KR20070022206A - System for audio signal processing - Google Patents

Abstract

A digital audio signal input (1), a digital audio signal processor (2, DSP) and a digital audio signal output (3), the digital signal processor (2, DSP) having a high pass frequency (f) ) A low pass with a filter 21, an amplifier 22 for the signal filtered by the HP filter and a low pass frequency f 'for filtering the signal after amplification by the amplifier 22 and providing an output signal. A pass-through (LP) filter 23, wherein the digital processor includes a setter 24, 25 for setting a high pass frequency or a low pass frequency, and a high pass frequency and a low pass frequency of each of the high pass filter and the low pass filter. And a matcher (26) for matching the two to each other.

Digital Audio Signal Input, Low Pass Frequency, Matching, Sound Reproduction System, Digital Processor

Description

System for audio signal processing

The present invention relates to the field of sound reproduction, and more particularly to the field of digital audio signal processing.

The present invention relates to a sound reproduction system comprising a digital audio signal input, a digital audio signal processor and a digital audio signal output.

The invention also relates to an audio signal processor for processing an incoming audio signal of an audio output signal. In particular, the present invention relates to digital signal processor (DSP) circuit programs.

The invention also relates to a method for processing a digital audio signal.

An acoustic reproduction system, such as a loudspeaker telephone system, includes an output transducer, often referred to as a loudspeaker, and an input for an audio signal. The loudspeaker generates sound pressure waves in response to an audio input signal representing the desired sound pressure wave.

The clarity of sound perceived by the listener is very important, especially in noisy environments. The simplest way to increase clarity is to increase the average SPL (sound pressure level), that is, increase the volume. However, simply raising the volume does not always lead to a clearer sound. In addition, too high power can cause loudspeaker overload, further reducing clarity.

Many attempts have been made to increase the clarity of the sound.

US patent application US 2002/0015503, for example, proposes increasing clarity by individually constraining gain factors for different frequency bands.

US Pat. No. 6,011,853 describes a system in which the frequency spectrum of noise is measured and the dialogue signal is equalized against the effect of noise at a particular frequency.

However, existing systems and methods are each very complex and require complex operations, thereby requiring complex programs or only limited advantages when complex circuits (hardware) or programs (software) are used. to provide.

Despite the above references, there is still a need in the art for improved systems and methods that provide improved clarity.

It is an object of the present invention to provide a sound reproduction system and method having improved clarity.

To this end, the sound reproducing system according to the present invention comprises a digital signal processor, the digital signal processor comprising: a high pass (HP) filter having a high pass frequency, an amplifier for amplifying the signal filtered by the HP filter; After amplification, a low pass (LP) filter is provided for filtering the signal and providing an output signal, and the digital processor includes a setter for setting a high pass frequency or a low pass frequency and a high pass of each of the high pass filter and the low pass filter. And a matcher for matching a pass frequency and a low pass frequency to each other.

The system according to the invention is based on the following aspects:

1. The incoming signal is amplified (by an amplifier) to increase the loudness,

2. Increasing the incoming signal may result in a signal higher than the maximum digital signal, in which situations the signal is often digitally clipped resulting in distortion of the signal.

3. Low frequencies are removed from the signal prior to amplification, which allows the rest of the signal to be amplified with a higher gain factor on average. This is done by an HP filter placed before amplification. Lower frequencies are of relatively low importance as far as clarity is concerned. Lower frequencies include most of the amplitude of the signal, so eliminating lower frequencies greatly reduces the amplitude of the signal, creating headroom for amplification, i.e., stronger to the remainder of the signal. Perform amplification. In particular, a large part of the amplitude of the dialogue is included in the lower frequencies, so that attenuating the lower frequencies enables a significant increase in headroom (ie amplification that does not encounter clipping levels).

4. Simply cutting off the lower frequencies and increasing the output can lead to increased clarity, but this does not always lead to an easily recognizable increase in clarity. Due to the use of a high pass filter, the signal contains a relatively high proportion of high frequency tones, which causes perceptually disturbing sharp signals, discoloration of the audio signal, and reduces clarity. Also, amplification can result in the introduction of overtones. The low pass (LP) filter after amplification restores balance and can additionally remove or at least reduce overtones produced by the amplifier to result in a more natural sound, thereby at least partially reducing discoloration of the signal, Increase clarity.

5. In the system according to the invention, the cutoff frequencies are matched, ie there is a relationship between the set HP cutoff frequency and the LP cutoff frequency, the values of the low pass frequency and / or high pass frequency matched in opposite directions, This increases the LP cutoff frequency when the high cutoff frequency decreases (decreases) and vice versa or otherwise, imparts a cutoff action of the HP filter and the LP filter, where, if one action is large, the remaining The same applies to the case where one action is relatively small, and so on. HP and LP filters are thus used as paired elements in dialogue or sound processing, which are not elements with extraneous parameters. The cutoff action of the high pass filter provides discoloration in the audio signal, and the cutoff action of the LP filter is matched to counteract this effect. The system has means for setting either the HP or LP cutoff frequency and for matching the LP or HP cutoff frequency to the set frequency.

