EP2437521A1 - Method for frequency compression with harmonic adjustment and corresponding device - Google Patents

Abstract

The method involves providing the audio signal into several frequency channels (31,32). The frequency associated with the fundamental frequency, in the frequency channel (32) is estimated as harmonic (20,30). The harmonics of the audio signal is moved or mapped from a frequency channel (31) in frequency channel (32). An independent claim is included for device for frequency compression of audio signal.

Description

The present invention relates to a method of frequency-compressing an audio signal having a fundamental frequency and at least one harmonic by providing the audio signal in a plurality of frequency channels and shifting or mapping the harmonics of the audio signal from a first frequency channel of the plurality of frequency channels to a second frequency channel of the plurality of frequency channels. Moreover, the present invention relates to a corresponding device for frequency compression. In particular, such a device can be used in a listening device. A hearing device is understood here to be any sound-emitting device that can be worn in or on the ear, in particular a hearing device, a headset, headphones and the like.

Hearing aids are portable hearing aids that are used to care for the hearing impaired. In order to meet the numerous individual needs, different types of hearing aids such as behind-the-ear hearing aids (BTE), hearing aid with external receiver (RIC: receiver in the canal) and in-the-ear hearing aids (ITE), e.g. Concha hearing aids or canal hearing aids (ITE, CIC). The hearing aids listed by way of example are worn on the outer ear or in the ear canal. In addition, bone conduction hearing aids, implantable or vibrotactile hearing aids are also available on the market. The stimulation of the damaged hearing takes place either mechanically or electrically.

Hearing aids have in principle as essential components an input transducer, an amplifier and an output transducer. The input transducer is usually a sound receiver, z. As a microphone, and / or an electromagnetic receiver, for. B. an induction coil. The output transducer is usually used as an electroacoustic transducer, z. B. miniature speakers, or as an electromechanical transducer, z. B. bone conduction, realized. The amplifier is usually integrated in a signal processing unit. This basic structure is in FIG. 1 shown using the example of a behind-the-ear hearing aid. In a hearing aid housing 1 for carrying behind the ear, one or more microphones 2 for receiving the sound from the environment are installed. A signal processing unit 3, which is also integrated in the hearing aid housing 1, processes the microphone signals and amplifies them. The output signal of the signal processing unit 3 is transmitted to a loudspeaker or earpiece 4, which outputs an acoustic signal. The sound is optionally transmitted via a sound tube, which is fixed with an earmold in the ear canal, to the eardrum of the device carrier. The power supply of the hearing device and in particular the signal processing unit 3 is effected by a likewise integrated into the hearing aid housing 1 battery. 5

Many hearing losses can be compensated by a frequency-dependent amplification in combination with a dynamic compression. However, there are also hearing losses where amplification has no effect or is detrimental. An example of this is hearing loss with so-called "dead regions". Dead regions are frequency ranges in which spectral components can no longer be audibly amplified.

One possible technique to deal with the above problem is frequency compression. In this case, beam components spectral components from a source frequency range, which is typically at higher frequencies and in which no amplification is to be applied (eg dead region), are pushed into a lower-lying target frequency range. Audibility is generally guaranteed in this target frequency range, which is why amplification can be used.

There are known hearing aids that support such frequency compression. In the compression method, for example, the properties of a filter bank for a easy implementation used. Individual channels are selectively copied to other channels, depending on their instantaneous power, for example, so that the frequency components contained in these channels reappear on the output in a different frequency range. Where the channels are shown determines a mapping rule, which is adjustable, so that different compression ratios can be realized.

FIG. 2 shows the principle of frequency compression by simply copying channels as it is already used for hearing aids. For example, the channel 14 '(indicated by its center frequency 14) is copied to the channel 11' (indicated by its center frequency 11). In the channel 14 'there is a tone 14 "(eg a harmonic) which is shifted to the tone 11" in the target channel 11'. The pitch of the sound 14 "to the center frequency 14 is identical to the pitch of the sound 11" to the center frequency 11.

This simple mapping rule brings with harmonic signals problems. Harmonic signals occur, for. For example, in voiced sounds in the language, such as vowels on. In this case, the uncompressed spectrum has a line-like structure, with spectral lines occurring at the speech fundamental frequency and at their integer multiples. The raster of the harmonic signals (line structure) is not considered in the simple mapping rule according to the prior art and therefore destroyed, d. H. the spectral lines are no longer guaranteed to be at an integer multiple of the speech fundamental frequency. This manifests itself in clearly perceptible artifacts (signal components occurring at integer multiples of the fundamental frequency are called "harmonics" for short).

