MXPA96004270A - Digital filter and apparatus to play sound with the digi filter - Google Patents

Abstract

The present invention relates to the digital filter comprising: an input terminal for admitting a digital signal, the filter responding to finite pulses for winding a pulse response in the digital signal, the finite pulse response filter comprising a plurality of delay units, a plurality of first adders, and a plurality of first coefficient multipliers, the pulse response being obtained measuring from a source to a measurement point, and a feedback means for attenuating an output supplied from a delay unit selected from the plurality of delay units, through a second coefficient multiplier and produce an attenuated output, and fed back the attenuated output to a second adder connected between a selected pair of adjacent delay units, placed before the selected delay unit of the plurality of delay units, from which its Minister Sali

Description

"DIGITAL FILTER AND APPARATUS TO PLAY SOUND USING THE DIGITAL FILTER" FIELD OF THE INVENTION The present invention relates to a digital filter, and more particularly to a digital filter having a finite impulse response type filter for winding an impuLso response and an apparatus for producing sound through the effect of the digital filter.

BACKGROUND OF THE INVENTION Many electronic instruments have been proposed to have a finite impulse response type filter (referred to as an FIR filter), as shown in Figure 1, composed of a signal processing unit. Those electronic instruments are for measuring an impulse response of a system and reproducing the response through the effect of the FIR filter. In Figure 1, a signal fed to an input terminal 81 is sent to many delay units 82 connected in series. The signals delayed by these delay units 82 are subsequently transferred in a synchronized clock signal with a sampling period. These delay units 82 provide leads conducted at their 1/0 terminals. The signals from these leads are sent to coefficient multipliers 83 where those signals are multiplied by the coefficients. The multiplied values are added in series by adders 84 and then collected in an output terminal 85. Therefore, in the FIR filter, by grading a coefficient for an impulse response to each coefficient multiplier 83, it is possible to carry out the operation of winding this impulse response. An apparatus for adding a reverberation placed to use the aforementioned FIR filter, for example, has a function of measuring an impulse response in a room and reproducing the response through the FIR filter. With this function, it is possible to hear a reproduced sound that has the same reverberation characteristics as that of the reproduced sound heard in a room where the sound is actually measured. That is, the reproduced sound has the same reverberation characteristic as that of the actual measured sound of the room. A type of helmet system for locating a sound image from the head placed to have the FIR filter, functions to measure an impulse response representing a transfer characteristic of the loudspeakers to both ears of the listener and reproduce the sound through FIR filter effect. This telephone helmet system therefore makes it possible to produce the same effect of locating a sound image as the loudspeaker system. If the FIR filter is used to accurately obtain a convergence time and a frequency response that corresponds exactly to the actually measured impulse response, the FIR filter is required to have a long derivation length which makes the total is expensive and on a large scale. Under this condition, an FIR filter having a relatively short derivation length is proposed. This FIR filter can produce an approximate characteristic to the frequency characteristics of the actual sound. However, this type of FIR filter does not offer a listener a sufficiently high sense of hearing the sound of the head. Or, this type of FIR filter does not produce a clear acoustic image, so that the resulting reproduction signal does not have a superior quality acoustic image location. In addition, a delay unit having a long delay time can only be used to reproduce a characteristic of the reverberation time and an initial reflection characteristic. This type of delay units compose a filter to extract only the impulses representative of the impulse response. Figure 2 shows a representative arrangement of an apparatus for processing an added reverberation signal. As shown in Figure 2, a signal input in an input terminal 91 is admitted to an input terminal of the delay unit 92. This delay unit 92 operates to send the signals SR] _, SR2 and SR3 through the terminals R] _, >; 2 and R3 output, during different delay time periods. Those signals SR1 'SR2 SR3 are attenuated by multipliers 93, 94 and 95 of coefficient. Then, those signals are added to an adder 96. This adder 9d supplies the signal SRQ as an output. This SRQ signal is reproduced as a pulse response having large pulse intervals in the portion responding to an initial reflection in a pulse response shown in Figure 3. Next, an SR4 signal supplied from an R terminal of output of the delay unit 92 to a delay unit 98 through the adder 97, is delayed as a delay signal by the delay unit 98. The delay signal is attenuated by a coefficient multiplier 99. The attenuated signal is added to the signal SR4 by the adder 97. The delay signal is also attenuated by a coefficient multiplier 100 and then supplied to an adder 109. In addition the signal SR4 supplied from the delay unit 92 to a unit 102 of delay through an adder 101, is delayed as a delay signal by the delay unit 102. The delay signal is attenuated by a coefficient multiplier 103 and then added to the signal SR4 by the adder 101. The delay signal is also attenuated by a coefficient multiplier and then supplied to an adder 109. In addition, a signal SR4, supplied from the delay unit 92 to the delay unit 106 through the adder 105, is delayed as a delay signal by this delay unit 106. Then, the delay signal is attenuated by a coefficient multiplier 107 and then added to the signal SR4 by the adder 105. The delay signal is also attenuated by a coefficient multiplier 108 and then added to an adder 109. The The added output signal of the adder 109 is supplied to a delay unit 111 through an adder 110. Then, the delayed signal is attenuated by a coefficient multiplier 114 and then returned to the adder 110. In addition, the output add-on of the adder 109 is supplied to a coefficient multiplier 112 through the adder 110. The signal attenuated by the multiplier 112 is supplied to an adder 113. The added signal of the adder 113 is supplied to a delay unit 116 through an adder 115. The delayed output is attenuated by a coefficient multiplier 119 and then returned to the adder 115. In addition, the added output of the adder 11 3 is supplied to a coefficient multiplier 117 through the adder 115 so that the output is attenuated by the multiplier 117. The attenuated signal is supplied to an adder 118. The signal Sp > 4, supplied from the output terminal R4 of the delay unit 92 to the adders 97, 101 and 105, composes a pulse response corresponding to a scattered sound shown in Figure 3 until the signal SR4 reaches the adder 118 Each delay unit can have a value of several milliseconds that are selected as a delay time. Dispersed sound, particularly in the initial portion of the scattered sound, is a much larger impulse response compared to the value 20 and a few seconds which is a sampling time interval of the signal processing apparatus that is normally selected to ensure an acoustic band Therefore, the reproduced characteristic becomes larger than the actual transmission characteristics even on a frequency axis.