There are a number of preferred embodiments:

In embodiments, the system includes a sensor for measuring a background noise level and includes an element having the measured noise level as an input and an HP cutoff frequency as an output, wherein the HP cutoff frequency is a background noise level. It increases with increasing, and the LP cutoff frequency decreases as the HP cutoff frequency increases. This reduction of the LP cutoff frequency can be performed by changing (decreasing) the cutoff frequency of a single LP filter as a function of the HP cutoff frequency, or alternatively, the system includes a set of other LP filters, and the HP cutoff frequency In dependence, the signal after amplification is directed to one of a set of LP filters. The latter embodiment is particularly useful when more complex LP filters are used, where more parameters are changed than just one or more variables, namely the LP cutoff frequency. This selection is performed using a lookup table and, depending on the HP cutoff frequency, the signal receives other digital calculations that are equivalent to other LP filters. This can be done in two steps, in the first step, the noise level is determined, which triggers the selection of the HP cutoff frequency, after which the LP cutoff frequency is selected from the HP cutoff frequency.

Preferably, the system and / or program comprises means for setting the cutoff frequency of the high pass filter, in accordance with the average amplification in the amplification stage. Average amplification is a measure of the average gain of the signal and thereby the loudness level of the emitted acoustic signal. When the HP cutoff frequency of the high pass filter increases with an increase in the average loudness level of the emitted signal, and additionally, the cutoff frequency of the low pass filter stepwise with a match, that is, a change in the HP cutoff frequency. It is advantageous when decreasing. At very high amplification levels (which can occur when the system is used in noisy and noisy environments), there is a great need for the filtering action of the HP filter that introduces relatively large unbalance in sound, and in addition, a large amplification level It can introduce a relatively large discoloration of the signal, where overtones of the signal after undesirable, unnatural amplification contain a lot of energy. This results in rough sound. The roughness of the sound sometimes and even frequently, as the inventors have felt, keeps the loudspeaker at a distance from the ear, especially in mobile phones. Apart from the fact that the loudspeaker is kept at a distance from the ear, since the signal is reduced and the noise is increased, it in itself leads to a significant reduction in the signal-to-noise ratio, and the fact that the sound itself is roughly perceived is a given message. Means a decrease in clarity. For voice messages, the roughness of the voice is often part of the message the speaker wants to convey to the listener, and sometimes even more important than the actual words of the messages. Thus, for the clarity of the message, viewed from a broader concept than just whether words are understood, it is important that a clear “natural” speech delivery is achieved. At lower amplification levels, the "rough acoustic" effect, ie discoloration of the signal, can be heard much less. In summary, at high amplification levels, there is a relatively large unbalance of the signal (much more energy at high frequencies), and additionally, a relatively large portion of the amplitude at high frequencies is due to artificial artifacts due to amplification. Eliminating (completely or partially) the higher frequencies results in significantly higher signal at higher frequencies, while at higher relatively low amplification levels, while higher frequencies eliminate unbalance and artifacts, thus resulting in a more natural sound. The amplitude is naturally occurring and, less likely, due to artificial artifacts, so it is desirable to set the cutoff frequency to a relatively high frequency. Improved sound reproduction is possible by setting the cutoff frequency of the high pass filter in accordance with the amplification and matching the cutoff frequency of the low pass filter at the same time. To some extent, this embodiment has a similar effect to the embodiment with the noise sensor mentioned earlier, but instead of combining the matched cutoff action of the HP and LP filters to the measured noise level, the matched LP and HP filter cutoffs. Action is tied to amplification levels. In general, higher amplification levels are used at higher background noise levels (user raises the volume in noisy environments), and so, conceptually, the user acts as a noise sensor. Using noise sensors as part of the system is from a desirable technical point of view, but this increases the cost and complexity of the system.

In embodiments, the system comprises means for measuring / setting the sample frequency fs of the incoming signal and setting the maximum cutoff frequency of the LP filter to fs / 2. Any signal above fs / 2 is not a component of the original signal but is due to overtones. Thus, the sample frequency fs actually determines the maximum cutoff frequency of the LP filter. Thus, in these embodiments, the cutoff frequency has an upper limit of fs / 2. The sample frequency fs may be determined by the bandwidth of the incoming signal. As an example, Internet audio-video bandwidth is often very small. Also, when the audio system itself has very low power, there are not many uses with very large bandwidth. Thus, in preferred embodiments, the system can be set from the incoming signal, or can be set as a function of power regulation of the audio system (eg, if the system can be hooked up so far, other physical amplifiers and Accordingly power restrictions may apply) means for setting the sample frequency of the signal. For high values of fs, for example 44 kHz, this has no significant effect on the cutoff behavior of the low pass filter, but low values of fs (e.g. fs / 2 equals 4 kHz, 5.5 kHz, 8 kHz, Not so for 11 kHz, 8 kHz, 11.025 kHz, 16 kHz, 22.05 kHz). However, this regulation 'colors' the sound because there are no frequencies above fs / 2, and in fact, the original signal received is already 'discolored'. The system of the preferred embodiment has means for determining the HP cutoff frequency according to the sample frequency fs set to cancel discoloration. Again, this may be done in a number of ways: The increase in HP cutoff frequency may be performed by changing (increasing) the cutoff frequency of a single HP filter as a function of the LP cutoff frequency, or alternatively, the system may The signal is directed to one of a set of HP filters, including a set of filters, and depending on the cutoff frequency. The latter embodiment is particularly useful when more complex filters are used that have one or more variables, that is, more parameters change than just the cutoff frequency. This selection can be performed using a lookup table, where, depending on the LP cutoff frequency, the signal receives other digital calculations that are equivalent to other HP filters.

