
noise signal components.
Perceptual Noise Compensation (PNC) is a signal processing method to compensate for the
perceptual effects of interfering noise by using psychoacoustic knowledge. The basic
principle behind PNC is to apply time-varying equalization such that spectral components
of the input audio signal are amplified which are masked by the interfering noise. The main
idea has been referred to as e.g. Noise Compensation, see, e.g., [3], Masking
Compensation, see, e.g., [4], Sound Equalization in Noisy Environments, see, e.g., [5], or
Dynamic Sound Control, see, e.g., [6].
Perceptual Noise Compensation processes an audio signal such that its timbre and
loudness, when presented in environmental noise, is perceived as similar or close to those
when presented unprocessed in quiet. The additive noise leads to a decrease of the
loudness of the desired signal due to partial or total masking effects. The resulting
sensation is known as partial loudness. Due to the frequency selective processing in the
human auditory system, the interfering noise effects the perceived spectral balance of the
desired signal and thereby its timbre.
The basic principles of PNC have been applied, e.g. in [3]. Recent developments have, for
example, been described in [9], [10], [ 1 ] and [6]. The rationale of the method is to apply
time-varying spectral weighting factors to the desired signal such that the sensation of
loudness and timbre is restored.
The spectral weighting method of the PNC splits the input audio signal into M frequency
bands, preferably according to a perceptually motivated frequency scale, having the
bandwidth of a critical band, e.g. the Bark or ERB scale. The derived sub-band signals
sm[k] are scaled with time-varying gain factors gm[k], with sub-band index m = 1...M and
time index k. The gains are computed such that the partial specific loudness N', e.g., the
loudness evoked at each auditory frequency band, of the processed signal in noise are
equivalent to the specific loudness of the unprocessed audio signal in quiet or a fraction b
thereof, as shown in Equation (1), with em[k] being the sub-band signals of the additive
noise:
wherein
is the loudness in quiet, and wherein
N p' [ k ] = f(g m [k] s r k ] , e [k])
is the partial loudness of the processed signal in noise e[k].
Loudness models compute the partial specific loudness N ' [m, k] of a signal s[k] when
presented simultaneously with a masking signal e[k].
The gains gm[k] can be computed using a model of partial loudness, see, for example [10].
n the following, reference is made to computational models of partial loudness. Loudness
models compute the partial specific loudness N'(s [k] + Qm[k]) of a signal s[k] when
presented simultaneously with a masking signal e[k]
N ' { k) f(s m [k] {k)) 2)
A particular implementation of a perceptual model of partial loudness is shown in Fig. 6. It
is derived from the models presented in [12] and [13] which itself drew on earlier research
by Fletcher, Munson, Stevens, and Zwicker with some modifications. Alternative methods
for the calculation of the specific loudness have been developed in the past, as, e.g.
described in [14].
The input signals are processed in the frequency domain using a Short-time Fourier
transform (STFT), for example, with a frame length of 2 1 ms, 50% overlap and a Harm
window function. Mimicking the frequency resolution and the temporal resolution of the
human auditory system, sub-band signals are obtained by grouping the spectral
coefficients. The transfer through the outer and middle ear is simulated with a fixed filter.
Additionally, the transfer function of the reproduction system can be incorporated
optionally, but is neglected here for simplicity.
Fig. 7 illustrates the transfer function modeling the path through the outer and middle ear.
The excitation function is computed for auditory filter bands spaced on the equivalent
rectangular bandwidth (ERB) scale or the Bark scale.
Fig. 8 illustrates a simplified spacing of auditory filter bands as an example for a
perceptually motivated spacing of the frequency bands.
In addition to the temporal integration due to the windowing of the STFT, a recursive
integration can be used, with different time constants during attack and decay. The specific
partial loudness, e.g., the partial loudness evoked in each of the auditory filter bands, is
computed from the excitation levels from the signal of interest (the stimulus) and the
interfering noise according to Equations (17)-(20) in [12]. These equations cover the four
cases where the signal is above the hearing threshold in noise or not, and where the
excitation of the mixture signal is less than 100 dB SPL or not. If no interfering signal is
fed into the model, e.g. e[k] = 0, the result equals the total loudness N [ ] of the stimulus
s[k] and should predict the information represented in the equal loudness contours (ELC),
as shown in Fig. 9 . There, Fig. 9 illustrates equal loudness contours, ISO226-2003, from
[15].
Examples of outputs of the model are shown in Figs. 10 and 11.
Fig. 0 illustrates specific partial loudness, exemplarily for frequency band 4, wherein the
function of noise excitation ranges from 0 to 100 dB.
Fig. 11 illustrates specific partial loudness in noise with 40 dB noise excitation.
US Patent 7,050,966 (see [ ]) describes a method for enhancing the intelligibility of
speech in noise and mentions the combination of ANC and PNC, however, no teaching is
given of how ANC and PNC can be advantageously combined.
The object of the present invention is to provide improved concepts for improving the
perceived quality of sound reproduction. The object of the present invention is solved by
an apparatus for improving the perceived quality of sound reproduction according to claim
1, by a headphone according to claim 13, by a method according to claim 16 and by a
computer program according to claim 17.
An apparatus for improving a perceived quality of sound reproduction of an audio output
signal is provided. The apparatus comprises an active noise cancellation unit for generating
a noise cancellation signal based on an environmental audio signal, wherein the
environmental audio signal comprises noise signal portions, the noise signal portions
resulting from recording environmental noise. Moreover, the apparatus comprises a
residual noise characteristics estimator for determining a residual noise characteristic
depending on the environmental noise and the noise cancellation signal. Furthermore, the
apparatus comprises a perceptual noise compensation unit for generating a noisecompensated
signal based on an audio target signal (a desired signal) and based on the
residual noise characteristic. Moreover, the apparatus comprises a combiner for combining
the noise cancellation signal and the noise-compensated signal to obtain the audio output
signal.
