JP5980133B2 - Zero cross detector - Google Patents

Description

  The present invention relates to a zero-cross detection device, and in particular, is suitable for use in a device that performs zero-cross detection in a digital amplifier that processes an audio signal by PWM modulation.

  In general, an audio apparatus often performs an operation of switching audio volume by volume adjustment processing, mute processing, or the like. However, when the volume is switched in a state where the amplitude of the audio signal is large, the amplitude of the signal changes abruptly, so that pop noise often occurs, which makes the user uncomfortable. When a speaker with low input resistance is used, if pop noise continues to occur, the speaker may eventually be destroyed.

  In order to avoid the occurrence of such pop noise, a method called zero cross detection has been proposed (see, for example, Patent Document 1). This zero cross detection is a method of detecting that the amplitude of the signal is close to the zero level and switching ON / OFF of the volume adjustment process and the mute process at that timing.

  When an analog audio signal is handled, the zero cross detection device detects a level near 0 [V] in amplitude and outputs a zero cross point trigger signal. When a digital audio signal (PCM signal) is handled, the zero cross detection device detects that the amplitude of the digital data has become a certain value or less and outputs a zero cross point trigger signal. By using this trigger signal for the operation timing of the volume adjustment process and the mute process, it is possible to minimize the pop noise generated during the operation of the volume adjustment process and the mute process.

Japanese Patent Laid-Open No. 10-177044

  As described above, when the audio signal is an analog signal or a PCM digital signal, the zero cross point can be detected by determining the amplitude value. However, there is a problem that the conventional zero-cross detection method cannot be applied to an audio signal subjected to PWM modulation (Pulse Width Modulation), which has been rapidly increasing recently, because the amplitude does not change.

  The present invention has been made to solve such a problem, and an object of the present invention is to enable zero cross detection on a PWM modulated audio signal.

In order to solve the above-described problem, in the present invention, it is determined whether the PWM wave duty of the PWM-modulated audio signal is 50%, and the PWM wave duty is 50%. In a configuration including a zero-cross detection unit that detects the determined timing as a zero-cross point , the zero-cross detection unit is a PWM wave of a PWM-modulated audio signal, and is one of two PWM waves that are in a phase-inverted relationship with each other. An integrator that calculates the average value of the voltage by integrating each period, and a comparator that compares the two average values calculated by the integrator and outputs a zero-cross point trigger signal if they match And so on.

  According to the present invention configured as described above, the zero cross point is detected based on the duty of the PWM wave, not the amplitude of the PWM-modulated audio signal. PWM modulation is a method of expressing the signal level by changing the duty of one cycle. Here, when the output of the digital amplifier has a positive amplitude, the duty that is positive increases (duty is greater than 50%), and when the output is a negative amplitude, the duty that is negative increases (duty is greater than 50%). Smaller). That is, in order for the output of the digital amplifier to become zero, it is a condition that the positive and negative duties are equal (duty is 50%). Therefore, by detecting the timing when the duty becomes 50%, zero-cross detection can be realized also for the audio signal modulated by PWM.

It is a figure which shows the principal part structural example of the audio apparatus containing the zero crossing detection apparatus by 1st Embodiment. It is a figure for demonstrating operation | movement of the zero cross detection apparatus by 1st Embodiment. It is a figure which shows the principal part structural example of the audio apparatus containing the zero cross detection apparatus by 2nd Embodiment. It is a figure for demonstrating operation | movement of the zero crossing detection apparatus by 2nd Embodiment. It is a figure which shows the principal part structural example of the audio apparatus containing the zero cross detection apparatus by 3rd Embodiment. It is a figure for demonstrating operation | movement of the counter by 3rd Embodiment.

(First embodiment)
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a first embodiment of the invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a configuration example of a main part of an audio device including a zero-cross detection device according to the first embodiment. As shown in FIG. 1, the audio apparatus according to the first embodiment includes a PWM modulation circuit 1, an FET driver circuit 2, FETs 3A and 3B, an LC filter 4, a speaker 5, and a zero cross detection device 10. Among these, the FET driver circuit 2, the FETs 3A and 3B, and the LC filter 4 constitute a digital amplifier (class D amplifier).

  The PWM modulation circuit 1 performs PWM modulation on an input audio signal and outputs a PWM wave of the audio signal that has been PWM modulated. Since this PWM modulation circuit 1 can apply a known technique, a detailed description is omitted.

  The FET driver circuit 2 controls on / off of the two FETs 3A and 3B connected in half bridge based on the PWM wave output from the PWM modulation circuit 1. That is, the FET driver circuit 2 generates another PWM wave having a phase inversion relationship with the PWM wave output from the PWM modulation circuit 1. Then, by supplying two PWM waves having a phase inversion relationship to the two FETs 3A and 3B, the on / off of the FETs 3A and 3B is controlled.

