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Claims  |
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What is claimed is:
1. A voice band multiplex transmission system comprising:
frequency generating means for generating first and second signals having
first and second frequencies respectively;
modulating means connected to said frequency generating means for frequency
modulating said first and second signals by the same input information
while maintaining the frequency ratio between the said first and second
frequencies, the first and second frequencies respectively belonging to
predetermined first and second frequency bands and bearing no harmonic
relation to each other in the band of a voice circuit;
means for superimposing the modulated first and second signals and an input
voice signal on one another and sending them out on a transmission line;
receiver means coupled to said transmission line and comprising pitch
extracting means for extracting a pitch frequency of said input voice
signal from the signal received from the transmission line;
first comb-characteristic filter means supplied with said received signal
to reject the extracted pitch frequency and frequencies of its integral
multiples;
first and second filter means respectively supplied with the output from
the first comb-characteristic filter means and having the first and second
frequency bands as their pass bands;
first and second frequency detecting means, each supplied with the output
from one of the first and second filter means to detect one frequency
larger than a predetermined threshold;
multiplying means for multiplying one of the detected frequencies of the
first and second frequency detecting means by said frequency ratio or a
reciprocal of the frequency ratio to coincide with the other detected
frequency;
demodulating means for producing a signal of a level corresponding to said
other detected frequency and the output frequency of the multiplying
means; and
voice output means for obtaining the voice signal from said received
signal.
2. A voice band multiplex transmission system according to claim 1, wherein
the voice output means comprises second comb-characteristic filter means
which is supplied with the received signal and permits the passage
therethrough of the extracted pitch frequency and frequencies of its
integral multiples.
3. A voice band multiplex transmission system according to claim 2, further
comprising rejection frequency generating means for producing, from the
detected frequency from one of the first and second frequency detecting
means, a frequency to be detected by the other frequency detecting means,
and variable band rejection filter means for removing from the output of
the second comb-characteristic filter means the frequency component
produced by the rejection frequency generating means.
4. A voice band multiplex transmission system according to claim 3, further
comprising interpolating means for interrpolating into the output of said
variable band rejection filter means a voice component of the frequency
produced by the rejection frequency generating means.
5. A voice band multiplex transmission system according to claim 1, wherein
the voice output means comprises variable band rejection filter means
which is supplied with the received signal and rejects the detected
frequencies of the first and second frequency detecting means.
6. A voice band multiplex transmission system according to claim 5, further
comprising rejection frequency generating means for producing, from the
detected frequency from one of the first and second frequency detecting
means, a frequency to be detected by the other frequency detecting means,
and applying the produced frequency as a rejection frequency to the
variable band rejection filter means.
7. A voice band multiplex transmission system according to claim 6, further
comprising interpolating means for interpolating into the output from the
variable band rejection filter means a voice component of the frequency
produced by the rejection frequency generating means.
8. A voice band multiplex transmission system according to claim 1, wherein
the voice output means comprises means for outputting the received signal
as it is.
9. A voice band multiplex transmission system according to any one of
claims 1 or 8, wherein the modulating means comprises a memory storing the
corresponding relationships between levels and frequencies and supplied
with the input information to output a frequency corresponding to the
level of the input information, means for digitally calculating a first
signal of the frequency outputted from the memory and a second signal of a
frequency having said frequency ratio to the outputted frequency, and
means for outputting the first and second digital signals after converting
them to first and second analog signals.
10. A voice band multiplex transmission system according to any one of
claims 1 to 8, wherein the modulating means comprises a first digital
oscillator supplied with the input information as a digital value
representing its level to oscillate at a frequency corresponding to the
digital value, a multiplier for multiplying the input digital value by the
frequency ratio, a second digital oscillator oscillating at a frequency
corresponding to the multiplied output value, an adder for adding together
the outputs from the first and second digital oscillators, and D-A
converter for converting the added value to an analog signal.
11. A voice band multiplex transmission system according to any one of
claims 1 to 8, wherein the modulating means comprises a first variable
frequency oscillator supplied with analog input information to oscillate
at a frequency corresponding to the level thereof, and a second variable
frequency oscillator supplied with the input information to oscillate at a
frequency of a multiple of said frequency ratio with respect to the
oscillation frequency of the first variable frequency oscillator in
accordance with the level of the input information.
12. A voice band multiplex transmission system according to claim 10,
wherein the first and second frequency detecting means each comprise means
for performing a discrete Fourier transform operation, and means for
determining the frequency of a single spectrum in the operation results
exceeding a threshold.