According to the present invention, concepts are provided for reproducing the audio
signals such that their timbre, loudness and intelligibility when presented in an
environmental noise are similar or close to those when presented unprocessed in quiet. The
proposed concepts incorporate a combination of Active Noise Cancellation and Perceptual
Noise Compensation. Active Noise Cancellation is applied to remove the interfering noise
signals as much as possible. Perceptual Noise Compensation is applied to compensate for
the remaining noise components. The combination of both can be efficiently implemented
by using the same transducers.
Embodiments of the present invention are based on the concept to process the desired
audio signal s[k] by taking psychoacoustic findings into account. By this, the adverse
perceptual effect of the residual noise components e[k] are subsequently compensated for
by processing the desired audio signals s[k] by taking psychoacoustic findings of the
Perceptual Noise Compensation into account.
Embodiments are based on the finding that ANC can physically cancel the interfering
noise only partially. It is imperfect and consequently some residual noise remains at the ear
entrances of the listener as shown in the schematic diagram of an exemplary
implementation of a sound reproduction system according to the state of the art in Fig. 12.
According to an embodiment, the residual noise characteristics estimator may be
configured to detennine the residual noise characteristic such that the residual noise
characteristic indicates a characteristic of noise portions of the environmental noise that
would remain when only reproducing the noise cancellation signal.
In a further embodiment, the residual noise characteristics estimator may be arranged to
receive the environmental audio signal. The residual noise characteristics estimator may be
arranged to receive information on the noise cancellation signal from the active noise
cancellation unit, and wherein the residual noise characteristics estimator is configured to
detennine the residual noise characteristic based on the environmental audio signal and
based on the information on the noise cancellation signal. The remaining noise estimate
may, e.g., indicate the noise portions of the environmental noise that would remain when
only reproducing the noise cancellation signal.
According to another embodiment, the residual noise characteristics estimator may be
arranged to receive the noise cancellation signal as the information on the noise
cancellation signal from the active noise cancellation unit. The residual noise
characteristics estimator may be configured to determine the remaining noise estimate
based on the environmental audio signal and based on the noise cancellation signal.
According to a further embodiment, the residual noise characteristics estimator may be
configured to determine the remaining noise estimate by adding the environmental audio
signal and the noise cancellation signal.
In another embodiment, the apparatus furthermore comprises at least one loudspeaker and
at least one microphone. The microphone may be configured to record the environmental
audio signal, the loudspeaker may be configured to output the audio output signal, and
wherein the microphone and the loudspeaker may be arranged to implement a feedforward
structure.
According to another embodiment, the residual noise characteristics estimator may be
arranged to receive the environmental audio signal, wherein the residual noise
characteristics estimator may be arranged to receive information on the noise-compensated
signal from the perceptual noise compensation unit. The residual noise characteristics
estimator may be configured to determine as the residual noise characteristic a remaining
noise estimate based on the environmental audio signal and based on the noisecompensated
signal. The remaining noise estimate may, e.g., indicate the noise portions of
the environmental noise that would remain when only reproducing the noise cancellation
signal.
In another embodiment, the residual noise characteristics estimator may be arranged to
receive the noise-compensated signal as the information on the noise-compensated signal
from perceptual noise compensation unit. The residual noise characteristics estimator may
be configured to determine the remaining noise estimate based on the environmental audio
signal and based on the noise-compensated signal.
According to a further embodiment, the residual noise characteristics estimator may be
configured to determine the remaining noise estimate by subtracting scaled components of
the noise-compensated signal from the environmental audio signal.
In another embodiment, the apparatus may furthermore comprise at least one loudspeaker
and at least one microphone. The microphone may be configured to record the
environmental audio signal, the loudspeaker may be configured to output the audio output
signal, and the microphone and the loudspeaker may be arranged to implement a feedback
structure.
According to another embodiment, the apparatus may furthermore comprise a source
separation unit for detecting signal portions of the environmental audio signal which shall
not be compensated for, e.g., speech or alarm sounds.
In a further embodiment, the source separation unit may be configured to remove the
signal portions of the environmental audio signal which shall not be compensated from
environmental audio signal.
According to an embodiment, a headphone is provided. The headphone comprises two earcups,
an apparatus for improving a perceived quality of sound reproduction according to
one of the above-described embodiments, and at least one microphone for recording the
environmental audio signal. In this context, concepts for the reproduction of audio signals
over headphones in noisy environments are provided.
In an embodiment, a method for improving a perceived quality of sound reproduction of an
audio output signal is provided. The method comprises:
Generating a noise cancellation signal based on an environmental audio signal, wherein the
environmental audio signal comprises noise signal portions, the noise signal portions
resulting from recording environmental noise.
Determining a residual noise characteristic depending on the environmental noise and the
noise cancellation signal.
Generating a noise-compensated signal based on an audio target signal and based on the
residual noise characteristic, and:
Combining the noise cancellation signal and the noise-compensated signal to obtain the
audio output signal.
Moreover, a computer program for implementing the above-described method when being
executed on a computer or signal processor is provided.
