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The present invention relates to a musical tone reproducing apparatus for reproducing musical
tones from musical tone signals, particularly to mobile communication devices such as mobile
phones, PHSs, and pagers, portable devices such as pocket watches, or toys and the like. The
present invention relates to a suitable tone reproduction device capable of performing a
performance with arbitrary performance data of two or more tones.
2. Description of the Related Art Recently, a ringing tone function that plays not a ringing tone
but a song melody at the time of an incoming call is popularly called "Ringtone (trademark of
Tokyo Communication Network Co., Ltd.)" or the like. This function is to decode and play
sequence data (musical score format data) stored in advance in the mobile phone's memory at
the time of an incoming call using the host CPU, tone generator IC, etc. However, with the
progress of technology, it has been gradually shifted to polyphonic (harmonic) playing.
When the output data of such a function is to be amplified on the cellular phone, it is reproduced
by the cellular phone casing which is an extremely small enclosure using an extremely small
amplifier and a speaker which can be mounted on the cellular phone. Unlike the case of playing
polyphony output data with an audio player, a special situation occurs. That is, there are
significant performance limitations in the dynamic range and the reproducible frequency band,
and general audio quality is difficult to reproduce.
The problem of the reproducible frequency band is that the volume of the bass region can not in
principle be large due to the small diameter of the speaker, etc. With a typical 15φ speaker, only
the frequency band of 4 to 6 kHz is reproduced at a satisfactory volume It is a problem that it
can not do. In addition, the problem of the dynamic range is that the sounder used for single tone
performance is about 93 dB-97 dB, while the speaker with superior frequency characteristics to
that of the sounder, which is adopted for multi-tone performance, has a dynamic of about 89 dB90 dB. The problem is that the range goes down. Furthermore, since the multi-tone reproduction
is performed, the dynamic range per sound further decreases, and the dynamic range per sound
decreases in inverse proportion to the increase in the number of simultaneous sounds.
Sound sources that can produce more simultaneous sounds are diversified in timbre variations,
and tend to use many sounds that decay after the volume is sustained, and even with one sound,
the volume difference immediately after the start of sound production and immediately before
the end is remarkable is there. Therefore, if the upper limit is set to the output level when the
sound source is at the maximum volume, such as when the same waveform is sounding at the
same phase when several simultaneous voices are generated, the volume at the time of single
tone performance can not be obtained, and conversely If the volume at the time of performance
is maintained appropriately, the output level will be excessive at the maximum volume and the
reproduced sound will be distorted.
It is conceivable to use a limiter or a compressor that compresses the dynamic range of musical
tones as a means for reproducing as rich a musical tone as possible from musical tone data
having a wide dynamic range by effectively utilizing a limited dynamic range. However, since the
limiter / compressor suppresses the sound volume based on the entire signal level, it also follows
the signal outside the reproducible frequency band to suppress the sound volume. Therefore,
there is a problem that the limited dynamic range can not be used effectively.
SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a
tone reproduction apparatus which can reproduce as much as possible of tones even if the
reproducible frequency range and dynamic range are narrow.
According to the present invention, there is provided a tone reproduction apparatus comprising a
compression circuit for compressing a dynamic range of a tone signal and a tone reproduction
unit for converting an output of the compression circuit into a tone. There is provided a musical
tone reproduction apparatus comprising an equalizer provided at a front stage of the musical
tone reproduction unit and having a frequency characteristic according to the frequency
characteristic of the musical tone reproduction unit.
BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be
described below with reference to a tone generator for a ringing tone in a cellular phone / PHS,
but a musical score information type music delivery data reproduction tone source, a karaoke
tone source etc For a simple sound source that can be played by control of arbitrary sequence
data, for example, a pager, a portable watch, if it uses a reproduction system using an amplifier
element whose frequency characteristic is not sufficiently flat or a speaker with a small aperture.
The same can be applied to toys and the like.
FIG. 1 is a block diagram of a tone reproduction apparatus according to an embodiment of the
present invention.
The tone generator circuit 10 outputs, for example, a digital musical tone signal obtained by
decoding sequence data.
The equalizer 12 has characteristics simulating the frequency characteristics of the power
amplifier 18 and the speaker 20, and removes or suppresses signals in a frequency band which is
not reproduced from the speaker 20 or not completely reproduced.
The limiter / compressor 14 limits or compresses the dynamic range of the output of the
equalizer 12. The D / A converter 16 converts the digital signal output from the limiter /
compressor 14 into an analog signal, and the power amplifier 18 amplifies the power of the
output of the D / A converter 16 and sends it to the speaker 20.
FIG. 2 shows an implementation of the device of FIG. The limiter / compressor 14 of FIG. 1 is
composed of, for example, an envelope follower 22 for detecting an envelope of the output of the
equalizer 12 and a variable gain amplifier 24. By controlling the gain of the variable gain
amplifier 24 with the inverse characteristic of the envelope detected by the envelope follower 22,
a smaller gain is obtained when the signal level is large, and a larger gain is obtained when the
signal level is small. Compress dynamic range.
The sound source 10, the equalizer 12, the envelope follower 22 and the variable gain amplifier
24 are realized by software of a DSP (digital signal processor). The codec decoding LSI that
implements the D / A conversion unit 16 includes a band pass filter 28 and a decoder (not
shown) in addition to the D / A converter 26.
FIG. 3 shows a detailed configuration of the envelope follower 22 of FIG. The input signal is first
subjected to an absolute value (30) and delayed by one sample (32), and the high pass
component is removed by the low pass filter 34 to generate an envelope detection output. The
difference (A−B) with the signal one sample before is calculated (36), and when the result is
positive, the time constant fc1 is set to the low pass filter 34, and when it is negative, the time
constant fc2 is set Ru. This makes the followability at the rise of the signal (beginning of sound)
better than at the time of signal attenuation.
FIG. 4 shows an implementation of the entire compressor 14. In FIG. 4, the low pass filter 34 of
FIG. 3 is realized by the adding element 40, 41 and the delay element 42 and the amplifying
elements 44, 46. The delay element 42 plays the role of the delay element in the low pass filter
34 as well as the role of the delay element 32 of FIG. The output of the envelope follower 22 (the
output of the adding element 40) is clipped by the clipper 48 at the upper and lower limits, and
is subtracted from “1” in the adder 50 to generate a signal having an inverse characteristic to
the envelope. And an integrator consisting of a delay element 56. The output of the integrator
controls the gain of the amplification element 58, and the output is summed 64 to the output of
the amplifier 62.
FIG. 5 shows an example of the input and output of the envelope follower 22. FIG. 6 shows the
frequency characteristics of the output signal of the adder 64. The frequency characteristic when
there is no equalizer 12 is shown in FIG. 7 for comparison. Comparing the two, when there is no
equalizer in the previous stage of the limiter / compressor, the frequency component of 1kHz or
more is suppressed for the component of 1kHz or less, but if the equalizer is provided in the
previous stage of the limiter / compressor It can be seen that components of 1 kHz or more can
be reproduced at a sufficient volume.
As described above, according to the present invention, musical tones as rich as possible can be
reproduced even if the frequency band and dynamic range are limited.
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