JPH0510427Y2 - - Google Patents

Description

[Detailed description of the invention] Technical field The present invention relates to an FM stereo tuner, in particular,
The present invention relates to an FM stereo tuner that maintains a good S/N ratio even when the received field strength decreases.

Background Art An example of a conventional FM stereo tuner will be explained with reference to FIG. In Figure 2, antenna 1
The FM stereo broadcast radio waves that arrive are converted into FM high frequency signals and supplied to the FM front end 2. The FM front end 2 selectively amplifies a desired broadcast signal from the supplied high frequency signals, converts it into an IF (intermediate frequency) signal, and supplies it to the IF amplifier 3. The IF amplifier 3 amplifies the IF signal, adjusts the signal waveform, removes noise signal components, and sends it to the signal strength detection circuit 4 and the detection circuit 5.
supply to.

A signal strength detection circuit 4 is supplied to the IF amplifier 3 in order to detect a level proportional to the received electric field strength.
A control signal based on the level of the IF signal is supplied to the detection circuit 5, the frequency characteristic control circuit 6, and the stereo demodulation circuit 7.

The detection circuit 5 detects the main signal from the IF signal,
A composite signal consisting of a sub signal and a pilot signal is demodulated. This signal is adjusted to a level corresponding to the control signal by a so-called electronic volume circuit (not shown), and is supplied to the noise canceller circuit 8 as a modulation signal. Note that the FM front end 2, IF amplifier 3, and detection circuit 5 form an FM detection section 9.

The noise canceller circuit 8 removes pulse noise mixed into the modulation signal from the vehicle engine, etc., and supplies it to the frequency characteristic control circuit 6.

The frequency characteristic control circuit 6 changes the frequency response according to the level change of the control signal.

The stereo demodulation circuit 7 demodulates left and right channel signals from the output signal of the frequency characteristic control circuit 6. Then, left and right channel signals are supplied as left and right demodulated outputs from the L and R output terminals with a degree of channel separation according to the control signal. The frequency characteristic control circuit 6 and the stereo demodulation circuit 7 form a demodulation section 10.

In such a configuration, when the received electric field strength drops below a certain value, control is first performed to continuously reduce the degree of separation in the stereo demodulation circuit 7 to reach monaural mode, and then the frequency characteristic control circuit 6 operates, and increases the amount of attenuation in the high frequency range in response to the decrease in the received electric field strength. Furthermore, when the received electric field strength decreases, the detection circuit 5
control is made to reduce the output level of
Although it is possible to maintain a nearly constant S/N even with fluctuations in received field strength, during manufacturing, variations in the gains of the FM front end 2 and IF amplifier 3, and the stereo demodulation circuit Since there are variations in the operating characteristics of the controlled circuits of No. 7, there is a problem in that the output noise suppression operating characteristics of the assembled tuner with respect to a drop in the input signal vary greatly.

Note that an example of the apparatus is disclosed in Japanese Utility Model Publication No. 59-31077.

Summary of the invention Therefore, the purpose of the invention is to provide an FM stereo tuner that suppresses the variation in the control operation characteristics described above in an FM stereo tuner that performs control operations to maintain the S/N ratio even when the received electric field strength decreases. To provide a stereo tuner.

In order to achieve the above object, in the FM stereo tuner of the present invention, when the difference between the signal level of the predetermined frequency band of the left and right channel signals output from the stereo demodulation circuit and the reference level exceeds a certain value, The configuration is such that feedback control is performed by supplying an output corresponding to the output to the controlled circuit as a control signal.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to FIG. In FIG. 1, the same reference numerals are used for parts equivalent to those of the circuit shown in FIG. 2, and a description of these parts will be omitted. The left and right channel signals of the stereo demodulation circuit 7 are supplied to a bandpass filter 22 via buffer amplifiers 20 and 21. The bandpass filter 22 passes only noise components in a predetermined frequency band that does not include left and right channel signals and a priori components, for example around 17 KHz or 20 KHz or more. Here, the spurious components are, for example, harmonics of carrier waves that are not included in the standard spectrum of the composite signal. The output of the bandpass filter 22 is amplified by an amplifier 23 and supplied to a detection circuit 24. Detection circuit 24
converts the level of the above noise component to DC voltage,
This is supplied to the comparison input terminal of the comparison circuit 25 as a noise signal. When the level of the input noise signal exceeds the reference voltage V〓, the comparison circuit 25 sets the reference voltage V〓
A difference signal corresponding to the level difference between the noise signal and the noise signal is supplied to the time constant circuit 26 as a control signal. A time constant circuit 26 determines the response speed of the feedback control loop, and its output is supplied as a control signal A to a stereo demodulation circuit 7 and a delay circuit 27. The stereo demodulation circuit 7 sets the degree of channel separation according to this control signal A. The output of the delay circuit 27 is supplied as a control signal B to the frequency characteristic control circuit 6 and the delay circuit 28. The frequency characteristic control circuit 6 sets the amount of high frequency attenuation according to the control signal B. The output of the delay circuit 28 is supplied as a control signal C to the electronic volume circuit of the detection circuit 5, and the level of the modulation signal is set. Delay circuits 27 and 28 operate so that feedback control is performed sequentially.