In the following, embodiments of the present invention are described in more detail with
reference to the figures, in which:
Fig. 1 is an apparatus for improving a perceived quality of sound reproduction
according to an embodiment,
Fig. 2 illustrates a headphone according to an embodiment,
Fig. 3 is a block diagram of an active noise cancellation implementation with a
feedforward structure,
Fig. 4 is a block diagram of an active noise cancellation implementation with a
feedforward structure with an additional error microphone
Fig. 5 is a block diagram of an active noise cancellation implementation with a
feedback structure,
Fig. 6 is a block diagram of a perceptual model of partial loudness,
Fig. 7 is an example of a transfer function through the outer and middle ear,
Fig. 8 is a simplified spacing of auditory filter bands,
Fig. 9 are equal loudness contours,
Fig. 10 is a specific partial loudness, exemplary for frequency band 4, and a
function of noise excitation ranging from 0 to 100 dB,
Fig. is a specific partial loudness in noise with 40 dB noise excitation,
Fig. 1 is a block diagram of an exemplary implementation of a sound reproduction
system with acoustic noise cancellation according to the state of the art with
feedforward structure,
Fig. 13 is a block diagram of a sound reproduction system with Perceptual Noise
Compensation according to the state of the art,
is a block diagram of an exemplary implementation of a sound reproduction
system with ANC and PNC according to an embodiment, where the primary
noise sensor is used for estimating the characteristics of the residual noise,
is a block diagram of an alternative implementation of a sound reproduction
system with ANC and PNC according to a further embodiment, where the
residual noise sensor is used for estimating the characteristics of the residual
noise,
is a block diagram of an exemplary implementation of a sound reproduction
system with ANC and PNC according to another embodiment, where the
primary noise sensor is used for estimating the characteristics of the residual
noise,
is a block diagram of an alternative implementation of a sound reproduction
system with ANC and PNC according to a further embodiment, where the
residual noise sensor is used for estimating the characteristics of the residual
noise,
is an apparatus for improving a perceived quality of sound reproduction
according to a further embodiment, wherein the apparatus comprises a
source separation unit,
illustrates a headphone according to an embodiment comprising two
apparatuses for improving a perceived quality of sound reproduction
according to the embodiment of Fig. 16,
illustrates a headphone according to an embodiment comprising a two
apparatuses for improving a perceived quality of sound reproduction
according to the embodiment of Fig. 7,
illustrates a test arrangement for modelling the transfer through the
headphones and ANC processing as a Linear Time Invariant system
according to an embodiment,
illustrates modelled LTI systems corresponding to the test arrangement of
Fig. 2 according to an embodiment, and
Fig. 23 illustrates a flow chart depicting the steps conducted to model the transfer
through the headphones and ANC processing as a Linear Time-Invariant
system according to an embodiment.
Fig. 1 illustrates an apparatus for improving a perceived quality of sound reproduction of
an audio output signal according to an embodiment. The apparatus comprises an active
noise cancellation unit 110 for generating a noise cancellation signal based on an
environmental audio signal. The environmental audio signal comprises noise signal
portions, wherein the noise signal portions result from recording environmental noise.
Moreover, the apparatus comprises a residual noise characteristics estimator 120 for
determining a residual noise characteristic depending on the environmental noise and the
noise cancellation signal. Furthermore, the apparatus comprises a perceptual noise
compensation unit 130 for generating a noise-compensated signal based on an audio target
signal and based on the residual noise characteristic. Moreover, the apparatus comprises a
combiner 140 for combining the noise cancellation signal and the noise-compensated
signal to obtain the audio output signal. In this context, environmental noise may be any
kind of noise which occurs in an environment, e.g. an environment of a recording
microphone, an environment of a loudspeaker or an environment where a listener perceives
emitted sound waves.
Embodiments of the apparatus for improving a perceived quality of sound reproduction of
an audio output signal are based on the finding that ANC can physically cancel the
interfering noise only partially. ANC is imperfect and consequently some residual noise
remains at the ear entrances of the listener as shown in the schematic diagram of the
exemplary implementation according to the state of the art illustrated in Fig. 12.
To overcome this disadvantage, according to some embodiments, the residual noise
characteristics estimator 120 may be configured to determine the residual noise
characteristic such that the residual noise characteristic indicates a characteristic of noise
portions of the environmental noise that would remain when only reproducing the noise
cancellation signal, e.g., when the noise cancellation signal would be reproduced, e.g., by a
loudspeaker.
An apparatus according to the above-described embodiment may be employed in a
headphone. Fig. 2 illustrates a corresponding headphone according to such an embodiment.
The headphone comprises two ear-cups 241, 242. The ear-cup 241 may, for example,
comprise at least one microphone 261 and an apparatus 251 for improving a perceived
quality of sound reproduction according to one of the above-described embodiments. In the
embodiment of the headphone of Fig. 2, the apparatus 251 for improving a perceived
quality of sound reproduction may be integrated into the ear-cup 241 . A loudspeaker of the
ear-cup 241 may reproduce the audio output signal of the apparatus 251 for improving a
perceived quality of sound reproduction. Likewise, the ear-cup 242 may, for example,
comprise at least one microphone 262 and an apparatus 252 for improving a perceived
quality of sound reproduction according to one of the above-described embodiments. In the
embodiment of the headphone of Fig. 2, the apparatus 252 for improving a perceived
quality of sound reproduction may be integrated into the ear-cup 242. A loudspeaker of the
ear-cup 242 may reproduce the audio output signal of the apparatus 252 for improving a
perceived quality of sound reproduction. Moreover, Fig. 2 illustrates a listener 280 wearing
the headphone.
The headphone implements ANC. In embodiments, one or more microphones are mounted
to the headphone of Fig. 2 for measuring the environmental noise and/or the residual noise
at the ear entrances. The microphone signals are used to generate the secondary signal for
canceling the noise. Additionally, PNC processing is conducted, which improves the
perceived sound quality by compensating for the remaining noise signal by applying timevariant
and signal-dependent spectral weights (filters) to the desired input signals. The
estimate of the residual noise characteristics needed for the PNC processing for computing
the filters is obtained from the microphone signals.
Different structures of implementations of ANC exists. A distinguishing feature between
such structures is the position of the noise sensor in the processed chain, leading to two
basic control structures, namely feedforward and feedback structure. The technical
background on implementations of ANC has already been described above.