13. A voice band multiplex transmission system according to any one of
claims 3 to 7, wherein the variable band rejection filter means comprises
means for performing a discrete Fourier transformation of the received
signal, means for removing those frequencies in the frequency spectra
obtained by the transformation which are equal to the detected frequencies
of the first and second frequency detecting means, and means for
performing an inverse discrete Fourier transformation of the frequency
spectra left unremoved.
14. A voice band multiplex transmission system according to any one of
claims 3 to 7, wherein the second variable band rejection filter means is
formed by a digital filter.
15. A voice band multiplex transmission system according to claim 10,
wherein the first comb-characteristic filter means comprises means for
performing a discrete Fourier transformation of the received signal, and
means for removing the pitch frequency and frequencies of its integral
multiples in the frequency spectra obtained by the transformation and
performing an inverse discrete Fourier transformation of the remaining
frequency spectra.
16. A voice band multiplex transmission system according to claim 10,
wherein the first comb-characteristic filter means comprises a
comb-digital filter for rejecting the fundamental frequency and its higher
harmonics, and means for controlling the number of unit delay elements
inserted in the digital filter in accordance with the pitch frequency so
that their total delay time may be equal to the period of the pitch
frequency.
17. A voice band multiplex transmission system according to claim 1,
wherein the modulating means are plurally provided, the modulating means
being respectively supplied with different input information and the
frequencies of the first and second signals of each modulating means being
disposed in different frequency bands, wherein the first and second filter
means, the first and second frequency detecting means, the multiplier
means and the demodulating means are plurally provided respectively
corresponding to the plurality of modulating means, and wherein the input
information is individually derived from the demodulating means.
18. A voice band multiplex transmission system according to claim 11,
wherein the first and second frequency detecting means each comprise means
for performing a discrete Fourier transform operation, and means for
determining the frequency of a single spectrum in the operation results
exceeding a threshold.
19. A voice band multiplex transmission system according to claim 11,
wherein the first comb-characteristic filter means comprises means for
performing a discrete Fourier transformation of the received signal, and
means for removing the pitch frequency and frequencies of its integral
multiples in the frequency spectra obtained by the transformation and
performing an inverse discrete Fourier transformation of the remaining
frequency spectra.
20. A voice band multiplex transmission system according to claim 12,
wherein the first comb-characteristic filter means comprises means for
performing a discrete Fourier transformation of the received signal, and
means for removing the pitch frequency and frequencies of its integral
multiples in the frequency spectra obtained by the transformation and
performing an inverse discrete Fourier transformation of the remaining
frequency spectra.
21. A voice band multiplex transmission system according to claim 13,
wherein the first comb-characteristic filter means comprises means for
performing a discrete Fourier transformation of the received signal, and
means for removing the pitch frequency and frequencies of its integral
multiples in the frequency spectra obtained by the transformation and
performing an inverse discrete Fourier transformation of the remaining
frequency spectra.
22. A voice band multiplex transmission system according to claim 11,
wherein the first comb-characteristic filter means comprises a
comb-digital filter for rejecting the fundamental frequency and its higher
harmonics, and means for controlling the number of unit delay elements
inserted in the digital filter in accordance with the pitch frequency so
that their total delay time may be equal to the period of the pitch
frequency.
23. A voice band multiplex transmission system according to claim 14,
wherein the first comb-characteristic filter means comprises a
comb-digital filter for rejecting the fundamental frequency and its higher
harmonics, and means for controlling the number of unit delay elements
inserted in the digital filter in accordance with the pitch frequency so
that their total delay time may be equal to the period of the pitch
frequency. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to a voice band multiplex transmission system which
permits talking on the telephone simultaneously with the transmission and
reception of some other information using the same telephone circuit.
If it is possible to transmit and receive some information while speaking
on the telephone, the function of the telephone can be expanded. There has
been proposed a method in which when a character, figure or the like is
written down by a pen on a tablet, the position of the pen at each moment
is outputted in the form of analog or digital data of x and y co-ordinates
and these two x and y data are received to reconstruct the original
character, figure or the like. Since the character, figure or the like is
written by hand, the x and y data are relatively low in speed. It would be
very convenient if these x and y data could be transmitted and received
via a telephone circuit while speaking on the telephone.