In the most preferred embodiments, the system combines both preferred embodiments. That is, it sets the sample frequency (fs) on the one hand, then determines the HP cutoff frequency based on the fs value, on the other hand measures the noise level, determines the HP cutoff filter value, and then LP Means for determining a cutoff filter frequency.

Preferably, the high pass frequency is between 300 and 2 kHz, the low pass frequency is greater than 2 kHz, and fs / 2 is the sample frequency.

Preferably, the amplifier is preferably arranged so as not to amplify a signal having a signal strength below the threshold.

Below the threshold (specific minimum amplitude), the signal is likely due to noise. Not amplifying these signals improves clarity because noise is reduced. In addition, the difference between silence and conversation can be better distinguished, which also increases clarity. Since signal noise may depend on the GSM device or GSM provider used by the far end user, if the user needs to set a threshold, for example, when forming multiple telephone calls, at run time, Alternatively, a threshold may be set at initialization time.

The high pass filter is preferably a primary or secondary filter, that is, a filter having a relatively gradual gradient. It is advantageous to remove most of the energy of the low frequency components of the incoming signal to provide headroom for amplification. However, a filter with a relatively steep gradient (step filter is an extreme example of such a filter) removes most of the lower frequency components that can lead to unnatural acoustic voices. The system includes means for allowing a user to change the order and / or cutoff frequency. Second-order high pass filters result in good speech intelligibility and / or signal loudness, while mid-order high pass filters preserve the more natural sound of the original signal.

In a preferred embodiment, the system comprises a high pass (HP) filter followed by an AGC followed by a limiter / clipper followed by a low pass (LP) filter. This embodiment is preferred in environments where signal loudness is of primary interest. The limiter scans the peaks of the audio signal and attenuates the audio portion around the peak if attenuation is necessary to limit the amount of clipping, but allows clipping for very large signals.

In another preferred embodiment, the system comprises an automatic volume leveler preceded by a high pass filter (HP) or followed by a high pass filter (HP), followed by a gain and clipper. Is followed by a low pass filter LP. This embodiment is good when low computational effort is desired.

Clipping is a simple operation in which any signal exceeding the threshold signal strength is reduced to the given threshold signal strength, i.e., the maximum signal strength is set. The advantage of this embodiment is that a simple system is used, and the disadvantage is that the signal is very severely distorted because any detail in the signal above the threshold signal is lost.

In all embodiments, HP and LP cutoff frequencies are associated. This forms the core of the present invention wherein HP and LP filter actions are matched, where discoloration due to application of one filter is at least partially compensated by the action of another filter. Often, the 'leading filter' is an HP filter, i.e. in embodiments where the HP cutoff frequency is set based on a parameter, followed by the LP cutoff frequency, but the incoming signal has a small sample frequency, LP filter.

In preferred embodiments, the system includes a measurement system, such as a microphone, for measuring background noise levels.

For one or more parameters, the dependence on the measured noise level is preferably nonlinear.

Within the concept of the invention, without being restricted to the embodiments given below, 'clipper', 'compressor', 'amplifier', 'filter', 'converter', 'comparator' and the like are widely understood, examples Any piece of hardware (such as a clipper, compressor, amplifier, etc.), any circuit or subcircuit designed to perform functions such as clipping, compression, amplification, etc., as described above, and clipping, compression, Any piece of software designed or programmed to perform operations such as filtering (a computer program or subprogram or set of computer programs or program code (s)) and a piece of hardware or software that acts alone or in combination Any combination of these. One program can combine multiple functions.

The invention also relates to any computer program comprising program code means for performing a method according to the invention when the program is run on a computer and a computer for performing the method according to the invention when the program is run on a computer. Any computer program product comprising program code means stored on a readable medium and any program product comprising program code means for use in the telephone system according to the invention for performing a dedicated operation for the invention. Can be implemented.

These and other aspects of the invention are described in more detail by way of example with reference to the accompanying drawings.

1 is a schematic diagram of a system including a loudspeaker and a DSP.

2 shows schematically a DSP according to the invention;

3 schematically illustrates the matching of an LP filter frequency to an HP filter frequency.

4 schematically illustrates an alternative way of matching the LP filter frequency against the HP filter frequency.

5 schematically illustrates the matching of HP filter frequencies to LP filter frequencies.

FIG. 6 schematically illustrates an alternative way of matching LP filter frequency to HP filter frequency. FIG.

FIG. 7 illustrates in schematic form the matching of LP and HP filter frequencies. FIG.

8 illustrates two examples of high pass filters that may be used in the present invention.

9 illustrates one type of embodiments of the present invention.

10 illustrates another type of embodiment.

11 illustrates AVL behavior of AVL.

12 illustrates a preferred embodiment in which parameters are adapted depending on the measured noise level.

