CN100478810C - Beat number detector - Google Patents

Abstract

The invention is a beat number detecting device, including: selecting device, to select special frequency form input audio signal; transient detecting device, to detect the transient from the output of the selecting device; timer, to determine integrator window length; period detecting device, to detect time interval and determine BPM mean; and display logic device, to determine the most credible BPM mean.

Description

Beat count detection device

technical field

The invention relates to a device for detecting the number of beats per unit time in music.

Background technique

Beats per minute (BPM) detectors have a variety of applications. A BPM detector is a device that detects the number of beats per minute in a piece of music.

In the application of DJ (popular music announcer), it is necessary to detect the BPM values of two different music sources to compare them, so as to seamlessly transition from one music source to another. In MIDI applications, the BPM detector can be used to synchronize the tempo of a MIDI sequencer to an existing track. In the music database system, dance music can be classified and retrieved according to the BPM speed value.

A common way to detect the number of beats in a piece of music is to first pick a beat position from the audio signal. In such a system, the user manually inputs data at the position of a beat. This just requires the user to listen to the audio signal while completing the knocking action, and no mistakes can occur from the beginning to the end of the audio signal, so the user is required to concentrate and stick to the end. The user is forced to operate for a long time, and the operation time is determined by the length of the music. The system is always subject to user actions.

Recently, a great deal of research and development has been conducted on devices and processing methods for detecting the number of beats of a musical piece.

For example, US Patent No. 5,614,687 discloses a device for detecting the number of beats, which is hereby incorporated by reference. In order to detect the number of beats in a musical piece, such known devices use a variable threshold. This variable threshold is obtained under the assumption that no transient events occur for a certain time interval. During this time, the maximum value of the audio signal is detected, and then it is multiplied by a constant factor, and the result of the product will be used as a threshold. In the following time interval, if the signal exceeds this threshold, it will be detected. , which indicates that an emergency has occurred. The time interval between two adjacent bursts is measured. This step is performed in three different frequency bands of the input signal.

The disadvantage of this existing device for detecting the number of beats is that at least 5 to 6 beats are needed to detect the BPM value.

Contents of the invention

It is therefore an object of the present invention to provide a device which can detect the number of beats of a musical piece with high precision in a detection time of only 3 beats.

The real-time operation of current BPM detectors is usually based on autocorrelation and variable threshold principles.

According to the beat detection device of the present invention, it is characterized in that it comprises: a band-pass filter for selecting a specific frequency from an input audio signal; a transient value detection device for detecting a transient value from the output of the band-pass filter; a device for determining the length of the integrator window; period detection means for examining the measured time interval and determining the BPM average; and display logic means for determining the most plausible measured value from the above period detection means BPM value.

According to the present invention, the detection accuracy of the BPM detector can be as high as ±0.1 BPM.

Description of drawings

Objects and features of the present invention will become more clear through the following detailed description and accompanying drawings.

Fig. 1 shows a beat detection device according to the present invention;

Fig. 2 is the structural block diagram of transient value detector;

Figure 3 shows a window integrator according to the invention;

Figure 4 shows the time relationship between delay length and constant time interval;

Figure 5 shows a threshold circuit according to the invention;

Figure 6 shows a process for detecting the maximum value of the measured energy value;

Fig. 7 shows the flow chart that the measured energy value in several retarders is linearly attenuated;

Figure 8 shows a flow diagram of the operation of the cycle detector; and

FIG. 9 shows a flow chart showing the operation of the logic unit.

Detailed ways

A specific embodiment of the present invention will be described in detail below according to the accompanying drawings.

FIG. 1 shows a tempo detection device according to the present invention.

In this BPM detection device, an analog audio signal is first input into the A/D converter 1 . Thus, through the A/D converter 1, the input analog audio signal is converted into a digital audio signal. The converted digital audio signal serves as input to each BPM detector 101, 102, . . . , 10n.

FIG. 1 shows all elements of the BPM detector 101 . Since all BPM detectors 101 , 102 , . . . , 10n differ only in the parameters of their respective bandpass filters, only BPM detector 101 is shown in detail.