COMPENDIUM OF THE INVENTION It is an object of the present invention to provide a digital filter adopted to reproduce a prolonged impulse response with high accuracy, while reducing in size and cost using an FIR filter with a short branch length. Another object of the present invention is to provide an apparatus for reproducing "adopted sound to have the aforementioned digital filter and thereby greatly improve a sense of location of a sound image of the head or a sense of forward location of an image. of sound, keeping the device with reduced size and cost. To carry out the object, in accordance with an aspect of the invention, a digital filter adopted to have an FIR filter with a finite derivation length to compose a pulse response from a pre-measured sound source to a measurement point, the FIR filter has delay units, and includes a feedback means that has coefficient multipliers and adders, the coefficient multipliers to attenuate the output of any of the delay units, the attenuated output being fed back to the adder that it is provided between the delay units placed before any delay unit. Here, multiple feedback means may be provided. The feedback means allows the FIR filter with a short branch length to reproduce a long impulse response with great fidelity. This makes it possible to greatly reduce the scale of signal processing. In case the signal processing unit is composed of ICAEs of digital signal processing, the number of ICAEs is greatly reduced. This results in reducing the assembly area and energy consumption of the ICAEs and the cost and size of the total unit. In addition, two or more feedback means make it possible to reproduce a longer impulse response with high fidelity. To carry out the object, in accordance with another aspect of the invention, an apparatus for reproducing sound including a digital filter adopted to have an FIR filter with a finite derivation length to compose an impulse response from a pre-sound source. -measured to a point of measurement, the FIR filter having delay units, including the digital filter a feedback means having coefficient multipliers and adders, the coefficient multipliers to attenuate the output of any of the delay units, being fed again the attenuated output to the adder that is provided between the delay units placed before any delay unit. Multiple feedback means may be provided. Therefore, even when the apparatus for reproducing sound needs a simple composition and is manufactured at low cost, the apparatus greatly improves the sense of location of a sound image of the head, a sense of advance location of an acoustic image, a quality of the acoustic image, or a sense of location of a sound image compared to the conventional FIR filter that maintains constant the constant sampling frequency and that has the same number of derivations and the same total derivation length.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram showing an FIR filter; Figure 2 is a circuit diagram showing a schematic arrangement of the conventional apparatus for processing an added reverberant signal; Figure 3 is a characteristic view showing a pulse response of the conventional apparatus for processing an added reverberant signal; Figure 4 is a circuit diagram showing a schematic arrangement for a digital filter in accordance with an embodiment of the present invention; Figure 5 is a characteristic view showing an impulse response obtained by the digital filter shown in Figure 4; Figure 6 is a characteristic view showing one. impulse response of a conventional digital filter that has no feedback section; Figure 7 is a circuit diagram showing a schematic arrangement of a digital filter in accordance with another embodiment of the present invention; Figure 8 is a functional diagram showing a schematic layout of a telephone headset system for locating a sound image from the head which is a mode of an apparatus for reproducing sound in accordance with the present invention; Figure 9 is a functional diagram showing an arrangement of a signal processing circuit used in the telephone helmet system for locating a sound image from the head; and Figure 10 is a model view for describing a principle of a loudspeaker system, which is another embodiment of the apparatus for reproducing sound in accordance with the present invention.