The invention is now described in more detail below with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. However, the invention may be embodied in many other forms and should not be construed as limited to the embodiments set forth herein. Overall, like reference numerals indicate like elements.

1 schematically illustrates a sound reproduction system. Such a system may be, for example, a handfree loudspeaker cellular radiotelephone for use in a motor vehicle. When implemented as a hand-free cellular telephone, conversational signals received from the far end, i.e. from a remote population, are transmitted from a cellular base station (not shown), received by a transceiver of the cellular telephone (not shown), and input waveforms. It is applied to input 1 for incoming far end signal as (W). In this example, the forward and reverse transmissions between the system, in this example, the telephone system, and far-end are assumed to be in digital form. If the original signals are in analog form, the system includes an A-to-D converter to generate a digital far-end signal fed to the input (1).

As shown in FIG. 1, the waveform is applied in digital format at the input 1 of, or connected to, a DSP (Digital Acoustic Processor) 2, which is connected to or includes a digital output 3. The digital signal output is supplied to the D-to-A converter 4 and thereby converted into an analog format and amplified by the amplifier 5 for use by the loudspeaker 6. Sound pressure wave W1 representing the conversation of a remote group is emitted by loudspeaker 5. Thus, the radiotelephone user hears sound pressure waveforms representing conversations in a remote group.

However, the listener not only listens to the sound produced by the loudspeaker, but also other sounds, which may make it difficult to understand the sound produced by the loudspeaker, ie with low clarity.

Raising the volume is the first and obvious choice for increasing clarity. However, the maximum output level of the loudspeaker is often limited, and simply raising the volume causes more noise and does not necessarily achieve better clarity of the signal.

In order to improve clarity, a number of cooperative operation measures are taken in the system and method according to the present invention.

2 very schematically illustrates a DSP (Digital Acoustic Processor) for use in a system according to the present invention. The DSP includes a high pass (HP) filter 21 having a cutoff frequency, for example, preferably between 300 Hz and 2 kHz, for example between 500 and 1500, more preferably between 800 and 1200 Hz. The high pass filter removes or reduces frequency components below the cutoff frequency f.

By the HP filter, most of the energy of the signal was thereby removed. This allows the residual signal to be amplified much more by the amplifier 22 (before proceeding to problems with digital clipping, ie, values above the maximum value). However, HP filtering increases high frequencies compared to low frequencies, resulting in a sharp sound. Application of low pass filter (LP) 23 after amplification at least partially restores the 'naturalness' of the sound. In the system according to the invention, the HP and LP cutoff frequencies f and f 'are matched, as schematically indicated by arrows between the HP and LP filters. The system has means for setting either one of the frequencies f or f ', and one of these frequencies after it is set.

In the system according to the invention, the cutoff frequencies and thereby the actions of the HP and LP filters are matched, ie they are correlated with each other. As described, for this purpose, the system, on the one hand, has setters 24 and 25 for setting the HP or LP cutoff frequency (f or f ') and match the LP or HP cutoff frequency to the set HP or cutoff frequency. And a matcher 26 for the purpose of assembly. In all embodiments, HP and LP cutoff frequencies are associated. This forms the core of the present invention wherein HP and LP filter actions are matched, where discoloration due to application of one filter is at least partially compensated by the action of another filter. "Matched" means that there is a relationship between f and f ', that is, a relationship of f = F (f') or f '= F' (f). Often, the 'lead filter' is an HP filter, ie in embodiments where the HP cutoff frequency is set based on measured or determined parameters and the LP cutoff frequency follows, but the incoming signal has a small sample frequency, Lead filter 'can be an LP filter.

FIG. 3 schematically illustrates a high pass filter having a high pass filter frequency f, which is dependent on parameters P such as environmental noise level, line noise level, parameters set by the user or amplification. get affected. The setter sets the HP cutoff frequency depending on this parameter or parameters. This effect is schematically indicated by the arrow between P and f and the cross arrow through frequency f. The system includes a matcher for matching the low pass frequency f 'of the low pass filter 23 to the HP filter frequency f. The term "matching" of the present invention should not be interpreted in the sense that f is equal to f ', and a relationship between value f and value f', e.g. f '= F' (f), where F is Function)) should be interpreted as being formed. This may be a fixed relationship, i.e. a fixed function F, or the function may be independent of itself with respect to other parameters, as shown in FIG. 3 by the letter P 'and by the arrow between this letter and the function F It is shown schematically. The parameters may be, for example, environmental noise level (N), line noise level or amplification (gain) of the amplifier between HP and LP filter. In this case, the relationship between f and f 'can be expressed by f, = f, P').

4 illustrates a variation on the scheme shown in FIG. 3. In this embodiment, a number of LP filters are used and depending on the set HP cutoff frequency f and possibly additional parameters P ', the signal is matched after amplification by the amplifier 22. Is used to guide one of the LP filters, each filter having a cutoff frequency f '. This is indicated schematically in FIG. 4 by the different arrows interconnecting the HP filter with one of the LP filters, and by the fact that the cutoff frequencies are different in the other LP filters.