In the state of the art, which is illustrated by Fig. 12, the interfering noise is not canceled
completely. The residual noise can be compensated in its adverse effects on the quality of
the reproduced audio signal by using PNC, a signal processing method based on
psychoacoustics. PNC applies time-varying equalization such that spectral components of
the input signal are amplified which are masked by the interfering noise. This is typically
achieved by using a spectral weighting method where the sub-band gains are computed by
taking psychoacoustic knowledge and the characteristics of the desired signal (the audio
target signal) and the interfering noise into account. More technical background on PNC
implementations has already been provided above. A sound reproduction with PNC
according to the state of the art is depicted in Fig. 13.
Figs. 14 and 15 illustrate sound reproduction systems according to embodiments. Both
implementations include a means for estimating the characteristics of the residual noise,
referred to as Residual Noise Characteristics Estimator (RNCE). A difference between the
two implementations is the control structure used for the ANC (feedforward structure and
feedback structure).
Fig. 14 illustrates an apparatus according to an embodiment, and, in particular, a
combination of PNC with ANC in a feedforward structure. The RNCE is based on the
primary noise sensor without a dedicated microphone for measuring the residual noise. The
apparatus of the embodiment of Fig. 14 comprises an active noise cancellation unit 1410, a
residual noise characteristics estimator 1420, a perceptual noise compensation unit 1430
and a combiner 1440, which may correspond to the active noise cancellation unit 110, the
residual noise characteristics estimator 120, the perceptual noise compensation unit 130
and the combiner 140 of the embodiment of Fig. 1, respectively.
The apparatus of the embodiment of Fig. 14 furthermore comprises a loudspeaker 1450
and a microphone 1405. The microphone 1405 is configured to record the environmental
audio signal. Moreover, the loudspeaker 1450 is configured to output the audio output
signal. In the embodiment of Fig. 14, the microphone and the loudspeaker are arranged to
implement a feedforward structure. A feedforward structure may, e.g., represent an
arrangement of a microphone and a loudspeaker, wherein the microphone does not receive
sound waves emitted by the loudspeaker.
Fig. 15 illustrates an implementation in feedback structure that takes advantage of a
dedicated microphone for measuring the residual noise. In particular, Fig. 15 illustrates an
apparatus for improving the perceived quality of sound reproduction, wherein the
apparatus again comprises an active noise cancellation unit 1510, a residual noise
characteristics estimator 1520, a perceptual noise compensation unit 1530 and a combiner
1540, which may correspond to the active noise cancellation unit 10, the residual noise
characteristics estimator 120, the perceptual noise compensation unit 130 and the combiner
140 of the embodiment of Fig. 1, respectively.
As in the embodiment of Fig. 14, the apparatus of the embodiment of Fig. 15 furthermore
comprises a loudspeaker 1550 and a microphone 1505. The microphone 1505 is configured
to record the environmental audio signal. Moreover, the loudspeaker 1550 is configured to
output the audio output signal. In contrast to Fig. 14, in Fig. 15, the microphone and the
loudspeaker are arranged to implement a feedback structure. A feedback structure may,
e.g., represent an arrangement of a microphone and a loudspeaker, wherein the microphone
does receive sound waves emitted by the loudspeaker.
Fig. 16 illustrates an apparatus according to an embodiment depicting more details than
Fig. 14. The apparatus of the embodiment of Fig. 16 comprises an active noise cancellation
unit 1610, a residual noise characteristics estimator 1620, a perceptual noise compensation
unit 1630 and a combiner 1640, a microphone 1605 and a loudspeaker 1650. The
microphone 1605 and the loudspeaker 1 50 implement a feedforward structure.
In the embodiment of Fig. 16, the residual noise characteristics estimator 1620 is arranged
to receive information on the noise cancellation signal from the active noise cancellation
unit 1610. This is indicated by arrow 1660. The residual noise characteristics estimator
1620 is configured to determine as the residual noise characteristic a remaining noise
estimate which may, e.g., indicate the noise portions of the environmental noise that would
remain when only the noise cancellation signal (and not, e.g. also a signal resulting from
PNC) would be reproduced.
As Fig. 16 implements a feedforward structure, the environmental audio signal may, e.g.,
only comprise noise signal components. The residual noise characteristics estimator 1620
may receive the noise cancellation signal from the active noise cancellation unit 1610 and
may, for example, add this noise cancellation signal (anti-noise) to the environmental audio
signal. The resulting signal may then be the noise estimate representing the environmental
noise that would remain when only reproducing the noise cancellation signal.
Fig. 17 illustrates an apparatus according to an embodiment depicting more details than
Fig. 15. The apparatus of the embodiment of Fig. 17 comprises an active noise cancellation
unit 1710, a residual noise characteristics estimator 1720, a perceptual noise compensation
unit 1730, a combiner 1740, a microphone 1705 and a loudspeaker 1750. The microphone
1705 and the loudspeaker 1750 implement a feedback structure.
In the embodiment of Fig. 17, the residual noise characteristics estimator 1720 is arranged
to receive information on the noise-compensated signal from the perceptual noise
compensation unit 1730. This is indicated by arrow 1770. The residual noise characteristics
estimator 1720 may be configured to determine as the residual noise characteristic a
remaining noise estimate which may, e.g., indicate the noise portions of the environmental
noise that would remain when only the noise cancellation signal (and not also a signal
resulting from PNC) would be reproduced.