As systems for transmitting and receiving some information while speaking
on the telephone, there have been proposed a system that transmits a voice
and other information using different circuits and a system that
frequency-divides or time-divides the same telephone circuit for
transmitting a voice and information individually. Of these conventional
systems, the system employing a plurality of independent circuits for
transmission requires circuits of the same number as the information to be
transmitted and received and hence is not preferred from the viewpoint of
efficient utilization of circuit.
The frequency division system is a system that divides a transmission
frequency band and transmits a voice signal and an information signal in
the divided, different bands. Accordingly, the voice frequency band for
voice transmission is narrower than an ordinary voice transmission
frequency band and a portion of the voice frequency component is removed,
resulting in deteriorated voice quality such as lowered loudness and
intelligibility. For enhancement of the voice quality, there has been
proposed a method that applies the transmitted voice to a non-linear
circuit by which the voice component in the removed band is synthesized
approximately. But it is doubtful to what extent the voice information
once lost can be recovered by such processing. Also there has been
proposed a method of compressing the information of a voice by the band
compression techniques, but this method is still in the stage of study
since various band compression techniques are confronted with a problem in
the voice quality.
The time division system is a system that transmits a voice and other
information while changing them over at short-time intervals and
interpolates the voice after reception. In this case, the voice quality is
deteriorated by cutting-off of the voice waveform and its discontinuous
connection. Another method that has been proposed is to compress the voice
waveform in terms of time, but this is still in the stage of study.
Further, there has been proposed a method of inserting the information
signal in a pause/silence period, but this method is not capable of
completely simultaneous transmission and further presents a problem in
that a voice switch is needed.
A system that superimposes the voice and the information on each other is
proposed in I.B.M. Technical Disclosure Bulletin 1964, 4, "Voice-Data
System". According to this system, a data signal is phase modulated and
the modulated output is transmitted after being superimposed on the
high-frequency portion of the voice. Since the high-frequency portion of
the voice is usually smaller in energy than the low-frequency portion, the
high-frequency portion of the voice is regarded as a noise with respect to
the modulated data signal, and the data is received and demodulated and
the data signal in the voice is removed therefrom utilizing the
demodulated output. In practice, however, the high-frequency portion of
the voice may sometimes have a relatively large amount of energy, and
consequently the data signal cannot correctly be demodulated in some
cases.
An object of the present invention is to provide a voice band multiplex
transmission system which permits simultaneous transmission of a voice and
some other information via the same voice circuit without partial removal
of a voice component, and consequently with good volume and
intelligibility and with substantially no deterioration of the voice
quality.
Another object of the present invention is to provide a voice band
multiplex transmission system which performs voice transmission of good
quality without losing a portion of the voice signal, and permits correct
demodulation of information transmitted simultaneously with a voice being
signal and superimposed thereon.
SUMMARY OF THE INVENTION
The frequency spectrum of a voiced sound is observed as a line spectral
series composed of the fundamental frequency or a so-called pitch
frequency of the voice and frequencies of its higher harmonics. In the
present invention, utilizing the harmonics structure of the voice, an
information signal desired to be transmitted together with a voice signal
is represented by a signal having a spectrum separable from the voice
spectrum and this signal is superimposed on the voice signal for
transmission. That is, first and second sine-wave signals of first and
second frequencies are frequency modulated by the same information signal
so that frequency variations of the sine-wave signals may correspond to
variations of the information signal, and the modulated outputs are
superimposed on the voice signal. In this case, the first and second
sine-wave signals are disposed in different first and second predetermined
frequency bands which do not overlap each other, and the frequency ratio k
between the first and second sine-wave signals is set to a value, for
example, 1.918, 2.119 or so, which cannot be represented by a simple
integral ratio so that it does not coincide with the harmonic relation of
the voice spectrum in the voice circuit band. As a consequence, at least
one of the first and second sine-wave signals does not overlap the voice
spectrum. It is desirable that the amount of variation of the information
signal for frequency modulating the sine-wave signals is such that the
modulated first and second sine-wave signals can be regarded as a single
line spectrum in a short time. But it is sufficient that even if the
spectra of the first and second sine-wave signals spread, their peaks
could be detected. In view of this, it is general that the frequency
variations of the sine-wave signals are less than 1 KHz in a second.
On the receiving side, the pitch of the voice is first extracted from the
transmitted superimposed signal. This pitch extraction may be performed by
known pitch extraction methods, for example, by a method of detecting a
maximum peak of the short time autocorrelation function of a signal. Based
on the pitch information thus detected, the pitch frequency and its
harmonic components are removed from the received superimposed signal.
