Narrow-band communication apparatus

A novel narrow-band communication apparatus comprises a digital signal processor for demodulating the output from a receiving section and outputting a modulation signal for an audio input, an electric field strength detector section for detecting electric field strength in the output from the receiving section, an indicator for indicating whether the narrow-band communication apparatus is in a receiving mode or in a transmission mode, and a power controller for controlling supply of power to, or an operating state of specified portions of, the narrow-band communication apparatus according to the output from the electric field strength detector as well as from the indicator.

This patent application is the subject of a submission under the Disclosure 
Document Program, which was filed thereunder by applicants on Aug. 1, 1994 
and identified as Disclosure Document No. 358784 with its accompanying fee 
paid under the provisions of all the Rules and Regulations including 37 
CFR $1.21(c). 
FIELD OF THE INVENTION 
The present invention relates to a narrow-band communication apparatus, and 
more particularly to a narrow-band communication apparatus employing 
digital modulation/demodulation therein wherein operation of elements 
constituting the apparatus, operations of which are not required, is 
stopped according to electric field strength detection as well as to a 
mode of transmitting or receiving, to effectively reduce dissipation power 
in the transmission system, and detection of electric field strength is 
also used for controlling gain in the receiving system to improve circuit 
efficiency as well as to provide improved characteristics of the 
apparatus. 
Also, the present invention relates to a narrow-band communication 
apparatus in which malfunction in AFC due to misrecongnition of an audio 
signal or data for a pilot signal never occurs even if a search is 
executed in a state where a strong audio signal or data exists near a 
pilot signal when AFC is executed in a narrow-band communication 
apparatus, or the like, employing digital modulation/demodulation therein. 
Also, the present invention relates to a narrow-band communication 
apparatus which reduces hearing discomfort caused by noise and enables 
high speed AGC operation without a receiving section being saturated when 
executing an AGC operation according to the measured power of a pilot 
signal in a narrow-band communication apparatus or the like, employing 
digital modulation/demodulation therein. 
BACKGROUND OF THE INVENTION 
FIG. 6 is a block diagram of a conventional type of narrow-band 
communication apparatus. The narrow-band communication apparatus based on 
the prior art employs a digital modulation technology and also employs a 
simplex system, and the receiving section is based on a triple super 
heterodyne configuration. 
In FIG. 6, the conventional type of narrow-band communication apparatus 
largely comprises an RF and IF receiving section for a triple super 
heterodyne reception, a local frequency generating section for receiving, 
a local frequency generating section for transmission, a reference 
frequency generating section, a transmitting section, a signal processing 
section for digital modulation/demodulation, an audio output section, and 
a microphone audio input section. 
The RF and IF receiving section for a triple super heterodyne receiver 
comprises a receiving frequency (RF) amplifier 601, a first mixer 602, a 
first intermediate frequency (IF) amplifier 603, a second mixer 604, a 
third mixer 605, a third intermediate frequency (IF) amplifier 606, a 
filter 607 and an A/D converter 608. 
The local frequency generating section for receiving comprises a first 
local oscillator 613, a second local oscillator 614, a third local 
oscillator 615, and an amplifier 621 for AGC. Also the local frequency 
generating section is realized by a local oscillator 620 for transmission, 
and the reference frequency generating section is realized by a reference 
frequency oscillator 619. 
The signal processing section for digital modulation/demodulation is 
realized by a transmitting/receiving digital modulator/demodulator 609 
using, for instance, a DSP (Digital Signal Processor). 
The audio output section comprises a D/A converter 610, an audio amplifier 
611, and a loudspeaker 612. Also the microphone audio input section 
comprises an A/D converter 625, a filter 626, a microphone 627, and an 
amplifier 628 for a microphone. 
The transmitting section comprises a transmission frequency (RF) power 
amplifier 622, a modulator 623, a D/A converter 624, a push-to-talk switch 
(PTT switch) 629, and a transmitter control circuit 630. 
In a narrow-band communication apparatus employing digital 
modulation/demodulation having the configuration as described above, 
current in the digital signal processing section 609, A/D converter 608, 
D/A converter 610, and A/D converter 625 is large, and power is dissipated 
even though transmission or reception is not always being executed, which 
causes a serious problem especially in a portable terminal unit using 
batteries or the like. 
To overcome the problems as described above, for instance, in Japanese 
Patent Laid-Open Publication No.345330/1992 is proposed a technique for 
lowering power consumption by executing so-called intermittent receiving 
in which a receiving system is intermittently operated, or by detecting a 
electric field strength and making the ON/OFF cycle longer when the 
detected electric field strength is weak. Another described technique 
disconnects power when electric field strength in a stand-by state is weak 
for a specified period of time. 
However, the technique for lowering power consumption, as described above, 
is applied only to a receiving system, and in a narrow-band communication 
apparatus employing digital modulation/demodulation and having the 
configuration as described above, dissipation power in the transmission 
system is large as swell, and effective power saving has not been 
achieved. Also, detection of electric field strength has been used only 
for control over intermittent receiving or control of power disconnection, 
and efficiency of the circuit is low. 
Also, in the conventional type of narrow-band communication apparatus as 
described above, when AFC (Automatic Frequency Control) is executed, as 
shown in FIG. 7, a pilot signal inserted at a center of an aural signal is 
used to measure the frequency. 
The audio signal is shifted by 225 Hz respectively at a center of a band as 
shown in FIG. 8, and if a search is executed for AFC in a state where a 
strong audio signal or data exists near a pilot signal, sometimes the 
audio signal or data may be mistaken for a pilot signal because of a 
relation between a filter's bandwidth and the shift rate of an auding 
signal. 