  Basically, the PWM wave is a rectangular wave, and the FETs 3A and 3B are turned on while the rectangular wave is “Hi”. Since the PWM waves supplied to the two FETs 3A and 3B are in a phase-inverted relationship with each other, only one of the FETs 3A and 3B is turned on. As a result, a rectangular wave is output from the FETs 3A and 3B so that the FET 3A on the power supply side is at a high level when the FET 3A is on and the low level when the FET 3B on the ground side is on.

  The LC filter 4 performs a low-pass filter process on the rectangular wave output from the FETs 3A and 3B, and outputs an audio signal obtained as a result to the speaker 5.

  The zero-cross detection device 10 determines whether or not the PWM wave duty of the PWM-modulated audio signal is 50%, and the timing at which the PWM wave duty is determined to be 50% is set as the zero-cross point. To detect. Specifically, the zero-cross detection device 10 includes an integrator 11 and a comparator 12.

  The integrator 11 integrates one period of the PWM wave output from the PWM modulation circuit 1 to calculate an average voltage value. The comparator 12 compares the average value of the voltage calculated by the integrator 11 with a reference value set in advance as the average value of the voltage when the duty of the PWM wave is 50%, and the two match. In this case, a zero cross point trigger signal is output.

  FIG. 2 is a diagram for explaining the operation of the zero-cross detection device 10 configured as described above. FIGS. 2A and 2B show Case 1 in which the duty of the PWM wave is smaller than 50%. FIGS. 2C and 2D show Case 2 in which the duty of the PWM wave is greater than 50%. FIGS. 2E and 2F show Case 3 in which the duty of the PWM wave is equal to 50%.

  FIGS. 2A, 2 </ b> C, and 2 </ b> E of cases 1 to 3 show one period of the PWM wave output from the PWM modulation circuit 1. 2B, 2D, and 2F of cases 1 to 3 show an average voltage Vave and a predetermined reference value Vref obtained as a result of integrating one period of the PWM wave by the integrator 11. ing. This reference value Vref is set to the same value as the average value of the voltage when the duty of the PWM wave is 50%.

  In case 1, since the duty of the PWM wave is smaller than 50%, the average value Vave of the voltage calculated by the integrator 11 is smaller than the predetermined reference value Vref. On the other hand, in case 2, since the duty of the PWM wave is larger than 50%, the average value Vave of the voltage calculated by the integrator 11 is larger than the predetermined reference value Vref. On the other hand, when the duty of the PWM wave is equal to 50% as in case 3, the average value Vave of the voltage calculated by the integrator 11 is equal to the predetermined reference value Vref. In this case, the comparator 12 outputs a zero cross point trigger signal.

  PWM modulation is a method of expressing the signal level by changing the duty of one cycle. Here, when the output of the digital amplifier has a positive amplitude, the duty that is positive increases (duty is greater than 50%), and when the output is a negative amplitude, the duty that is negative increases (duty is greater than 50%). Smaller). That is, in order for the output of the digital amplifier to become zero, it is a condition that the positive and negative duties are equal (duty is 50%).

  Therefore, the zero cross detection device 10 is configured by using the integrator 11 and the comparator 12, thereby detecting the timing at which the duty of the PWM wave becomes 50%, thereby realizing the zero cross detection even for the PWM modulated audio signal. can do.

  In the first embodiment, the configuration in which the zero-cross detection device 10 is connected from the output terminal of the PWM modulation circuit 1 has been described. However, the present invention is not limited to this. For example, the zero cross detection device 10 may be connected from the output ends of the FETs 3A and 3B. However, since the amplitude of the rectangular wave output from the FETs 3A and 3B is larger than the amplitude of the rectangular wave output from the PWM modulation circuit 1, a predetermined reference value Vref is set larger or the integrator 11 It is necessary to insert a voltage dividing resistor in the previous stage.

(Second Embodiment)
Hereinafter, a second embodiment of the present invention will be described with reference to the drawings. FIG. 3 is a diagram illustrating a configuration example of a main part of an audio device including the zero cross detection device according to the second embodiment. 3 that have the same reference numerals as those shown in FIG. 1 have the same functions, redundant description thereof will be omitted here.

  As shown in FIG. 3, the audio apparatus according to the second embodiment includes a PWM modulation circuit 1, an FET driver circuit 2, FETs 3 </ b> A and 3 </ b> B, an LC filter 4, a speaker 5, and a zero cross detection device 20. The zero-cross detection device 20 includes two integrators 21A and 21B and a comparator 22.

  The two integrators 21A and 21B respectively integrate two PWM waves output from the FET driver circuit 2, that is, one cycle of two PWM waves having a phase inversion relationship with each other to calculate an average voltage value. To do. The comparator 22 compares the two voltage average values calculated by the two integrators 21A and 21B, and outputs a zero cross point trigger signal when they match. The comparator 22 can be constituted by a differential amplifier, for example.