Next, the operation of the apparatus will be explained with reference to FIG. FIG. 3A shows the standard spectrum of the demodulated composite signal, where the left and right channel signal components CH are indicated by the 0-15KHz region,
A leak component of the pilot signal P exists at a position of 19KHz. Further, FIG. 3A shows an example when the received electric field strength is relatively large, and FIG. 3B shows an example when the received electric field strength is relatively small, and the noise component N is shown. There is. FIG. 3C shows an example where there is no signal input to the tuner. As shown in these figures, when the received electric field strength is high, there are only left and right channel signals and spurious signals, but as the received electric field strength decreases, the level of noise components increases. So, for example 17KHz
When the level of a noise component in the vicinity or above 20KHz is detected and the level of this noise component exceeds a predetermined value set as the reference voltage V, a control signal is generated from the comparator circuit 25 and feedback control is started. It is. First, the stereo demodulation circuit 7 sequentially lowers the channel separation degree in accordance with the control signal A to reduce stereo noise and improve the S/N.
Even if the output of the stereo demodulation circuit 7 becomes monaural, if the level of the noise signal does not become lower than the reference voltage V, the frequency characteristic control circuit 6 changes the frequency characteristic in accordance with the control signal B to Noise is suppressed. Further, when the level of the noise signal does not become lower than the reference voltage V, the detection circuit 5 lowers the output level in accordance with the control signal C. Note that the time constant circuit 26 makes it possible to obtain a control signal of a level corresponding to the average level of the fluctuations when the received electric field strength is constantly changing due to multipath or the like. By appropriately setting the circuit constants of the delay circuits 27 and 28, it is also possible to overlap the control operation timing of each circuit.

Effects of the invention As explained above, in the FM stereo tuner of the invention, the noise level in the signal output from the stereo demodulation circuit is extracted, and based on this noise level, the detection circuit, the frequency control circuit, and the stereo demodulation circuit are Since the feedback control configuration is used to control the circuit, it is possible to maintain a good S/N ratio, and it is preferable because it suppresses variations in the output noise suppression operation characteristics of the manufactured tuner in response to a drop in the input signal. .

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention;
Fig. 2 is a circuit diagram showing a conventional example, and Figs. 3 A to C are
2 is a diagram showing frequency spectra of composite signals and noise output from buffer amplifiers 20 and 21. FIG. Explanation of symbols of main parts, 2...FM front end, 3...IF amplifier, 5...detection circuit, 6...
... Frequency characteristic control circuit, 7 ... Stereo demodulation circuit, 20, 21 ... Buffer amplifier, 22
... Bandpass filter, 25 ... Comparison circuit, 2
6...Time constant circuit.

Claims (1)

[Claims for Utility Model Registration] (1) An FM detector that extracts a modulated signal from a received FM high-frequency signal and outputs the modulated signal at a level that corresponds to a supplied control signal; an FM stereo tuner comprising a demodulator that obtains left and right demodulated output signals from the modulated signal with a frequency characteristic and a stereo separation degree, the level of noise components in a predetermined frequency band of the left and right demodulated output signals and a reference level. 1. An FM stereo tuner comprising control signal generating means for outputting the control signal according to the difference between the two. (2) The control signal generating means includes a buffer amplifier receiving the left and right demodulated output signals as input, a bandpass filter that passes only noise components in the predetermined frequency band from the output signal of the buffer amplifier, and An FM stereo tuner comprising: a detection circuit that detects an output; and a differential circuit that outputs a difference signal as the control signal when the output level of the detection circuit exceeds the reference level.