This embodiment is more complicated than the embodiment where a single LP filter is used. However, the latter embodiment is particularly useful when more complex LP filters are used that have one or more variables, that is, more parameters are changed than just the LP cutoff frequency. This selection can be performed with a lookup table, and depending on the HP cutoff frequency set, the signal receives other digital calculations that are equivalent to other LP filters. This can be done in two steps, where the noise level is determined in the first step, which triggers the selection of the HP cutoff frequency, after which the LP cutoff frequency is selected from the HP cutoff frequency. This choice may depend on additional parameters.

5 is a function of the LP filter cutoff frequency f 'with the LP filter frequency set as a function of the sample frequency fs, with the HP filter cutoff set, ie f = F (f'), and the matcher 26 Schematically illustrates an embodiment of matching this frequency variation nf, i.e., calculating f based on the set frequency f '. In some embodiments, the function F itself depends on other parameters P ', such as, for example, the noise level.

6 illustrates a variation of the embodiment illustrated in FIG. 5. In the present embodiment, a number of HP filters are used, and depending on the set LP cutoff frequency f ', and possibly additional parameters P', the signal after amplification by the amplifier 22, Guided to one of the HP filters, each filter has a cutoff frequency f. This is illustrated schematically in FIG. 6 by another arrow interconnecting the LP filter with one of the HP filters and by the fact that the cutoff frequency is different in the other HP filters.

This embodiment is more complicated than the embodiment where a single HP filter is used. However, the latter embodiment is particularly useful when more complex HP filters are used that have one or more variables, that is, more parameters than just the HP cutoff parameter, eg the order of the filter is also changed. This selection can be performed using a lookup table, and depending on the LP cutoff frequency set, the signal receives equivalent digital calculations in other HP filters. This can be done in two steps, where in the first step, the sample frequency fs is determined, which triggers the selection of the LP cutoff frequency, after which the HP cutoff frequency is selected from the LP cutoff frequency. This choice may depend on additional parameters.

7 illustrates in graphical form the relationship between the LP cutoff frequency f 'and the HP cutoff frequency f. This relationship is found by drawing a vertical line for a particular value of a parameter (eg, noise) between the HP (f) and LP (f ') cutoff frequency graphs and reading the corresponding curve values (indicated by double arrows). do. As schematically shown, the graphs may be continuous (f ' ₁ (P) _LP and f' ₁ (P) _HP ) or as such, for example when multiple separate LP or HP filters are used. , May be stepped (f ' ₂ (P) _LP and f' ₂ (P) _HP ). The curves themselves may depend, for example, on parameters P and / or P 'such as measured noise level (N), line noise level, selected amplification level (when set by the user), sample frequency (fs). Can be.

Amplification is performed by amplifier 22 (see FIG. 2) /

This amplifier 22 is preferably a compression amplifier. A compression amplifier is an amplifier that not only amplifies the signal but also amplifies the average sound level, that is, sounds with small amplitude are amplified more than sounds with a high sound level, thus reducing the signal amplitude range. This consists of a number of ways, eg an clipper / limiter arrangement, a clipper / compressor or an AVL (automated voltage leveler) followed by a gain and clipper. Numerous other techniques can be used, including using lookup tables to perform amplification and compression. The amplitude range, in particular the upper limit of the range, may be set by the manufacturer or may be influenced by the user, for example, by a loudness setter (a knob that the user can use to adjust the loudness). Compared to simple, linear amplification of the signal (i.e. the same amplification factor for all sound levels), compressed amplification leads to more acoustic clarity, especially when environmental noise is high, and that silence speech portions To be shielded by. Words can be distinguished more easily, thus improving the clarity of the sound.

However, this may also lead to distortion of the sound due to nonlinear amplification of the acoustically introduced overtones (high frequency components at twice, three times the original frequency, etc.) which increases the sharpness of the sound. This is perceived as unpleasant to the listener and, in fact, in terms of importance, in a wider sense, reduces the clarity of the voice message, because the sharpness of the spoken words sometimes forms an important aspect of the voice message. This effect exists even in the absence of clipping, although clipping itself also introduces overtones and even distorts the signal more.

Clarity in its broadest sense is not only related to words, but also to the message the speaker wants to convey to the listener. Especially at higher average amplification, the sharpness of the sound makes the whole sound harsh, thus significantly reducing the sophistication of the emotions the speaker wants to convey.

Application of the low pass filter after the compression amplifier as in the system of the present invention shown in FIG. 2 reduces the perceived sharpness of the voice, thereby at least restoring the original emotional content of the spoken words, ie a much more natural sound. To provide.

In most Western languages, the pitch of a word affects the emotional impact of a word, but not the meaning of the word itself. However, there are languages in which the pitch of a word plays a much larger role, resulting in completely different meanings for one and the same "word" depending on the pitch of the word. When these languages are used (which cannot be eliminated), the use of low pass filters is much more advantageous. The invention is particularly advantageous when used in or in conjunction with automatic conversation recognition systems for languages in which the pitch of the spoken word affects the meaning of the words. The foregoing is directed to spoken words, ie voice, and likewise applies when an acoustic recognition system is used to reproduce music. Also, in music, the sharpness of the sound is very important, as well as how the music is perceived, as well as whether the tunes can be heard. Thus, the present invention is very important for systems in which voice messages such as telephone systems are relayed, but not limited to such a system, systems for reproducing music can likewise benefit from the present invention.

