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Description 9 Title of the Invention
PWM signal demodulation method
3. Detailed Description of the Invention The present invention relates to a PWM signal
demodulation method for converting a PWM (Pulse Width Modulation) signal into an analog
audio signal. More particularly, the present invention relates to a demodulation scheme that
removes noise (e.g., high frequency noise) caused by the PWM signal and directly drives the
speaker by switching up without passing through a low pass filter. In general, pulse width
modulation (PWM) is used to improve the efficiency of an amplifier in audio signal processing. In
this case, a switching amplifier can be used as the amplifier, and the power supply efficiency is
very high, and the shape of the amplifier device can be miniaturized. 0 In recent years, the PWM
method is applied to various devices, for example, voice synthesizers Applied. This device is
implemented by digital processing, does not require a special circuit, and can output a pulse
width modulated signal by simple processing. The applicant previously described in Japanese
Patent Application No. 54-170857 (filed on December 27, 1959), in the case of outputting digital
speech information synthesized by a speech synthesizer, by pulse width modulation (pwM) A new
sound EndPage: provided a voice output method. According to this method, it can be converted to
an analog signal simply by passing low-pass filters without passing through a 音 声 converter,
and a voice synthesizer can be realized simply by connecting an amplifier without requiring a
particular S / C converter outside LSj. The characteristic O but the above PWM signal
demodulation scheme requires a low pass filter to remove noise (for example, harmonic noise)
generated by the PWM signal, also requires an audio amplifier, and the circuit configuration is
complicated and costly. There is a drawback in that it becomes high and hinders the downsizing
and thinning of the device. Furthermore, speech synthesis, which is currently under development,
mainly uses programs to generate speech, in other words, research on software that controls
speech synthesizers, and devices are replaced by do-air. It has become possible to configure it
with 3 Lsi chips. Therefore, the four converters, low-pass filters, audio amplifiers, etc. externally
attached to the chip have necessarily become a factor that reduces the size and thickness of the
equipment and reduces the production efficiency. However, if the low pass filter is removed and
the speaker is directly driven by the PWM signal, the carrier frequency component is reproduced
in the speaker and appears as high frequency noise or distortion, and the carrier frequency
component causes a large power loss in the current amplifier. There were drawbacks such as the
occurrence of
The present invention has been made in view of the above-mentioned various drawbacks of the
prior art. In driving a speaker by using a PWM signal as a signal source, the speaker has low-pass
filter characteristics, in particular mechanical characteristics. In the demodulation method of the
PWM signal according to the present invention, the noise generated (for example, high frequency
noise) is removed and the speaker can be directly driven by the switching amplifier without
passing through the low pass filter audio amplifier. FIG. 1 is a system configuration diagram of an
example of a speech synthesizer adopting the system according to the present invention, which
will be described in detail with respect to one embodiment. System configuration is shown. This
system constitutes a low power consumption system using the C-MOS microcomputer 1 and Ls +
2 for speech synthesis. Power can be supplied to the switching amplifier 3 and the others can be
turned off. That is, Lsi2 for voice synthesis can be commanded from microcomputer 1 and voices
can be produced according to the command 0. Furthermore, this system adopts a voice output
method by pulse width modulation (PWM), a converter, low-pass The audio can be reproduced
by the speaker 4 simply by connecting the switching amplifier 3 without the need for the filter
and the audio amplifier. 5 is an external ROM. This system takes advantage of PWM's inherent
power efficiency and low-pass filter. An inexpensive system is provided with a simplified circuit
configuration with a small number of terminals that does not require an audio amplifier. FIG. 2 (a
= C) shows an example of the switching amplifier used in the present system. In (a), the PWM
signal is input to the base of the transistor 6 constituting the switching amplifier 3, and the
speaker 4 is supplied with power to the collector between the emitter and the ground. (B) is an
example in which a speaker is connected to the collector side, and (c) is a transistor in which the
transistor 6.7 is connected in a complementary manner. Unlike ordinary audio amplifiers,
switching amplifiers do not require any bias resistors to drive in the saturation region of the
transistor, and when the system of the present invention constitutes, for example, a voice clock,
one Lsi for voice synthesis, its clock generation One ceramic vibrator, one control
(microcomputer) Lsi, one clock generation crystal thereof, and one transistor shown in FIG. 3 (a,
b), for example, can be used.