This problem can be solved by setting the shift rate of an audio signal to 
a large value, but the maximum effective band is limited to 4 KHz (in case 
of 220 MHz in USA), so that, if the shift rate is set to a large value, 
the audio signal can not be transmitted appropriately. 
Also, the above problem can be solved by making the filter band for a pilot 
signal narrow, but then another problem occurs that timing for locking to 
the pilot signal becomes difficult. In brief, a finer search is required, 
and furthermore the response speed of the filter becomes slower, and 
consequently a longer time is required for searching. 
In a tone-in-band receiver, generally demodulation of an audio signal is 
executed by referring to a pilot signal. Namely, a constant audio output 
can be obtained regardless of the electric field strength by measuring 
power P of a pilot signal and multiplying the demodulated aural element by 
1/P. This is so-called AGC (automatic Gain Control) operation. 
FIG. 4A shows characteristics of audio output and SINAD (signal plus noise 
plus distortion to noise plus distortion ratio) against electric field 
strength. As shown in the figure, audio output at a constant level is 
obtained because of AGC. 
However, in a weak electric field, SINAD is low, so that, if the audio 
output is provided at a constant level, signals containing heavy noise are 
outputted at a constant level and it causes hearing discomfort. In 
addition, sometimes measurement of power of the pilot signal can not be 
executed correctly, and consequently noise level is raised as indicated by 
a dotted line in the figure. The phenomenon as described above also occurs 
when a receiving signal drops out due to Rayleigh fading or the like. 
In the AGC operation, measurement of signal power is executed for a pilot 
signal having passed through a narrow-band filter for a pilot signal as 
described above, so that 100 ms or more is required only for, for 
instance, response by the pilot signal filter and a long time is required 
before the AGC operation is started. 
Furthermore when a strong input is received, a pilot filter can not pass 
the pilot signal through before the AGC operation is locked, and for this 
reason the AGC operation is not started and the receiving section becomes 
saturated. 
In brief, the conventional type of narrow-band communication apparatus has 
the following problems. First, an intermittent cycle is controlled or 
power is turned ON/OFF according to electric field strength detected 
during intermittent reception in a receiving system to reduce dissipation 
power in the apparatus, but this technique for reducing dissipation power 
has been applied only to a receiving system. In the conventional type of 
narrow-band communication apparatus employing an digital 
modulation/demodulation, any effective measure for power saving has not 
been applied to a transmission system although dissipation power in the 
transmission system is also large. Detection of electric field strength 
has been applied only to intermittent reception control or to power ON/OFF 
control, and consequently, circuit efficiency is rather low. 
Second, if a search is executed, when executing AFC, in a state where a 
strong audio signal or data exists near a pilot signal, sometimes the 
audio signal or data may be mistaken for the pilot signal because of the 
relation between the filter bandwidth and audio signal shift rate. If the 
shift rate is set to a large value to overcome this problem, the audio 
signal can not be transmitted appropriately because there is a limit to 
the maximum effective bandwidth. If the bandwidth of a pilot signal is 
made narrower, timing for catching a pilot signal becomes difficult. This 
makes it necessary to execute a finer search, and consequently, the 
response speed of the filter becomes slower, and a longer time is required 
for searching. 
Third, when AGC is executed according to the measured power of a pilot 
signal, an audio output at a constant level is obtained through AGC, but 
in a weak electric field region, SINAD is low. Hence, signals containing 
heavy noise are generated at a constant level, which causes serious 
hearing discomfort. Also pilot signal power can not be measured correctly, 
so that the noise level is raised, which makes hearing discomfort more 
serious. 
Fourth, in the AGC operation, signal power measurement is executed for a 
pilot signal having passed through a pilot signal filter, so that a long 
time is required before the AGC operation is started. When, for instance, 
a strong input is received, the pilot signal can not pass through the 
pilot signal filter before the AGC operation is locked, and the AGC 
operation is not started, so that the receiving section becomes saturated. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a narrow-band 
communication apparatus employing digital modulation/demodulation wherein 
operation of elements constituting the apparatus, operation of which is 
not required, is stopped according to electric field strength detection as 
well as to a transmitting or receiving mode to effectively lower power 
consumption also in the transmitting system, and furthermore electric 
field strength detection is applied to control of gain in the receiving 
system for improving circuit efficiency as well as for improving 
characteristics of the apparatus. 
It is another object of the invention to provide a narrow-band 
communication apparatus which does not generate AFC malfunction due to 
misrecognition of an audio signal or data for a pilot signal, when 
executing AFC in a narrow-band communication apparatus or the like 
employing digital modulation/demodulation, even if a strong audio signal 
or data exists near the pilot signal. 
It is still another object of the present invention to reduce hearing 
discomfort due to noise when executing AGC operation according to a pilot 
signal power measurement and also to enable high speed AGC operation 
without saturating the receiving section in a narrow-band communication 
apparatus or the like employing digital modulation/demodulation therein. 
In a narrow-band communication apparatus according to the present 
invention, when demodulating the output from a receiving section or 
modulating the audio input with a digital signal processing means, 
electric field strength of the output is detected by an electric field 
strength detecting means, and a power control means provides control over 
supply of power to, or the operating state of, specified portions of the 
narrow-band communication apparatus according to the output from the 
electric field strength detecting means and/or the output from an 
indicating means for indicating in which of receiving mode or transmitting 
mode the narrow-band communication apparatus currently is in. 