DESCRIPTION OF THE PREFERRED MODALITIES Next, the description will be directed towards a digital filter and an apparatus for reproducing sound in accordance with one embodiment of the present invention. First, the digital filter according to one embodiment of the invention will be described with reference to Figures 4 to 7. This mode is related to a digital filter 1 adopted so that a pulse response from a pre-measured sound source to a measurement point is composed through the effect of an FIR filter having a finite derivation length, this impulse response is wrapped in a digital speech input signal and the resulting signal is sent. The digital filter 1 includes a feedback section 6. In the feedback section 6, an output of any delay unit 3n is attenuated by a coefficient multiplier 4. The attenuated output is fed back to an adder 5 placed between a delay unit 32 and a delay unit 33, placed before the delay unit 3n. In Figure 4, a signal input in an input terminal 2 passes through the units 3] _, 32, 33, ..., 3p_2, 3n _] _, 3n, delay, multipliers 71 '72' 73 '74' --- '7m-3' 7m-2 '7m-l And 7m e coefficient, and the adders 8] _, 82, 83, ..., 8n_3, 8n_2, 8n _] _ , and 8n, where the signal is rolled, an impulse response corresponding to a total delay time of all the delay units. On the other hand, in the feedback section 6, a signal collected from the last delay unit 3n is attenuated by the coefficient multiplier 4 and then fed back to the adder 5 placed between the delay units 32 and 33. This signal is delayed by a group of delay units composed of units 3? _, 32, 33, ..., 3n_2, 3n_? and 3n delay. Then, the coefficients are coiled in the signal through the effect of the multipliers 74, ..., 7m-3 '7m-2' 7m-l and 7m of coefficient. In this digital filter 1, the feedback section 6 repeats the aforementioned feedback process to generate an infinitely continuous impulse response. Then, a speech signal corresponds to the characteristic of the impulse response as shown in Figure 5, it is sent to an output terminal 9. In Figure 5, a repetitive portion is formed by the feedback section 6. This figure illustrates the first and second repetitive waveforms. Actually, the waveforms are repeated when infinitely attenuated. On the other hand, for comparison purposes, Figure 6 shows a characteristic of the impulse response in the case where no feedback section 6 is provided. In accordance with the arrangement as shown in Figure 4, a response portion to be added is the impulse response from which a portion is removed around the edge or leading edge. In the reverberant characteristic of a room, for example, the portion around the leading edge of the impulse response is occupied by an initial reflection. However, the second half of the impulse response is removed due to the restriction caused by the derivation length of the FIR filter. This second half of the impulse response is mainly based on scattered sound, which has the different characteristic from the portion around the leading edge of the impulse response. Therefore, the addition of the impulse response from which the portion around the leading edge of the response is removed indicates the addition of the impulse response portion that closes to the scattered sound. The resulting response becomes more natural to be heard. Therefore, this digital filter 1 makes it possible to represent a prolonged impulse response even with the FIR filter having a short branch length. This can greatly reduce the scale of signal processing. If the signal processing unit is composed of ICAEs for digital signals, the necessary number of ICAEs is greatly reduced. Obviously, this results in the reduction of an assembly area of energy consumption and the decrease in the cost of the total system. In turn, the description will be oriented towards a digital filter according to another embodiment of the present invention with reference to Figure 7. This embodiment is related to a digital filter adopted so that a pulse response from a pre-sound source - measurement towards a measurement point is composed through the effect of an FIR filter with a finite derivation length, and this impulse response is wrapped in a digital speed input signal and then the resulting signal is sent. In contrast to the above embodiment, two feedback sections 25 and 28 are provided. In the feedback section 25, an output of any delay unit 22n. _? _ Is attenuated by a coefficient multiplier 23. The attenuated output is fed back to an adder 24 placed between the adjacent delay units placed before the delay unit 22_ ^. In the feedback section 28, an output of any delay unit 22n is attenuated by a coefficient multiplier 26. This attenuated output is fed back to an adder 27 placed between the delay units 222 and 223 located before the delay unit 22n. In Figure 7, an input signal in an input terminal 21 passes through the 22] _, 222, 223, ..., 22n_x, ..., 22n_2, 22n_ ^ and 22n, delay units, the coefficient multipliers 29] _, 292, 293, 294, ..., 29m__, ..., 29m_2, 29m _] _, 29m and the adders 30j_, 3Ü2, 3O3, ..., 30n_ ?, ..., 30n_3, ..., 30n_2, 30n _] _, and 30n that make up the FIR filter. Through the FIR filter, an impulse response corresponding to a total delay time of all the delay units is wound onto the input signal. On the other hand, in the feedback section 25, a signal collected from the delay unit 22n _] _ is attenuated by the coefficient multiplier 23. Then, the signal is applied to the adder 24.

