Audio amplifier

This invention relates to an audio amplifier.

In an audio amplifier, if a power amplifier of a final stage is configured with a so-called D-class amplifier, a whole is able to be digitized, and is able to be configured as a digital audio amplifier.

FIG. 3 shows one example of such digital audio amplifier. Namely, a digital audio signal S 11 is supplied to an over-sampling circuit 12 from an input terminal 11 , a sampling frequency thereof is over-sampled to be a digital signal S 12 of 8 times, this digital signal S 12 is supplied to a ΔΣ modulation circuit 14 through a variable attenuator circuit 13 for volume control, and is re-quantized to be bit-reduced digital signal S 14 . Further, this digital signal S 14 is supplied to a PWM modulation circuit 15 , and converted to a PWM signal S 15 , then this PWM signal S 15 is supplied to a power amplifier 16 operating in D-class.

This power amplifier 16 is configured with a switching circuit for power amplifying by switching a power source voltage in accordance with the PWM signal S 15 , and a low pass filter for outputting a D/A converted and power amplified analog audio signal by smoothing the switching output. Further, by the power amplifier 16 , the power amplified audio signal is supplied to a speaker 30 through an output terminal 17 .

Further, in a system controller (not illustrated), a volume control signal SVOL is formed, and this signal SVOL is supplied to the variable attenuator circuit 13 as a control signal. Accordingly, when a switch for the volume control is operated, an attenuation level of the variable attenuator circuit 13 is changed, and a volume of a reproduced sound outputted from the speaker 30 is changed.

Further, in this case, the ΔΣ modulation circuit 14 includes a feedback loop for a quantizing error, so that even if a content of the digital signal S 12 supplied from the variable attenuator circuit 13 to the ΔΣ modulation circuit 14 is zero, a digital signal S 14 having something value is accordingly outputted from the ΔΣ modulation circuit 14 , and the digital signal S 14 is accordingly outputted from the speaker 30 as a noise sound having a specified frequency.

Consequently, in a dither signal forming circuit 18 , a dither signal SDI of a minute level is formed, this dither signal SDI is supplied to the ΔΣ modulation circuit 14 , and is superimposed on the digital signal S 12 upon re-quantization. Accordingly, even in a case where the content of the digital signal S 12 outputted from the variable attenuator circuit 13 is zero, an actual content of the ΔΣ modulation circuit 15 does not become zero, so that it is suppressed to output the noise sound.

Further, in a case when the digital signal S 11 to be supplied to the input terminal 11 is switched or disconnected by the switching of the source devices supplying the digital signal S 11 , the synchronization of the digital signal S 11 is temporary disturbed, and this disturbance of synchronization is accordingly outputted from the speaker 30 as the noise sound.

For the sake, the digital signal S 11 supplied to the input terminal 11 is supplied to an asynchronous detection circuit 19 , and a disturbance of synchronization of the digital signal S 11 is detected. Further, this detection signal SDET is supplied to the circuits 12 to 14 as a muting signal, and when the synchronization of the digital signal S 11 is disturbed, the contents of the signals S 12 and S 14 are set to be zero, and as the result, the reproduced sound outputted from the speaker 30 is muted.

The above is one example of an audio amplifier where the power amplifier 17 in the final stage is configured with a D-class amplifier (See Japanese Laid-open Patent Application OP2002-158543, for example).

By the way, in case of the audio amplifier as shown in FIG. 3 , when a muting is performed by the detection signal SDET of the asynchronous detection circuit 19 , not only the digital signal S 12 is muted, but also the dither signal SDI is simultaneously muted in the ΔΣ modulation circuit 14 . Accordingly, the dither signal SDI is abruptly cutoff upon muting, and a noise signal is generated by this abrupt cutoff, so that this is outputted from the speaker 30 as a noise sound.

Further, though the dither signal SDI has a minute level, the presence/absence of the dither signal SDI is able to be recognized as a difference in a noise level. Therefore, when the muting is set to be on, the noise level changes because the dither signal SDI is muted, but in a case where the content of the input digital signal S 11 is zero (or a minute level), the change in the noise level is recognized, and this causes uncomfortable feeling.