It can be seen from FIG. 1 that the BPM detector 101 includes a bandpass filter 2 , a transient value detector 3 , a timer 4 and a period detector 5 . The band-pass filter 2 is connected with the A/D converter 1 to receive the input digital audio signal. Bandpass filter 2 is designed as a very narrow bandwidth filter (high Q). The center frequency of the bandpass filter 2 is a specific frequency, such as 80Hz. For different BPM detectors 101, 102, . . . , 10n, each BP filter is set to only one center frequency. The different center frequencies to which each BP filter in the different BPM detectors are set are preferably in the range of very high and low frequencies in the spectrum of audible sounds, typically with Useful for monitoring rhythm instruments such as hi-hats and bass drums.

The output of the bandpass filter 2 is connected to a transient value detector 3 . The filtered digital signal obtained from the bandpass filter 2 is input to and analyzed by the transient detector 3 .

Figure 2 is a block diagram of the transient value detector.

It can be seen from FIG. 2 that the transient value detector 3 includes a window integrator 201 , a threshold circuit 202 , a switch 203 , a timer 205 and a local maximum value detector 204 . Window integrator 201 receives the filtered digital signal from bandpass filter 2 . The detailed processing of the digital signal will be described below with reference to FIG. 3 .

Figure 3 shows a windowed integrator according to the invention.

The integrator 201 includes a multiplier 301 for receiving the input digital signal output from the bandpass filter 2 . It can be seen from FIG. 3 that the digital signal output from the band-pass filter 2 is first multiplied by itself in the multiplier 301 . The output terminal of the multiplier 301 is connected with a delay 302 and an adder 304 . The output of the multiplier 301 is first sent to the delayer 302 and delayed by a length of tw. The tw length is the length of the window integrator. The output terminal of the delayer 302 is connected with the multiplier 303 .

In this way, the delayed digital signal output by the delayer 302 is multiplied by “-1”, and the output terminal of the multiplier 303 is connected to the adder 304 . It can be seen from FIG. 3 that the output signal of the multiplier 303 and the input signal of the delayer 302 are added through the adder 304 , and the result of the addition in the adder 304 is then sent to the input end of the delay element 305 .

The output terminals of the delay element 305 are respectively connected with the input terminals of the adder 304 , the constant multiplier 306 and the threshold circuit 202 . In this way, the output of the delayer 302 is added to the input of the delayer 302 and the output of the delay element 305 after being multiplied by "-1", to obtain a rectangle with a width of tw for measuring the average energy value in a frequency band Integral window.

Then, the output of the delay element 305 is multiplied by a constant factor "C", that is, the measured energy value can be adjusted by the constant factor "C". The output terminal of the constant multiplier 306 is connected to the switch 203 . Thus, the output of the constant multiplier 306 is transferred to the switch 203 . In this embodiment, the switch 203 is a gate circuit. But the present invention is not limited thereto.

Timer 4 controls the switch 203 to close once in a time interval of ts. The measured energy value adjusted by the constant factor "C" is output in a constant time period of ts. When the gate is closed, the measured energy value is passed to the local maximum detector 204 .

Timer 4 is also connected to timer 205 . Every ts Timer4 keeps incrementing the value of Timer 205 which will be used in Local Maximum Detector 204 .

FIG. 4 shows the time relationship between the delay length and the constant time interval.

The digital signal output from the multiplier 301 is delayed by the time length tw by the delayer 302 . Timer 4 generates a constant time period ts. In order to make the integration windows overlap as shown in FIG. 4 , it is necessary to select the constant time period ts to be shorter than the delay length tw. For example, in this embodiment, constant time ts=0.5*delay time length tw.

Figure 5 shows a threshold circuit according to the invention.

To monitor the average energy level in the frequency band, a standard peak-and-hold circuit is used to generate a signal threshold. The threshold circuit 202 includes a comparator 501 , a delay 502 , a constant multiplier 503 , a delay 504 and a constant multiplier 505 . It can be seen from FIG. 5 that the output of the delayer 504 is multiplied by the constant factor C _Rel and then input to the comparator 501 . In the comparator 501 , the energy value output from the delayer 504 and then multiplied by the constant factor C _Rel is compared with the measured energy value obtained from the window integrator 201 . The larger value is sent to the delayer 504, and the value is delayed by the delayer 502 for a time length of 5*ts. The output of delayer 502 is multiplied by a constant factor C by constant multiplier 503 and sent to local maximum detector 204 .

FIG. 5 shows that after the output of the comparator 501 is delayed by 5*ts, its value is multiplied by a constant factor C less than 1.0. However, the length of the delay is not limited to the above-mentioned length of time. This length of time can be modified without departing from the scope of the present invention.

