JPH0773225B2 - Anti-fading medium wave receiver - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anti-fading medium wave receiver which prevents fluctuations in demodulation output volume due to fading of an input amplitude modulated wave, and more particularly to an automatic wave receiver conventionally provided to prevent fading. Volume control (AV
The circuit C) is designed to prevent a relatively fast fluctuation in output volume that cannot be suppressed by the circuit.

[0002]

2. Description of the Related Art In the daytime, the sky waves of mid-wave band audio broadcasting are absorbed as they pass through the D layer in the ionosphere that occurs in the sky, but at night, the D layer disappears, so that the E wave in the sky is further absorbed. The sky waves reflected by the layers and the F layers reach a considerable distance, and so-called fading occurs in which the electric field strength fluctuates due to the interference between the sky waves and the surface waves. The 50% time ratio of the received electric field strength based on the ionospheric reflection of the medium wave broadcast wave becomes almost constant 2 hours after sunset, but the fluctuation of the received electric field strength due to such fading generally has a Rayleigh distribution. ,
Occasionally, it may exhibit coherent fluctuations, which includes level fluctuation components at high frequencies of 1 Hz or higher. In addition, even in the same channel interference due to the same carrier frequency, based on the frequency difference within the allowable deviation of the carrier frequency,
As shown in FIG. 1, the fluctuation of the received electric field strength is apparently similar to that of coherent fading. As a measure to prevent the fluctuation of the demodulation output volume based on the fluctuation of the received electric field strength due to such fading, conventionally, along with the method based on the configuration arrangement of the receiving antenna such as spatial diversity reception, the automatic feedback control of the intermediate frequency amplification gain by the negative feedback is performed. The technique by the receiver circuit itself of the volume control (AVC) circuit was used. Therefore, the automatic volume control (AVC) that has been conventionally used to prevent a change in the reception output volume due to a change in the reception electric field strength such as a large change due to a difference in the transmitting station at the time of tuning in a medium wave receiver. The circuit is
Taking advantage of the fact that the received output audio signal does not substantially contain signal components below 10 Hz, the DC component representing the strength of the carrier component in the detected output signal is negatively fed back to the intermediate frequency amplifier circuit to obtain the intermediate frequency. The amplification gain is automatically controlled.

FIG. 2 shows a schematic configuration of a conventional medium-wave receiver in which fading is prevented by using such an automatic volume control circuit. In the medium wave receiver having the configuration shown in the figure, the input wave signal from the receiving antenna 1 is converted into a high frequency (RF)
It is led to the mixer 3 through the amplifier 2, mixed with the oscillation output of the local oscillator 4 and converted into an intermediate frequency signal, and the intermediate frequency (I
F) After being amplified by the amplifier 5, it is guided to the wave detector 6, and the detected output is supplied to the speaker 8 through the low frequency amplifier 7, and the automatic sound volume control (AVC) having a loop filter composed of a CR time constant circuit. Supply also to the circuit 9,
The quasi-DC component representing the level change of the carrier component is extracted from the audio signal being output for detection, and the quasi-DC component is negatively fed back to the intermediate frequency amplifier 5 to be its gain control voltage V _c , and the intermediate frequency amplification is performed. The gain G (Vc) is controlled to prevent a change in output volume due to a change in input wave electric field strength, that is, fading. A CR low-pass filter with a time constant of about 0.1 second is generally used as a loop filter for extracting the quasi-DC component in the detection output, but this CR low-pass filter has a simple circuit configuration and is inexpensive. And
It operates satisfactorily and stably against changes in the received electric field strength during channel selection.

[0004]

However, when the present inventors actually receive medium-wave broadcasting by such a conventional medium-wave receiver, the received sound quality may be considerably deteriorated at night. The deterioration of the sound quality is that the level change of the carrier component in the detection output due to fading is not limited to the quasi-DC component, and includes a high frequency component of 1 Hz to a few Hz, and the AVC circuit changes relatively quickly. It is recognized that it is not working well following it.

Therefore, in order to prevent the deterioration of the received sound quality, the response speed of the AVC circuit should be increased so that the AVC circuit can immediately respond to the level change of the high frequency component. However, if the CR time constant of the CR low-pass filter used in the conventional AVC circuit is made small and the response speed is increased, the lower limit of the audio frequency band of the mid-wave broadcasting is more than ten.
Since the negative feedback control of the AVC circuit extends up to Hz and the sound quality of the detection output is further deteriorated, the CR time constant cannot be reduced.

