JP2001230672A - Device reproducing one-bit signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid a sound quality from being lowered by a noise peak existent inside a reproducing band, for a one-bit signal reproducing device for converting a ΔΣ modulated one-bit input signal to an analog signal by a low-pass filter(LPF). SOLUTION: For this one-bit signal reproducing device, the one-bit signal from a ΔΣ modulated one-bit signal generating circuit 1 is converted to an analog output and reproduced by a D/A converting filter 4 composed of the LPF. Moreover, a noise floor in the plural specified frequencies of the one-bit signal is extracted by a band-pass filter(BPF) 2 and by deciding the noise level of the extracted noise floor, the presence/absence of the peak of the noise and a frequency, where that peak exists, are detected by a noise level detecting circuit 3. Then, when the noise peak is detected, the D/A converting filter 4 switches a cutoff frequency, so that the frequency of the detected noise peak cannot be contained in the reproducing band of the one-bit signal reproducing device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a 1-bit signal reproducing apparatus for reproducing a 1-bit signal obtained by ΔΣ modulation into an analog signal by a low-pass filter.

[0002]

2. Description of the Related Art A 1-bit signal obtained by ΔΣ modulation can widen an effective frequency band or a dynamic range by appropriately setting coefficients of an integrator and an adder. Has an excellent feature that frequency characteristics according to a sound source or the like can be set. For this reason, the 1-bit signal is adopted in the new standard of CD and DVD.

[0003] On the other hand, the 1
The bit signal is not only used for recording an acoustic signal and transmitting the signal between devices. As a higher quality audio amplifier than a conventional PWM (pulse width modulation) switching amplifier, a high-speed sampling 1-bit switching amplifier can be applied to the audio field.

[0004] In addition, a 1-bit signal is easily converted into an analog signal by taking a form of a PDM (pulse density modulation) signal and passing through an analog low-pass filter. Due to such characteristics, a 1-bit signal is converted to an analog signal by a simple configuration different from a D / A converter used for analog conversion of a conventional multi-bit signal.

[0005]

By the way, the reproduction band of the recent hi-fi audio tends to be a wide band up to 100 kHz. However, ΔΣ modulation 1
A device (circuit) that converts a bit input signal into an analog signal
In some cases, a noise peak may occur in a high frequency region of a noise floor in a reproduction band due to the characteristics of a noise shaping circuit on the 1-bit signal generation side.

In general, when a 1-bit signal is observed by an FFT (frequency analyzer) or the like, noise is distributed in a band below the signal band as shown in FIG. The noise distributed as described above is called a noise floor because the spectrum of the signal appears to rise above the noise floor.

The characteristic curve of the noise shaping depends on the number of stages of the ΔΣ modulator (the N-stage modulator is referred to as Nth order), that is, the number of integrators. In general, as shown in FIG. 12, the higher the order, the flatter the characteristic is in the low frequency range and the more steeply rising characteristics are obtained in the high frequency range. Higher orders are achieved in audio equipment for which a wide reproduction band is desired. However, as shown in FIG. 13, this technique is obtained by shifting and overlapping the peaks and valleys that occur in the output characteristics of each integrator. Therefore, strictly speaking, peaks and valleys undulate at a low level in a high-frequency portion in the reproduction band, but as shown in FIG. 8, a portion corresponding to the peak becomes large and a sharp peak is formed. May appear as Such a peak corresponds to the above-mentioned noise peak.

[0008] Such a noise peak has a different frequency at which the noise peak occurs (for example, a relatively low 30 kHz or a comparatively low frequency) depending on the characteristics of the device (ΔΣ modulation circuit) on the signal source side. 80 kHz or 50 kHz in between.) Therefore, in a reproducing apparatus in which the reproduction band is fixed to 100 kHz or less, there is a problem that the noise peak as described above adversely affects the reproduction characteristics of the reproduction system system, thereby lowering the reproduction sound quality.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to prevent the noise peak as described above from being present in a reproduction band and to avoid a decrease in sound quality due to the noise peak. I have.

[0010]

According to the present invention, there is provided a 1-bit signal reproducing apparatus having a low-pass filter for converting a ΔΣ modulated 1-bit input signal into an analog signal. And ΔΣ modulation 1
A band-pass filter that extracts a noise floor at a plurality of specific frequencies of the bit input signal, and a detection that detects the presence or absence of a noise peak and a frequency at which the peak exists by determining a noise level in the extracted noise floor Means, wherein the low-pass filter has cut-off frequency switching means for switching a cut-off frequency so as not to include the frequency of the detected noise peak in the reproduction band of the 1-bit signal reproducing device. I have.

