Patent Description:
In conventional headphones with active noise cancellation, a microphone inside the headphone detects external environmental noise which is then processed to generate an inverted signal that cancels the environmental noise in the audio signal generated for the headphone wearer. The measured noise signal is used to generate a feedback signal that is processed through an amplifier to adjust the level and then inverted and applied to a speaker of the headphone for cancellation of the noise signal. Filtering is applied to preserve the intended audio signal. Most active noise cancellation techniques employed today are analogue with variations in implementation schemes, filters, and the placing of the microphone and speaker.

More recently digital noise cancellation techniques have been developed. Conventional digital noise cancellation techniques are primarily based on sub-band filtering and generation of the main frequency tones and their harmonics to cancel a large portion of the environmental noise. These techniques provide reasonably effective noise cancellation for a large part of the typical noise to which users are subject to in practice. However, existing noise cancellation techniques have major limitations on the audio signal bandwidth that they can handle, the quality of the intended audio signal to be played for the user, and the reduction level of the noise, whereby the best-in-class products typically cannot reduce the noise level more than 10dB.

It would be desirable to improve the performance of active noise cancellation without compromise in the audio quality of the intended sound generated for the user.

<CIT> and <CIT> describe an active noise cancellation system with a linear prediction filter in order to remove noise from a sound signal picked up by microphones.

<CIT> describes a system for location selective processing of an acoustic signal based on capturing the signal with a least two microphones.

<CIT> describes an adaptive prediction filter for reducing the impact of tonal components in a target input signal on the quality of acoustic feedback.

<CIT> describes a method and apparatus for active noise cancellation in audio listening systems without use of a dedicated microphone to capture the noise heard by the user.

<CIT> describes a headphone having a microphone for active noise cancellation of ambient noise.

<NPL>, Graz discloses various feedforward and feedback active noise cancellation (ANC) systems for headphones, including an adaptive feedforward ANC system with an additional microphone in front of the loudspeaker for measuring an error signal, and a feedback ANC system with a prediction filter.

In view of the foregoing, it is an object of the present invention to provide a headphone with an active noise cancellation system that is effective in cancelling environmental noise and that has minimal or no effect on the quality of the audio signal intended for the wearer.

It is advantageous to provide an active noise cancellation system that is easy to implement and that is cost effective.

Objects of this invention have been achieved by providing the headphone according to claim <NUM> and the method of generating a headphone audio signal according to claim <NUM>.

The term "headphone" as used in the present description and claims is intended to encompass any electrically powered mobile sound reproducing device that is worn by a person over, close to, or in a person's ear or pair of ears. For instance one earphone or a pair of earphones are understood herein as falling within the meaning of the term "headphone".

Disclosed herein is a headphone comprising a casing, a feedfoward active noise cancellation system, a microphone arranged to sense environmental noise, and a speaker system, the speaker system mounted in the casing, the active noise cancellation system comprising an active noise cancellation circuit connected to a microphone arranged to sense environmental noise, the active noise cancellation circuit comprising:.

wherein the predicted plurality (D) of inverted digital environmental noise samples has a prediction depth time TPD corresponding to a total latency of the active noise cancellation circuit including the ADC and the DAC.

In an embodiment, the active noise cancellation circuit comprises a casing frequency response filter arranged in the digital signal path before or after the prediction filter, to compensate the effects of location of the microphone on the sensed environmental noise.

In an embodiment, the active noise cancellation circuit comprises a summing circuit arranged to add an audio signal intended for playing to a user to the digital or analog inverted environmental noise signal.

In an embodiment, the active noise cancellation circuit comprises an amplifier to adjust the gain of the summed audio and inverted environmental noise signals.

In an embodiment, each of the ADC and DAC has a total latency of less than µs.

In an embodiment, the microphone and the active noise cancellation circuit are mounted in the casing.

Also disclosed herein is a method of generating a headphone audio signal comprising at least an inverted environmental audio noise signal generated by feedforward active noise cancellation, wherein the method includes the steps:.