• Bereitstellen des Audiosignals in mehreren Frequenzkanälen
  und
• Verschieben oder Abbilden der Harmonischen des Audiosignals von einem ersten Frequenzkanal der mehreren Frequenzkanäle in einem zweiten Frequenzkanal der mehreren Frequenzkanäle, sowie
• Schätzen einer ersten Frequenz, die zu der Grundfrequenz ebenfalls harmonisch ist, in dem zweiten Frequenzkanal, wobei
• die Harmonische auf die geschätzte erste Frequenz verschoben oder abgebildet wird.

The object of the present invention is thus to further reduce artifacts in frequency compression. According to the invention this object is achieved by a method for frequency compression of an audio signal having a fundamental frequency and at least one harmonic, by

• Providing the audio signal in multiple frequency channels
  and
• Shifting or mapping the harmonics of the audio signal from a first frequency channel of the plurality of frequency channels in a second frequency channel of the plurality of frequency channels, as well
• Estimating a first frequency which is also harmonic to the fundamental frequency in the second frequency channel, wherein
• the harmonic is shifted or mapped to the estimated first frequency.

• einer Signalverarbeitungseinrichtung zum Bereitstellen des Audiosignals in mehreren Frequenzkanälen und
• einer Verschiebeeinrichtung zum Verschieben oder Abbilden der Harmonischen des Audiosignals von einem ersten Frequenzkanal der mehreren Frequenzkanäle in einen zweiten Frequenzkanal der mehreren Frequenzkanäle, sowie mit
• einer Schätzeinrichtung zum Schätzen einer ersten Frequenz, die zu der Grundfrequenz ebenfalls harmonisch ist, in dem zweiten Frequenzkanal, wobei
• die Harmonische durch die Verschiebeeinrichtung auf die geschätzte erste Frequenz verschoben oder abgebildet wird.

In addition, the invention provides a device for frequency compression of an audio signal having a fundamental frequency and at least one harmonic

• a signal processing device for providing the audio signal in a plurality of frequency channels and
• a shifting device for shifting or mapping the harmonics of the audio signal from a first frequency channel of the plurality of frequency channels into a second frequency channel of the plurality of frequency channels, and with
• an estimator for estimating a first frequency, which is also harmonic to the fundamental frequency, in the second frequency channel, wherein
• the harmonic is shifted or imaged by the shifter to the estimated first frequency.

Advantageously, a harmonic correction occurs during or after the shifting or mapping of the harmonics into another frequency channel. This means that the harmonic is set to a frequency position, which is also an integer multiple of the fundamental frequency. Thus, the harmonic represents a harmonic even after moving. This significantly reduces the artifacts.

In one embodiment, the first frequency channel is completely shifted into the second frequency channel. This allows, for example, a frequency channel from a dead region in an audible range of a hearing aid wearer move. If there is a harmonic in the first frequency channel, then it is completely shifted with the frequency channel. Their distance from the center frequency of the channel remains unchanged.

One of the harmonics associated with the frequency channel may be estimated, and the shifted harmonic may then be further shifted to the first frequency in the second frequency channel. This means that the move takes place in two steps. First, the entire frequency channel is shifted and then within the frequency channel, the original harmonic is pushed back to a harmonic frequency position.

The further shifting to the first frequency in the second shift step can be effected, for example, by amplitude modulation. This can be realized in the time domain by a simple multiplication by a factor exp (j · ω · t).

Preferably, the harmonic in the first frequency channel represents a dominant frequency. Thus, its position can be estimated relatively accurately before and after the shift.

In an alternative embodiment, the harmonic is mapped to the estimated first frequency by obtaining a signal synthetically generated in the second frequency channel, the amplitude of the harmonics in the first frequency channel, and the estimated frequency of the second frequency channel. It is therefore not necessary here to make a second shift step, for example by amplitude modulation, since a synthetic signal is used at the appropriate, harmonic point. However, this has the disadvantage that under some circumstances phase information is lost.

The inventive device for frequency compression has a signal processing device, which preferably has a polyphase filter bank. This makes it possible to generate only positive frequency components in the channels.

The device according to the invention is particularly advantageously used in a listening device and in particular in a hearing aid. Thus, a frequency compression in hearing aid users can be realized with fewer artifacts.

FIG 1

den prinzipiellen Aufbau eines Hörgeräts gemäß dem Stand der Technik;

FIG 2

das Prinzip der Frequenzkompression durch einfaches Kopieren von Kanälen gemäß dem Stand der Technik;

FIG 3

eine Kompression gemäß dem Stand der Technik;

FIG 4

eine Kompression gemäß der vorliegenden Erfindung; und

FIG 5

einen Ausschnitt eines unkomprimierten Spektrums und einen Ausschnitt eines komprimierten Spektrums.