As Fig. 17 implements a feedback structure, the environmental audio signal which
represents the recorded sound waves in the environment of the microphone also comprises
the noise-compensated signal. The residual noise characteristics estimator 1720 may
receive the noise-compensated signal from the perceptual noise compensation unit 1730,
and may subtract scaled components of the received noise-compensated signal from the
environmental audio signal. For example, the scaled components of the received noisecompensated
signal may be determined by scaling the received noise-compensated signal
by a predetermined scale factor. The resulting signal may then be the noise estimate
representing the environmental noise that would remain when only reproducing the noise
cancellation signal. The predetermined scale factor may, for example, be a signal level
difference between an average signal level of a signal when being emitted at the
loudspeaker and an average signal level of the signal when being recorded at the
microphone.
Some of the advantages of combining ANC and PNC are:
• Improved sound quality: additionally compensating for the residual noise is an
improvement over ANC, and, vice versa cancellation of the low-frequency noise
components prior to PNC guarantees your listening experiences at low payback
levels.
• Cost-efficient implementation: ANC and PNC can use the same transducers (both,
microphones and loudspeakers). The RNCE can be obtained from a noise sensor,
e.g. a residual noise sensor or from the primary noise sensor by taking the ANC
suppression characteristics into account.
Two different ways for obtaining the noise estimate may be used. These two ways depend
on the structure of the ANC implementation:
· If the implementation of the ANC features a microphone for measuring the
residual noise, the noise estimate is obtained from this sensor and the crosstalk of
the desired signal into the sensor needs to be suppressed.
• If the ANC is implemented in a feedforward structure with only one microphone
for sensing the primary noise, the noise estimate can be obtained from this sensor
using a model of the transfer through the headphone (including mechanical
dumping of the external noise due to passive absorption by the headphone and the
ANC.
In general, the noise estimation may comprise:
1. The cancellation of the crosstalk of the music playback into the microphone.
2. The modelling of the transfer function/attenuation of the outer noise through the
ear-cup and the ANC processing.
3. Optionally, a signal analysis, possibly combined with a source separation
processing, in order to avoid compensation/marking of certain outside sounds
which are desired to be perceived by the headphone listener, e.g. speech and alarm
sounds.
To achieve crosstalk suppression, the PNC scales the desired signal with sub-band gain
values which are monotonically increasing with increasing noise sub-band level. If the
music playback is picked-up by the microphone and adds to the noise estimate, the
resulting feedback can potentially lead to over-compensation and excessive amplification
of the corresponding sub-band signals. Therefore, the crosstalk of the music playback into
the microphones needs to be suppressed.
Before the environmental noise reaches the ear entrances, it is damped by the passive
attenuation of the ear-cups and by the ANC processing. The transfer through the
headphone is modelled by the function ¾r, see equation (3):
e [k} = f P ( d [k) ) (3)
wherein d[k] denotes an external noise and wherein e[k] denotes a noise estimate.
The transfer can be modelled as a Linear Time-Invariant (LTI) system or as a non-linear
system. Such system identification methods use a series of measurements of the input and
output signals and determine the model parameters such that an error measure between
output measurements and predicted output is minimized.
In the first case (modelling as an LTI system), the system is described by its impulse
response or magnitude transfer function.
Fig. 1 illustrates a test arrangement for modelling the transfer through the headphones and
ANC processing as a Linear Time-Invariant system according to an embodiment. In Fig.
21, a test signal is fed into a first loudspeaker 2 110. The test signal should have a broad
frequency spectrum. In response, the first loudspeaker 2 110 outputs sound waves which
are then recorded by a first microphone 2120 arranged on an ear-cup 242 of a headphone
as a first recorded audio signal. The first recorded audio signal records sound waves that
have not yet passed through the ear-cup 242. Moreover, ANC processing has not yet been
conducted.
The test signal can be considered as an excitation signal of a first LTI system. Moreover,
the first recorded audio signal can be considered as an output signal of the first LTI system.
In an embodiment, an impulse response of the first LTI system is calculated based on the
test signal and based on the first recorded audio signal as a first impulse response. For this
purpose, the test signal should have a broad frequency spectrum. Furthermore, the first
impulse response is transferred to the frequency domain, e.g. by conducting STFT (Short-
Time Fourier Transform), to obtain a first frequency response. In an alternative
embodiment, the first frequency response is directly determined based on frequencydomain
representations of the test signal and the first recorded audio signal.
Moreover, to obtain a second recorded microphone signal, a second microphone 2130
records sound waves that have passed through the ear-cup 242 and after ANC has been
conducted. To conduct ANC, an ear-cup loudspeaker 272 of the ear-cup 242 is employed
to output so-called "anti-noise" for cancelling the sound waves from the first loudspeaker.
Again, the test signal can be considered as an excitation signal of a further, second LTI
system. The second recorded microphone signal can be considered as an output signal of
the second LTI system. According to an embodiment, an impulse response of the second
LTI system is calculated based on the test signal and based on the second recorded audio
signal as a second impulse response. Furthermore, the second impulse response is
transferred to the frequency domain to obtain a second frequency response. In an
alternative embodiment, the second frequency response is directly determined based on
frequency-domain representations of the test signal and the first recorded audio signal.
This is explained in more detail with reference to Fig. 22. The second LT system 2220 can
be considered to comprise two LTI systems, namely the first LTI system 2210, already
described with respect to Fig. 2 1 and a third LTI system 2230. The first LTI system 2 10
receives the test signal (output by the first loudspeaker 2 110) as an excitation signal.
Moreover, the first LTI system 2210 outputs the first recorded audio signal (recorded by
the first microphone 2120). The third LTI system 2230 receives the first recorded audio
signal as an excitation signal and outputs the second recorded audio signal (recorded by the
second microphone).
To model ANC and the influence of the transfer of the sound waves through the ear-cups,
the third LTI system 2230 is determined. In an embodiment, the frequency response of the
third LTI system 2230 is calculated as a third frequency response based on the first
frequency response of the first LTI system 2210 and based on the second frequency
response of the second LTI system 2220.
