JPS61242133A - Radio receiver - Google Patents

Abstract

PURPOSE:To reduce an S/N in both a stereo demodulation mode and a monaural action mode by detecting that the stereo demodulator has a stereo demodulating action or that the stereo demodulation mode is switched to the monaural action mode in response to the reduction of the field intensity. CONSTITUTION:A voltage control resistor 33 which obtains the resistance value from the noise level of a reception signal in the stereo demodulation mode of an FM demodulator 26 is set in a transmission route for noise signals. At the same time, the resistance value is set at a low level in the demodulation mode. Then the noise of the wave detection output is detected at a low threshold level. In a weak field intensity mode the resistance value is set at a high level in response to the monaural action of the demodulator 26. Then the noise of the wave detection output is detected at a high threshold level. Thus it is possible to reduce the S/N in response to deletion of noises of high or low levels when the stereo demodulation is possible at the high field intensity and when the stereo demodulation is impossible at the low field intensity.

Description

[Detailed description of the invention] 〔Technical field〕 The present invention relates to a radio receiver.

[Background technology]

Currently, FM stereo broadcasting is being carried out, but if you compare the S/N ratio during stereo demodulation and monaural demodulation, it is difficult to compare the S/N ratio between stereo demodulation and monaural demodulation. Gijutsusha PP43-44) K is listed as 5K.

The deterioration during stereo demodulation is approximately 21.7 dB (theoretical value).

For this reason, if the electric field strength decreases to a weak electric field strength while receiving a stereo broadcast, it will be very difficult to hear, so people skilled in the art use a system called separation control.
/N ratio improvement measures have been adopted.

That is, when the electric field strength decreases, the stereo demodulator automatically switches from stereo demodulation to monaural demodulation, thereby reducing the deterioration of the S/N ratio.

On the other hand, the FM receiver has a noise canceller that detects and rectifies the noise component contained in the detection output, and uses this rectified output to block the transmission of the FM detection output to the FM stereo demodulator when pulse-shaped noise is received. There are a number of things that are provided.

The noise canceller is configured to perform the above operation when a high level of noise is received. The reason is that when detecting low level noise and performing the above operation,
Since the FM detection output is frequently cut off, the occurrence of distortion due to this cut-off operation increases, and the S/N ratio deteriorates, so this is to reduce this.

Therefore, it is not possible to remove low-level noise during either stereo demodulation or monaural operation, and S/
N ratio improvement@G (there was a limit.

The present inventor has developed the above-mentioned noise canceller and separation control tube. Considering the above, the KS/N ratio should be further improved (considered).Since the signal level is high when the electric field strength is strong, even if the noise canceller is activated in response to low level 7 noise, I noticed that the S/N ratio did not deteriorate much.Also, when the electric field strength becomes weak and monaural operation is performed, the above noise removal operation is performed in response to high level noise rather than low level noise. It has been found that by doing so, deterioration of the S/N ratio can be reduced.

[Purpose of the invention]

An object of the present invention is to provide a radio receiver that can improve the S/N ratio both during stereo demodulation and monaural operation.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[Summary of the invention]

A brief summary of the invention disclosed in this application is as follows.

That is, it detects when the stereo demodulator is performing stereo demodulation operation or has switched to monaural operation due to a decrease in electric field strength, and detects the level change of the detection signal and the noise component included in the FM detection output. By controlling the sensitivity of the noise canceller according to the level, in other words, by controlling the hold level for the threshold,
S/ during both stereo demodulation and monaural operation.
This achieves the object of the present invention, which is to reduce the N ratio.

〔Example〕

Hereinafter, one embodiment of a radio receiver to which the present invention is applied will be described with reference to FIGS. 1 to 4. The first factor is a circuit diagram of a radio receiver, FIG. 2 is a waveform diagram illustrating circuit operation, and FIG. 3 is a resistance characteristic diagram illustrating circuit operation.

The feature of this embodiment is that the S/N ratio is improved by changing the noise detection sensitivity of the noise canceller circuit in response to switching from stereo broadcast reception to monaural reception. The change in noise detection sensitivity described above is determined by the voltage control resistor (hereinafter referred to as VCR).
This is performed by a voltage control circuit or the like composed of transistors Q and %Q.

The operation of one circuit will be explained below in order, but antennas 1 to F
Since the circuit configuration and circuit operation up to the M detector 10 are widely known among those skilled in the art, their explanation will be simplified.

1 is a receiving antenna, 2 is a high frequency amplifier, 3 is a mixing circuit,
4 is a local signal oscillator, which performs a predetermined circuit operation when receiving an FM stereo signal, and generates a frequency of 10.7M.
The Hz intermediate frequency signal fi is supplied to the next stage intermediate frequency amplification circuit 5.