If, for instance, the indicating means does not indicate receiving mode, 
nor transmitting mode, it is not necessary for both the transmitting 
system and receiving system to be run, so that operations of the systems 
are stopped for reducing power consumption. If the indicating means 
indicates the transmitting mode, operation of the transmitting system is 
immediately started to enable transmission without any trouble. 
Furthermore, if the indicating means indicates the receiving mode, and it 
is determined by the electric field strength detecting means that the 
electric field strength of a received signal in the receiving section is 
more than a specified value, the receiving system's operation is enabled 
so that signals can be received. 
As described above, power supply to and operating state of elements 
constituting an apparatus, operation of which is not required, is 
controlled according to electric field strength output from a receiving 
section as well as to whether the apparatus is in transmitting mode or in 
receiving mode, so that reduction of dissipation power can efficiently be 
realized not only in the receiving system but also in the transmitting 
system. 
In a narrow-band communication apparatus according to the present 
invention, gain control for a receiving section is executed by an 
automatic gain control means according to an automatic gain control signal 
outputted from a digital signal processing means as well as to a result of 
detection by an electric field strength detecting means. 
When executing gain control only according to an automatic gain control 
signal outputted through demodulation in a digital signal processing 
means, sometimes the receiving section may become saturated, for instance, 
when a strong input is received first, which in turn disables normal 
receiving. In the narrow-band communication apparatus according to the 
invention, the electric field strength detecting means is implement, and 
normal receiving is enabled by providing coarse gain control so that the 
receiving section will not become saturated before demodulation in the 
digital signal processing means is started and an accurate automatic gain 
control signal is outputted. With this feature, electric field strength 
detection is also applied to gain control for a receiving system, so that 
circuit efficiency can be improved and also improved characteristics of 
the apparatus are provided. 
In a narrow-band communication apparatus according to the present 
invention, the receiving section is intermittently run by an intermittent 
control means to reduce dissipation power in the apparatus. 
In a narrow-band communication apparatus according to the present 
invention, in a digital signal processing means for demodulating the 
output from a receiving section and outputting a modulated signal for an 
audio input, after an indicating means indicates the transmitting mode, a 
specified period of time is measured by a timer circuit, and only a pilot 
signal is outputted as a modulated signal within the specified period of 
time by a modulated output control means and sent through a transmitting 
section. On the other hand, at the receiving side, a frequency control 
means measures frequency making use of a pilot signal and generates a 
frequency control signal for automatically controlling frequency in the 
receiving section as well as in the transmitting section. The frequency 
control means is previously synchronized to the pilot signal by a 
synchronism control means within the specified period of time, however, 
when only a pilot signal is transmitted. 
With this feature, automatic frequency control malfunction in which, for 
instance, an audio signal or data or the like is mistaken for a pilot 
signal due to the effect of modulation, is eliminated. 
In a narrow-band communication apparatus according to the present 
invention, in a digital signal processing means for demodulating the 
output from at least a receiving section, pilot signal power included in a 
receiving signal is measured by a pilot signal power measuring means, and 
when the measured power of the pilot signal is not more than a specified 
value, the audio output level is corrected by a level correcting means. 
For instance, if the power of a pilot signal becomes very weak due to the 
effect of fading or the like, the audio output level is corrected, and 
also an audio output level becomes lower in association with a drop of 
SINAD in contrast to the fact that audio output is provided at a constant 
level even in a weak electric field region where SINAD is low in the 
conventional technology. For this reason, hearing discomfort, caused by 
the fact that signals containing heavy noise are outputted at a constant 
level in the conventional technology, is eliminated. 
In a narrow-band communication apparatus according to the present 
invention, in a digital signal processing means for demodulating at least 
the output from a receiving section, the power of a pilot signal included 
in a received signal is generally measured by a pilot signal power 
measuring means. An automatic gain control signal is outputted by an 
automatic gain control signal output means to an automatic gain control 
means according to the output of the pilot signal power measuring means 
for controlling gain in the receiving means, but power in specified 
channels, each for a receiving signal, is measured by channel power 
measuring means before the output of the pilot signal power measuring 
means is obtained. An automatic gain control signal is supplied to the 
automatic gain control means according to the result of the above 
measurement for executing gain control in the receiving section. 
For instance, even if a strong input is received, power in one channel is 
measured by the channel power measuring means and gain control is executed 
so that the receiving section will not become saturated according to the 
power measurement above. For this reason, the problem in the conventional 
technology, that the receiving section becomes saturated because the 
automatic gain control operation is not started, is eliminated, and high 
speed automatic gain control operation can be realized. 
Furthermore in a narrow-band communication apparatus according to the 
present invention, in a digital signal processing section for demodulating 
the output from a receiving section and outputting a modulated signal for 
an audio input, current is detected by a current detecting means, and 
fluctuation of current detected by the current detecting means is consumed 
in a load circuit. With this feature, a current consumption in the digital 
signal processing means can be maintained at a constant level, and 
degradation of S/N ratio in components such as an A/D converter due to the 
effect of change in current consumption can be prevented. 
Other objects and features of this invention will become understood from 
the following description with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Detailed description is made hereinafter for the narrow-band communication 
apparatus according to the present invention in the order of Embodiment 1 
and Embodiment 2. 