In an embodiment, the second frequency response of the second LTI system 2220 is
divided by the first frequency response of the first LTI system 2210 to obtain the third
frequency response of the third LTI system 2230.
Fig. 23 illustrates a flow chart depicting the steps to model the transfer through the
headphones and ANC processing as a Linear Time-Invariant system according to an
embodiment.
In step 2310, a test signal is fed into a first loudspeaker. The first loudspeaker outputs
sound waves in response to the test signal.
In step 2320, a first microphone arranged on an ear-cup of a headphone records the sound
waves to obtain a first recorded audio signal.
In step 2330, a first frequency response of a first LTI system is determined based on the
test signal as an excitation signal of the first LTI system and based on the first recorded
audio signal as an output signal of the first LTI system.
In step 2340, a second microphone records a second recorded audio signal after the sound
waves have been passed through the ear-cup and after ANC has been conducted.
In step 2350, a second frequency response of a second LTI system is determined based on
the test signal as an excitation signal of the second LTI system and based on the second
recorded audio signal as an output signal of the second LTI system.
In step 2360, a third frequency response of a third LTI system is determined based on the
first frequency response of the first LTI system and based on the second frequency
response of the second LTI system.
In an alternative embodiment, the first impulse response and the first frequency response of
the LT system and the second impulse response and the second frequency response of the
LTI system are not determined. Instead, the frequency response of the third LTI system is
determined based on the first recorded audio signal as an excitation signal of the third LTI
system and based on the second recorded audio signal as an output signal of the third LTI
system.
In embodiments, the third frequency response may be transformed from the frequency
domain to the time domain to obtain the impulse response of the third LTI systems.
In some embodiments, the frequency response and/or the impulse response of the third LTI
system, which reflects the effect of the ANC and of the transfer of the sound waves
through the ear-cup, is available for a residual noise characteristics estimator. In some
embodiments, a residual noise characteristics estimator may determine the frequency
response and/or the impulse response of the third LTI system.
The residual noise characteristics estimator may use the frequency response and/or the
impulse response of the third LTI system to determine a residual noise characteristic of the
environmental audio signal. For example, the residual noise characteristics estimator may
multiply a frequency-domain representation of the environmental audio signal and the
frequency response of the third LTI system to determine the residual noise characteristic.
The frequency-domain representation of the environmental audio signal may, for example,
be obtained by conducting a Fourier transform on a time-domain representation of the
environmental audio signal. In an alternative embodiment, the noise characteristics
estimator may determine a convolution of a time-domain representation of the
environmental audio signal and the impulse response of the third LTI system.
A variety of approaches for identification of non-linear systems exist, e.g. Volterra series
or Artificial Neural Networks (ANN) or Markov chains.
For example, Artificial Neural Networks (ANN) may be trained by receiving the first
recorded audio signal of Fig. 2 1 and Fig. 22 as an input signal and the second recorded
audio signal of Fig. 2 1 and Fig. 22 as an output signal.
If the ANC is implemented in feedforward structure with only one microphone for sensing
the primary noise, and since the anti-noise is known, the noise estimate can be derived
from adding the noise and the anti-noise.
The spectral envelope is derived from the time signal of noise estimate the STFT (Short-
Time Fourier Transform) or an alternative frequency transform or filter-bank. Using a
regression method for approximating the transfer path, e.g. using ANN, the noise
estimation can be implemented to directly estimate the spectral envelope preferably using
features extracted from the noise measurement, e.g. obtained from the primary noise
sensor, computed in the frequency domain.
The derived noise estimate is optionally post-processed by smoothing the trajectories of
sub-band envelope signals, e.g. smoothing along the time axis, and by smoothing the
spectral envelope, e.g. smoothing along the frequency axis.
In order not to compensate for semantically meaningful sound, e.g. speech and alarm
sounds, and intelligent signal analysis is performed. The microphone signal is divided into
the environmental noise which is compensated for and semantically meaningful sound
which are excluded from noise estimate, either by applying a source separation processing
or by detecting the presence of semantically meaningful sounds and manipulating the noise
estimate in cases of positive detections.
In the latter case, the manipulation of the noise estimate is performed such that if sounds
are detected which need to be presented to the listener the noise estimation is paused and
thereby both PNC and ANC are disabled. The noise estimate is not updated in the
microphone signals capture outside sounds which are not supposed to be compensated for.
Fig 8 illustrates a corresponding apparatus according to an embodiment. The apparatus of
the embodiment of Fig. 18 comprises an active noise cancellation unit 1810, a residual
noise characteristics estimator 1820, a perceptual noise compensation unit 1830 and a
combiner 1840, which may correspond to the active noise cancellation unit 110, the
residual noise characteristics estimator 120, the perceptual noise compensation unit 130
and the combiner 140 of the embodiment of Fig. 1, respectively. The apparatus
furthermore comprises a source separation unit 805 which is configured to detect signal
portions of the environmental audio signal which shall not be compensated. The source
separation unit 1805 is moreover configured to remove the signal portions of the
environmental audio signal which shall not be compensated from environmental audio
signal.
Fig. 9 illustrates a headphone according to an embodiment comprising an apparatus for
improving a perceived quality of sound reproduction according to the embodiment of Fig.
16. As in Fig. 2, the ear-cup 241 comprises a microphone 261 and an apparatus 251 for
improving a perceived quality of sound reproduction. Fig. 19 moreover illustrates a
loudspeaker 271 of the ear-cup 241. Reference sign 291 denotes an inner side 291 of the
ear-cup 241 . The inner side 291 of the ear-cup 241 is the side of the ear-cup that is in
contact with an ear 28 of a listener 280 wearing the headphone as illustrated in Fig. 19. In
the embodiment of Fig. 19, the microphone 261 is arranged such that the loudspeaker 271
of the ear-cup 241 is located between the microphone 261 and the inner side 291 of the
ear-cup 241. Thus, the ear-cup 241 of Fig. 19 implements the feedforward structure of Fig.