Therefore, the cutoff frequency characteristic of the low-pass filter of the AVC circuit for extracting the change component of the carrier level in the detection output is made steeper than the frequency characteristic of the CR time constant circuit,
For example, if a low-pass filter with a multi-pole four-terminal network as described later in the present invention is used, the cutoff frequency characteristic of the AVC circuit itself can extract only the carrier level change component excluding the audio signal component. However, if feedback control of the amplification gain of the intermediate frequency amplifier circuit is performed by such a loop filter having a sharp cutoff characteristic of the multipole, circuit operation such as oscillation due to the phase delay characteristic unavoidable in the multipole loop filter will occur. Be stable. Therefore, the AVC circuit for feedback-controlling the amplification gain of the intermediate frequency amplification circuit is
The cutoff frequency characteristics are gentle, but the phase delay characteristics are also gentle,
Only a CR time constant filter capable of stable loop control can be used.

Furthermore, when the medium-wave broadcasting is actually received, the occurrence of beats due to the deviation of the carrier frequency in the same channel interference considerably deteriorates the received sound quality. That is, the carrier frequency of medium-wave broadcasting is 9 worldwide.
Although it is standardized in kHz intervals, frequency deviations of several Hz or less are allowed, so when co-channel interference occurs, beats below the audible frequency corresponding to the sum or difference of both carrier frequency deviations are generated. As described above, a coherence level change occurs that is similar to the fading that occurs relatively fast.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional problems and to provide a relatively fast reception volume due to a change in the reception electric field strength caused by the distant propagation of sky waves at night in the medium-wave broadcasting band. An object of the present invention is to provide a fading-preventing medium-wave receiver that prevents a level change, that is, so-called fading and generation of a beat at the time of co-channel interference that is similar to fading, so that desired reception sound quality is not deteriorated.

That is, the anti-fading medium-wave receiver according to the present invention performs the fading prevention by the AVC circuit described above in the conventional manner, and the intermediate speed for the relatively fast fading which cannot be essentially responded by the conventional AVC circuit. Extract the excess or deficiency of amplification gain compensation of the frequency amplification circuit from the detection output, and extract the detection output audio signal as the reception output by removing the change in the level of the carrier component due to the excess or deficiency of the gain compensation by timing and calculation. The intermediate frequency signal converted from the input amplitude-modulated wave is amplitude-detected and demodulated, and the detection output is negatively fed back to the intermediate-frequency amplifier circuit via an automatic volume control (AVC) circuit equipped with a time constant circuit. In the medium-wave receiver that automatically controls the output volume that changes due to the fading of the input wave to a substantially constant level, A low frequency level fluctuation component Y corresponding to the fluctuation of the carrier level extracted from the wave output through a low pass filter having a cutoff frequency of at least 8 Hz, and a delay time corresponding to the delay of signal passing in the low pass filter. The main signal of the detection output is provided with an operation circuit for guiding the detection output X and the low-frequency control output Z of the AVC circuit respectively through delay circuits to perform multiplication / division operation of (Z / Y) · X. It is characterized in that X, Y and Z are multiplied / divided by the arithmetic circuit to remove the fading and the arithmetic output is output as the reception output of the medium wave receiver.

[0010]

Therefore, in the fading prevention medium wave receiver of the present invention, in addition to the effect of fading prevention by the conventional AVC circuit, the same high speed fading that cannot be essentially suppressed by the conventional AVC circuit is performed. It is possible to obtain a special operation effect that a reception output with good sound quality that is stably removed together with the same co-channel interference beat is obtained.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings. As described above, the conventional anti-fading medium wave receiver of the present invention leaves the conventional AVC circuit as a gain control circuit that suppresses fluctuations in the detection output level due to a large change in the received input electric field strength during channel selection. Is an operation circuit newly inserted in the detection output circuit for removing the carrier level fluctuation due to the fast fading remaining in the detection output.