This invention is to solve the above problems.

According to the present invention, an audio amplifier is configured to include, for example, a sampling rate converter circuit for performing a sampling rate conversion of a first digital audio signal with a first clock synchronized thereto and a second clock having a predetermined frequency into a second digital audio signal synchronized with the second clock, a ΔΣ modulation circuit for re-quantizing the second digital audio signal into a third digital audio signal having reduced number of bits, a PWM modulation circuit for converting the third digital audio signal into a PWM signal, a D-class power amplifier to be supplied the PWM signal outputted from the PWM modulation circuit, a dither signal forming circuit for superimposing a dither signal on the third digital audio signal by supplying the dither signal to the ΔΣ modulation circuit, and a muting signal forming circuit, wherein an input side of the sampling rate converter circuit is stopped by a muting signal upon muting.

Accordingly, the dither signal is continuously supplied to the ΔΣ modulation circuit even during muting, and the digital audio signal including the dither signal is supplied to the D-class power amplifier after converting into the PWM signal.

FIG. 1 is a system chart designating one mode of the present invention.

FIG. 2 is a chart for describing the present invention.

FIG. 3 is a system chart for describing the present invention.

FIG. 1 shows one example of a digital audio amplifier 10 according to the present invention, and a digital audio signal S 11 is supplied to an over-sampling circuit 12 through an input terminal 11 . Further, the digital signal S 11 from the input terminal 11 is supplied to a PLL 21 , a clock SPLL synchronized with the digital signal S 11 and having a frequency of n times of its sampling frequency is formed, and thus generated clock SPLL is supplied to the over-sampling circuit 12 as a clock for over-sampling. In this case, the magnification n of the over-sampling is set to be a value as shown in FIG. 2 corresponding to the sampling frequency of the digital signal S 11 .

Thus, in the over-sampling circuit 12 , the digital signal S 11 supplied thereto is over-sampled to a digital signal S 12 synchronized with the signal S 11 , and having a frequency of n times the sampling frequency.

And, this digital signal S 12 is supplied to the sampling rate converter circuit 23 as a conversion input. Further, the clock SPLL from the PLL 21 is supplied to the sampling rate converter circuit 23 as a clock for conversion input side.

Further, the clock forming circuit 22 is configured with a crystal oscillation circuit, and a dividing circuit, and a clock SGEN having a frequency of 49.152 MHz (=48 kHz×1024) with a stable frequency and a phase is derived from this clock forming circuit 22 . Then, this clock SGEN is supplied to the sampling rate converter circuit 23 as a clock of conversion output side. Thus, in the sampling rate converter circuit 23 , the digital signal S 12 supplied thereto is converted into a digital signal S 23 having a sampling frequency which is a frequency of 384 kHz (=48 kHz×8), for example, with a stable frequency and a phase.

Then, the sampling rate converted digital signal S 23 is supplied to the variable attenuator circuit 13 for the volume control, the level thereof is controlled by the control signal SVOL from the system controller (not illustrated), the level controlled digital signal S 12 is supplied to the ΔΣ modulation circuit 14 , and is re-quantized into a bit-reduced digital signal S 14 . By the way, in the dither signal forming circuit 18 , the dither signal SDI of a minute level is formed at this time, and this dither signal SDI is superimposed on the digital signal S 23 to be supplied to the ΔΣ modulation circuit 14 .

Then, the digital signal S 14 re-quantized by the ΔΣ modulation circuit 14 is supplied to the PWM modulation circuit 15 , and is converted into a PWM signal S 15 . This PWM signal S 15 is then supplied to the power amplifier 16 operating in D class, and power amplified, and after that, thus amplified output is supplied to the speaker 30 through the output terminal 17 .

At this time, the clock SGEN from the forming circuit 22 is supplied to the circuits 13 to 15 , and 18 as their clocks. Accordingly, the output side of the sampling rate converter circuit 23 and the circuits 13 to 15 , and 18 are to be operated in synchronism with the clock SGEN.