FIG. 1 is a block diagram of a narrow-band communication apparatus 
according to Embodiment 1 of the present invention. The narrow-band 
communication apparatus according to this embodiment is a communication 
apparatus employing digital modulation/demodulation technology. The 
apparatus has an electric field strength detecting means based on an 
analog system and a power control means for controlling dissipation power 
as a means for reducing dissipation power, and drives the power control 
means according to (1) an operating mode of the narrow-band communication 
apparatus or (2) electric field strength direction for reducing 
dissipation power in the apparatus. Also, an AGC amplifier is driven and 
controlled according to an output of the electric field strength detecting 
means to effect coarse AGC so that the receiving section will not be 
saturated before operation of a signal processing section for executing 
digital demodulation is started. 
In this figure, the narrow-band communication apparatus according to the 
present embodiment largely comprises, in addition to the means for 
reducing dissipation power described above, an RF and IF receiving section 
of a triple super heterodyne receiver, a local frequency generating 
section for reception, a local frequency generating section for 
transmission, a reference frequency generating section, a transmitting 
section, a digital modulation/demodulation signal processing section 109, 
an audio output section, a microphone audio input section or the like. 
The RF and IF receiving section of a triple super heterodyne comprises a 
received frequency (RF) amplifier 101, a first mixer 102, a first 
intermediate frequency (IF) amplifier 103, a second mixer 104, a third 
mixer 105, a third intermediate frequency (IF) amplifier 106, a filter 
107, an A/D converter 108, and an amplifier for AGC 121. In the receiving 
section, an electric wave, received through an antenna 131 and a switch 
132, is processed under gain control by the AGC amplifier 121 based on AGC 
output b from the signal processing section 109 for digital 
modulation/demodulation according to triple super heterodyne system 
principles, and is supplied as a third intermediate frequency input a into 
the signal processing section 109 for digital modulation/demodulation. 
The local frequency generating section for reception comprises a first 
local oscillator 113, a second local oscillator 114, and a third local 
oscillator 115. The local frequency generating section for transmission is 
realized by the local oscillator 120. Also, the reference frequency 
generating section is realized by the reference frequency oscillator 119. 
The signal processing section for digital modulation/demodulation is 
realized by a digital modulator/demodulator 109 employing, for instance, a 
DSP (Digital Signal Processor) therein. 
The audio output section comprises a D/A converter 110, an audio signal 
amplifier 111, and a loud speaker 112. The microphone audio input section 
comprises an A/D converter 125, a filter 126, a microphone 127, and an 
amplifier 128 for a microphone. 
The transmitting section comprises a transmitting frequency (RF) power 
amplifier 122, a modulator 123, a D/A converter 124, a push-to-speak 
switch (PTT) 129, and a transmitter control circuit 130. 
Furthermore the dissipation power reducing means added in the present 
invention comprises an electric field strength detecting circuit 116, a 
level discriminator or comparator (an electric field strength comparator) 
117, and a dissipation power control circuit 118. 
The electric field strength detecting circuit 116 is a circuit for 
detecting electric field strength of a signal received by the receiving 
section. In the electric field strength comparator 117, the level of 
electric field strength of a received signal, detected by the electric 
field strength detecting circuit 116, is compared to a specified value to 
make a determination as to whether a received signal is one having an 
electric field strength less than a specified value or there is no 
receiving signal, and the result is reported to the dissipation power 
control circuit 118. 
In a narrow-band communication apparatus employing digital 
modulation/demodulation as in the present embodiment, current in the 
transmitting/receiving digital modulator/demodulator 109, A/D converter 
108, D/A converter 110, and A/D converter 125 is large. Elements 
constituting the apparatus each require a large current for operating when 
the apparatus is in the receiving mode or in the transmission mode. 
For this reason, the dissipation power controlling circuit 118, according 
to the present invention, controls supply of power to, or an operating 
state of, specified portions of the narrow-band communication apparatus 
according to the detected electric field strength of the received signal 
by the electric field strength comparator 117 as well as to an instruction 
by the PTT switch indicating whether the narrow-band communication 
apparatus is in the receiving mode or in the transmission mode. 
Namely if an instruction by the PTT switch 129 indicates that the current 
operating mode is the receiving mode and at the same time electric field 
strength in the receiving section is not more than a specified value, it 
is not necessary for the transmitting/receiving digital 
modulator/demodulator 109 to operate. It is not necessary for the A/D 
converter 108, D/A converter 110, audio signal amplifier 111, and A/D 
converter 125, each around the transmitting/receiving digital 
modulator/demodulator 109, to operate, so that operation of these elements 
is stopped to reduce dissipation power. 
Then, if an instruction by the PTT switch 129 indicates the transmission 
mode, the dissipation power control circuit 118 immediately starts 
operation of the transmitting/receiving digital modulator/demodulator 109 
as well as of the A/D converter 125 to enable transmission without any 
trouble. 
Also, if an instruction by the PTT switch indicates the receiving mode, and 
the electric field strength of a received signal in the receiving section 
is not less than a specified value, when the electric field strength 
comparator 117 detects the state, the dissipation power control circuit 
118 enables operation of the A/D converter 108, transmitting/receiving 
digital modulator/demodulator 109, D/A converter 110, and audio signal 
amplifier 111 to enable receiving by the narrow-band communication 
apparatus. 
It should be noted that, when AGC in the receiving section is executed 
according to operation (AGC output b) by the transmitting/receiving 
digital modulator/demodulator 109, if a strong input is received first, 
the components 101 to 108 in the RF and IF receiving section are 
saturated, which disables the normal operation for receiving. 