In the feedback section 28, a signal collected from the delay unit 22n is attenuated by the coefficient multiplier 26. Then, the attenuated signal is applied to the adder 27. After the delay time has passed since when the signal is admitted to the input terminal 21 until when the signal is collected from the feedback sections 25 and 28, the coefficient multipliers placed after the adders 24 and 27 function to wind the coefficients in these signals. The signals rolled in the coefficient are then sent. In the impulse response, therefore, the repetitive portion is composed of the overlapped signals of the feedback sections 25 and 28. The content of the repetitive portion repeats infinitely as the different impulse responses are alternately attenuated. In accordance with the arrangement as shown in Figure 7, the response portion to be added is the impulse response from which the portion around the leading edge is removed. Here, in the reverberant feature of the room, the portion around the leading edge of the impulse response is occupied by an initial reflection. However, the second half of the impulse response that is removed due to the restriction caused by the length of the FIR filter bypass is based mainly on the scattered sound. The second half has a different characteristic of the portion around the leading edge of the impulse response. The addition of the response from where the portion around the leading edge is removed indicates the addition of the impulse response near the scattered sound. The resulting response becomes more natural in view of the sense of listening. As described above, the digital filter 20 allows to present a prolonged impulse response even when the FIR filter has a short branch length. This filter 20 makes it possible to greatly reduce the scale of signal processing. If the signal processing unit is composed of ICAEs for digital signals, the required number of ICAEs can be greatly reduced. This results in the reduction of the assembly area and energy consumption and decrease the cost of the total system. In turn, the description will be oriented towards an apparatus for reproducing the sound in accordance with an embodiment of the present invention and with reference to Figures 8 and 9. This embodiment is related to a helmet system 40 for locating a sound image from the head, including digital filters 51L, 51R, 52L and 52R within a signal processing circuit 42. These filters 51L, 51p > , 52L AND 52p > are the same as the digital filter 1 described above. In Figure 8, an input of the analog talk signal in a conversation input terminal 41 is converted into a digital signal through the effect of an analog-to-digital (A / D) converter 42. The digital signal is applied to the signal processing circuit 43 where the signal is filtered for the location of a sound image from the head. The digital conversation signal, filtered by the signal processing circuit 43 for placing the acoustic image from the head, is divided as a stereophonic signal in two systems L and R. The digital signals are converted into the analog signals through the effect of 44 ^ and 44p converters. digital to analog (D / A). The analog signals are amplified in energy by the amplifiers 5L and 45R and then supplied to the sound bodies 46 ^ and 46_ mounted on a headband 46a. For example, the signal processing circuit 43 performs a process of pre-measuring the impulse responses of four systems in total from two sound sources placed in front of a listener to the listener's ears and rolling up the characteristics of the responses of impulse. This processing is implemented by the arrangement as shown in Figure 6. In Figure 9, the input of the signals on the first and second input terminals 50 ^ and 5Ü2 are supplied to the digital filters 51L, 5IR and 52L, 52R, respectively. The outputs of digital filters 51L and 52L are added by a 53L adder. The outputs of the digital filters 51R_ and 52R are added by an adder 53R. The added output of summer 53L is sent to a 5 L output terminal. The added output of adder 53R is sent to an output terminal 54R. As described above, each of the digital filters 51L, 51p_ and 52L, 52R is composed of the same FIR filter of the digital filter 1 mentioned above. The digital filter includes a feedback section in which an output of any delay unit of the FIR filter is attenuated by a coefficient multiplier and the attenuated output is fed back to an adder placed between the adjacent delay units located before the delay unit from where the output is collected. Since the FIR filter can be simplified and reduced in size, the helmet system 40 for locating a sound image from the head is reduced in size and becomes economical.