The leveling or compression action may be performed before HP filtering of the incoming signal or after HP filtering. Amplification itself (ie gain) is performed after HP filtering. Clipping is performed in conjunction with or after the gain. When use of the clipper is made, the low pass filter is placed after the clipper.

As mentioned above, the relationship between HP and LP cutoff frequency may be fixed or may depend on a number of parameters.

The amplification stage and / or the clipping stage also result in discoloration of the signal, and in more sophisticated embodiments of the system, the matcher for matching HP and LP frequencies may be one of the elements between the HP and LP filters, such as amplifiers and / or clippers. An input for at least one parameter, wherein the relationship between HP and LP cutoff frequency depends on said parameter (s), ie f '= F (f, P) or f = F' (f ' , P '), where P and P' represent the parameter (s) of these intermediate elements. By way of example, the nonlinearity introduced by the clipper and / or leveling amplifier introduces higher overtones, so depending on the presence or efficiency of these elements, the LP filter cutout action of the LP filter can be increased.

8 provides two examples of high pass filters that may be used in the system according to the present invention.

The left side of the figure illustrates the primary filter and the right side illustrates the secondary filter. The high pass filter shown has a cutoff frequency f of about 1 kHz. First or second order high pass filters (with relatively moderate gradients of about 5-15 dB per octave) are suitable. Removing too many low frequency components results in a very unnatural sound (or unnatural strange sound). Therefore, the order of the high pass filter is preferably limited to two. This also reduces the computational power required. The user can change the high pass filter from primary to secondary and vice versa, or it is desirable to include an automatic switching mechanism depending on the signal that the system enters. The use of the second order results in high dialogue clarity (in a limited sense, ie only words) and / or signal loudness, while the primary HP filter better preserves the natural sound of the original signal.

The HP filter may be composed of, for example, biquads whose coefficients are listed in Table 1, according to the following format.

Table 1-Filter Coefficients of HP Filters Shown in FIG. 3

Lower frequencies mainly contribute to the specific sound of the voice, but less contribute to speech clarity. This feature forms one aspect of the present invention.

By attenuating lower frequencies, the signal amplitude is reduced, creating a greater headroom for amplifying the residual signal containing relatively more frequencies that contribute to dialogue clarity.

When amplifying conversational signals, then even, even when compressing and clipping them, the conversational integrity is better than without using an HP filter, mainly for two reasons:

The signal contains relatively more frequencies that contribute to dialogue clarity.

Low frequencies are less hard clipped, resulting in fewer harmonics (due to clipping) that interfere with dialogue clarity.

However, removing too many low frequencies can cause very abnormal acoustic voices. Thus, the HP filter is preferably a first order, ie Butterworth (first order IIR) filter. This has the advantage of using small computational power.

9 shows details of a system according to an embodiment of the invention.

The DSP may be an HP filter (e.g., as shown in Figure 8, in this example for example a Butterworth primary or secondary filter with a cutoff frequency between 50 and 2 Hz) and then to AGC. The limiter / clipper is subsequently followed by a low pass filter LP having a cutoff frequency between 2 kHz and fs / 2.

In this example all audio streams may be mono. The sample rate frequency can be, for example, one of 8 Hz, 11.025 Hz, 16 Hz, 22.05 Hz, 32 Hz, 44.1 Hz or 48 Hz.

For example, for an input signal with a very low sample frequency of 8 Hz, the LP frequency cutoff frequency is set to fs / 2, or 4 Hz, and the HP cutoff frequency is set to 400 Hz, for example, and sampling at 16 Hz For frequency, the HP cutoff frequency is set to 200 Hz. The sample frequency fs sets the LP cutoff frequency, and the HP cutoff frequency matches the LP cutoff frequency.

In an embodiment, the AGC (Automatic Gain Control) action is blocked based on means in which only changes by the block are gain factors. In this way, computational power is kept to a minimum.

The gain can be calculated as follows, for example.

First, the running root mean square (RMS) value of the input signal is calculated. This RMS value gently averages based on the most recent "history" of the signal waveform. The peak value is then calculated using the look ahead time to predict this signal peak.

At the RMS and peak values, the crest factor is calculated. The so-called "depeak" factor is used to specify the intensity with which the algorithm can clip the peak signal (higher values can yield higher clipping). Then, the gain is calculated and compared to the maximum allowable gain that can be set by the user, the minimum of the two being selected. Although not shown here, the maximum allowable gain setting will be the input of the high pass filter, and the cutoff frequency is a function of the maximum allowable gain setting (or more generally one or more characteristics of the filter, which is also away from the value of the cutoff frequency or yes). For example, optionally switching from primary to secondary).

The maximum allowable gain setting may be an input for the high pass filter, and the cutoff frequency is a function of the maximum allowable gain setting (or more generally one or more characteristics of the filter, which is also away from the value of the cutoff frequency or is, for example, primary). And optionally switching from secondary to secondary).

Figure 9 illustrates one form of embodiment of the present invention, and Figure 10 is for a different form of embodiments.

Basically these embodiments include a number of elements or steps, which

1. Automatic Volume Leveler (AVL): AVL is a signal-dependent processing block that maintains the volume of incoming signals at a reasonably constant level.