To solve this problem, in the present embodiment, gain control is executed 
by the AGC amplifier 121 according to the AGC output b from the 
transmitting/receiving digital modulator/demodulator 109 as well as to the 
level of electric field strength of a received signal detected by the 
electric field strength detecting circuit 116. Namely, coarse gain control 
is executed to enable normal operation for receiving before operation of 
the transmitting/receiving digital modulator/demodulator 109 for 
demodulation is started and an accurate automatic gain control signal is 
outputted so that the components 101 to 108 in the receiving section will 
not be saturated. Thus the problem described above is solved. 
The above description assumes that, when the apparatus is in the receiving 
mode (when an instruction by the PTT switch indicates the receiving mode), 
the received frequency (RF) amplifier 101, first mixer 102, first 
intermediate frequency (IF) amplifier 103, second mixer 104, third mixer 
105, third intermediate frequency (IF) amplifier 106, filter 107, first 
local oscillator 113, second local oscillator 114, third local oscillator 
115, electric field strength detecting circuit 116, electric field 
strength comparator 117, dissipation power control circuit 118, reference 
frequency oscillator 119, local oscillator 120, and AGC amplifier 121 are 
operating normally, causing these components to operate intermittently for 
reducing dissipation power in the apparatus is a known technique. This 
technique can be applied in the present invention. 
FIG. 2 and FIG. 3 are block diagrams each of a narrow-band communication 
apparatus according to Embodiment 2 of the present invention. In FIG. 2 
and FIG. 3, the narrow-band communication apparatus according to the 
present invention largely comprises an RF and IF receiving section of a 
triple super heterodyne receiver, a local frequency generating section for 
receiving, a local frequency generating section for transmission, a 
reference frequency generating section, a transmitting section, a digital 
modulation/demodulation signal processing section 309, an audio output 
section, a microphone input section, and a peripheral circuit. 
In FIG. 2, the RF and IF receiving section 241 of a triple super heterodyne 
receiver comprises a received frequency (RF) amplifier 201, a first mixer 
202, a first intermediate frequency (IF) amplifier 203, a second mixer 
204, a third mixer 205, and a third intermediate frequency (IF) amplifier 
206. It should be noted that the third intermediate frequency in this 
embodiment is 5 KHz. 
In the receiving section 241, an incoming waveform received through an 
antenna 231 and a select switch 232 is received according to triple super 
heterodyne system principles under gain control (signal B from )by an AGC 
amplifier 321 providing on AGC output b from a digital 
modulation/demodulation signal processing section 309, outputs signal A, 
and supplies it as a third intermediate frequency input a through the 
filter 307 and the A/D converter 308 to the digital 
modulation/demodulation signal processing section 309. 
The local frequency generating section 242 for receiving comprises a first 
local oscillator 213, a second local oscillator 214, and a third local 
oscillator 215. The local frequency generating section 220 for 
transmission is realized by a local oscillator. Furthermore the reference 
frequency generating section 219 is realized by a reference frequency 
oscillator. 
All of the local oscillators 213, 214, and 215 in the local frequency 
generating section 242 lock (synchronize) to the reference frequency 
oscillator 219 by means of, for instance, PLL (phase locking loop) to 
supply a local frequency to the receiving section 241. 
The local oscillator 220 for transmission is a circuit which generates a 
local frequency (a carrier frequency or a pilot frequency) for the 
transmitting section 243. In FIG. 2, the local frequency generating 
section 242 has an independent configuration, but it may also be used as a 
PLL for the receiving section 241 in the local frequency generating 
section 242. Furthermore TCXO (temperature-controlled crystal oscillator), 
satisfying a legally required value, is available as the reference 
frequency oscillator 219. 
The transmitting section 243 comprises a transmission frequency (RF) power 
amplifier 222, a modulator 223, and a distortion reducing circuit 230. In 
response to modulation output d from the digital modulation/demodulation 
signal processing section 309, a transmitting signal is transmitted 
through the select switch 232 and the antenna 231 according to the 
modulated output signal D supplied through a filter 323 and a D/A 
converter 324. 
Also in FIG. 3, the digital modulation/demodulation signal processing 
section is realized by the transmitting/receiving digital 
modulator/demodulator 309 employing, for instance, a DSP (Digital Signal 
Processor). Also the digital modulation/demodulation signal processing 
section has, as periphery circuits for the transmitting/receiving digital 
modulator/demodulator 309, the filter 307, the A/D converter 308, the D/A 
converter 321 for AGC, the D/A converter 322 for AFC, the filter 323 and 
the D/A converter 324. It should be noted that AFC output from the 
transmitting/receiving digital modulator/demodulator 309 is supplied via 
the D/A converter 322 for AFC to the reference frequency generating 
section 219. 
The receiving audio output section comprises a D/A conversion/mute circuit 
310 for audio output, an encoder for sub-audio data, a decoder (CPU) 311, 
and a push-to-talk switch (PTT) 329. It should be noted that in the 
figure, f indicates receiving audio output, g indicates sub-audio data 
output, h indicates control input for AFC, i indicates timer output, j 
indicates sub-audio data input, and k indicates PTT switch input. Herein, 
the PTT switch 329 is a switch indicating that the narrow-band 
communication apparatus is in the receiving mode or in the transmission 
mode, and is set by an operator. 
Furthermore, the microphone audio input section comprises an A/D converter 
325 for an audio modulation signal, a microphone 327, and an amplifier 328 
for a microphone. It should be noted that in the figure, l indicates 
microphone audio input, and m indicates DC power input. To the amplifier 
328 for a microphone is provided, in addition to a microphone audio input 
from the microphone 327, a DTMF tone input. Furthermore to the A/D 
converter 325 is supplied, in addition to the output from amplifier 328 
for a microphone, a data communication input. 