In the above configuration, when a ΔΣ modulated 1-bit input signal is input, the noise floor at a plurality of specific frequencies of the signal is detected by the band-pass filter, and the noise level of the noise floor is determined by the detecting means. Is done. As a result, when the noise peak and its frequency are detected, the cutoff frequency of the low-pass filter is switched. At this time, the cutoff frequency is switched so that the frequency of the noise peak is not included in the reproduction band of the 1-bit signal reproduction device.

Therefore, since the noise peak does not exist in the reproduction band, the reproduction characteristic of the 1-bit signal reproduction device is improved.

In the above 1-bit signal reproducing apparatus,
Preferably, the low-pass filter includes a coil, and the cut-off frequency switching means switches a cut-off frequency by changing an inductance of the coil. As described above, since the cutoff frequency is switched by changing the inductance of the coil, the cutoff frequency switching means can be realized with a relatively simple configuration. In particular,
A configuration is conceivable in which some coils are provided so as to be connectable to and disconnectable from a low-pass filter depending on whether or not a noise peak is detected.

Further, the 1-bit signal reproducing apparatus described above has a Δ
The switching amplifier provided in the preceding stage of the low-pass filter for amplifying the modulated 1-bit input signal, and attenuating means for attenuating the output of the switching amplifier and outputting the output to the band-pass filter may be provided. In such a 1-bit signal reproducing apparatus, it is possible to amplify a 1-bit signal, and even if an amplified 1-bit signal is used, the signal level is attenuated by the attenuating means. The noise floor can be extracted by the band-pass filter and the noise peak can be detected by the detecting means.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS.

As shown in FIG. 1, a 1-bit signal reproducing apparatus according to the present embodiment converts a 1-bit signal (Δ か ら -modulated 1-bit input signal) output from a ΔΣ-modulated 1-bit signal generating circuit 1 into an analog signal. It is configured to play. The ΔΣ modulation 1-bit signal generation circuit 1 includes a signal source 1a for generating an analog audio signal, and a ΔΣ modulation A / for converting the audio signal output from the signal source 1a into a 1-bit digital signal (1 bit signal). And a D converter 1b.

The 1-bit signal is a ΔΣ modulation A / D converter 1
FIG. 2A shows the noise shaping characteristic of FIG.
As shown in the figure, the noise floor characteristic shows that the noise level rises before 100 kHz. However, as shown in FIG.
Within a band of 00 kHz or less, a noise peak may be generated at a position (at least one position) of a peak frequency fp where a noise peak occurs.

In order to avoid the above-mentioned noise peak, the present 1-bit signal reproducing apparatus sets a band-pass filter (BPF in the figure) 2 and a noise level so as to set the reproduction bandwidth to a band lower than the peak frequency fp. The detection circuit 3 and D
/ A conversion filter 4.

The band-pass filter 2 is configured, for example, as shown in FIG. 3 in order to detect the presence / absence of a noise peak in the above-mentioned noise floor and the peak frequency fp for the 1-bit signal.

[0020] The band-pass filter 2, and OP amp (operational amplifier) 11, 12, a resistor R ₁ to R _4, which is a capacitor C _1, C ₂ Metropolitan. An input voltage Vin is input to one input terminal of the OP amplifier 11 via a resistor R ₃ , and between the input terminal and the output terminal.
And resistor R ₂ and capacitor C ₂ are connected in parallel.
The other input terminal of the OP amplifier 11 is connected to a ground line.

The output terminal of the OP amplifier 11 and the OP amplifier 1
A resistor R ₁ and a capacitor C ₁ are connected in series between one of the two input terminals. The OP amplifier 12
Resistor R ₄ is connected between the input terminal and the output terminal. The other input terminal of the OP amplifier 12 is connected to a ground line. Then, an output voltage Vout including a noise peak is output to the output terminal of the OP amplifier 12.