In an embodiment, the ADC has a total latency of less than <NUM>.

In an embodiment, an analog audio signal including the active noise cancellation signal is fed to a speaker system, to play to the user the intended audio signal and at the same time cancelling environment noise.

In an embodiment, the prediction filter is configured to predict said plurality D of future samples of environmental noise signal, so that said plurality divided by said clock frequency D/fs is substantially equal to the prediction time depth TPD.

In an embodiment, the multiple N is in a range of <NUM> to <NUM>.

In an embodiment, the number of the predicted noise samples in the anticipated future noise signal is advantageously equal to TPD*fs, in which TPD is the total latency of the active noise cancellation system and fs is the clock frequency of the ADC.

In an embodiment, the total latency TPD of the active noise cancellation system is in a range of <NUM> to <NUM>.

In an embodiment, the clock frequency fS of the ADC is higher than <NUM>, for instance in a range from <NUM> to <NUM>.

Further objects and advantageous features of the invention will be apparent from the claims and the following detailed description of embodiments of the invention in relation to the annexed drawings in which:.

Referring to the figures, a headphone <NUM> according to embodiments of the invention that is configured to be worn against, in, or close to a person's ear, comprises a casing <NUM>, an active noise cancellation system <NUM> mounted in the casing <NUM>, and a speaker system <NUM> mounted in the casing <NUM>. The speaker system <NUM> may comprise various sound transducers to reproduce sound from an audio signal supplied to the transducer as is per se well known in the art.

The casing <NUM> comprises an outer side 4a corresponding to an external environmental noise receiving side, an ear side 4c configured to direct the sound produced by the speaker system <NUM> towards the person's ear, and an inner portion 4b housing components of the the speaker system <NUM>. Components of the active noise cancellation system are preferably also mounted inside the casing <NUM>, however in variants, components of the active noise cancellation system may also be mounted in part or wholly outside of the casing <NUM> that houses the speaker system, for instance in a separate housing such as in a head strap joining two headphone devices or in a cabled control connected to the headphone device.

The active noise cancellation system <NUM> comprises a microphone <NUM> and an active noise cancellation circuit <NUM>. In a preferred embodiment, the microphone may be positioned proximate the outer side 4a of the casing configured to capture external (environmental) noise that is to cancelled. In variants however, the microphone may also be positioned within the casing at various positions or outside the casing in a separate support such as a headphone head band.

In an embodiment, the active noise cancellation circuit includes an Analog-to-Digital Converter (ADC) <NUM>, a prediction filter <NUM>, a Digital-to-Analog converter (DAC) <NUM>, a clock <NUM>, and an amplifier circuit <NUM> connected to the speaker system <NUM>. The active noise cancellation system may further include a casing frequency response filter circuit <NUM>.

The prediction filter <NUM> comprises a digital prediction filter circuit <NUM> and a prediction filter coefficients training algorithm <NUM>.

Referring to <FIG> and <FIG>, exemplary embodiments of an active noise cancellation system of a headphone according to the invention are illustrated schematically. The active noise cancellation system incorporates the microphone <NUM>, analog-to-digital converter <NUM>, prediction filter training algorithm <NUM> for extracting the optimal coefficients for the prediction filter, a prediction filter <NUM> for predicting a plurality D of inverted noise samples of the anticipated environmental noise E. , a digital summing circuit <NUM>, a digital-to-analog converter <NUM>, an amplifier <NUM> to adjust the noise levels, and a speaker system <NUM> to play the audio and inverted noise signals. In an advantageous embodiment, the plurality of inverted noise samples D may advantageously be in the range of <NUM> to <NUM> samples depending on the sampling frequency. This range of samples facilitates prediction of future environmental noise for a duration of the predicted future environmental noise samples of up to about <NUM> for instance.