Fig. 6 shows a procedure for detecting the maximum value of the measured energy value.

Referring to FIG. 6 , the output of the window integrator 201 divided into different time segments is fed into a delay line 601 . The delay line 601 includes 10 delayers 611 , 612 , . . . , 620 . The energy value output by the fifth delay 615 is called X(n). The first 5 and last 5 measured energy values are called X(n-5), X(n-4), X(n-3), X(n-2), X(n-1), X (n+1), X(n+2), X(n+3), X(n+4), and X(n+5). Suppose X(n) is the local maximum energy value.

Referring to Fig. 6, in step S61, check whether the measured energy value X(n) is among 11 energy values X(n-5), X(n-4), ..., X(n+5) maximum value. If it is checked that the measured energy value X(n) is the maximum value among the 11 energy values, the process goes to step S62. However, if it is checked that the energy value X(n) output from the delayer 615 is not the maximum value among the 11 energy values, the process ends in step S61.

In step S62, it is checked whether the energy value X(n) exceeds the threshold output by the threshold circuit. In order to avoid BPM detection when no audio signal exists, it is also checked whether the energy value X(n) is lower than the specified minimum energy standard MinLevel. If the energy value X(n) is greater than the threshold value and greater than the prescribed minimum energy value MinLevel, the flow goes to step S63. Otherwise, the process ends at S62.

In step S63, the two measured energy values before and after the energy value X(n) also need to meet two conditions:

X(n-2)<X(n-1)&X(n+1)>X(n+2) (1)

Wherein X(n-2) is the energy value of the output of delayer 613, X(n-1) is the energy value of delayer 614 output, and wherein X(n+1) is the energy value of delayer 616 output, and wherein X (n+2) is the energy value output by the delayer 617 .

In step S63, it is checked whether the energy value X(n-1) exceeds the energy value X(n-2), and whether the energy value X(n+1) exceeds the energy value X(n+2). If it is checked in step S63 that the condition defined by formula (1) is satisfied, the flow goes to step S64. Otherwise, the process ends in step S63.

This system is designed for the measurement of the possible maximum BPM value of 158BPM. However, considering that some percussion instruments in the music signal may produce a transient value equivalent to 2 times or 4 times the actual BPM value, the minimum time interval between the two detected transient events should not be less than 90ms.

Therefore, we assume the minimum time interval tmin=90ms of the present invention.

In step S64, the value of the timer 205 is compared with the minimum time value tmin. If it is checked that the value of the timer 205 is smaller than the minimum time value tmin, the flow ends in step S64. In this way, all local maxima arising during the tmin period from previously detected transients are ignored. However, if the value of the checked timer 205 is greater than the minimum time value tmin, the flow proceeds to step S65.

In step S65, the energy values output by the 3rd, 4th, 6th and 7th delayers 613, 614, 616, 617 are subjected to linear attenuation processing. The time value Δt can be determined by this linear decay process. The detailed processing of the linear attenuation will be described with reference to FIG. 7 . Then, the flow goes to step S66.

In step S66 , the calculated time value Δt is added to the value of the timer 205 . In this way, the measured time period tp is equal to the sum of the timer value and the calculated time value Δt. Afterwards, the measured time value of the timer 205 is transmitted to the period detector 5 . Then, the timer 205 is re-initialized to Δt.

Since the presence of the local maximum is detected only in a time period of length ts, it is clear that the local maximum position can only be determined with an accuracy of ±0.5*ts (because the timer is incremented with ts in each step). To get a more precise location of the local maximum, a four-point linear decay is calculated using the first two and last two measured energy values of delayer 615 .

Fig. 7 shows a flow chart for linear attenuation of measured energy values in several retarders.

Referring to FIG. 7, the X-axis represents time, and the Y-axis represents energy values. It can be seen from Figure 7 that the two measured values X(n-2) and X(n-1) and the two measured values X(n+1) and X(n+2) are connected by a straight line . Thus, the intersection of these two lines indicates the exact location of the local maximum:

Δt=[5*X(n+1)-4*X(n+2)+X(n-1)-2*X(n-2)]/[X(n-1)-X(n- 2)-X(n+2)+X(n+1)] (2)

Fig. 8 shows a flowchart of the operation of the cycle detector.