In the above-described bandpass filter 2,
As shown in FIG. 4, the cutoff frequency fc ₁ · fc ₂ is fc ₁ = 1.
/ (2πR ₁ C ₁ ) and fc ₂ = 1 / (2πR ₂ C ₂ ) (fc ₁ <fp <fc ₂ ). Thereby, a portion including a noise peak on the noise floor is extracted. Pass bandwidth of the band pass filter 2 is determined by the combination of a cut-off frequency fc ₁ in cutoff characteristics of the high-pass filter indicated by the broken line in the figure, a cutoff frequency fc ₂ in the cutoff characteristics of the low-pass filter shown by the solid line in FIG.

As shown in FIG. 5, the noise level detection circuit 3 includes an OP amplifier (operational amplifier) 3a and a comparator 3
b. In the noise level detection circuit 3, the output voltage Vout of the bandpass filter 2 amplified by the OP amplifier 3a is compared with a prescribed reference voltage level by a comparator 3b, and a detection output corresponding to the comparison result is obtained. Specifically, a high-level noise detection signal is output when the output voltage Vout is equal to or higher than the reference voltage level, and a low-level noise non-detection signal is output when the output voltage Vout is lower than the reference voltage level.

The noise peak has a large level difference of 20 dB (10 times) or more as compared with the average level of the noise floor. Therefore, the reference voltage level in the comparator 3b does not need to be set strictly. The output is set to a high level when a noise peak is input, and is set to a low level when a normal noise floor level is input. It should just be adjusted so that.

The D / A conversion filter 4, as shown in FIG. 6 is a fourth order Butterworth filter, the coils L ₁ ~
L ₃ , capacitor C and relay RLY ₁ · RLY
₂ (cutoff frequency switching means).

The coils L _{1 to} L ₃ are connected in series, a relay RLY ₁ is connected between both ends of the coil L ₂ , and a relay RLY is connected between both ends of the coil L _3.
2 is connected. The capacitor C is provided in the output stage. Relay RLY ₁ is closed by the drive current supplied from the current driving amplifier (not shown) to a drive coil provided inside. On the other hand, relay RLY ₂
The drive coil is closed when a drive current is supplied from another current drive amplifier (not shown) to a drive coil provided inside. In order to close the relays RLY ₁ and RLY ₂ at the same time, a current drive amplifier for supplying a drive current to the drive coils of the relays RLY ₁ and RLY ₂ is separately provided.

In the above D / A conversion filter 4,
When the output of the comparator 3b is low level,
When a portion including a noise peak is not detected, the relay R
LY ₁・ RLY_TwoDrive coil is not driven.
Leh RLY₁・ RLY_TwoIs open. At this time,
Ductance value is coil L₁~ L_ThreeIs determined by
Determines the cutoff characteristics (cutoff frequency) of the filter.
It is. At this time, the widest reproduction band is secured.

As shown in FIG. 7, the 1-bit signal reproducing apparatus having the D / A conversion filter 4 having the configuration shown in FIG. 6 is used to pass a plurality (three in this case) of different peak frequencies fp. Bandpass filters 2 having different pass bandwidths are provided. Thus, if there are multiple noise peaks on the noise floor,
Each bandpass filter 2 extracts a portion (noise level) including a noise peak in the noise floor, and the noise level is detected by each noise level detection circuit 3 at the noise level. Then, the relays RLY ₁ and RLY ₂ connected to the noise level detection circuit 3 that has detected the noise peak are individually closed.

For example, the node having the highest peak frequency
The peak is bandpass filter 2 (BPF₁) To extract
The noise level is detected
Detected by circuit 3. Then, the relay RLY_TwoOnly closed
Coil L_ThreeIs disconnected and bypassed.
Therefore, at this time, the inductance value ₁
・ L_TwoThe playback bandwidth is determined by this value.
Fill so that it is below the high peak frequency.
The cutoff characteristics (cutoff frequency) of the data are determined.

Subsequently, when a noise peak having the second highest peak frequency is extracted by the band-pass filter 2 (BPF ₂ ), the presence of the noise peak is detected by the noise level detection circuit 3. Then, since only the relay RLY ₁ is closed, the coil L ₂ is has been bypassed disconnected. Therefore, at this time, since the inductance value is determined by the coil L ₁ · L _3, this value, so as to be positioned on the lower side than the high peak frequency regeneration zone is the second, determined cutoff characteristics of the filter (cutoff frequency) Is done.