The environmental noise E. received by the microphone <NUM> is converted by the transducer of the microphone into an electrical signal that is fed into the Analog-to-Digital Converter (ADC) <NUM> that converts the analog signal of the environmental noise to a digital signal. It may be noted that the microphone location can be in different positions in or on the headphone, or separate from the headphone, whereby the signal generated by the microphone can be adjusted for its specific location by a transfer function applied by a filter system of the headphone. In other words, the position dependent variation of the microphone transducer output signal can be compensated by a filter system that acts as a transfer function on the microphone output signal. The microphone filter can be applied on the analog signal before the ADC <NUM> or on the digital signal after the ADC <NUM>.

The Analog-to-Digital converter (ADC) <NUM> is per se known, but is preferably configured or selected among ADCs having a conversion cycle of less than <NUM> of total latency and preferably a resolution of <NUM> bits or more.

The digital signal of the E. is fed into a prediction filter <NUM> that stores and executes a training algorithm to extract coefficients of the prediction filter circuit <NUM>. Various generic training algorithms used in various generic prediction filters such as Recursive Least Squares (RLS) filters or Kalman filters can be used for this purpose. Because of the typical natural changes of environment noise in most environments in which users are located, the coefficients of prediction filter may be configured to be updated at discrete time intervals TU of up to <NUM> seconds or less, where the time interval TU is preferably smaller than <NUM> second.

In a non-limiting example, a prediction filter coefficients training program may comprise a general NLMS (Normalized Least Mean Square) algorithm receiving a digital input signal of microphone and an expected output signal of the prediction filter <NUM>, the expected output signal comprising predicted samples of digital signal. The prediction filter may for instance be a Finite Impulse Response (FIR) filter. The coefficients of the Finite Impulse Response (FIR) filter for the prediction are then generated by the coefficients training algorithm. Typically, <NUM> coefficients will be enough for proper prediction. These filter coefficients will be used in prediction filter circuit <NUM>. The prediction filter circuit <NUM> and prediction filter coefficients training program <NUM> may for instance be implemented and executed in a field-programmable gate array (FPGA) (e.g. Artix <NUM> Series of Xilinx) to meet the speed and latency requirements of the system.

The digital noise samples from the ADC, along with the prediction filter coefficients are fed into the prediction filter circuit <NUM>. The prediction filter circuit may for instance be based on a Finite Impulse Response (FIR) or Infinite Impulse Response (IIR) general schemes of prediction filters. In embodiments of the invention, the prediction filter is operated at multiple N times higher clock frequency (N×fs) than the clock frequency (fs) of the ADC <NUM> and DAC <NUM>, because it needs to generate D samples in the future in one clock time (<NUM>/fs). Multiple N is preferably greater than <NUM>, for instance in a range of <NUM> to <NUM>. The number of the predicted noise samples in the anticipated future noise signal may advantageously be equal to TPD*fs, in which TPD is the total latency of the active noise cancellation system (as depicted in <FIG> and <FIG>). Depending on the total delays of all the modules <NUM>, <NUM>, <NUM>, <NUM>, <NUM> in the digital path, the total latency TPD is preferably in the range of <NUM> to <NUM>. For optimal implementation of a high performance system, the clock frequency fS is preferably higher than <NUM>, for instance in a range of values from <NUM> to <NUM>.

Digital noise samples and the predicted noise samples may in addition be processed through a casing frequency response filter <NUM>. Casing frequency response filter <NUM> compensates the effects of the headphone casing <NUM> on environmental noise signals with respect to the location of microphone <NUM>. The casing frequency response filter allows the microphone to be installed anywhere in the headphone or even the noise environment, and can be calibrated to compensate the difference between the noise signal received by microphone and the noise signal received by the listener's ear using this casing frequency response filter. In embodiments where the microphone is installed inside the headphone such that it receives essentially the same acoustic signal that is generated for the listener's ear, the casing frequency response filter may have a transfer function set to <NUM>, corresponding to no filtering effect or to -<NUM> corresponding to no filter effect but with an inverted signal.