Referring to FIG. 8, the process starts at step S801. First, in step S801, the measured time interval tp is first converted into a BPM value using the following formula:

BPMnew＝60/tp (3)

Where BPMnew is a newly determined BPM value, and tp is the measured time interval output from the local maximum detector.

It is assumed that the time interval to be measured can be obtained from a beat which is 1/2, 1/4 or 2 times the actual BPM value of the music piece to be detected. Given that the BPM detector is only used to determine BPM values within the range of 60 to 158 BPM, it is assumed that BPM values below or above this range are likely to be multiples of the actual BPM value. For this reason, in order to make the value of BPMnew fall within a desired range, the newly determined BPM value BPMnew is multiplied by a factor of 2, 4, or 0.5 according to the BPMnew value.

After that, the flow goes to step S802. In step S802, it is checked whether the newly determined BPM value BPMnew is lower than 60 BPM. If it is lower than 60 BPM, the flow goes to step S803, otherwise the flow goes to step S804.

In step S803, the newly determined BPM value is doubled using the following formula:

BPMnew＝2*BPMnew (4)

Then, the flow goes to step S804. In step S804, it is checked again whether the newly determined BPM value BPMnew is lower than 60 BPM. If it is lower than 60 BPM, the flow goes to step S805, otherwise, the flow goes to step S806.

In step S805, the newly determined BPM value BPMnew using formula (4) is doubled. Then, the flow goes to step S806.

In step S806, it is checked whether the newly determined BPM value BPMnew is greater than 158 BPM. If it is greater than 158BPM, the flow goes to step S807, otherwise, the flow goes to step S808.

In step S807, the newly determined BPM value BPMnew is halved by using the following formula:

BPMnew＝0.5*BPMnew (5)

Then, the flow goes to step S808. In step S808, the average value BPMavr of the previously measured BPM values can be obtained by using the following formula:

BPMavr=SUM/NUMBER (6)

Among them, BPMavr is the average value of the previously measured BPM values; SUM is the sum of the previous BPM values; NUMBER is the number of accumulated BPM values.

After that, the flow goes to step S809. In step S809, the newly detected BPM value BPMnew is compared with the average BPMavr of the previously measured BPM values and it is determined whether the absolute value of the difference between the two values is smaller than the current value ΔBPMmax.

If the absolute value of the difference between the newly detected BPM value BPMnew and the average BPMavr of the previously measured BPM values is greater than the current value of ΔBPMmax, the new detection is considered to be a failure. The failure flag FAIL is set to 1.

If the absolute value of the difference is within the range of the current value ΔBPMmax, the flow proceeds to step S813. Otherwise, the process goes to step S810.

In step S813, the newly detected BPM value BPMnew is added to the sum of the previous BPM values. And, the number of accumulated BPM values increases by 1. Then the flow goes to step S814.

In step S810, it is checked whether the failure flag FAIL is smaller than the value "2". If the failure flag FAIL is less than "2", the flow proceeds to step S812. In step S812, the failure flag FAIL is incremented by 1 and the flow ends in step S812. Otherwise, if the failed flag FAIL is not less than 2 after checking in step S810, then the flow goes to step S811. In step S811, both the sum of the previous BPM values SUM and the number of accumulated BPM values NUMBER are initialized to "0". Then the flow ends in step S811.

Returning to step S814, it is checked whether the number NUMBER of the accumulated BPM values is greater than or equal to "3". If it is less than 3, the flow ends at step S814. Otherwise, the flow goes to step S815 and the failure flag FAIL is reset to "0". Then the flow goes to step S816.

In step S816, formula 6 is used to recalculate the average BPMavr of the previously measured BPM values. Finally, both the average value BPMavr of the previously measured BPM values and the number NUMBER of accumulated BPM values are sent to the display logic unit 6 .

FIG. 9 shows a flow chart showing the operation of the logic unit.

1 and 9, the display logic unit 6 first receives the average BPMavr of the previously measured BPM values and the number of accumulated BPM values from the cycle detectors of each BPM detector 101, 102, ..., 10n NUMBER.

In step S901, a variable "i" is used as a loop indicator variable and is first initialized to "2". Also, the variable ibest is initialized to "1".