2. (First-order) HP (high pass) filter: These filters remove some of the lower frequencies to create amplification headroom.

3. Gain: Increases the SPL (Sound Pressure Level) of the signal, ie amplifies the signal.

4. Clipper in the preferred embodiment, preferably when a simple system is desired. Hard Clipper: The signal is clipped to a predetermined size to ensure linear operation of the analog amplifier (after D / A conversion). Instead of a hard clipper that simply clips a signal above the clipping level, a soft clipper may be used that not only clips the signal above the clipping signal but also weakens the signal at a level close to the clipping level. A smooth clipper is used to recover some of the dynamics of the signal and increase clarity.

5. LP (Low Pass) Filters: Filters restore or at least improve the balance between mid and high frequencies, unbalance produces abnormal sounds, signal sounds are audible, and these LP filters are more audible. To produce sound. The cutoff frequencies of the HP and LP filters are matched.

In this example, the input is a conversational input, but note that the input can be any acoustic signal.

Representative AVL characteristics are also shown in FIG. 11.

The left graph shows the step change in amplitude of the three input signals. The graph on the right shows that the AVL gain increases faster for large changes in amplitude. This is a preferred embodiment which further improves clarity.

12 shows an example of a device similar to that shown in FIG. 10 in which a sensor (measurement system) 130 is used to measure background noise, such as an individual microphone, by way of example. The device includes a setter 131 for setting the cutoff frequency of the HP-filter. Although not shown, it is noted that other parameters such as gain, clipping, settings of the AVL may be dependent on the measured noise level.

The noise measurement provides, in preferred simple embodiments, single data representing the total noise S or noise figures for different noise bands S _f1 , S _f2 , S _f3 . When the noise figure are measured for different noise bands, and the average or total noise is instantaneous or S _av = calculated for ΣS _fi or moment can be weighted according to the dB (A) scale for S _av = Σw _i S _fi Where w _i are weighting coefficients on the dB (A) scale. The noise level is measured by amplitude measurement.

The HP cutoff frequency is adapted to the measured noise. The higher the noise level, the larger the cutoff frequency. In adaptive embodiments, the cutoff frequency may advantageously be ranged over a wider range than in non-adaptive embodiments. The HP cutoff frequency f is advantageously ranged generally from 50 Hz (for practically noisy situations) to generally up to 2 kHz for high noise level situations.

Thus, the HP filter cutoff frequency is updated in accordance with the amount of environmental noise and generally ranges from a very low value of 50 Hz (no environmental noise) to 2 kHz (solid environmental noise), for example. More low frequencies are then removed in high noise environments to create more headroom to amplify the signal. A maximum value of 2 kHz is recommended to avoid the removal of frequencies that contribute to speech clarity. Filter coefficients are calculated at run time.

The relationship between the cutoff frequency and the noise level is preferably set as follows: f _{cut -off} = f ₀ + Δf (S), where f ₀ is the lower noise limit (eg 50, 100 or 300 Hz) and Δf Is greater than the linear function of the noise level (proportional to S ⁱ , i is greater than 1).

The matcher 26 matches the LP cutoff frequency to the set HP cutoff frequency, i.e. f 'is set to f' = F (f).

In the absence of noise, the cutoff frequency is set to, for example, an upper limit of f _s / 2.

For maximum noise, the cutoff frequency is set to the lower limit of 2 kHz, for example.

12 schematically shows that the match itself may depend on the gain level and the sample frequency fs. In preferred embodiments, the matcher can operate in two ways, namely, depending on the sample frequency and the measured noise, with the LP cutoff frequency matched to the HP cutoff frequency or vice versa.

In these embodiments, the algorithm used is designed to be adaptively driven and driven by the amount of environmental (near-end) noise. This results in a user friendly system feature that allows a user to use his system (eg GSM) when changing conditions regarding environmental noise without requiring any additional interaction to control the GSM volume level.

When used in the adaptive processing mode, the parameters of the processing blocks are adapted to incoming samples according to environmental noise. The algorithm adapts the parameters according to the environmental noise. The amount of noise can be measured by individual microphones or set using a system (GSM) microphone (single microphone application).

When the environmental noise is reduced in volume, the parameters are preferably adapted very quickly so that the naturalness and gentleness of the incoming signal are restored.

The term 'no environmental noise' does not mean complete silence, but there are regular noises such as fan noise and background music. In a typical environment, the background noise is typically around 50 dB (A). The term 'high-grade environmental noise' refers to noise such as noise passing inside a train or subway or a dance club. These noises can measure up to 100 dB (A).

Noise can be measured by measuring spectral amplitude information of environmental noise and calculating a single value representing the amount of noise.

The relationship between the cutoff frequency and the measured noise (or more generally any parameter upon which the cutoff frequency depends) is not necessarily linear.

Tests using linear interpolation show that the algorithm affects too much for 'moderate' environmental noises.

Using higher, for example second or third order interpolation, the impact is smaller than linear interpolation for the same environmental noise. For high environmental noises, the amount of influence is the same.

The system of the preferred embodiments includes an adapter for adapting an additional one of the additional parameters depending on the background noise level measured in addition to the cutoff frequency. These parameters are for example the order of the high pass filter.