  FIG. 4 is a diagram for explaining the operation of the zero-cross detection device 20 configured as described above. 4A and 4B show Case 1 in which the duty of the PWM wave output from the PWM modulation circuit 1 is smaller than 50%. FIGS. 2C and 2D show Case 2 in which the duty of the PWM wave output from the PWM modulation circuit 1 is greater than 50%. FIGS. 2E and 2F show Case 3 in which the duty of the PWM wave output from the PWM modulation circuit 1 is equal to 50%.

  4A, 4C, and 4E of cases 1 to 3 show one cycle of two PWM waves output from the FET driver circuit 2. FIG. 4B, 4D, and 4F of cases 1 to 3 are obtained as a result of integrating one period of two PWM waves output from the FET driver circuit 2 by the integrators 21A and 21B, respectively. Average values Vave1 and Vave2 of the two voltages are shown. Here, one voltage average value Vave1 is calculated based on the PWM wave output from the PWM modulation circuit 1, and the other voltage average value Vave2 is calculated based on the PWM wave whose phase is inverted. .

  In case 1, since the duty of the PWM wave output from the PWM modulation circuit 1 is smaller than 50%, the two voltage average values Vave1 and Vave2 calculated by the two integrators 21A and 21B are different values. The same applies to the case 2 in which the duty of the PWM wave output from the PWM modulation circuit 1 is greater than 50%. On the other hand, when the duty of the PWM wave output from the PWM modulation circuit 1 is equal to 50% as in the case 3, the two voltage average values Vave1 and Vave2 calculated by the two integrators 21A and 21B are equal. . In this case, the comparator 22 outputs a zero cross point trigger signal.

  As described above, also in the second embodiment, the zero cross detection device 20 is configured using the integrators 21A and 21B and the comparator 22, thereby detecting the timing at which the duty of the PWM wave becomes 50%. Zero-cross detection can also be realized for a PWM modulated audio signal.

(Third embodiment)
Hereinafter, a third embodiment of the present invention will be described with reference to the drawings. FIG. 5 is a diagram illustrating a configuration example of a main part of an audio apparatus including a zero cross detection apparatus according to the third embodiment. In FIG. 5, those given the same reference numerals as those shown in FIG. 1 have the same functions, and therefore redundant description is omitted here.

  As shown in FIG. 5, the audio apparatus according to the third embodiment includes a PWM modulation circuit 1, an FET driver circuit 2, FETs 3 </ b> A and 3 </ b> B, an LC filter 4, a speaker 5, and a zero-cross detection device 30. The zero-cross detection device 30 includes a counter 31 and a comparator 32.

  The counter 31 obtains one period of the PWM wave output from the PWM modulation circuit 1, and obtains a time interval from the rising edge to the falling edge of the PWM wave and a time from the falling edge to the rising edge of the PWM wave. Each interval is calculated.

  FIG. 6 is a diagram for explaining the operation of the counter 31. As shown in FIG. 6, the counter 31 uses a reference pulse with a short cycle, the number of pulses m from the rising edge to the falling edge of the PWM wave, and the number of pulses n from the falling edge to the rising edge of the PWM wave. And count. These pulse numbers m and n are output as time intervals in the high and low intervals, respectively.

  The comparator 32 compares the two time intervals (number of pulses m and n) calculated by the counter 31, and outputs a zero cross point trigger signal when they match. Also in the third embodiment configured as described above, zero-cross detection can also be realized for a PWM-modulated audio signal.

  In the third embodiment, the configuration in which the zero-cross detection device 30 is connected from the output terminal of the PWM modulation circuit 1 has been described. However, the present invention is not limited to this. For example, the zero cross detection device 30 may be connected from the output ends of the FETs 3A and 3B.

  In addition, each of the first to third embodiments described above is merely an example of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed as being limited thereto. It will not be. That is, the present invention can be implemented in various forms without departing from the gist or the main features thereof.

DESCRIPTION OF SYMBOLS 10 Zero cross detection apparatus 11 Integrator 12 Comparator 20 Zero cross detection apparatus 21A, 21B Integrator 22 Comparator 30 Zero cross detection apparatus 31 Counter 32 Comparator

Claims (1)

A zero-cross detector that determines whether or not the PWM wave duty of the PWM-modulated audio signal is 50%, and detects the timing at which the PWM wave duty is determined to be 50% as a zero-cross point equipped with a,
The zero-crossing detecting unit integrates one period of two PWM waves that are PWM waves of the PWM-modulated audio signal and have a phase inversion relationship with each other, and calculates an average value of the voltage; ,
A zero cross detection apparatus comprising: a comparator that compares two average values calculated by the integrator and outputs a trigger signal of the zero cross point when the two values match .

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