In addition, each of the filters 51L, 51p > and 52 ^, 52p > The digital array can adopt the same arrangement as the digital filter 20 as shown in Figure 7. As described above, the headset system 40 is adopted so that the digital filter to wind up in the impulse response that is extend from the sound source to both ears include the feedback section to have an output of any delay unit through the effect of the coefficient multiplier and feed again the attenuated output to the adder placed between the adjacent delay units remaining before that unit of delay from where the exit was collected. Therefore, it is possible to improve the sense of location of a sound image from the head or the sense of the forward location of a sound image with simple composition at low cost. In turn, the description will be oriented towards the apparatus to reproduce sound in accordance with another embodiment of the present invention. This modality is related to a loudspeaker system that includes two loudspeakers to fix the acoustic images in different locations. The principle of this loudspeaker system will be described with reference to Figure 10. In a case where the sound sources S] _ and Sr are used to reproduce an acoustic image equivalent to that of the SQ sound source for the ears of a listener M, the sources S_ and Sr of sound are represented as follows: SI =. { (Hol x Hrr) - (Hor x Hrl)} /. { (Hll x Hrr) - (Hlr x Hrl)} . S Sr =. { (Hor x Hll) - (Hol x Hlr)} /. { (Hll x Hrr) - (Hlr x Hrl)} . S where Hll and Hlr are transfer functions from the sound source S ^ to the ears of the listener M, Hrl and Hrr are transfer functions from the source Sr to both ears of the receiver M, and Hol and Hor are transfer functions from the sound source S0 to the ears of the listener M. If the impulse response derived by converting the transfer functions into time axes is wound up on the signals sent to the sound sources S ^ and Sr, the acoustic image is placed in the site of the sound source S0. If this impulse response is composed through the digital filter, the digital filter must be an FIR filter having a long derivation length. The loudspeaker system according to this embodiment includes the same digital filter of the aforementioned digital filters 1 and 20. Therefore, it is possible to place a sound image outside of two loudspeakers with a simple constitution at low cost. If the digital filter is applied to a system that simulates a sound field, a long spare time with a small number of taps can be reproduced. Therefore, reverberation is conventionally generated by adopting the steps of capturing characteristic pulses in a perspective view of the impulse response, and taking more time intervals than the sampling times. In this modality, on the other hand, the reverberation is generated by taking more dense impulses distributed in each sampling time. This makes it possible to reproduce a sound field with high fidelity.