When the noise peak having the third highest peak frequency is extracted by the band-pass filter 2 (BPF ₃ ), the presence of the noise peak is detected by the noise level detection circuit 3. Then, the relay RLY ₁・ R
Since LY ₂ are both closed, the coil L ₂ · L ₃ it is has been bypassed disconnected. Therefore, at this time, since the inductance value is determined only by the coil L _1, this value, so as to be positioned lower than the high peak frequency in the third regeneration zone is a side, cut-off characteristics of the filter (cutoff frequency) is determined .

As described above, the D / A conversion filter 4 sets the cutoff characteristic so that the reproduction band is as initially set when no noise peak is generated, and when the noise peak is generated. As shown in FIG. 8, the cutoff characteristic (cutoff frequency) is switched so that the reproduction band is located on the lower side of the noise peak. As a result, the frequency of the noise peak is not included in the reproduction band of the 1-bit signal reproduction device. Therefore, although the playback band narrows,
Since noise peaks that adversely affect the reproduction system are not included, the quality of the analog output (reproduced sound) reproduced by the 1-bit signal reproduction device can be improved. Further, when there are a plurality of noise peaks having different frequencies, as described above, by providing a processing system for detecting each noise peak, the cutoff characteristics can be switched according to each noise peak. The reproduction band is set so as not to include the frequency of each noise peak.

Next, another 1-bit signal reproducing apparatus according to the present embodiment will be described. As shown in FIG. 9, the 1-bit signal reproducing apparatus is configured to amplify the 1-bit signal output from the ΔΣ modulated 1-bit signal generating circuit 1 and then reproduce the amplified signal as an analog signal. The 1-bit signal reproducing apparatus includes a switching amplifier (SW amplifier in the figure) 21, an attenuator 22, a band-pass filter 2
, A noise level detection circuit 3 and a D / A conversion filter 4
And

The switching amplifier 21 has a rectangular waveform 1
An amplifier that switches a bit signal as it is to amplify it into a large amplitude signal. For example, as shown in FIG.
ET driver 21a and power MOSFET circuit 21b
And an amplifier mainly comprising: Power MOSFET
The circuit 21b is _{supplied with} a voltage of + V _CC as a power supply and is connected to a ground line. In the switching amplifier 21, the power MOSFET circuit 21b
Are switched by being driven by the FET driver 21a, and a 1-bit switching (amplification) output having a level of approximately + V _CC or a ground (zero) level is output.

The attenuator 22 as the attenuating means comprises resistors R ₂₁ and R ₂₂ connected in series. Since the 1-bit signal described above is amplified by the switching amplifier 21, the attenuator 22, by dividing the 1-bit switching output which is input from the switching amplifier 21 in the voltage dividing ratio of the resistors R ₂₁ · R _22, band Pass filter 2
The 1-bit switching output is attenuated to a level suitable for input to the switch.

Thus, similarly to the 1-bit signal reproducing apparatus of FIG. 1, it is possible to extract a portion including a noise peak in the noise floor by the band-pass filter 2 and to detect the noise peak by the noise level detection circuit 3.
As a result, even in the present 1-bit signal reproducing apparatus, the cutoff characteristic of the D / A conversion filter 4 is switched by detecting the noise peak, and the reproduction characteristic can be improved.

As described above, even when the 1-bit signal reproducing device is a 1-bit signal amplifying system including the switching amplifier 21, the reproduction sound quality can be improved.

Since the one-bit signal input to the D / A conversion filter 4 is amplified by the switching amplifier 21, the D / A conversion filter 4 handles a large current. On the other hand, by increasing the wire diameter of the coils L _{1 to} L ₃ in the D / A conversion filter 4, a Butterworth filter having the configuration shown in FIG. 6 can be used.

Further, it goes without saying that the configuration of FIG. 7 for detecting a plurality of noise peaks can also be applied to the present 1-bit signal reproducing apparatus.

[0040]

As described above, the 1-bit signal reproducing apparatus of the present invention comprises a low-pass filter for converting a ΔΣ modulated 1-bit input signal into an analog signal, and a noise at a plurality of specific frequencies of the ΔΣ modulated 1-bit input signal. A band-pass filter for extracting a floor; and a detecting means for detecting the presence or absence of a noise peak and a frequency at which the peak is present by determining a noise level in the extracted noise floor. This is a configuration having cut-off frequency switching means for switching the cut-off frequency so as not to include the frequency of the detected noise peak in the reproduction band of the bit signal reproducing device.