Next, description is made for components inside the transmitting/receiving 
digital modulator/demodulator 309. In FIG. 3, the transmitting/receiving 
digital modulator/demodulator 309 comprises a frequency shift circuit 351, 
a channel filter 352, a coarse filter 353 for a pilot signal, a 
narrow-band filter 354 for a pilot signal, a pilot signal power measuring 
circuit 355, an I/F circuit 356, a delaying circuit 357 for tone-in-band 
demodulation, a phase/level correcting circuit 358, a power detecting 
circuit 359 for high speed AGC, a zero-crossing detecting circuit 360, a 
squelch gate circuit 361, a filter 362 for audio output, a filter 363 for 
a sub-audio data demodulation, a sub-audio data demodulation circuit 364, 
a frequency measuring circuit 365 for AFC, an AND circuit and a lock 
circuit 366 for AFC synchronization, a timer circuit 367, a sub-audio 
modulation circuit 368, a filter 369 for sub-audio data modulation signal, 
a band pass filter (BPF) 370 for an audio modulation signal, an audio 
modulation signal processing circuit 371 (Tone-in-Band generating 
circuit), a filter 372 for an audio modulation signal, a mute circuit 373, 
an AND circuit 374, a pilot signal controller 375, an AND circuit 376, a 
current detecting circuit 377, and a current load circuit 378. 
The frequency shift circuit 351 shifts the audio (or data) spectrum in TONE 
IN BAND, but executes processing to return it to the original position. A 
pass bandwidth in the channel filter 352 is .+-.2 KHz. A pass bandwidth in 
the rough filter 353 for a pilot signal is about .+-.400 Hz. Furthermore, 
a pass bandwidth in the narrow-band filter 354 for a pilot signal is about 
.+-.100 Hz. 
The phase/level correcting circuit 358 is a circuit which corrects phase 
and level with reference to a receiving signal in TONE IN BAND, a 
so-called pilot signal. The power detecting circuit 359 for high speed AGC 
is a power detecting unit for high speed AGC and measures all powers in 
one channel. 
The I/F circuit 356 sets an audio output level according to a data table in 
the I/F circuit 356 based on pilot signal power measurement by the pilot 
signal power measuring circuit 355 when the pilot signal has substantially 
weakened transitionally due to, e.g., fading, or when it is determined 
that the electric field is weak. 
The timer circuit 367 is used in a transmission pre-time. The audio 
modulation signal processing circuit 371 generates TONE IN BAND. The mute 
circuit 373 is a mute circuit for an audio signal as well as for sub-audio 
data, and is driven and controlled according to output from the timer 
circuit 367. The AND circuit 374 adds an audio signal (output from the 
mute circuit 373) to a sub-audio data modulation signal (output from the 
filter 369). The AND circuit 376 adds a pilot signal to a modulation 
signal (output from the AND circuit 374). 
Next, description is made for various operations in the narrow-band 
communication apparatus according to the present invention. 
(1) AFC Operation 
In the present invention, AFT is realized by measuring frequency by means 
of making use of a pilot signal (Refer to FIG. 7) inserted at the center 
of a band of an audio signal. 
It should be noted that the digital signal processing means, as defined in 
this embodiment, is the transmitting/receiving digital 
modulator/demodulator 309, a timer circuit measuring a specified time 
after the PTT switch 329 gives an instruction for transmission mode is the 
timer circuit 367, and the modulation output control means allowing output 
of only a pilot signal as a modulation signal within the specified period 
of time is realized with the mute circuit 373, AND circuit 374, pilot 
signal controller 375, and AND circuit 376. 
The frequency control means, generating a frequency control signal (C) for 
executing automatic frequency control in the receiving section 241 as well 
as in the transmission section 243 by means of measuring a frequency 
making use of a pilot signal, is the frequency measuring circuit 365 for 
AFC. The synchronism control means, synchronizing the frequency measuring 
circuit 365 for AFC (output therefrom) to the pilot signal within the 
specified period of time when only a pilot signal is transmitted, is an 
AND circuit for AFC synchronization and lock circuit 366. 
At first, description is made for an operation for transmission. When the 
PTT switch 329 is turned ON and the apparatus is set in the transmission 
mode, the timer circuit 367 starts an operation for time measurement. 
During the specified period of time when this timer circuit 367 executes 
an operation for time measurement, the timer circuit 367 supplies a 
control signal to the mute circuit 373 to make the circuit operate. In 
this step an audio signal, a DTMF tone signal, and a data communication 
signal are not subjected to modulation. Namely, during a specified period 
of time since start of transmission when the timer circuit 367 executes 
time measurement, only a pilot signal is outputted and transmitted as a 
modulated signal D. 
Next, description is made for an operation for receiving. In the receiving 
side, during a period of time when only the pilot signal is transmitted, a 
pilot signal is searched, and the output from the frequency measuring 
circuit 365 for AFC (AFC output signal C) is locked by the AND circuit for 
AFC synchronization and the lock circuit 366. In this case, once the AFC 
output signal C is locked, during a period of time when only a pilot 
signal is transmitted, the AFC output signal C is stopped. 
It should be noted that the search for a pilot signal having passed through 
the coarse filter 353 is executed through the zero-crossing detecting 
circuit 360, squelch gate circuit 361, sub-audio data demodulation filter 
363, sub-audio demodulation circuit 364, an encoder for sub-audio data, 
and the decoder (CPU) 311. 