This operation is performed by using, for example, a variable frequency
comb filter and controlling its rejection frequency with the extracted
pitch frequency. In this way, the voice component is removed from the
received superimposed signal. The signal having the voice component
removed therefrom is split into the aforementioned first and second
frequency bands, and in each band, one frequency having a level larger
than a predetermined threshold is detected. Since at least one of the
first and second sine-wave signals is disposed so that it may not overlap
the voice spectrum, at least one of the first and second sine-wave signals
can surely be detected from the output of the comb filter that is used to
remove the voice component. The detected sine-wave signal which has been
detected in either one of the high-frequency band and low-frequency band
is converted by multiplying its frequency value by 1/k or k into the
frequency value of the other sine-wave signal in the other frequency band.
The original information is demodulated from one or both of the converted
signal and the detected signal which is not converted. As for the voice
signal, the superimposed signal may be outputted as it is. From the
viewpoint of enhancement of the voice quality, the frequency components of
the first and second sine-wave signals are removed from the superimposed
signal, based on the detected frequency value, and then the superimposed
signal is outputted, or the voice component is taken out from the
superimposed signal, using a variable comb filter which permits the
passage therethrough of the pitch frequency and its harmonic frequencies.
The voice component that is lost by applying the superimposed signal to
the filter for removing the first and second sine-wave components from the
superimposed signal is very small, and accordingly the resulting quality
deterioration is very slight.
Since no distinct harmonics structure is observed in the spectrum of an
unvoiced sound, the principles described above do not apply in this case.
But since the power of the unvoiced sound is usually small, the
information signal can be detected even in the unvoiced sound period by
increasing the power of the first and second sine-wave signals and setting
thresholds for detection to be large.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating an embodiment of a voice band
multiplex transmission system of the present invention;
FIG. 2 is a block diagram showing an example of a variable digital
sine-wave generator;
FIGS. 3A to 3E, inclusive, are graphs showing examples of frequency spectra
occurring at respective parts in the embodiment of FIG. 1;
FIG. 4 is a block diagram illustrating an example of a variable comb filter
used in the embodiment of FIG. 1;
FIG. 5 is a block diagram showing the receiving side in another embodiment
of the voice band multiplex transmission system of the present invention;
FIG. 6 is a block diagram illustrating a part of another example of the
receiving side of the voice band multiplex transmission system of the
present invention;
FIG. 7 is a block diagram showing a part of a modified form of the
embodiment of FIG. 1 which employs the receiving side shown in FIG. 6;
FIG. 8 is a block diagram illustrating an example of an analog frequency
modulator;
FIG. 9 is a block diagram showing an example of a variable band rejection
filter employed in the embodiment of FIG. 5; and
FIG. 10 is a block diagram illustrating another embodiment of the present
invention as being applied to the case where a plurality of input
informations are superimposed on a voice.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 there is illustrated an embodiment of the voice band multiplex
transmission system of the present invention. Input information A to be
transmitted together with a voice is applied via a terminal 11 to a
frequency modulator 12, wherein first and second sine-wave signals are
frequency modulated by the input information A; namely, variations in the
input information are detected as frequency changes in the sine-wave
signals. In this embodiment, the frequency modulation is performed by
digital processing, and in the case of the input information A being an
analog signal, it is applied to an analog-to-digital converter
(hereinafter referred to as the A-D converter) 13 for conversion into a
digital signal B. The digital signal B is provided to a digital frequency
modulator 14 to derive therefrom, as modulated outputs,
frequency-modulated sine-wave signals C of two frequencies. In the
frequency modulator 14, for example, a digital sine-wave generator 15 is
provided, which is supplied with the output digital signal B from the A-D
converter 13, as a coefficient for determining the frequency of the
sine-wave generator 15, to yield a sine-wave signal of a frequency
corresponding to the output digital signal B. The digital signal B is
multiplied by a constant k in a multiplier 16 to provide a multiplied
output, by which the output frequency of a digital sine-wave generator 17
is controlled. The output sine-wave signals from the sine-wave generators
15 and 17 are added together in an adder 18. The added output is applied
to a digital-to-analog converter (hereinafter referred to as the D-A
converter) 19 for conversion into an analog signal D.