16. Likewise, the ear-cup 242 comprises another apparatus 252 for improving a perceived
quality of sound reproduction and another microphone 262 being arranged such that the
loudspeaker 272 of the ear-cup 242 is located between the microphone 262 and an inner
side 292 of the ear-cup 242. The inner side 292 of the ear-cup 242 is the side of the ear-cup
242 that is in contact with an ear 282 of a listener 280 wearing the headphone as illustrated
in Fig. 19. Thus, the ear-cup 242 of Fig. 19 also implements the feedforward structure of
Fig. 16.
Fig. 20 illustrates a headphone according to an embodiment comprising an apparatus for
improving a perceived quality of sound reproduction according to the embodiment of Fig.
17. As in Fig. 2, the ear-cup 241 comprises a microphone 261 and an apparatus 251 for
improving a perceived quality of sound reproduction. Fig. 20 moreover illustrates a
loudspeaker 271 of the ear-cup 241. Reference sign 291 denotes an inner side 291 of the
ear-cup 241. The inner side 291 of the ear-cup 241 is the side of the ear-cup that is in
contact with an ear 281 of a listener 280 wearing the headphone as illustrated in Fig. 20. In
the embodiment of Fig. 20, the microphone 261 is arranged such that the microphone 261
of the ear-cup 241 is located between the loudspeaker 271 and the inner side 291 of the
ear-cup 241. Thus, the ear-cup 241 of Fig. 20 implements the feedback structure of Fig. 17.
Likewise, the ear-cup 242 comprises another apparatus 252 for improving a perceived
quality of sound reproduction and another microphone 262 being arranged such that the
microphone 262 of the ear-cup 242 is located between the loudspeaker 272 and an inner
side 292 of the ear-cup 242. The inner side 292 of the ear-cup 242 is the side of the ear-cup
242 that is in contact with an ear 282 of a listener 280 wearing the headphone as illustrated
in Fig. 20. Thus, the ear-cup 242 of Fig. 20 also implements the feedback structure of Fig.
17.
Headphones according to other embodiments may comprise more than two microphones,
e.g., four microphones. For example, each ear-cup may comprise two microphones, one of
them being a reference microphone and the other one being an additional error
microphone, the additional error microphone being used for improving the ANC as
mentioned in Fig. 4.
Although some aspects have been described in the context of an apparatus, it is clear that
these aspects also represent a description of the corresponding method, where a block or
device corresponds to a method step or a feature of a method step. Analogously, aspects
described in the context of a method step also represent a description of a corresponding
block or item or feature of a corresponding apparatus.
The inventive decomposed signal can be stored on a digital storage medium or can be
transmitted on a transmission medium such as a wireless transmission medium or a wired
transmission medium such as the Internet.
Depending on certain implementation requirements, embodiments of the invention can be
implemented in hardware or in software. The implementation can be performed using a
digital storage medium, for example a floppy disk, a DVD, a CD, a ROM, a PROM, an
EPROM, an EEPROM or a FLASH memory, having electronically readable control
signals stored thereon, which cooperate (or are capable of cooperating) with a
programmable computer system such that the respective method is performed.
Some embodiments according to the invention comprise a non-transitory data carrier
having electronically readable control signals, which are capable of cooperating with a
programmable computer system, such that one of the methods described herein is
performed.
Generally, embodiments of the present invention can be implemented as a computer
program product with a program code, the program code being operative for performing
one of the methods when the computer program product runs on a computer. The program
code may for example be stored on a machine readable carrier.
Other embodiments comprise the computer program for performing one of the methods
described herein, stored on a machine readable carrier.
In other words, an embodiment of the inventive method is, therefore, a computer program
having a program code for performing one of the methods described herein, when the
computer program runs on a computer.
A further embodiment of the inventive methods is, therefore, a data carrier (or a digital
storage medium, or a computer-readable medium) comprising, recorded thereon, the
computer program for performing one of the methods described herein.
A further embodiment of the inventive method is, therefore, a data stream or a sequence of
signals representing the computer program for performing one of the methods described
herein. The data stream or the sequence of signals may for example be configured to be
transferred via a data communication connection, for example via the Internet.
A further embodiment comprises a processing means, for example a computer, or a
programmable logic device, configured to or adapted to perform one of the methods
described herein.
A further embodiment comprises a computer having installed thereon the computer
program for performing one of the methods described herein.
In some embodiments, a programmable logic device (for example a field programmable
gate array) may be used to perform some or all of the functionalities of the methods
described herein. In some embodiments, a field programmable gate array may cooperate
with a microprocessor in order to perform one of the methods described herein. Generally,
the methods are preferably performed by any hardware apparatus.
The above described embodiments are merely illustrative for the principles of the present
invention. It is understood that modifications and variations of the arrangements and the
details described herein will be apparent to others skilled in the art. It is the intent,
therefore, to be limited only by the scope of the impending patent claims and not by the
specific details presented by way of description and explanation of the embodiments
herein.
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Claims
An apparatus for improving a perceived quality of sound reproduction of an audio
output signal, comprising:
an active noise cancellation unit ( 110; 1410; 1510; 1610; 1710; 1810) for
generating a noise cancellation signal based on an environmental audio signal,
wherein the environmental audio signal comprises noise signal portions, the noise
signal portions resulting from recording environmental noise,
a residual noise characteristics estimator (120; 1420; 1520; 1620; 1720; 1820) for
determining a residual noise characteristic depending on the environmental noise
and the noise cancellation signal,
a perceptual noise compensation unit (130; 1430; 1530; 1630; 1730; 1830) for
generating a noise-compensated signal based on an audio target signal and based on
the residual noise characteristic, and
a combiner (140; 1440; 1540; 1640; 1740; 1840) for combining the noise
cancellation signal and the noise-compensated signal to obtain the audio output
signal.