That is, between the point a and the point b of the detection output circuit in the schematic configuration of the medium wave receiver shown in FIG.
A fading prevention circuit composed of an arithmetic circuit having the configuration shown in FIG. In the fading prevention circuit shown in the figure, the detection output from the terminal a connected to the detector 6 is guided to a low pass filter (LPF) 10 to block, for example, an audio signal component exceeding 8 Hz and a carrier level not exceeding 8 Hz. The fluctuation component Y of is extracted and guided to the arithmetic circuit 13, and the detection output from the terminal a is guided to the delay circuit 11 having a delay time equal to the time required for the signal to pass through the low pass filter (LPF) 10. The differential detection output X including the audio signal whose timing is aligned with the carrier level fluctuation component Y is led to the arithmetic circuit 13 and further the intermediate frequency amplification consisting of the quasi DC component extracted from the conventional automatic volume control (AVC) circuit 9 is performed. calculating also a delay control signal Z having uniform carrier level fluctuation component Y and timing the gain control voltage V _C is guided from the terminal c to the delay circuit 12 having the same delay time as the delay circuit 11 Leading to the road 13. As a result, in the arithmetic circuit 13, the detection output audio signal and the timing including the fast carrier level fluctuation component which cannot completely compensate the intermediate frequency amplification gain by the gain control signal from the conventional AVC circuit 9 which is composed of the quasi DC component And a ratio of a quasi-DC component Z which has been gain-compensated by the conventional AVC circuit 9 and a fast level fluctuation component Y of several Hz which cannot be gain-compensated by the conventional AVC circuit 9, A carrier level fluctuation compensation signal capable of almost completely removing fading from the detected output audio signal by compensating the excess or deficiency of the intermediate frequency amplification gain compensation by the AVC circuit 9 again is obtained. Therefore, the arithmetic circuit 13 performs an analog operation of X · (Z / Y) to almost completely eliminate the carrier level fluctuation of several Hz actually caused by fading and co-channel interference, and thus the detection of good sound quality is performed. The output audio signal can be taken out from the terminal 6 extremely stably by using a low-pass filter having a desired cutoff frequency characteristic without causing operational instability as in the loop control of the intermediate frequency amplification gain. The operation level range of the arithmetic circuit 13 corresponds to the range of change of the received input electric field strength due to fading, and if the operation level range is, for example, 30 dB, good input / output characteristics can be obtained.

Next, an example of a concrete detailed configuration of each circuit constituting the fading prevention circuit shown in FIG. 3 will be sequentially described.

Low-pass filter (LPF) 10 An example of a concrete circuit configuration of the low-pass filter (LPF) 10 for separating a voice signal component in a detection output and extracting a carrier level fluctuation component Y due to fading As shown in FIG. As a low-pass filter that requires the extraction of fading frequency components of several Hz or less while blocking the audio signal components of more than 10 Hz in the detection output in the medium wave receiver, as shown in the figure, Frequency 8Hz, delay time 53
It is preferable to use a sixth-order Bessel type active filter exhibiting a constant delay characteristic of mS.

Delay circuits 11 and 12 In order to give a delay of 53 mS to the delay detection output X and the delay control signal Z in order to align the timing with the carrier level fluctuation component Y that has passed through the low pass filter 10 having the delay time of 53 mS described above. As the delay circuits 11 and 12, there are provided delay circuits for sequentially delaying signals by using a large number of charge transfer devices (BBDs) as shown in FIG. 5, and for writing and reading to and from a digital memory as shown in FIG. It is preferable to use any one of a delay circuit that delays a signal at once by a time difference.

In the delay circuit using the BBD shown in FIG. 5, for example, a low-pass filter (a low-pass filter) that allows signals up to the audio frequency to pass before and after the BBD delay circuits 15 and 18 in which 4096 stages of BBDs are continuously connected ( LPF) 14, 16 and 17, 19 are respectively arranged in two lines to drive the BBD delay circuit in parallel by the single inter-element transfer clock signal with a frequency of 38.6 kHz from the clock oscillator 20, and from the input end (a). The audio signal supplied via the DC blocking capacitor C
The signal is delayed by 53 mS and taken out from the output end (a) as the delay detection output X, and the control signal supplied to the input end (c) is taken out from the output end (c) as the delay control signal Z. In addition, BBD
In the delay circuit, signal charges are delayed due to repetitive delay between multi-stage elements, so that noise may be mixed.

On the other hand, the delay circuit using the digital memory shown in FIG. 6 has the same input terminal as the delay circuit shown in FIG.
A control circuit 22 and a clock pulse generation circuit 23 drive control the main circuit 21 that delays the detection output and control signal supplied from (a) and (c) and takes them out from the output terminals (a) and (c). is there.