Then, at the beginning of the next reception, a search for a pilot signal 
in a normal transmission signal is restarted by each component in the path 
above. With this operation, it never occurs that an audio signal or data 
or the like is mistaken for a pilot signal due to the effect by modulation 
to cause a malfunction in AFC. Also, a pass band of the coarse filter 353 
for a pilot filter is .+-.400 Hz, so that there is no problem concerning 
timing. Also, even if the output of an AFC output signal C is stopped, the 
phase/level correcting circuit 358 is working, so that displacement of 
frequency due to such effects as Rayleigh fading can be compensated. 
(2) Operation For Correcting an Audio Output Level 
Also in this embodiment, a so-called AGC operation is executed so that 
constant audio output can be obtained irrespective of electric field 
strength. Namely, pilot signal power, having passed through the 
narrow-band filter 354, is measured by the power measuring circuit 355 (a 
pilot signal power measuring means), and in the power detecting circuit 
359 for high speed AGC, a voltage level for AGC is set according to the 
measured power. The AGC voltage level is supplied as an AGC signal B 
through the D/A converter 321 to the AGC amplifier 321. Thus, gain control 
is executed. 
Furthermore in this embodiment, in the output side of the pilot signal 
power measuring circuit 355 is provided the I/F circuit 356, and moreover 
are provided the delay circuit 357 for TONE IN BAND, and the phase/level 
correcting circuit (level correcting means) 358 for correcting phase and 
level of an output signal from the delay circuit 357 according to the 
output from the I/F circuit 356. 
Namely, when pilot signal power becomes very weak due to effects by, for 
instance, fading, power measurement by the pilot signal power measuring 
circuit 355 drops to below a specified level, but then a signal according 
to the data table in the I/F circuit 356 is supplied to the phase/level 
correcting circuit 358 to correct the audio output level. It should be 
noted that an operation start level (the specified level described above) 
for this function may be set to a level seemingly most suited when actual 
communication is started. 
For this reason, characteristics of audio output and SINAD (SINAD 
sensitivity) against electric field strength is as shown in FIG. 4B, and 
also the audio output level drops in association with drop of SINAD in 
contrast to the fact that audio level is provided at a constant level even 
in a weak electric field region where SINAD is low in the conventional 
technology. With this feature, the problem that a signal including heavy 
noise elements is outputted at a constant level is solved, and listening 
is more comfortable. 
FIG. 5A shows a comparison of a received audio signal when a signal 
strongly affected by fading is received to an example of the conventional 
technology. In the example of the conventional technology (Refer to FIG. 
5A-a), noise and the audio signal are at the same level and it is 
uncomfortable for hearing. In this embodiment (Refer to FIG. 5A-b), 
however, noise is reduced and discomfort is eliminated. 
Furthermore, FIG. 5B shows a comparison of a received audio signal when a 
signal affected by weak fading is received to an example of the 
conventional technology. In the example of the conventional technology 
(Refer to FIG. 5B-a), a signal including heavy noise elements is outputted 
at a constant level and it is uncomfortable for hearing, but in the 
present embodiment (Refer to FIG. 5B-b), the volume of sound is reduced to 
an appropriate level, and also the noise becomes not so uncomfortable. 
(3) High Speed AGC Operation 
In this embodiment, to realize high speed AGC operation, in addition to the 
pilot signal power measuring circuit 355 for measuring the power of a 
pilot signal having passed through the narrow-band filter 354 which is 
generally installed for AGC operation, a power detecting circuit (channel 
power measuring means) 359 for high speed AGC to measure power for one 
channel, the power having passed through the channel filter 352, is 
provided. 
Namely, before the AGC operation is locked (synchronized), the power for 
one channel having passed through the channel filter 352 is measured by 
the power detecting circuit 359 for high speed AGC, and an AGC voltage 
level is set according to the measured voltage. Also after the AGC 
operation is locked, the pilot signal, having passed through the 
narrow-band filter 354, is measured by the pilot signal power measuring 
circuit 355, and the AGC voltage level is set according to the measured 
power in the power detecting circuit 359. 
In the embodiment, the pass band of the channel filter 352 is 4 KHz, which 
is substantially wider than .+-.100 Hz, which is a pass band of the 
narrow-band filter 354, so that response speed in the AGC operation can be 
improved by several ms. 
For instance, even if strong input is received, power for one channel is 
measured by the power detecting circuit 359, and an AGC voltage level is 
set according to the measured power to a degree where the receiving 
section is not saturated. For this reason, the problem in the conventional 
technology that the receiving section 241 is saturated because the AGC 
operation is not started is solved, and high speed AGC operation can be 
realized. 
(4) Processing of Power-Supply Current 
The narrow-band communication apparatus according to the present embodiment 
treats signals having power in the range of from several V to several mV. 
In addition, a S/N ratio of 40 dB or more is required to an input signal 
with a power of several mV. 
For instance, if a receiving signal is a weak signal, an interface input 
level in an A/D converter is at a level of several mV. The level of 
microphone audio, input by the microphone 327 in a transmission system, is 
around several mV. Furthermore, if a received signal is a strong signal, 
an interface input level in an A/D converter is around 2V, and modulation 
output to a transmitting section is also around 2V. 
So it is assumed herein that the maximum signal level is 2 V and the 
minimum signal level is 2 mV. Also, if it is assumed that the S/N ratio of 
40 dB is secured at the minimum signal level, the dynamic range D of the 
maximum signal level as well as of a noise level is expressed by the 
following equation; 
EQU D=20 log (2/2.times.10.sup.-3)+40=100 dB 
For this reason, 100 dB or more is required as a dynamic range D of the 
maximum signal level and noise level. It is difficult to realize this 
value even if there is a slight cause, such as power noise. 