Further, the digital signal S 11 supplied to the input terminal 11 is supplied to the asynchronous detection circuit 19 , also the clock having a frequency equal to the sampling frequency of the input digital signal S 11 and synchronized thereto is derived from the PLL 21 , this clock is supplied to the asynchronous detection circuit 19 , and a disturbance of the synchronization of the digital signal S 11 supplied to the input terminal 11 is detected.

Then, the detection signal SDET is supplied to the over-sampling circuit 12 , an input side of the sampling rate converter circuit 23 , and the variable attenuator circuit 13 as a muting signal, and when a synchronization of the digital signal S 11 is disturbed, the content of the signal S 12 is set to be zero, and also the operation of the input side in the sampling rate converter circuit 23 is stopped.

According to the above-mentioned configuration, the digital audio signal S 11 supplied to the input terminal 11 , in spite of its sampling frequency, the sampling frequency is converted its sampling rate into the digital signal having a sampling frequency of 384 kHz by the sampling rate converter circuit 23 , and after that, it is power amplified after converted into the PWM signal S 15 , then supplied to the speaker 30 .

Further, as a result of switching or disconnecting the digital signal S 11 to be supplied to the input terminal 11 by the switching of the source devices supplying the digital signal S 11 , the synchronization of the digital signal S 11 is temporary disturbed, and this disturbance of synchronization is detected by the asynchronous detection circuit 19 , and by the detection signal SDET, the over-sampling circuit 12 and the input side of the sampling rate converter circuit 23 are stopped. Accordingly, the digital signal S 12 is to be shut out during an interval of the detection signal SDET.

However, even the operation of the input side in the sampling rate converter circuit 23 during the interval of the detection signal SDET is stopped, the digital signal S 23 is continuously outputted from the sampling rate converter circuit 23 , because the output side thereof is supplied with the clock SGEN, and the operation is continuous.

However in this case, as the operation of the input side in the sampling rate converter circuit 23 is stopped and the detection signal SDET is also supplied to the variable attenuator circuit 13 , the content of the digital signal S 23 outputted from the variable attenuator circuit 13 is zero.

Then, such digital signal S 23 is supplied to the ΔΣ modulation circuit 14 , and the detection signal SDET is not supplied to the ΔΣ modulation circuit 14 , during the interval of the detection signal SDET, the digital signal S 14 the content of which is zero is outputted from the ΔΣ modulation circuit 14 , and this digital signal S 14 is supplied to the PWM modulation circuit 15 . Accordingly, during the interval of the detection signal SDET, a muting is activated to the input audio signal S 11 . That is, the interval of the detection signal SDET is the muting interval.

As described above, the digital audio amplifier shown in FIG. 1 , the muting is performed, however, even during the muting interval, the dither signal SDI is supplied to the ΔΣ modulation circuit 14 , and accordingly, even if the content of the digital signal S 23 supplied thereto is zero, it never happens to output from the ΔΣ modulation circuit 14 the signal components which become a noise sound of a particular frequency.

Further, even during the muting interval, the dither signal SDI is supplied to the ΔΣ modulation circuit 14 , the noise levels become equal between when the muting is off and when the muting is on. Accordingly, in a case when the content of the input digital signal S 11 is zero (or, a minute level), if a muting is activated, there is no fear of being recognized the change in the noise level and uncomfortable feeling.

Further, when changing from a state where the muting is off to a state where the muting is on, and when changing from a state where the muting is on to a state where the muting is off, the dither signal SDI is always continuing, so that there is no fear of generating noise signals, and also there is no fear of outputting a noise sound from the speaker 30 .

(List of Abbreviations Used in this Specification)

D/A Digital to Analog

PLL: Phase Locked Loop

PWM: Pulse Width Modulation

According to the present invention, even if a muting is on when the content of the input digital audio signal is zero, or a minute level, there is no fear of uncomfortable feeling due to the recognition of the change in the noise level. Further, when the muting is made on from the muting off state, or when the muting is released from the muting on state, there is no fear of generating noise signals, and also there is no fear of outputting a noise sound from the speaker.