Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
2003-05-12
2004-09-21
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S118000, C341S120000
Reexamination Certificate
active
06795004
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a device for carrying out delta-sigma modulation of an audio signal which has been caused to undergo oversampling, and more particularly to a delta-sigma modulation device for correcting signal distortion produced when delta-sigma modulated signal is amplified by amplifier, and a signal amplifier apparatus comprising such delta-sigma modulation device.
BACKGROUND ART
Hitherto, as a method of improving S/N ratio in the audible band of audio signal, delta-sigma modulation system is known. In this delta-sigma modulation system, by the noise shaping technology, e.g., PCM digital signal of 16 bits or more is converted into quantization signal having lower number of bits from one bit to several bits to have ability to reproduce it into analog signal by local D/A converter of several bits.
Here, reproduction of audio signal utilizing the delta-sigma modulation system will be explained. A reproducing apparatus
9
which reproduces audio signal comprises, as shown in
FIG. 1
, a delta-sigma modulator
90
, a Pulse Width Modulator
91
, a switching module
92
, a LPF (Low Pass filter)
93
, and a speaker
94
.
The delta-sigma modulator
90
comprises, as shown in
FIG. 2
, an input terminal supplied with an audio signal, a subtracter
191
for carrying out subtraction between audio signal and delay signal, an integrator
192
for integrating an output of the subtracter
191
, a quantizer
193
for quantizing an output of the integrator
192
into a digital signal having lower number of bits, and a delay circuit
194
for delaying output S
1
of the quantizer
193
by one clock (operating clock of the quantizer) to input it as a delay signal to the subtracter
191
. In delta-sigma modulator
90
, there is generated an audio signal distributed so that quantization noise is caused to be less in the Nyquist band which is the band having a frequency up to one half of sampling frequency of input audio signal and quantization noise is caused to be many in the frequency band more than the Nyquist band. For example, input audio signal is digital audio data having 24 bits and sampling frequency of 768 kHz (=48 kHz×16), and output audio signal is digital signal having 6 bits and sampling frequency of 768 kHz. The output audio signal S
1
is inputted to the Pulse Width Modulator
91
. The Pulse Width Modulator
91
carries out Pulse Width Modulation with respect to inputted audio signal.
The Pulse Width Modulated audio signal S
2
is amplified by the switching module
92
into a pulse signal of a predetermined magnitude, and an audio signal of the audible band is outputted from the speaker
94
through the LPF
93
.
Since the switching module
92
is ordinarily operative at a power supply voltage higher than input voltage, e.g., 20V~50V, distortion takes place in an output signal. Moreover, the output signal also experiences influence of counter-electromotive force by the coil of the LPF
93
or the speaker
94
so that it is difficult that there results ideal waveform.
For example, as shown in
FIG. 3
, in the above-described reproducing apparatus
9
, signal S
3
amplified by the switching module
92
is such that delay of signal takes place by T
rise
at the rising and by T
fall
at the falling with respect to the signal S
2
which has been caused to undergo Pulse Width Modulation by the Pulse Width Modulator
91
. As a result, there takes place the problem that pulse width of the signal S
2
and that of the signal S
3
are different from each other. There is the problem that ringing takes place at the rising and the falling in the signal S
3
so that waveform like the signal S
2
cannot be obtained.
Further, in the class D amplifier which drives the speaker
94
by signal S
3
amplified by the switching module
92
, since distortion of the signal S
3
results in distortion of audio signal as it is so that such distortion is outputted from the speaker
94
, there is the problem that the audio characteristic such as distortion ratio and/or S/N ratio, etc. is deteriorated.
In order to eliminate this distortion, a class D power amplifier as described in, e.g., the Japanese Patent Application Laid Open No. 214259/1997 publication is proposed. In this class D amplifier, waveform distortion by power supply ripple taking place at the power switch and/or waveform distortion by overshoot or ringing, etc. are suppressed.
This class D amplifier comprises a delta-sigma modulation unit as described above, a power switch for power-amplifying an output signal of the delta-sigma modulation unit, a first LPF for smoothing an output of the power switch, a delay for delaying an output signal of the delta-sigma modulation unit, an attenuator for attenuating amplitude of an output signal of the power switch, an adder for calculating difference between an output signal of the delay and an output signal of the attenuator, a second LPF for smoothing an output signal of the adder, and a selector circuit for selecting reference level of a quantizer on the basis of an output signal of the second LPF. Input signal is converted into binary signal at the delta-sigma modulation unit. This binary signal is power-amplified at the power switch, and is smoothed at the first LPF. The signal thus obtained is delivered to load. On the other hand, the delay delays an output of the delta-sigma modulation unit. The attenuator attenuates amplitude of an output signal of the power switch equivalently to amplitude of an output signal of the delay. Further, the adder calculates difference between an output signal of the delay and an output signal of the attenuator to smooth it at the second LPF to thereby detect waveform distortion taking place at the power switch. The selector circuit selects reference level of 1 bit quantizer in accordance with this waveform distortion quantity. As a result, the delta-sigma modulation unit converts input signal into binary signal so as to cancel waveform distortion. In this way, waveform distortion is cancelled.
Since the class D power amplifier described in the above-mentioned publication serves to change the reference level of the quantizer, in the case where a signal of high level is inputted to the quantizer, there is the problem that signal is clipped. Moreover, in the class D power amplifier, in the case where level of a signal inputted to the quantizer is suppressed to be low in order not to cause this clip, there is the problem that S/N ratio is deteriorated.
In addition, in the class D amplifier adapted for driving speaker by signal amplified by the switching module, since distortion of signal results in distortion of audio signal as it is so that it is outputted from the speaker, there is the problem that the audio characteristic such as distortion ratio and/or S/N ratio, etc. is deteriorated.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a novel delta-sigma modulation device and a novel signal amplifier apparatus which can solve problems that conventional apparatuses as described above have.
Another object of the present invention is to provide a delta-sigma modulation device which corrects in advance distortion taking place at the switching module, and a signal amplifier apparatus using such delta-sigma modulation device.
A further object of the present invention is to provide a signal amplifier apparatus which corrects in advance distortion taking place at the switching module by a feedback circuit.
A delta-sigma modulation device according to the present invention proposed in order to attain objects as described above is directed to a delta-sigma modulation device comprising a quantizer and operative to feed output or quantization error of this quantizer back to the input side as a feedback signal, the delta-sigma modulation device comprising a correction circuit for correcting output signal or quantization error of the quantizer to allow it to be a feedback signal, thus to correct, by the correction circuit, distortion taking place by circuit or circuits connected to the succeeding stage of this d
Masuda Toshihiko
Ohkuri Kazunobu
Maioli Jay H.
Nguyen Linh Van
Sony Corporation
Tokar Michael
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