In this embodiment, change of current consumed in a digital circuit is 
detected by the current detecting circuit 377, and a factor of current 
caused by fluctuation is dissipated by the DC current load circuit 378 to 
keep the dissipated current in the transmitting/receiving digital 
modulator/demodulator 309 at a constant level. Hence, the S/N ratio in the 
A/D converter due to change in current consumption is prevented. 
It should be noted that in this embodiment the transmitting/receiving 
digital modulator/demodulator 309 is realized by a digital signal 
processor (DSP), and the operations (1) to (4) described above are 
realized in software in the DSP. 
As described above, with the narrow-band communication apparatus according 
to the present invention, when demodulation of the output from a receiving 
section and modulation of audio input are executed by a digital signal 
processing means, electric field strength of the output from a receiving 
section is detected by an electric field strength detecting means, while 
the supply of power to or operating state of specified portions of the 
narrow-band communication apparatus is controlled according to the output 
from the electric field strength detecting means and/or to the output from 
an indicating means for indicating whether the narrow-band communication 
apparatus is in the receiving mode or in the transmission mode. Thus, 
supply of power to, or operating state of, apparatus components, operation 
of which is not required, is controlled according to detected electric 
field strength in the output from the receiving section as well as to an 
operating mode of transmission or receiving. Thus, it is possible to 
provide a narrow-band communication apparatus which can reduce dissipation 
power not only in the receiving system but also in the transmission 
system. 
Also, when gain control for a receiving section is executed according to 
only an automatic gain control signal outputted through demodulation 
processing in a digital signal processing means, sometimes normal 
operation for receiving is disabled because the receiving section is 
saturated, for instance, when a strong input is received at first. With 
the narrow-band communication apparatus according to the present 
invention, however, gain control for a receiving section is executed by an 
automatic gain control means according to an automatic gain control signal 
outputted from a digital signal processing means as well as a result of 
detection by an electric field strength detecting means. Thus, it is 
possible to provide a narrow-band communication apparatus which enables 
reception by executing coarse gain control preventing the receiving 
section from being saturated before demodulation processing in the digital 
signal processing means is actuated and an accurate automatic gain control 
signal is outputted. This improves circuit efficiency and provides 
improved characteristics by applying detected electric field strength also 
for gain control in the receiving system. 
With the narrow-band communication apparatus according to the present 
invention, a receiving section is operated intermittently by a 
intermittent control means, so that dissipation current in the apparatus 
can be reduced. 
With the narrow-band communication apparatus according to the present 
invention, after an indicating means indicates the transmission mode in a 
digital signal processing means for outputting a demodulation signal for 
output from a receiving section and a modulation signal for audio input to 
the receiving section, a specified time is measured by a timer circuit. 
Only a pilot signal is transmitted as an output of a modulation signal 
within the specified period of time through the transmitting section by a 
modulation output control means. On the other hand, in the receiving side, 
frequency is measured by a frequency control means by making use of a 
pilot signal to generate a frequency control signal for executing 
automatic frequency control in the receiving section as well as in the 
transmitting section. However, a frequency control means is synchronized 
previously by a synchronism control means to the pilot signal during a 
specified period of time when only a pilot signal is transmitted. For this 
reason, malfunction in automatic frequency control, such as mistaking an 
audio signal or data or the like for a pilot signal due to modulation 
effect never occurs. 
With the narrow-band communication apparatus according to the present 
invention, in a digital signal processing means for demodulation of at 
least the output from a receiving section, the power of a pilot signal 
included in a received signal is measured by a pilot signal power 
measuring means, and an audio output level is corrected by a level 
correcting means when the measured power of the pilot signal is less than 
a specified value. Hence, discomfort in hearing experienced in the 
conventional technology, for instance, when a pilot signal is considerably 
weakened due to fading or the like, can be eliminated. 
With the narrow-band communication apparatus according to the present 
invention, in a digital signal processing means for demodulating at least 
the output from a receiving section, the power of a pilot signal to be 
included in a receiving signal is measured by a pilot signal power 
measuring means. An automatic gain control signal is outputted by an 
automatic gain control signal output means to an automatic gain control 
means according to the measurement described above to execute gain control 
for the receiving section. However, received signal power for specified 
channels is measured by a channel power measuring means before measurement 
by the pilot signal power measuring means is obtained, and an automatic 
gain control signal is outputted to an automatic gain control means 
according to the measurement to execute gain control for the receiving 
section. For this reason, the problem as observed in the conventional 
technology that the receiving section is saturated because the automatic 
gain control operation is not started is solved, and high speed automatic 
gain control operation can be realized. 
With the narrow-band communication apparatus according to the present 
invention, in a digital signal processing means for demodulating the 
output from a receiving section and outputting a modulation signal for 
audio input, current consumption is detected by a current detecting means, 
and a factor of the current caused by fluctuation and detected by a 
current detecting means is consumed by a load circuit. Thus, current 
consumption in the digital signal processing means can be kept at a 
constant level and the S/N ratio in the component such as an A/D/D 
converter due to change in the dissipation current can be prevented. 
Although the invention has been described with respect to a specific 
embodiment for a complete and clear disclosure, the appended claims are 
not to be thus limited but are to be construed as embodying all 
modifications and alternative constructions that may occur to one skilled 
in the art which fairly fall within the basic teaching herein set forth.