The intermediate frequency amplification circuit 5 includes amplifiers 6a to 6C, level detectors 73 to 7c, an addition circuit 8 for adding up each output signal of the level detectors 7a to 7c, and an electric field strength meter 11Yffi based on the addition output VaK. It is constituted by a drive circuit 9.

Note that the deflection of the field strength meter 11 is proportional to the field strength of the received radio waves.

The detection output Vd obtained from the FM detector 10 is
Noise canceller circuit 2 via coupling capacitor CI
0. It is supplied to the bypass filter circuit 12.

The noise canceller circuit includes a signal transmission path I from T1 to T, and a control path I that opens and closes the switch S1 to reduce noise transmission when receiving noise. In the signal transmission path 11, 22 is an amplifier, a low-pass filter 23 and an amplifier 24 constitute an active filter, and the audio output Vs (composite signal)
get.

The switch S is closed when the electric field strength is good and the S/N ratio is good, but when the S/N ratio deteriorates, it is opened in response to the noise level and the time width in which the noise appears. , which temporarily blocks transmission of the audio output Vs. The resistor R+ and the capacitor C1 constitute a time constant circuit which, when the switch S is switched to the open state, maintains the voltage level of the audio output Va immediately before this switching takes place.

A buffer amplifier 25 supplies the audio output Vs to a 1kFM demodulation circuit 26 via a coupling capacitor C3. The FM demodulation circuit 26 is well known to those skilled in the art, and at the same time it obtains demodulated R and L signals, a lamp P indicating stereo reception is turned on. The circuit configuration of the FM demodulation circuit 26 is described in detail in the rFM tuner manuals PP145 to 166. And the stereo display output is
While the stereo output signal R%L is obtained, it is at a high level, and when the electric field strength decreases and the switch is made to monaural, it changes to a low level. In this example, this level change is the noise level. It is used as a detection signal.

The noise level detection operation at the time of strong electric field strength and at the time of weak electric field strength will be explained below.

31 is a bypass filter, which supplies frequency components with a frequency of 100 KHz or more to the next stage. Therefore, when the audio component is removed, the electric field strength is reduced, and the high frequency noise component increases, the output level also increases.

32 is a buffer amplifier, and the amount of transmission of the noise component is controlled by the ratio of the resistance Ra to the resistance value Rb of vCR 33.

At the time of strong electric field strength, the signal v of the detection output Vd
/ 6th construction As shown in Figure 2 (2), the noise level is high, and the noise level N is low. In this case, the output level of the buffer amplifier 32 is also at a low level and the field strength output Vm is at a high level, but since the phase is inverted by the inverter 40, a low level signal is supplied to the transistor Q.

Since the FM demodulator 26 performs a stereo demodulation operation, a high level signal is supplied to the transistor QtK, which turns on IEf+, increasing the current controlling the VCR 33 and decreasing its resistance value. The signal level supplied from the VCR 33 to the buffer amplifier 34 is Rb/
It is determined by Ra+RbK, and becomes a high level when the electric field strength is strong.

What should be noted here is that while the FM demodulator 26 performs stereo demodulation, a signal of approximately the same level is supplied to the transistor Q2, and the resistance value Rh also maintains approximately the same value, but the buffer amplifier 32 The output level changes in response to changes in electric field strength, in other words, in response to the level of noise components. Therefore, the level of the rectified output Vt of the rectifier circuit 35 changes in proportion to the noise level even when the electric field strength is strong.

The change in the level of the output voltage Vt mentioned above means that the change in the threshold level used for noise detection is proportional to the noise level. In other words, during low noise, the output voltage Vt is almost constant as shown in Figure 3. When the voltage level I/c is reached, the peak detector 36 does not supply a trigger signal to the mono multi 37, the mono multi 37 becomes inactive, and the switch S continues to be closed. Therefore, the FM demodulator 26 performs a stereo demodulation operation to obtain an R signal and an L signal.

And, since the threshold level vth for noise detection is a low level as shown in the second factor,
Even if a small pulse-like noise N is received, the switch S is switched to an open state by the monomulti 37, and the transmission of the signal Vs is temporarily blocked by the noise N. However, since the signal Vs is at a high level and is combined with the Lemille holding by the time constant circuit, the occurrence of distortion is reduced, causing almost no problem in terms of hearing.

On the other hand, if the electric field strength decreases, or even if it does not decrease, the noise component increases for some reason.

The level of the rectified output Vt increases due to the noise component, and the threshold level vth also increases f.

In this case, when a slightly high-level noise component before switching from stereo to mono multi is received, mono multi 37 operates and the above noise removal operation is performed. Note that 38 is when mono multi 370 is received. The cutoff circuit 39, which is a constant circuit, prevents the switch S from being continuously opened when the width of the noise 80 is extremely wide or when the noise N is continuously supplied.

