JPH04337934A - Modulation circuit - Google Patents

Abstract

PURPOSE:To enable modulation while limiting only the level of a prescribed frequency band. CONSTITUTION:Since a modulation circuit 1 is composed of a high-frequency limiter circuit 2 to limit the the signal level of a high frequency part in an audio input signal, stereoscopic composite wave signal generator 36 to generate a stereoscopic composite wave signal according to the output signal of this high-frequency limiter circuit 2, modulation part 37 to modulate this stereoscopic composite wave signal and high-frequency amplifier 38 to amplify this modulated output signal, the modulation is enabled while limiating only the signal level of the high frequency part in the audio input signal so as to reduce a change in the signal level of the other frequency band. Namely, the degradation of an S/N in the modulated output signal can be reduced, and the malfunction of a noise canceler circuit caused by an over signal contained in the high frequency part of the audio input signal can be recduced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a modulation circuit used mainly in audio signal reproducing devices such as car stereos.

0002

[Prior Art] When connecting the output terminal of a CD player or CD changer (CD continuous automatic playback device) to the input terminal of a car receiver (in-vehicle receiver) to amplify the signal and drive a speaker, etc., the car receiver side It would be good if the CD player or CD changer was equipped with an audio signal input terminal to which the audio signals from the CD changer could be directly input, but if it was not provided, direct connection could not be made.

Therefore, as shown in FIG. 5, for example, an FM car receiver 31 which is not provided with the audio signal input terminal,
When connecting a CD player 32, etc. to the CD player 32, etc., the left channel (hereinafter referred to as Lch) of this CD player 32, etc.
The audio signal of the right channel (hereinafter referred to as Rch) is once modulated by the FM stereo modulator 33, etc., and this modulated signal is received by the FM car receiver 31 and demodulated, and the Lc
Generally, audio is played back by the h and Rch speakers 34.

[0004] Furthermore, it is known that such an FM car receiver 31 has a built-in so-called noise canceller circuit (not shown). This noise canceller circuit is a circuit that detects pulse noise contained in an FM demodulated signal and suppresses this noise. This noise canceler circuit operates at a relatively high frequency such as car ignition noise, and is configured so that it does not operate with stereo demodulated signals. However, when a modulated output signal with an excessively high level is generated, the waveform of the stereo demodulated signal is distorted during demodulation, and this distortion may cause the noise canceller circuit to malfunction.

FIG. 6 shows a configuration diagram of an example of the FM stereo modulator 33. The conventional FM stereo modulator 33 is C
An audio input signal such as D was first input to an input attenuator 35 to lower the signal level, and then input to a stereo composite wave signal generator 36 to generate a stereo composite wave signal. This stereo composite wave signal is subjected to FM modulation in the modulation section 37, and then the high frequency amplifier 3
After being amplified by step 8, the signal is input to the FM car receiver 31. In this way, by first lowering the signal level of the audio input signal and then synthesizing it into a stereo signal and modulating it, the F
Distortion of the stereo demodulated signal in the M car receiver 31 can be reduced, and malfunctions of the noise canceler circuit caused by this distortion can be reduced.

[0006]

[Problems to be Solved by the Invention] However, an excessively high frequency FM modulation signal is transmitted to the FM car receiver 31.
When input to a stereo demodulated signal, the noise canceller circuit may malfunction, resulting in generation of harsh noise. In particular, when a CD audio output signal with a high high frequency level is input to the FM car receiver 31 via the FM stereo modulator 33, there is a high possibility that the noise canceler circuit will malfunction. Furthermore, since the input attenuator 35 lowers the signal level of the audio input signal at approximately the same rate over the entire frequency band, the S/N ratio may deteriorate as a result.

[0007]

[Means for Solving the Problems] In order to solve the above problems, the present invention provides a signal level limiting circuit that limits the signal level of a predetermined frequency band of an audio input signal, and an output signal of this signal level limiting circuit. It is characterized by being configured to modulate.

[0008]

[Operation] The level of the audio input signal is limited to a predetermined frequency band and then modulated.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a configuration diagram of an example of a modulation circuit according to the present invention, FIG. 2 is a configuration diagram of an example of a high-frequency limiter circuit of the modulation circuit, and FIG. 3 is a circuit diagram of an example of a high-frequency limiter circuit.
FIG. 4 is an input/output characteristic diagram of the limiter section. Incidentally, the same parts as in the conventional example are given the same numbers, and the explanation thereof will be omitted.