As the digital sine-wave generator 15, use can be made of a generator
disclosed, for instance, in Bernard Gold et al, "Digital Processing of
Signals", pp. 146-147, McGraw-Hill, 1969. As shown in FIG. 2, and in the
same manner as in this literature, page 146, FIG. 5.13, a signal available
at an output terminal 42 of the digital sine-wave generator is applied to
multipliers 44a and 44c via a delay unit 43 which has a delay equal to one
sampling period, i.e. one arithmetic operation period .tau., and the
output from the multiplier 44c is added with the output from a multiplier
44d in an adder 45, whose output is provided to a delay unit 46 having a
delay .tau.. The output from the delay unit 46 is applied to multipliers
44b and 44d, and the output from the multiplier 44b is added with the
output from the multiplier 44a in an adder 47, whose added output is
provided to the output terminal 42. In the multipliers 44a, 44b, 44c and
44d, the input signals thereto are respectively multiplied by constants A,
B, C and D. When A=D=cosbT and B=-C=sinbT, sinnbT is obtained at the
output terminal 42. The constant A=D is produced by a calculator 48 in
connection with the output from the A-D converter 13 and the constant B=-C
is produced by a calculator 49. These constants A=D and B=-C are
respectively supplied to the multiplier 44a and the multipliers 44b, 44c
and 44d to yield at the output terminal 42 a digital sine-wave signal of
the frequency corresponding to the digital value of the input information.
The sine-wave generator 17 can also be formed similarly.
The sine-wave signals of two frequencies derived from the frequency
modulator 12 have a frequency ratio k of, for example, 1.918, 2.119 or so,
which is so determined as not to coincide with the harmonics relationship
of voice frequency spectra in a voice band, and are separately disposed in
two predetermined frequency bands which do not overlap each other. The
sine-wave signals vary their frequencies in response to the input
information A while maintaining the abovesaid frequency ratio k
therebetween. As shown in FIG. 3A, when the frequency f.sub.1 of the first
sine-wave signal is disposed in a frequency band F.sub.1, the frequency
f.sub.2 of the second sine-wave signal is kf.sub.1, which is disposed in a
frequency band F.sub.2 higher than the frequency band F.sub.1. The
frequencies f.sub.1 and f.sub.2 vary in response to variations in the
input information A, and since the speed of variation of the input
information A is low, the speed of the frequency variations is limited so
that each of the two signals can be regarded as substantially one
sine-wave signal or even if their frequencies spread due to sideband
waves, the peak values of the spreading frequencies can be detected on the
receiving side.
The frequency modulated output D derived from the frequency modulator 12 is
superimposed on an analog voice signal E which is applied from a terminal
21 and has a frequency spectrum such, for example, as shown in FIG. 3B,
providing an analog superimposed signal F having a spectrum depicted in
FIG. 3C, which is sent out onto a transmission line 23 from a transmission
output terminal 22. The analog superimposed signal F transmitted over the
transmission line 23 and received at a reception input terminal 24 is
separated into the input information A and the analog voice signal E. This
separation is carried out by digital processing in the present embodiment.
The analog superimposed signal F at the terminal 24 is converted by an A-D
converter 25 to a digital superimposed signal G. The digital superimposed
signal G is applied to a variable comb filter 26 which is equipped with a
characteristic for suppressing a predetermined frequency and its harmonic
components and a pitch extractor 27 which extracts a voice fundamental
frequency, that is, what is called the pitch frequency. The pitch
extractor 27 may be a known one, which extracts the pitch frequency H in
the digital superimposed signal G by detecting a maximum peak of the short
time autocorrelation function of the digital superimposed signal G, as set
forth, for example, in L. R. Rabiner et al, "Digital Processing of Speech
Signals", Prentice-Hall, 1978, pp. 150-158, "4.8 Pitch Period Estimation
Using the Autocorrelation Function". This voice fundamental frequency H
controls the characteristic of the comb filter 26. The comb filter 26 is
designed to reject all integral multiples of the pitch frequency in the
band of the received signal F.
As such a comb filter 26, a variable-frequency, digital comb filter can be
employed. For instance, as shown in FIG. 4, the digital superimposed
signal G is supplied from a terminal 51 to a subtractor 52 and, at the
same time, applied thereto via a cascade connection of unit delay elements
53, each having a delay equal to one computation period, that is, one
sampling period .tau.. On the other hand, the extracted pitch frequency is
set in a register 54 and then decoded by a decoder 55, by the output of
which any one of a plurality of switches 56 is turned ON. By control of
the switches 56, the number m of unit delay elements 53 to be inserted
between the terminal 51 and the subtractor 52 is varied so that the total
amount of delays, m.tau., of the inserted unit delay elements may be equal
to the extracted pitch period. As for the fixed frequency characteristic,
this kind of digital comb filter is shown, for example, in Bernard Gold et
al, "Digital Processing of Signals", pp. 85, FIG. 3.23(a), McGraw-Hill,
1969.