EndPage: 2 FIG. 3 shows the frequency characteristics and impedance characteristics of the
speaker for explaining the PWM signal demodulation method of the present invention 0 in the
diagram, sp and Ip are the characteristics and impedance characteristics of the speaker according
to the present invention method, SQ , And Q are the conventional speaker characteristics and
impedance characteristics. In the figure, the horizontal axis represents frequency, the vertical
axis represents sound pressure and impedance, and f2 represents the carrier frequency of the
PWM signal. As can be seen from the speaker characteristic SQ in the figure, according to the
conventional speaker, the band of the carrier frequency f2 of the PWM signal is also reproduced,
and these components generated due to the PWM signal appear as noise and distortion. There is
a problem that the component causes a large power loss in the current amplifier. That is, the
impedance of the f2 band drops from the impedance characteristic IQ. In order to eliminate the
above-mentioned drawbacks of the present invention, in particular, in the case of driving a
speaker using a PWM signal as a signal source, the PWM having the low-pass filter (LPF)
characteristic is the most efficient unique PWM. The modulation noise can be cut. Hereinafter,
the PWM modulation noise removal method will be described in detail. (I) First, the first method
is to remove noise generated by the PWM signal by an inductive impedance inserted in series
with the speaker and the like constraints, and to drive the speaker directly without passing
through a low pass filter. That is, as shown in FIG. 4, the frequency component of the audio
signal is designed to be the same as the reproduction band (fo "f +) of the speaker. Here, it is fo =
600 Hz, L = 4 KHz, and PWM modulation frequency f2 = 16 KHz (by the way, the aperture of the
speaker = 50−). In order to obtain a speaker having such characteristics, for example, the voice
coil impedance of the speaker, in particular, the reactance component is increased. In order to
increase the reactance component of the voice coil of the speaker, one using an alnico magnet
having a pole piece of pure iron or one using an external magnet type ferrite magnet is suitable.
Further, the reactance component can be increased by increasing the number of turns of the
voice coil. The reproduction band of such a speaker is not affected at all by the component of the
carrier wave frequency f2 due to the PWM modulation, and the sound source component is not
removed even with a speaker of a narrow reproduction band. (Ii) The second method is to
eliminate noise noise caused by the PWM signal by weighting the vibration system of the
speaker, and to drive the speaker directly with the switching amplifier without passing through a
low pass filter and an audio amplifier.
That is, in this method of giving the speaker the LPF characteristics, for example, the diaphragm,
the coil, or a portion supporting these components is made heavier, or the diaphragm is provided
with corrugation. This can provide the same effect as (i). (Iii) A third method is to make the
speaker have appropriate rigidity, remove noise due to the PWM signal, and drive the switching
amplifier directly to the speaker. For example, the cut-off frequency of the low pass filter can be
varied by changing the position of the corrugation in order to have an appropriate stiffness, and
the unique PWM modulation noise can be cut most efficiently. Further, if the rigidity of the
diaphragm is increased, it is also possible to effectively use only the piston vibration region and
suppress the divided vibration. In this way, the speaker that uses the PWM signal as the signal
source can have LPF characteristics, in particular mechanical characteristics. As described above,
according to the PWM signal demodulation method of the present invention, by applying
mechanical treatment to the speaker, the unique PWM modulation noise can be removed most
efficiently, which means that the low pass filter Since the speaker can be driven only by the
switching amplifier without any intervention, it is possible to obtain an inexpensive and
extremely simple system having good operability. In particular, it is convenient to use a device
(for example, a speech synthesizer) that converts a PWM signal into an analog signal.
4. Brief description of the drawings EndPage: 3 Fig. 1 shows a system configuration of an
example speech synthesizer adopting the PWM signal demodulation method of the present
invention, and Figs. 2 (a) to 2 (C) show the same system respectively. FIG. 3 is a circuit diagram
showing an example of a switching amplifier, FIG. 3 is a diagram showing speaker characteristics
and impedance characteristics in comparison with the conventional characteristics according to
the present invention system, and FIG. 4 explains removal of PWM modulation noise according to
the present invention system It is a characteristic curve figure for explaining. In the figure, 1:
control microcomputer, 2 Ls 1% 3 'switching amplifier for voice synthesis, 4: speaker agent
patent attorney Aihiko Fukushi EndPage: 4
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jps57212898, description
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