Claims (10)

CLAIMS;

1. A digital filter comprising: an input terminal for admitting a digital signal; a finite impulse response filter for winding an impulse response in the digital signal, the finite impulse response filter comprises a plurality of delay units, a plurality of first adder and a plurality of first coefficient multipliers, the pulse that Answer is obtained by measuring from a source to a measurement point previously; and a feedback means for attenuating an output of any of the delay units through a second coefficient multiplier to again feed the attenuated output to a second adder placed between those adjacent to the delay units and placed before the delay unit power the output.

2. A digital filter comprising: an input terminal for admitting a digital audio signal; a finite impulse response filter for winding an impulse response in the digital audio signal, the finite impulse response filter comprises a plurality of delay units, a plurality of first adders and a plurality of first coefficient multipliers, the impulse response is obtained by measuring from an audio source to a previous measurement point; and a feedback means for attenuating an output of any of the delay units through a second coefficient multiplier and again feeding the attenuated output to a second adder placed between the adjacent units of the delay units and placed before that the delay unit powers the output.

3. A digital filter according to claim 1, wherein a plurality of feedback means is provided.

4. A digital filter according to claim 2, wherein a plurality of feedback means is provided.

5. A digital filter according to claim 1, wherein the number of delay units is the same as the same number of first adders.

6. A digital filter according to claim 2, wherein the number of the delay units is the same as the number of the first adders.

7. A digital filter according to claim 1, wherein a coefficient of compliance with the impulse response is provided to the first coefficient multiplier.

8. A digital filter according to claim 2, wherein a coefficient of compliance with the impulse response is provided to the first coefficient multiplier.

9. An apparatus for reproducing sound comprising: a digital filter for filtering an input audio signal; and a loudspeaker to send sounds; wherein the digital filter comprises: an input terminal for admitting a digital audio signal; a finite impulse response filter for winding an impulse response in the digital audio signal, the finite impulse response filter comprises a plurality of delay units, a plurality of first adders and a plurality of first coefficient multipliers, the impulse response is obtained by measuring from a sound source to a previous measurement point; and a feedback means for attenuating an output of any of the delay units through a second coefficient multiplier and re-feeding the attenuated output to a second coefficient multiplier and re-feeding the attenuated output to a second adder placed between an adjacent unit of the delay units and which is placed before the delay unit powers the output.

10. An apparatus for reproducing sound comprising: a signal processing means for locating a sound image in a predetermined position, the signal processing means having a digital filter; and a loudspeaker to send sound; wherein the digital filter comprises: an input terminal for adm: .tir a digital audio signal; a finite impulse response filter for winding an impulse response in the digital audio signal, the finite impulse response filter comprises a plurality of delay units, a plurality of first adders and a plurality of first coefficient multipliers, the impulse response is obtained by measuring from a sound source to a previous measurement point; and a feedback means for attenuating an output of any of the delay units through a second coefficient multiplier and feeding the attenuated output back to a second adder placed between the adjacent units of the delay units and which is placed before that the delay unit powers the output. - li ¬ l i. A sound reproduction apparatus comprising: a head fixing means for fixing the sound reproducing apparatus in a head of a user; a loudspeaker to send a sound, the loudspeaker is provided in the head fixing means; and a processing means of sena_.es to place the sound image on the outside of the head, the signal processing means has a digital filter; wherein the digital filter comprises: an input terminal for admitting a digital audio signal; a finite impulse response filter for winding an impulse response in the digital audio signal, and the finite impulse response filter comprises a plurality of delay units, a plurality of first adders and a plurality of first coefficient multipliers, the impulse response is obtained by measuring from a sound source to a previous measurement point; and a feedback means for attenuating an output of any of the delay units through a second coefficient multiplier and feeding the attenuated output back to a second adder placed between the adjacent units of the delay units and which are placed before that the delay unit powers the output.