In FIG. 1, there is obtained a signal I wherein the voice fundamental
frequency H and its harmonic components in the digital superimposed signal
G have been removed therefrom by the comb filter 26. The signal I is
applied to a digital low-pass filter or digital low band-pass filter 28
whose pass band is the frequency band F.sub.1, and is also applied to a
digital high-pass filter or digital high band-pass filter 29 whose pass
band is the frequency band F.sub.2, producing output signals J and K,
respectively. The filter output signals J and K are respectively supplied
to frequency detectors 31 and 32. The frequency detectors 31 and 32 each
have a function that detects one frequency component larger than a
predetermined threshold and determines its frequency. Such a frequency
detector can be so arranged, for example, as to obtain the frequency
spectrum of the input signal thereto by a discrete Fourier transform
operation and to detect, by processing, the frequency of that frequency
component in the calculated frequency spectrum which is larger than the
predetermined threshold. In this case, since the signals I applied to the
filters 28 and 29 have removed therefrom the frequency spectrum component
of a voice, they correspond to the frequencies f.sub.1 and f.sub.2
=kf.sub.1 of the two sine-wave signals derived from the frequency
modulator 12. In each of the frequency detectors 31 and 32, only one
frequency spectrum can exceed the threshold. The frequency (identified as
a signal M) detected by the frequency detector 32 is converted by a
frequency converter 33 to 1/k of the frequency in accordance with the
value of the ratio k determined by the frequency modulator 12.
Accordingly, the frequency represented by an output signal N from the
frequency converter 33 coincides with the frequency f.sub.1 (identified as
a signal L) detected by the frequency detector 31.
The signals L and N are supplied to a frequency demodulator 34 for
conversion into a voltage corresponding to the frequency. This conversion
can be performed, for example, by preparing a table for the conversion of
the input frequency to the corresponding output voltage and looking up in
the conversion table the frequency f.sub.1 represented by the signals L
and N to output the corresponding voltage. The demodulated output signal
from the frequency demodulator 34 coincides with the digital signal B of
the input information on the transmitting side and is converted by a D-A
converter 35 to the original analog signal A, which is derived at an
output terminal 36.
As described above, since the two frequency signals of the frequencies
f.sub.1 and f.sub.2 in the two frequency bands F.sub.1 and F.sub.2 are
employed for the transmission of the input information A and since their
frequency ratio k is selected not to bear a harmonic relation to the
frequency spectrum of a voice signal, even if the signal of one of the
frequencies is rejected by the comb filter 26, that is, even if the signal
of the frequency, for example, f.sub.1 coincides with the voice spectrum,
the signal of the other frequency kf.sub.1 is detected at the output of
the comb filter 26, as shown in FIG. 3D. Accordingly, the information
signal A can be detected accurately and stably without any interference by
the voice signal E.
The digital output signal G converted from the received signal is applied
to a variable comb filter 37 which has a characteristic that permits the
passage therethrough of the pitch frequency and its harmonic components.
The characteristic of the variable comb filter 37 is controlled by the
pitch frequency H extracted by the pitch extractor 27. The variable comb
filter 37 may comprise an arrangement that, for example, employs an adder
in place of the subtractor 52 in FIG. 4. In the manner described above,
the voice spectrum is derived from the variable comb filter 37. The output
signal P from the variable comb filter 37 is converted by a D-A converter
38 to an analog signal Q to provide the original analog voice signal E at
an output terminal 39.