Next, a description will be given of the circuit operation when the FFI field strength decreases and switching from stereo to mono multi is performed.

When the electric field strength decreases below a predetermined level and becomes weak, the detection output Vd becomes a low level as shown in FIG. 2 (BIK).Then, the %FM demodulator 26 automatically switches from stereo demodulation to monaural , pilot lamp P
The signal that lights I changes from high level to low level.

The pilot lamp P1 goes out, and the bias voltage supplied to the transistor Q is cut off, turning it off.

On the other hand, although the field strength output Vm is at a low level, a high-level pyame voltage whose phase is inverted by the inverter 40 is supplied to the transistor Q.

Therefore, the resistance value Rb of the VCR 33 becomes high as shown by the dotted line K in FIG. 3 [F]), and the resistance value changes in response to changes in the electric field strength. Also, the resistance Ra.

At the same time as the voltage division ratio with Rb increases, the output level of the bypass filter 31 also increases in proportion to the noise level, so the rectified voltage Vt of the rectifier circuit 36 also increases.

The above-mentioned level change in the rectified voltage Vt means that the threshold level for noise detection has changed to a high level as shown in Fig. 2 0, and that level also increases when the electric field strength decreases. As the electric field strength becomes stronger, it will shift to a lower level.
A very noteworthy circuit operation takes place as described below.

That is, when pulse-like noise N is received as shown in FIG.

is opened, and transmission of the signal Vs on the single line between them is blocked.

However, since the S/N ratio is improved by about 20 dB when switching to monaural, and the voltage level of the signal Vs is maintained by the time constant circuit, the distortion caused by the noise N is significantly improved.

Moreover, the most noteworthy point is that if the electric field strength decreases and low-level noise N' is continuously received, the threshold level will shift as described above, so the noise NK will cause a monomultiplier. 37 does not operate and switch S does not open. Therefore, switch Sl
No distortion occurs due to opening/closing.

If the threshold level is not changed as described above, the switch S will be opened and closed by continuous low-level noise NK, and the occurrence of distortion will increase. According to the circuit configuration and circuit operation, the occurrence of the above-mentioned disadvantageous distortion can be reduced beforehand, and the S/N ratio in monaural mode can be improved.

〔effect〕

(1) A voltage control resistor is provided in the noise signal transmission path to obtain a resistance value that is different from the noise level of the received signal and the FM demodulator during stereo demodulation, and the above resistance value is made low during stereo demodulation to achieve a low threshold. By detecting noise in the detection output at the hold level, when stereo demodulation is possible at high field strength, low level and high level noise can be detected by S.
It can be removed without reducing the /N ratio.

(2) Due to the above f1+ circuit configuration, 4C at weak electric field strength
As the FM demodulator performs monaural operation, the above resistance l
By making the resistance value high and detecting noise in the detection output at a high threshold level, high-level noise is removed when stereo demodulation is not possible due to weak electric field strength, and S /N ratio can be reduced.

Although the invention made by the present inventor has been specifically explained above based on examples, the present invention is not limited to the above-mentioned examples, and it is understood that various changes can be made without departing from the gist of the invention. Needless to say. For example, VCR33
may be provided after the rectifier circuit 35. Furthermore, the monomulti 37 may be replaced with a flip-flop 71 circuit.

[Application field]

In the above explanation, the invention made by the present inventor was mainly applied to an FM radio receiver, which is the background field of application, but the invention is not limited to this, and for example, it can be applied to various communication devices. can be used.

Furthermore, it can also be used for FM chainer.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a radio receiver to which the present invention is applied; FIG. 2 (AJQ31 is a waveform diagram explaining the circuit operation of the above radio receiver; FIG. 26...F shows a VOH characteristic diagram explaining the circuit operation of a radio receiver.
M demodulator, 33...VCR135...rectifier circuit, 3
6...Peak detection circuit, 37...Mono multi, S,
...Switch, Qt s Qt...Transistor,
Ra...resistance, Vd...detection output, Vm...field strength output, N, N...noise, vth...threshold level, Vt...rectified output, Vs...
Audio output signal. -N knee i/ Fig. 2 (A> (f3) Fig. 3 (A) Electric field degree '/ma'!f-explosion

Claims (1)

[Claims] 1. A resistor provided in a noise transmission path that detects the noise component of the detection output and transmits this noise component;
a resistance control circuit corresponding to stereo demodulation operation and monaural operation of the FM stereo demodulator to which the detection output is supplied, and whose resistance value is controlled in accordance with the level of electric field strength; The voltage of the noise component contained in the detection output is divided by the control circuit, and the voltage of the divided noise component is used as a threshold value whose level changes depending on the electric field strength during stereo demodulation operation and monaural operation. a switch circuit for controlling transmission of the FM stereo signal to the FM stereo demodulator, and performs noise removal in response to electric field strength using the threshold value.