In summary, the present invention can be described as follows:

A sound reproduction system, comprising a digital audio signal input (1), a digital audio signal processor (2, DSP) and a digital audio signal output (3), wherein the digital signal processor (2, DSP) has a high pass frequency (f). A high pass (HP) filter 21 with a low pass frequency for filtering the signal after the amplification by the amplifier 22 and the amplifier 22 for the signal filtered by the HP filter, f ') and a low pass filter (LP) filter 23, wherein said digital processor comprises a setter 24, 25 for setting a high pass frequency or a low pass frequency and a high pass filter and a low pass filter, respectively. And a matcher (26) for matching the high pass frequency and the low pass frequency to each other.

Matching the HP and LP cutoff frequencies matches the effect of these filters on the perceived speech. In particular, this reduces the sharpness of the sound that can otherwise be heard.

The present invention can be used in a variety of systems. The present invention is particularly useful for hands free mobile phones. However, it is applicable to all sound reproducing systems, especially those that run in a system with a limited voltage source and / or a small loudspeaker. The list of possible applications is:

Handsets (mobile phones, DECTs, etc.);

Portable systems, eg portable DVD player;

PDAs;

Vehicle kits;

TVs; Computers;

Web terminals;

-Responders.

It will be understood by those skilled in the art that the present invention is not limited to those specifically shown and described above. The present invention is concerned with each and every noise characteristic feature and each and every combination of characteristic features. Reference numerals in the claims do not limit their protective scope. The use of the verb “comprising” and its terms does not exclude the presence of elements other than those mentioned in the claims. Elements expressed in the singular do not exclude the presence of a plurality of such elements.

The invention has been described with reference to specific embodiments which should not be construed as limiting, exemplifying the invention. The invention can be implemented in hardware, firmware or software, or a combination thereof. Other embodiments are within the scope of the following claims.

Many other variations may be made within the spirit of the invention.

Claims (16)

In a sound reproduction system, Digital audio signal input (1), digital audio signal processor (2, DSP) and digital audio signal output (3), The digital audio signal processor (2, DSP) is connected to a high pass (HP) filter 21 having a high pass frequency f, an amplifier 22 for the signal filtered by the HP filter and the amplifier 22. A low pass (LP) filter 23 having a low pass frequency f 'for filtering the signal after the amplification by and providing an output signal, The digital processor is configured to set the high pass frequency or the low pass frequency of the setter 24, 25 and the high pass frequency and the low pass frequency of each of the high pass filter and the low pass filter with respect to each other. And a matcher (26) for the acoustic reproduction system. The method of claim 1, A sensor for measuring the background noise level, An element having the measured noise level as an input and an HP cutoff frequency as an output, And the HP cutoff frequency increases as the background noise level increases, and the LP cutoff frequency decreases as the HP cutoff frequency increases. The method of claim 2, And a single LP filter having a variable cutoff frequency. The method of claim 2, And a set of LP filters having a different LP cutoff frequency, wherein the matcher is arranged to send a signal after amplification to one of the set of LP filters, depending on the HP cutoff frequency. The method according to claim 1 or 2, The setter is arranged to set the cutoff frequency of the high pass filter in dependence on the average amplification in the amplification stage. The method according to claim 1 or 2, The setter is arranged to set the cutoff frequency f 'of the LP filter at fs / 2, fs is the sample frequency, and the matcher matches the high pass frequency f to the low pass frequency f'. Sound reproducing system. The method of claim 6, A sound reproduction system comprising a single HP filter having a variable cutoff frequency. The method of claim 6, And a set of HP filters having a different HP cutoff frequency, wherein the matcher is arranged to transmit a signal prior to amplification to one of the set of HP filters, depending on the LP cutoff frequency. The method of claim 1, Wherein the HP cutoff frequency (f) is a frequency between 300 Hz and 2 kHz. The method of claim 1, Wherein the LP cutoff frequency is above 2 kHz and fs / 2 and fs is the sample frequency. In a digital audio signal processor, Filter a high pass (HP) filter 21 having a high pass frequency f, an amplifier 22 for the signal filtered by the HP filter and a signal after amplification by the amplifier 22, and output signal A low pass filter having a low pass frequency f 'for providing The digital audio signal processor matches setters 24 and 25 for setting the high pass frequency or the low pass frequency and the high pass frequency and low pass frequency of each of the high pass filter and the low pass filter with respect to each other. A digital audio signal processor comprising a matcher (26). A method of processing digital acoustic signals, Components below the HP cutoff frequency f are removed before frequency amplification, and after amplification, the frequency components above the LP cutoff frequency are removed, and the values of the HP cutoff frequency and the LP cutoff frequency f ' Matched, digital acoustic signal processing method. The method of claim 12, The HP cutoff frequency is between 300 Hz and 2 kHz. The method of claim 12, The noise level (N) is measured and the HP cutoff frequency (f) is determined depending on the measured noise level. In a computer program, A computer program comprising program code means for performing the method as described in any one of claims 11 to 14 when the program is run on a computer. In a computer program product, A computer program product comprising computer program code means stored on a computer readable medium for performing the method of any one of claims 11-14.

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