The voice signal may also be extracted using a circuit arrangement of the
type shown in FIG. 5 in which parts corresponding to those in FIG. 1 are
identified by the same reference numerals. The received digital
superimposed signal G is applied directly to a variable band rejection
filter 41 without being passed through the variable comb filter. The
signals L and M representing the frequencies f.sub.1 and f.sub.2 detected
by the frequency detectors 31 and 32 are provided to the variable band
rejection filter 41 to set the frequencies f.sub.1 and f.sub.2 as those to
be rejected. Consequently, the frequency components f.sub.1 and f.sub.2 of
the digital superimposed signal G are removed and the filtered output
signal R is applied to the D-A converter 38. As the variable band
rejection filter 41, use can be made of a filter, for example, of the type
performing such processing that obtains the frequency spectrum of the
superimposed signal G by the discrete Fourier transform calculation,
reduces to zero the components of those frequencies f.sub.1 and f.sub.2 in
the spectrum represented by the signals L and M and subjects the remaining
spectrum to an inverse discrete Fourier transform operation. In the case
where the first and second sine-wave signals superimposed on each other
are not so jarring with respect to the voice, the received signal F may
also be provided directly as a voice output at the output terminal 39, as
indicated by the broken line in FIG. 5.
One of the frequencies f.sub.1 and f.sub.2 of the first and second
sine-wave signals may coincide with the voice spectrum, and in such a
case, only one of the frequency detectors 31 and 32 detects the frequency.
In this case, the other frequency is produced from the detected one and
these frequency components are removed by the band rejection filter 41 in
FIG. 5. To this end, for example, as depicted in FIG. 6, the signals L and
M respectively representing the frequencies f.sub.1 and f.sub.2 detected
by the frequency detectors 31 and 32 are supplied to a superimposed
frequency generator 57. In the superimposed frequency generator 57, the
signals L and M are respectively applied to OR gates 58 and 59 and
multipliers 61 and 62. In the multiplier 61, the frequency f.sub.1
represented by the signal L is multiplied by k to kf.sub.1 =f.sub.2,
whereas in the multiplier 62, the frequency f.sub.2 represented by the
signal M is multiplied by 1/k to f.sub.2 /k=f.sub.1. The multiplied
outputs from the multipliers 61 and 62 are respectively provided to the OR
gates 58 and 59, the outputs from which are both fed to the variable band
rejection filter 41. In this way, even in the case where only one of the
frequencies f.sub.1 and f.sub.2 is detected by one of the frequency
detectors 31 and 32, the other frequency is produced and components of the
both frequencies f.sub.1 and f.sub.2 in the voice are removed by the band
rejection filter 41. For example, in the example shown in FIG. 3, the
frequency f.sub.1 is produced from the detected frequency f.sub.2 and the
output from the band rejection filter 41 takes the form depicted in FIG.
3E in which the frequency f.sub.1 in the voice spectrum has been removed
therefrom. The superimposed frequency generator 57 may be formed not only
as a digital circuit but also as an arrangement that obtains the same
function as the digital circuit by processing.
In the case where the frequency is detected by only one of the frequency
detectors 31 and 32, the other frequency coincides with the voice
spectrum; accordingly, in FIG. 6, the frequency component in the voice
spectrum coincident with the frequency is also removed by the variable
band rejection filter 41, resulting in the quality of the voice being
degraded a little. To avoid this, as occasion demands, the output from the
filter 41 is applied to an interpolator 63, wherein the frequency
component removed from the voice spectrum is interpolated in the filter
output. This interpolation is carried out in the following manner: For
example, the spectrum envelope of the voice spectrum frequency
characteristic is obtained and that one of the frequencies f.sub.1 and
f.sub.2 which coincides with the voice spectrum is inserted, as a level
crossing the spectrum envelope at the frequency, into the output R from
the filter 41. The signal thus interpolated is applied to the D-A
converter 38.
Also in the embodiment illustrated in FIG. 1, when one of the frequencies
f.sub.1 and f.sub.2 of the first and second sine-wave signals coincides
with the voice spectrum, this sine-wave signal is not removed by the
variable comb filter 37. Accordingly, in the case where there is a fear
that the sine-wave signal coincident with the voice spectrum will be
offensive to the ear, the sine-wave signal component can be eliminated in
the same manner as described previously in connection with FIG. 6. For
example, as shown in FIG. 7 in which parts corresponding to those in FIGS.
1 and 6 are identified by the same reference numerals, a voice spectrum
signal P taken out by the variable comb filter 37 is supplied to the
variable band rejection filter 41. The frequencies detected by the
frequency detectors 31 and 32 are applied to a rejection frequency
generator 60 to produce a frequency coincident with the voice spectrum.
The rejection frequency generator is similar to the superimposed frequency
generator 57 shown in FIG. 6, but instead of taking OR by OR gates 58, 59,
the detected outputs from the frequency detectors 31, 32 are simply
multiplied at the multipliers 61, 62 and supplied to the band rejection
filter 41. In this case, as well as the case of FIG. 6, the multiplier 61
can be used in common with the frequency converter 33. This frequency
component in the voice spectrum is removed by the band rejection filter
41. The output R from the filter 41 may be supplied to the D-A converter
38 directly or after being subjected to interpolation by the interpolator
63 for the same reason as referred to previously with regard to FIG. 6.