An apparatus according to claim 1, wherein the residual noise characteristics
estimator (120; 1420; 1520; 1620; 1720; 1820) is configured to determine the
residual noise characteristic such that the residual noise characteristic indicates a
characteristic of noise portions of the environmental noise that would remain when
only reproducing the noise cancellation signal.
An apparatus according to claim 1 or 2,
wherem the residual noise characteristics estimator (120; 1420; 1620; 1820) is
arranged to receive the environmental audio signal,
wherein the residual noise characteristics estimator (120; 1420; 1620; 1820) is
arranged to receive information on the noise cancellation signal from the active
noise cancellation unit ( 1 10; 1410; 1610; 1810), and
wherein the residual noise characteristics estimator (120; 1420; 1620; 1820) is
configured to determine as the residual noise characteristic a remaining noise
estimate based on the environmental audio signal and based on the information on
the noise cancellation signal.
4. An apparatus according to claim 3,
wherein the residual noise characteristics estimator (120; 1420; 1620; 1820) is
arranged to receive the noise cancellation signal as the information on the noise
cancellation signal from the active noise cancellation unit ( 10; 1410; 1610; 1810),
and
wherein the residual noise characteristics estimator (120; 1420; 1620; 1820) is
configured to determine the remaining noise estimate based on the environmental
audio signal and based on the noise cancellation signal.
5. An apparatus according to claim 4, wherein the residual noise characteristics
estimator (120; 1420; 1620; 1820) is configured to determine the remaining noise
estimate by adding the environmental audio signal and the noise cancellation signal.
6. An apparatus according to one of claims 3 to 5,
wherein the apparatus furthermore comprises at least one loudspeaker (1450; 1650)
and at least one microphone (1405; 1605),
wherein the microphone (1405; 1605) is configured to record the environmental
audio signal,
wherein the loudspeaker (1450; 1650) is configured to output the audio output
signal, and
wherein the microphone (1405; 1605) and the loudspeaker (1450; 1650) are
arranged to implement a feedback structure.
7. An apparatus according to claim 1 or 2,
wherein the residual noise characteristics estimator (120; 1520; 1720; 1820) is
arranged to receive the environmental audio signal,
wherein the residual noise characteristics estimator (120; 1520; 1720; 1820) is
arranged to receive information on the noise-compensated signal from the
perceptual noise compensation unit (130; 1530; 1730; 1830), and
wherein the residual noise characteristics estimator (120; 1520; 1720; 1820) is
configured to determine as the residual noise characteristic a remaining noise
estimate based on the environmental audio signal and based on the noisecompensated
signal.
8. An apparatus according to claim 7,
wherein the residual noise characteristics estimator (120; 1520; 1720; 1820) is
arranged to receive the noise-compensated signal as the information on the noisecompensated
signal from the perceptual noise compensation unit (130; 1530; 1730;
1830), and
wherein the residual noise characteristics estimator (120; 1520; 1720; 1820) is
configured to determine the remaining noise estimate based on the environmental
audio signal and based on the noise-compensated signal.
9. An apparatus according to claim 8, wherein the residual noise characteristics
estimator (120; 1520; 1720; 1820) is configured to determine the remaining noise
estimate by subtracting scaled components of the noise-compensated signal from
the environmental audio signal.
10. An apparatus according to one of claims 7 to 9,
wherein the apparatus furthermore comprises at least one loudspeaker (1550; 1750)
and at least one microphone ( 05; 1 05),
wherein the microphone (1505; 1705) is configured to record the environmental
audio signal,
wherein the loudspeaker (1550; 1750) is configured to output the audio output
signal, and
wherein the microphone (1505; 1705) and the loudspeaker (1550; 1750) are
arranged to implement a feedback structure.
. An apparatus according to one of the preceding claims, wherein the apparatus
furthermore comprises a source separation unit (1805) for detecting signal portions
of the environmental audio signal which shall not be compensated.
12. An apparatus according to claim 1 , wherein the source separation unit (1805) is
configured to remove the signal portions of the environmental audio signal which
shall not be compensated from environmental audio signal.
13. A headphone comprising two ear-cups (241, 242), wherein each of the ear-cups
(241, 242) comprises:
an apparatus (251, 252) for improving a perceived quality of sound reproduction
according to one of the preceding claims,
a loudspeaker (271, 272), and
at least one microphone (261, 262) for recording the environmental audio signal.
14. A headphone according to claim 13, wherein each of the loudspeakers (271, 272) of
the ear-cups (241, 242) is arranged between one of the microphones (261, 262) of
one of the ear-cups (241, 242) and an inner side (291, 292) of said ear-cup (241,
242).
15. A headphone according to claim 13, wherein each of the microphones (261 , 262) of
the ear-cups (241, 242) is arranged between one of the loudspeakers (271 , 272) of
one of the ear-cups (241, 242) and an inner side (291, 292) of said ear-cup (241,
242).
16. A method for improving a perceived quality of sound reproduction of an audio
output signal, comprising:
generating a noise cancellation signal based on an environmental audio signal,
wherein the environmental audio signal comprises noise signal portions, the noise
signal portions resulting from recording environmental noise,
determining a residual noise characteristic depending on the environmental noise
and the noise cancellation signal,
generating a noise-compensated signal based on an audio target signal and based on
the residual noise characteristic, and
combining the noise cancellation signal and the noise-compensated signal to obtain
the audio output signal.
A computer program for implementing the method of claim 16 when being
executed on a computer or signal processor.