JPH01309507A - Low frequency characteristic variable circuit for receiver - Google Patents

Abstract

PURPOSE:To improve the S/N by adding a sound output signal to a low frequency amplifier through a low pass filter to be selected. CONSTITUTION:A noise component only of an output from a frequency discriminator 1 is amplified and rectified by a noise amplifier 2 and the result is fed to a comparator section 3. Moreover, an input terminal of a low pass filter 5 is connected to the output of the frequency discriminator 1 via a buffer for an attenuator and the output terminal is connected to an input terminal of a low frequency amplifier 6 having a wide band characteristic. Thus, as the RF level is decreased, the noise level is increased and the control signal is outputted from each comparator as comparators 31, 32 and comparators 31, 32, 33 and capacitors C1, C2 and capacitors C1, C2, C3 are connected to ground in this order. Thus, the low sound is emphasized at a frequency with bad S/N, the noise is decreased and the reception sensitivity is improved practically.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention automatically changes the frequency characteristics of an audio output circuit in accordance with the input electric field strength in a receiver, particularly in a receiving section of an FM radio, to improve S/N. This invention relates to a low frequency characteristic variable circuit for improving the low frequency characteristics.

(Prior art) When the input electric field decreases in an FM radio and the S/N deteriorates, a noise squelch circuit is activated to cut out the noise along with the signal, or the S/N is reduced by using a diversity reception method.
Efforts are being made to improve /N.

(Problem to be solved by the invention) However, if only the noise squelch circuit is used,
The audio signal is also interrupted, and it is difficult to set the operating point of the noise squelch to an appropriate point. Diperity reception also has disadvantages such as increased cost.

(Means for Solving the Problems) The present invention was developed against this background.The human voice distribution has a peak around 400Hz to IKHz, while the noise component of the receiver has a peak from 300Hz to IKHz. Focusing on the fact that the thickness is approximately constant for 3KHz, when the RF level of the receiver is low, the high frequency part of the low frequency output circuit (generally 2KH)
The S/N ratio is improved by attenuating the signal (z~3 K I-1z ) and automatically changing the degree of attenuation in accordance with the degree of input RF I-1z. Examples will be described in detail below with reference to the drawings.

(Embodiment) FIG. 1 is a block diagram showing an embodiment of the present invention, in which 1 is a frequency discriminator, 2 is a noise amplifier, 3 is a comparator section, and 4 is a block diagram showing an embodiment of the present invention.
['' is an analog switch, 5 is a low-pass filter, 6 is a low frequency amplifier, and 7 is a speaker.

The audio signal from the frequency discriminator 1 is applied to the low frequency amplifier 6 via a low pass filter g5, where it is amplified and then sent to the speaker 7.
The sound is interrupted by this.

In addition, only the noise component of the output from the frequency discriminator 1 is amplified and rectified by the noise amplifier 2, and then applied to the comparator section 73. The comparator section 3 includes a plurality of comparators 31 to 3.
4; a different reference voltage is applied to each of 3. Comparator 31 is given a reference voltage smaller than the other comparators, and is a noise amplifier? DC voltage from or slightly larger than its reference voltage and smaller than the reference voltage of the next comparator 32 (RF voltage or a relatively large value)
L; t, the control voltage is output only from the comparator 31.

This control voltage operates the corresponding switch of the analog switch 4, and one end of the congunzer C1 of the low-pass filter 5 is grounded. Since the other switches are not activated, capacitors "J'" C2 to C4 are open.

As mentioned above, the input end of this low-pass filter 5 is connected to the output side of the frequency discriminator 1 via a buffer such as a filter, and the output end is connected to the low-frequency amplifier 6 having wideband characteristics. Connected to the input end.

The cutoff frequency f c H of the low-pass filter 5 is approximately 1
/ 2 n C+ RI, and the high frequency range is attenuated by 6 dB10 ct. As the RF level becomes smaller, the noise level increases, and control signals are output from each comparator such as comparators 31 and 32, 31, 32, and 33, and CI and C2,
Grounding is performed in the order of C1, C2, and C3.

FIGS. 2 and 3 are explanatory diagrams of these operations.

The S/N characteristic of the narrowband FM radio is as shown in Figure 2. Now, if the S/N has deteriorated by a' (dB) from point A to point a, and the comparator 31 is set to operate, At point a, the noise level decreases from the original dotted curve to the solid curve, and the S/N improves. Similarly, at points c and d, the comparator 32,
It can be seen that when 33.3/l is operated sequentially, the interval between the dotted line and the solid line of the noise curve becomes large, and the S/N ratio is considerably improved. Figure 3 shows the attenuation characteristics of a low-pass filter.The constants of each filter element are adjusted so that the cutoff frequency is approximately constant, and the comparator is operated sequentially to 6dB1.
0ct, 12dB10ct, 18dB10ct, 24d
It is configured to be B10ct. In this way, as the RF level becomes smaller, the amount of attenuation in the high range becomes larger, and the S/N can be improved.

Note that in the case of an FM radio, it is appropriate to select the cutoff frequency for changing the frequency characteristics in the range of 2 KHz to 3 KHz in terms of maintaining the volume.

(Effects of the Invention) As explained above, the present invention utilizes a noise amplifier to
N's: In places where the treble is emphasized, clarity is improved, and where the S/N ratio is poor, the bass is emphasized and noise is reduced, so reception sensitivity is improved for practical purposes. There is.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIGS. 2 and 3 are operation explanatory diagrams. 1... Frequency discriminator, 2... Noise amplification section, 3...
Comparator section, 4... Analog switch, 5...
Low-pass filter, 6...Low frequency amplifier, 7... Speaker. Patent applicant Japan Radio Co., Ltd.

Claims (1)

[Claims] Low frequency amplifier 6 that amplifies the audio output signal of the receiver demodulator
and a speaker 7, a noise amplifier 2 that amplifies and rectifies the noise output from the demodulation section, and compares the DC output of the amplifier with reference voltages provided in a plurality of comparators and outputs the comparison results. It comprises a comparator section 3 and a low-pass filter 5 selected by the output of the comparator section 3, and the audio output signal is transmitted to the selected low-pass filter 5.
A low frequency characteristic variable circuit for a receiver, characterized in that the low frequency characteristic variable circuit is configured to be applied to the low frequency amplifier 6 through the low frequency amplifier 6.