In FIG. 1, the frequency modulator 12 may also be an analog modulator. For
instance, as shown in FIG. 8, analog input information A is applied from
the terminal 11 to voltage-controlled oscillators 64 and 65. As the
voltage-controlled oscillators 64 and 65, use is made of such oscillators
which have therebetween an oscillation frequency ratio k when the voltage
at the terminal 11 is zero and which have linear frequency-control voltage
characteristics that intersect the voltage axis at the same point. In the
case where the input information is a digital signal, it is converted to
an analog signal for input to the voltage-controlled oscillators 64 and
65.
In the case of performing digital processing by the frequency modulator 12,
the digital processing can also be carried out by an electronic computer.
In such a case, the output from the A-D converter 13 in FIG. 1 is applied
to the electronic computer which has prestored a table of the level and
the frequency of the input information A and in which the table is read
out by the input digital information to output by calculation, as digital
values, a sine-wave signal of the frequency and a sine-wave of a frequency
k times the frequency. In this case, the frequency detectors 31 and 32 are
adapted so that they check, for each operation period, whether the outputs
from the filters 28 and 29 are above or below a threshold, start counting
of clock pulses upon detection of the filter outputs exceeding the
threshold, and obtain the periods of detected frequencies from the count
values at the same time of the inputs exceeding again the threshold after
becoming lower than the threshold. The operations can easily be performed
by processing with a computer.
The variable band rejection filter 41 used in FIG. 5 is not always limited
specifically to the arrangement for the processing utilizing the Fourier
transformation but may also be formed, for example, by a digital filter.
The digital band rejection filter is shown, for instance, in the
aforementioned literature "Digital Processing of Signals", p. 42, FIG.
2.20. FIG. 9 illustrates this digital band rejection filter. In FIG. 9,
the input at an input terminal 66 is fed to an adder 67, and its output is
applied to a cascade connection of delay elements 68, each having a delay
equal to the sampling period .tau.. The outputs from the delay elements 68
are respectively supplied to multipliers 69, wherein they are multiplied
by constants -k.sub.1, -k.sub.2, . . . and -k.sub.m, respectively. The
multiplied outputs are provided to the adder 67. The output from the adder
67 and the outputs from first r stages of the delay elements 68 are
respectively multiplied by constants L.sub.0, L.sub.1, L.sub.2, . . . and
L.sub.i in multipliers 71 and their multiplied outputs are added together
by an adder 72, whose added output is provided as a filtered output at an
output terminal 73. The constants -k.sub.1 to -k.sub.m and L.sub.0 to
L.sub.i are modified by the frequencies f.sub. 1 and f.sub.2 to be
eliminated. The relationships between the frequencies and the constants
are prestored in a memory 74; the memory 74 is read out by the output
frequencies of the frequency detectors 31 and 32 or the output frequency
of the superimposed frequency generator 57; and the constants thus read
out from the memory 74 are imparted to the corresponding ones of the
pluralities of multipliers 69 and 71.
In the case where the detected frequency of the frequency detector 31 and
the output frequency of the frequency converter 33 do not coincide with
each other due to noise or by some other cause, the output frequency of
the frequency converter 33 can preferentially be supplied to the
demodulator 34. The reason is that since a voice is usually low in level
on the high-frequency side, the signal of the frequency f.sub.2 is less
likely to be erroneous than the signal of the frequency f.sub.1. In the
case of non-coincidence, it is also possible to retain the values in the
immediately preceding period. Furthermore, instead of multiplying the
output frequency of the frequency detector 32 by 1/k by the frequency
converter 33 as explained before, the frequency converter 33 may be
provided after the frequency detector 31 so as to multiply the output
frequency therefrom by k.
The present invention is applicable not only to transmitting a piece of
input information, as information other than a voice, together with a
voice signal but also to simultaneous transmission of pieces of input
information and a voice signal. FIG. 10 shows the case of transmitting two
pieces of input information simultaneously with a voice signal. In FIG.
10, parts corresponding to those in FIGS. 1 and 5 are identified by the
same reference numerals with suffixes "x" and "y" respectively
corresponding to input information Ax | | |