Assume that the average value BPMavr of the BPM values measured in front of all BPM detectors 101, 102, ..., 10n and the number NUMBER of the accumulated BPM values are stored in the array variables BPMavr (i) and NUMBER (i) . This means that the average BPMavr of the measured BPM values in the BPM detector 101 is stored in BPMavr(1) and the number of accumulated BPM values is stored in NUMBER(1). The average value BPMavr of the measured BPM values in the BPM detector 102 is stored in BPMavr(2) and so on.

To determine which BPM detector currently stores the maximum NUMBER, a loop is used to get an indication of the maximum NUMBER.

In step S902, the contents of variables NUMBER(i) and NUMBER(ibest) are compared. Only when the value NUMBER(i) is greater than the value NUMBER(ibest), the content of the loop indicator variable "i" is stored in ibest. Next, the loop variable "i" is incremented by "1". Then the flow goes to step S903. In step S903, it is checked whether "i" is equal to or greater than "n", where "n" is the number of BPM detectors plus 1. If "i" is smaller than "n", the process returns to step S902. Otherwise, the process goes to step S904.

In step S904, it is checked whether the value of NUMBER(ibest) in the BPM detector storing the largest NUMBER value is equal to or greater than "3". If it is less than 3, the flow ends in step S904. However, if it is equal to or greater than 3, the flow advances to step S905. Then, the determined BPM value BPMavr(ibest) is output to the display unit 7 . In this way, the number of beats of a piece of music is accurately detected.

The above-described embodiment is specially adapted to detect the number of beats of a piece of music. Given that this device is mainly used in "DJ equipment", the BPM detector is designed to have a tempo range between 60 and 158BPM. However, the present invention is not limited thereto, and it is also applicable to other detection methods, such as methods for detecting beat numbers of other audio signals.

Many other changes and modifications can be made without departing from the scope and spirit of the invention. It should be understood that the invention is not limited to the particular embodiments, but that the scope of the invention is defined by the appended claims.

Claims (10)

1. A beat detection device, comprising: a bandpass filter (2) for selecting specific frequencies from the input audio signal; A transient value detection device (3), used for detecting transient values from the output of the bandpass filter (2); Timer (4), is used for determining integrator window length; Period detection means (5) for checking the measured time interval and determining the average value of BPM; and Display logic means (6) for determining the most plausible measured BPM value from the period detection means (5) described above. 2. The beat number detection device according to claim 1, wherein the above-mentioned transient value detection device (3) comprises: A window integrator (201), used to generate a rectangular integration window to detect the average energy value in the frequency band; Threshold means (202), for generating a signal threshold to monitor the average energy value within the frequency band; local maximum detection means (204) for determining the maximum energy value from the output of said window integrator (201); and A switch (203), configured to provide the energy value output from the window integrator (201) to the local maximum detection device (204) under the control of the timer (4). 3. The beat number detection device according to claim 2, wherein the above-mentioned transient value detection device (3) also includes: The timer (205) is used for calculating time according to the above-mentioned timer (4) and detecting the time interval between two measured transient values. 4. The beat number detection device according to claim 2, wherein the above-mentioned transient value detection device (3) further comprises: A constant multiplier (306) for multiplying the energy value output from the window integrator (201) and outputting it to the local maximum detection means (204). 5. The beat number detection device according to claim 2, wherein the constant time interval (ts) generated by the timer (4) is shorter than the delay length (tw) generated by the window integrator (201). 6. The beat number detection device according to any one of claims 2 to 5, wherein the above-mentioned local maximum detection device (204) further comprises: A plurality of delayers (611, ..., 620), used to delay the energy value output from the above-mentioned window integrator (201); the above-mentioned local maximum detection device (204) On top of that, linear attenuation processing is performed, and the detected time interval is output to the above-mentioned cycle detection device (5). 7. The beat number detection device according to claim 1, wherein the period detection device (5) determines the average value of the previous BPM values measured according to the sum of the previous BPM values and the number of accumulated BPM values. 8. The beat number detection device according to claim 1, wherein said period detection device (5) compares the difference between the newly detected BPM value and the average value of the previously detected BPM values. 9. The number of beat detection device according to claim 1, wherein the above-mentioned display logic device (6) determines that the average value of the BPM values detected in front of the output from the BPM detector is the most credible BPM value, wherein the BPM The number of accumulated BPM values in the detector is the largest among the BPM detectors (101, . . . , 10n). 10. The beat number detection device according to claim 1, further comprising display means (7) for displaying in synchronization with generation of the measured BPM value.

Images