The present invention will be further explained with reference to the accompanying drawings, in which:

FIG. 1

the basic structure of a hearing aid according to the prior art;

FIG. 2

the principle of frequency compression by simply copying channels according to the prior art;

FIG. 3

a compression according to the prior art;

FIG. 4

a compression according to the present invention; and

FIG. 5

a section of an uncompressed spectrum and a section of a compressed spectrum.

The embodiments described in more detail below represent preferred embodiments of the present invention.

For a better understanding of the invention, however, is first based on FIG. 3 a frequency compression according to the prior art shown in detail. Thus, frequencies are compressed according to a frequency mapping curve (eg, SPINC, BARK, ...). Starting basis is, for example, a line spectrum, as in FIG. 3 is shown above. The amplitude response a is plotted against the frequency f. The line spectrum has numerous harmonics 20 which form the spectral fine structure of the harmonic signal. The amplitudes of the harmonics 20 can be controlled by a spectral envelope 21 connect. The distance f ₀ between two harmonics 20 corresponds to the fundamental frequency in the entire spectral range. The spectrum should now be compressed above a frequency f _c . Compression is channel-wise by copying selected channels of the original spectrum to lower-level channels. However, the channels usually have a different bandwidth than the distance f ₀ between the harmonics. As a result, the harmonics 20 land when shifting to frequency positions outside the in FIG. 3 above shown line grid. FIG. 3 below shows such a compressed spectrum. The distances f ₁ , f ₂ between the individual lines 22, which represent the shifted harmonics, are no longer constant and in particular not equal to f ₀ . Although the envelope 23 of the compressed spectrum shows in the compressed region the shifted formands 24 and 25 as they originate from the original spectrum, the spacing of the lines 22 is uneven, thus destroying the spectral fine structure and thus the structure of the harmonic signal. Corresponding artifacts are the result.

A clear improvement, in particular for speech signals, can be achieved if, in addition to the simple mapping rule according to the prior art, a harmonic correction is performed, which is based on FIG. 4 is explained in more detail. In the upper part of the figure is again the original spectrum with its harmonics 20 and the envelope 21 as in FIG. 3 shown above. The spacing of the individual harmonics 20 corresponds to the fundamental frequency f ₀ in the entire original spectrum.

In FIG. 4 Below, the goal sought by the invention is exemplified. The spectrum is compressed above the cutoff frequency f _c . The envelope 23 of the compressed spectrum has the same shape as that of FIG FIG. 3 below. Ie. also the formands 24 and 25 can be seen in the compressed area. The lines 26 of the spectrum in the compressed area above f _c have each other the same distance f ₀ as the lines or harmonics 20 in the uncompressed area. This means that the fine structure of the spectrum of the harmonic signal is unaffected by the compression. Accordingly, there are fewer artifacts.

For the purpose of frequency compression with harmonic correction, first the frequency structure of the harmonic raster of the uncompressed signal is estimated, ie the positions of the harmonics in the frequency domain are determined. Based on FIG. 5 this is explained in more detail above, which again shows a section of an uncompressed spectrum and below the section of a compressed spectrum. The section of the displayed spectrum here has a line or harmonic 30. This lies in a frequency channel 31, which in turn has a center frequency f ₃₁ . Below the first frequency channel 31 is a second frequency channel 32, which has the center frequency f ₃₂ . For compression, the first frequency channel 31 is now shifted to the second frequency channel 32, copied or mapped. This represents a first step 33 of the frequency compression. This step 33 corresponds to the compression according to the prior art of FIG. 3 , Accordingly, the harmonic 30 of the first frequency channel 31 is pushed onto the line 34, which is assigned a frequency f ₃₄ (also referred to as second frequency hereinafter). The distance Δf between the frequencies f ₃₁ and f ₃₀ is identical to the distance between the frequencies f ₃₂ and f ₃₄ . However, the frequency f ₃₄ does not correspond to a harmonic of the fundamental frequency. Rather, would be at the frequency position f ₃₅ in the second frequency channel 32 is a harmonic. This can be determined, for example, by a first frequency estimate in the target frequency range, ie in the second frequency channel 32, to which the first frequency channel 31 is imaged or shifted. The line 34 must therefore be shifted to the frequency f _{35 in} order to obtain the fine structure of the harmonic signal. For this purpose, the frequency structure of the still uncorrected compressed spectral components is estimated in a second estimation. In the simplified example of FIG. 5 , in which only one channel is shifted, so the frequency f _{34 of} the line 34 is estimated or determined after the shift in the first step 33. From the two frequency estimates, the frequency offset, ie the distance between the frequencies f ₃₄ and f ₃₅ can be determined. The offset is compensated by means of a modulation in a second step 36, whereby the harmonic raster is restored. The line 34 is pushed to the frequency f ₃₅ , resulting in the line 35 results.