In the embodiment of <FIG>, the casing frequency response filter outputs the final predicted sample of inverted noise for cancelling the environmental noise from the sound directed to the listener's ear. The inversion of the digital signal for the purpose of cancelling the environmental noise may be performed by the casing frequency response filter.

In a variant as illustrated in <FIG>, the casing frequency response filter <NUM> may be positioned before the prediction filter <NUM>, whereby the prediction filter circuit <NUM> outputs the final predicted sample of inverted noise for cancelling the environmental noise from the sound directed to the listener's ear.

The final predicted sample of noise is added by a summing circuit <NUM> to the user audio signal sample (e.g. music, speech) received from an audio signal source <NUM>. The output of the summing circuit <NUM> is processed by the Digital-to-Analog Converter (DAC) <NUM> working at fs clock frequency into an analog signal. The output of DAC is an analog inverted noise plus user audio signal. The DAC <NUM>, which is per se known, is preferably configured or selected among DAC's having a total conversion latency of less than <NUM>.

The analog inverted noise plus user audio signal may be fed to an amplifier with a fixed gain for adjusting the gain of the analog signal to the speaker system, and the amplified signal may be played through the speaker system <NUM>. The volume control of the audio signal is controlled by a volume control of the audio signal source before adding to the inverted environmental noise signal since the amplitude of the environmental noise to be cancelled is independent of the amplitude of the audio signal played to the user.

The acoustic audio signal of speaker system cancels the instantaneous environmental noise and only the user audio signal from the audio signal source <NUM> will be heard by the user.

In a variant (not shown), the summing circuit may be an analog summing circuit provided after the DAC arranged to add an analog audio signal to the analog inverted environmental signal output by the DAC.

A method of generating a headphone audio signal according to embodiments of the invention may include the following steps.

The analog audio signal may then be amplified by an amplifier to generate an amplified audio signal that is fed to a speaker system, to play to the user the intended audio signal and at the same time cancelling environment noise. It may be noted that the volume control of the audio signal is controlled by a volume control of the audio signal source before adding to the inverted environmental noise signal since the amplitude of the environmental noise to be cancelled is independent of the amplitude of the audio signal played to the user.

The total latency of the digital circuitry including the ADC and the DAC corresponds to a prediction depth time TPD. The prediction filter is configured to predict D samples in the future, so that D/fs becomes equal to TPD, which allows for the best possible reduction of the environmental noise.

The method may further include processing the digital environmental noise signal through a casing frequency response filter circuit to adjust for the location of the microphone.

Claim 1:
A headphone comprising a casing (<NUM>), a feedforward active noise cancellation system, a microphone (<NUM>) arranged to sense environmental noise, and a speaker system (<NUM>), the speaker system mounted in the casing, the active noise cancellation system (<NUM>) comprising an active noise cancellation circuit connected to the microphone (<NUM>) and to the speaker system, the active noise cancellation circuit comprising
- an analog-to-digital converter (ADC) (<NUM>) working at a clock frequency (fs) and arranged to convert the sensed environmental noise to a digital environmental noise signal,
- a prediction filter (<NUM>) working at a multiple N times higher clock frequency (N × fs) than the clock frequency (fs) of the ADC (<NUM>) and configured for predicting a plurality D of inverted digital environmental noise samples and generating a digital inverted environmental noise signal, wherein the prediction filter (<NUM>) is configured for running a prediction filter training algorithm on the digital audio noise signal to extract prediction filter coefficients and updating the prediction filter coefficients into the prediction filter;
- a digital-to-analog converter (DAC) (<NUM>) to convert the digital inverted environmental noise signal to an analog inverted environmental noise signal for cancelling the environmental noise,
wherein the predicted plurality (D) of inverted digital environmental noise samples has a prediction depth time TPD corresponding to a total latency of the active noise cancellation circuit including the ADC and the DAC.