In FIG. 1, a modulation circuit 1 according to the present invention has Lc which limits the signal level of high frequency of an audio input signal.
A conventional stereo composite wave signal generation unit 36 that combines a high frequency limiter circuit 2 provided for h and Rch, respectively, and the output signals of this high frequency limiter circuit 2 to generate a stereo composite wave signal.
, a conventional modulation section 37 and a high frequency amplifier 38, and the output signal of this high frequency amplifier 38 is a conventional FM
The signal is input to the car receiver 31.

Further, the high frequency limiter circuit 2 is composed of a pre-emphasis section 3, a limiter section 4, and a de-emphasis section 5, as shown in FIG. The pre-emphasis section 3 enhances the level of the high frequency part of the audio input signal, the limiter section 4 limits the level of the output signal of the pre-emphasis section 3 to within a predetermined value, and the de-emphasis section 5 enhances the level of the high frequency part of the audio input signal. The signal level of the high frequency portion of the output signal of the limiter section 4 is returned to approximately the original level. With this configuration, the overall frequency characteristics of the output signal of the de-emphasis section 5 can be made almost the same as the original audio input signal, so that the S/N ratio is less likely to deteriorate, and moreover, the high frequency portion Since the excessive signals included can be limited, malfunctions of the noise canceler circuit can be reduced.

FIG. 3 is a circuit diagram showing an example of the high frequency limiter circuit 2, which includes a conventional pre-emphasis section 3 composed of a differential amplifier 3a, a resistor and a capacitor, a differential amplifier 4a, and resistors Rs and Rf. , R1 to R4, a conventional limiter section 4 consisting of diodes D1 and D2, and a conventional de-emphasis section 5 consisting of a resistor and a capacitor. Vc is supplied to this circuit.

FIG. 4 is an input/output characteristic diagram of the limiter section 4. According to the limiter section 4, the output voltage is -(R2/R1
) x (1/2) x Vc to (R4/R3) x (1/2)
The amplification degree is Rf/Rs within the range of ×Vc, but the amplification degree is limited to (Rf//R4)/Rs outside this range. That is, the pre-emphasis section 3
enhances a predetermined high frequency part to a predetermined level, limits the signal level of the output of the pre-emphasis section 3 with the limiter section 4, and finally increases the signal level of the output of the pre-emphasis section 3 with the limiter section 4.
By returning the signal level to approximately the original level in the de-emphasis circuit 5, which has a polarity opposite to that of the high-frequency limiter circuit 2, the frequency characteristics of the output signal of the high-frequency limiter circuit 2 are almost the same as the original audio input signal as a whole, and the high-frequency Since the high frequency level can be limited only when the high frequency level is higher than a predetermined value, it is possible to configure a modulation circuit in which the SN ratio of an audio output signal is less likely to be deteriorated and the noise canceller circuit is less likely to malfunction.

Note that the present invention is not limited to the high-frequency limiter circuit 2, but the modulation circuit 1 may be configured to use a middle-frequency or low-frequency limit circuit. It goes without saying that the present invention can be used not only for the FM car receiver 31 but also for home FM receivers and the like.

[0015]

Effects of the Invention Since the audio input signal is level-limited and modulated only in a predetermined frequency band, changes in signal levels in other frequency bands are reduced.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an example of a modulation circuit according to the present invention.

FIG. 2 is a configuration diagram of an example of a high-frequency limiter circuit of the same modulation circuit.

FIG. 3 is a circuit diagram of an example of a high frequency limiter circuit.

FIG. 4 is an input/output characteristic diagram of a limiter section.

FIG. 5 is a configuration diagram of an example of a car receiver using a conventional modulator.

FIG. 6 is a configuration diagram of an example of a conventional modulation circuit.

[Explanation of symbols]

1... Modulation circuit, 2... High frequency limiter circuit, 3... Pre-emphasis section, 4... Limiter section, 5... De-emphasis section.

Claims (1)

[Claims] 1. A modulation circuit comprising: a signal level limiting circuit for limiting the signal level of a predetermined frequency band of an audio input signal; and configured to modulate an output signal of the signal level limiting circuit.