In the main circuit 21, the input terminals (a) and
The detection output and control signal from (c) are fed to amplifiers 24 and 25.
To the multiplexer 26 via the
Under the control of 31, the signals are sequentially converted into alternating time series signals, and then a single sample Honda (S / H) 27, analog / digital converter (A / D) 28, and buffer memory 29 are sequentially converted. Sampling frequency 2 via
It is sampled at 3.8kHz, converted into a 12-bit digital signal, and written to the random access memory (RAM) 30 alternately in sequence. Random access memory (RAM) 30
The number of memory stages is 1280 × 2, and each memory address is divided into the detection output X and the control signal Z by 1 bit “0” and “1”, and after writing to each memory address, the counter 31 is used. Set by counting clocks 53
Each sampled value is sequentially delayed by reading after mS.
Each delayed signal read from the RAM 30 is a separate digital / analog converter (D / A) 32, 33, a low pass filter (LPF) 34 for passing only an audio frequency signal,
35 and attenuators 36 and 37 are taken out from output terminals (a) and (c), respectively. The BBD shown in FIG.
A comparison between the delay circuit and the RAM delay circuit shown in FIG. 6 shows that the RAM delay circuit shown in FIG. 6 has a more complicated structure, but a good result is obtained because less noise is mixed.

Arithmetic Circuit 13 An example of the configuration of an analog arithmetic unit used as the arithmetic circuit 13 is shown in FIG.
Shown in. In the illustrated analog arithmetic unit, the respective inputs, that is, the differential detection output X, the low-pass filter output carrier level fluctuation Y and the delay control signal Z are respectively passed through operational amplifiers A ₁ and A ₃ and A _2. To the emitter-coupled transistor pair T _r1, T _r4 and T _r2 , T _r3 , respectively, and the transistor pair T _r2 , T _r3 is used (Z /
Y) and the differential detection output are multiplied by the transistor pair T _r1, T _r4 , and the calculated result is obtained by the operational amplifier A ₄
It is taken out as a calculation output via. The operating frequency range of this calculator exceeds 10kHz, and it fully satisfies the voice frequency band with an upper limit of 4.5kHz, and the operating level range also exceeds 30dB, which works well for fading prevention.

[0020]

As is apparent from the above description, according to the present invention, in addition to the effect of fading prevention by the conventional AVC circuit in the medium-wave receiver, the conventional AVC circuit can be used.
With the circuit, it is possible to stably remove fast fading, which is essentially uncontrollable, along with co-channel interference beats that cause similar level fluctuations, and to obtain a reception output with good sound quality. Can play.

[Brief description of drawings]

FIG. 1 is a signal waveform diagram showing how the received electric field strength fluctuates due to co-channel interference in the medium-wave broadcasting band.

FIG. 2 is a block diagram showing a schematic configuration of a conventional medium wave receiver.

FIG. 3 is a block diagram showing a schematic configuration of a fading prevention circuit according to the present invention.

FIG. 4 is a circuit diagram showing a detailed configuration example of a low-pass filter in the fading prevention circuit.

FIG. 5 is a block diagram similarly showing an example of a detailed configuration of a delay circuit in the fading prevention circuit.

FIG. 6 is a block diagram similarly showing another example of the detailed configuration of the delay circuit.

FIG. 7 is a circuit diagram showing an example of a detailed configuration of an arithmetic circuit in the fading prevention circuit.

[Explanation of symbols]

 1 Receiving antenna 2 High frequency amplifier 3 Mixer 4 Local oscillator 5 Intermediate frequency amplifier 6 Detector 7 Low frequency amplifier 8 Speaker 9 AVC circuit 10, 14, 16, 17, 19, 34, 35 Low pass filter 11, 12 Delay circuit 13 Operation circuit 15, 18 BBD circuit 20 Clock oscillation circuit 21 Main circuit 22 Control circuit 23 Clock pulse generation circuit 24, 25 Amplification circuit 26 Multiplexer 27 Sample holder 28 Analog-to-digital converter 29 Buffer memory 30 Random access memory 31 Counter 32, 33 Digital-to-analog converter 36, 37 Attenuator

Claims (1)

[Claims] 1. An intermediate frequency signal converted from an input amplitude-modulated wave is amplitude-detected and demodulated, and a detection output is negatively fed back to an intermediate-frequency amplifier circuit via an automatic volume control (AVC) circuit having a time constant circuit. In a medium-wave receiver that automatically controls the output volume that changes due to fading of the input wave to a substantially constant level, fluctuations in the carrier level extracted from the detection output through a low-pass filter having a cutoff frequency of at least 8 Hz. Corresponding to the low frequency level fluctuation component Y, and the detection output X and the AVC through a delay circuit having a delay time corresponding to the delay of signal passage in the low pass filter.
Deriving the low frequency control output Z of the circuit respectively (Z / Y)
.A multiplication circuit for performing multiplication / division operation X is provided, and the operation signal obtained by multiplying / dividing the main signals X, Y, Z of the detection output by the operation circuit to remove fading is received by the medium wave receiver. An anti-fading medium-wave receiver characterized by being configured to output as an output.