The modulation can be achieved for example on the basis of the analytical signal by multiplication with a suitable complex rotational factor. Thus, the shift by an angular frequency ω1 corresponds to a multiplication by the factor exp (j · ω1 · t). The resulting modulation corresponds to an amplitude modulation.

Advantageously, this method can be used in a polyphase filter bank that generates only the complex-valued, analytical signal (only positive frequency component of a Fourier transformation) in the channels. In this case, by modulating with the modulation term exp (j * ω1 * t), each channel can be cyclically modulated so that the frequency components in it are correspondingly cyclically shifted by the angular frequency ω1.

1. 1) Es existiert eine dominante Frequenz, die gut geschätzt werden kann, d. h. es existiert ein starker tonaler Anteil in diesem Kanal. Damit kann eine gute Korrektur des harmonischen Rasters erreicht werden.
2. 2) Es existiert keine dominante Frequenz, d. h. das Signal in dem Kanal ist rauschartig. Die Frequenzschätzung führt zu einer mehr oder weniger zufälligen Momentanfrequenz. Dies wiederum führt bei der Abbildung auf eine Zielfrequenz zu einer Phasenrandomisierung bzw. zufälligen Modulation in dem Kanal, was bei rauschartigen Kanälen kaum Einfluss auf den Höreindruck bewirkt.

Basically, two cases have to be distinguished when estimating the (dominant) frequency:

1. 1) There is a dominant frequency that can be well estimated, ie there is a strong tonal component in this channel. Thus, a good correction of the harmonic grid can be achieved.
2. 2) There is no dominant frequency, ie the signal in the channel is noisy. The frequency estimate leads to a more or less random instantaneous frequency. This in turn leads to a target frequency in the mapping to a phase randomization or random modulation in the channel, which causes little impact on the listening experience in noise-like channels.

The above-described embodiment is based on actually shifting the harmonic 30 as a signal component of the audio signal. According to an alternative embodiment, the compressed spectral components are generated semi-synthetically. The information about the frequency position of the semi-synthetically generated spectral components is obtained from the estimate of the uncompressed harmonic structure, ie the frequency 35 is determined as in the example above. At the frequency f ₃₅ , however, a synthetic signal is now generated. The amplitude of this synthetic signal is adjusted to correspond to the amplitude of the original harmonic 30, ie the associated amplitude is obtained from the source spectrum. This also allows a frequency compression can be achieved in which the harmonic grid is maintained.

The mapping rule of source frequency to target frequency for frequency compression is performed in audiology in a known manner. The harmonic correction or compliance with the harmonic structure of the compressed spectral components is then achieved according to the invention. Thus, the artefacts of the simple mapping rule according to the prior art are massively reduced.

Claims (9)

A method of frequency compression of an audio signal having a fundamental frequency and at least one harmonic (20, 30) - Providing the audio signal in a plurality of frequency channels (31, 32) and Shifting or mapping the harmonics (20, 30) of the audio signal from a first frequency channel (31) of the plurality of frequency channels in a second frequency channel (32) of the plurality of frequency channels,
marked by - estimating a first frequency (f ₃₅ ), which is also harmonic to the fundamental frequency, in the second frequency channel (32), wherein - the harmonic (20, 30) is shifted or mapped to the estimated first frequency (f ₃₅ ). The method of claim 1, wherein the first frequency channel (31) is completely shifted into the second frequency channel (32). The method of claim 2, wherein a second frequency (f ₃₄ ) associated with the shifted harmonics is estimated, and the shifted harmonic (20, 30) in the second frequency channel (32) is further shifted to the first frequency (f ₃₅ ). The method of claim 3, wherein said further shifting to said first frequency (f ₃₅ ) is by amplitude modulation. Method according to one of the preceding claims, wherein the harmonic (20, 30) in the first frequency channel (31) represents a dominant frequency. The method of claim 1, wherein the harmonic (20, 30) is mapped to the estimated first frequency (f ₃₅ ) by synthesizing one in the second frequency channel (32) Signal receives the amplitude of the harmonics (20, 30) in the first frequency channel (31). Apparatus for frequency compression of an audio signal having a fundamental frequency and at least one harmonic (20, 30) with - A signal processing means for providing the audio signal in a plurality of frequency channels (31, 32) and a shifting device for shifting or mapping the harmonics (20, 30) of the audio signal from a first frequency channel (31) of the plurality of frequency channels into a second frequency channel (32) of the plurality of frequency channels,
marked by - estimation means for estimating a first frequency (f ₃₅ ), which is also harmonic to the fundamental frequency, in the second frequency channel (32), - the harmonic (20, 30) is shifted or imaged by the shifter to the estimated first frequency (f ₃₅ ). Apparatus according to claim 7, wherein the signal processing means comprises a polyphase filterbank. Audio device with a device according to claim 7 or 8.

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