As a result, the cutoff frequency is switched so that the frequency of the above-mentioned noise peak is not included in the reproduction band of the 1-bit signal reproducing apparatus. The playback characteristics of the playback device are improved. Therefore, it is possible to avoid a decrease in sound quality due to a noise peak.

In the above 1-bit signal reproducing apparatus,
The low-pass filter includes a coil, and the cut-off frequency switching means switches the cut-off frequency by changing the inductance of the coil, so that the cut-off frequency switching means can be realized with a relatively simple configuration.
Therefore, there is an effect that the reproduction characteristics of the 1-bit signal reproduction device can be improved with a relatively simple configuration.

Further, the above 1-bit signal reproducing apparatus has a Δ
Σ Amplifying the modulated 1-bit input signal by providing a switching amplifier provided before the low-pass filter and attenuating means for attenuating the output of the switching amplifier and outputting the output to the band-pass filter. Even if a bit signal is used, the signal level is attenuated by the attenuating means, so that a noise floor can be extracted and a noise peak can be detected in the same manner as a 1-bit signal that is not amplified. Therefore, there is an effect that the reproduction characteristics can be improved even in the 1-bit signal amplification system.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a 1-bit signal reproducing device according to an embodiment of the present invention.

FIG. 2A is a graph showing a frequency characteristic of a noise level, and FIG. 2B is a graph showing a frequency characteristic of a noise level having a noise peak.

FIG. 3 is a circuit diagram showing a configuration of a bandpass filter in the 1-bit signal reproducing device.

FIG. 4 is a graph showing cutoff characteristics of the bandpass filter.

FIG. 5 is a block diagram showing a configuration of a noise level detection circuit in the 1-bit signal reproduction device.

FIG. 6 is a circuit diagram showing a configuration of a D / A conversion filter in the 1-bit signal reproducing device.

FIG. 7 is a block diagram showing a configuration of a main part of the 1-bit signal reproducing apparatus corresponding to a plurality of noise peaks.

FIG. 8 is a graph showing a characteristic having a noise peak of noise shaping of the ΔΣ modulator.

FIG. 9 is a block diagram showing a configuration of a 1-bit signal reproducing device according to another embodiment of the present invention.

FIG. 10 is a block diagram showing a configuration of a switching amplifier in the 1-bit signal reproducing device of FIG.

FIG. 11 is a graph showing a noise distribution state in a 1-bit signal.

FIG. 12 is a graph illustrating noise shaping characteristics that vary depending on the number (order) of integrators in the ΔΣ modulator.

FIG. 13 is a graph showing noise shaping characteristics of a higher order ΔΣ modulator.

[EXPLANATION OF SYMBOLS] 2 bandpass filters 3 noise level detecting circuit (detecting means) 4 D / A conversion filter (lowpass filter) 21 switching amplifier 22 attenuator (attenuator) L ₁ ~L ₃ coil RLY ₁ · RLY ₂ relay (Cut frequency switching means)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J022 AA00 AB00 BA02 CA07 CC02 CF02 CF07 CG01 5J064 AA01 BA02 BB07 BC07 BC11 BC14 BC18 BD03

Claims (3)

[Claims] 1. A 1-bit signal reproducing apparatus comprising a low-pass filter for converting a ΔΣ modulated 1-bit input signal into an analog signal, wherein the band-pass filter extracts a noise floor at a plurality of specific frequencies of the ΔΣ modulated 1-bit input signal. And detection means for detecting the presence or absence of a noise peak and the frequency at which the peak is present by determining the noise level in the extracted noise floor, wherein the low-pass filter has a reproduction band of a 1-bit signal reproduction device. And a cut-off frequency switching means for switching a cut-off frequency so as not to include the frequency of the detected noise peak. 2. The 1-bit signal reproducing apparatus according to claim 1, wherein said low-pass filter includes a coil, and said cut-off frequency switching means switches a cut-off frequency by changing an inductance of said coil. 3. A switching amplifier, which amplifies a ΔΣ modulated 1-bit input signal and is provided in a stage preceding the low-pass filter, and attenuating means for attenuating an output of the switching amplifier and outputting the output to the band-pass filter. 3. The 1-bit signal reproducing apparatus according to claim 1, wherein: