JPS6128427Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise canceling device for a stereo receiver, and more particularly to a noise canceling device for canceling switching noise generated when switching between stereo and monaural reception modes in a multiplex demodulation circuit.

In general, in FM stereo receivers, when the electric field strength is weak and S/N deteriorates significantly, or when the detuning occurs due to some reason, the receiver is forced to switch from stereo reception mode to monaural reception mode. /N has a circuit configuration that suppresses deterioration. In a multiplex demodulation circuit, it is inevitable that switching noise will occur when switching between reception modes, but in order to suppress this noise as much as possible, the integrated circuit elements of the multiplex demodulation circuit are selected. This is the current situation. Such a method has drawbacks such as the time and effort required for selection and the insufficient number of integrated circuit elements after selection.

An object of the present invention is to provide a noise canceling device for a stereo receiver that can eliminate switching noise between stereo and monaural reception modes in a multiplex demodulation circuit with a simple configuration.

The noise canceling device of the present invention is based on the fact that high-frequency noise components included in the left and right channel outputs, which are multiplex demodulation outputs, appear in an antiphase relationship with each other, and can similarly be switched between monaural and stereo. This was done in consideration of the fact that the noise is mainly high-frequency noise components, so they are output in opposite phases to each other. The channel signal output lines are short-circuited, and the left and right channel signal output lines are short-circuited and then gradually opened for a predetermined period in response to a stereo reception mode command signal.

The present invention will be explained below with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention.
is an MPX (multiplex) that receives the FM detection output (not shown) and separates and demodulates the left and right channel signals to the left and right channel signal output lines 2 and 3, respectively.
This is a demodulation circuit. The left and right channel signals are output via capacitors C ₁ and C ₂ , resistors R ₁ and ₂ , and further resistors R ₃ and R ₄ , respectively.

Switching between stereo and monaural reception modes is performed by a control circuit ₄ consisting of a transistor Q ₁ and a collector resistor R ₅ .
The on/off control of is performed in response to each reception mode command signal applied to its base. That is, in the normal stereo reception state, the command signal is at a high level, and therefore the transistor Q ₁
is turned on and a low-level control signal is applied to the MPX circuit 1 to enter the stereo reception mode, and left and right channel signals are separated and led out to output lines 2 and 3, respectively.

Furthermore, when the electric field strength decreases or becomes out of tune, a forced monaural reception mode command signal, that is, a low-level signal, is applied to the base of transistor Q ₁ , so transistor Q ₁ is turned off and MPX circuit 1
A high level control signal is applied to the resistor _R6 and the collector resistor _R5 to set the forced monaural mode, and the sum of the left and right channel signals (L+R) is output to both output lines 2 and 3, resulting in S/ Prevents N deterioration. Incidentally, the circuit for generating the monaural and stereo reception mode command signals is not particularly shown in the drawings, but a well-known configuration is used.

Furthermore, in the present invention, in order to control the output lines 2 and 3 of both channels to be short-circuited or open, a P-channel FET (field effect transistor) Q ₂ and this transistor are connected between both lines.
Transistor _Q3 that generates the gate control signal for _Q2
Furthermore, a time constant circuit consisting of a resistor R ₇ and a capacitor C ₃ is provided. and transistor
The collector output of _Q1 becomes the base control input of transistor _Q3 via resistor _R8 .

In such a configuration, when the base command signal of transistor Q ₁ is at a high level and the stereo reception mode is operated, the base of transistor Q ₃ is at a low level, so transistor Q ₃ is off, and therefore the FETQ ₂ gate signal is at high level. Therefore, FETQ ₂ is off, both channel output lines 2 and 3 are open, and left and right channel signals are output independently. In this state, when the reception mode command signal becomes low level, the transistor _Q1 is turned off and the MPX circuit 1 operates in monaural mode. At this time, when switching modes, switching noise as shown in Fig. 2 occurs instantaneously.
They are generated in opposite phases from the MPX demodulator 1 to the channel output lines of each other, but at the same time as transistor Q ₁ turns off, transistor Q ₃ turns on, turning on FET Q ₂ , and both output lines 2 and 3 connect to resistors R ₁ and R. ₂
Due to the on-resistance of FETQ 2 and FETQ ₂ , a short circuit occurs and both signals are mixed. Therefore, the anti-phase noise is almost eliminated, resulting in the result as shown in FIG.

Also, when switching from monaural mode to stereo mode, FETQ ₂ turns off and opens both output lines, even though it operates in the opposite way.
The impedance of FETQ ₂ is gradually increased in accordance with the delay time based on the time constant of the time constant circuit formed by R ₇ and capacitor C ₃ to maintain the on state and then transition to the off state. Therefore, for a certain delay time from the moment of switching from monaural to stereo mode, the opposite-phase switching noise shown in Figure 2 will occur.
The noise is mixed by FETQ ₂ , and the noise is eliminated as shown in FIG.

As described above, according to the present invention, switching noise that occurs when switching between stereo mode and monaural mode in the MPX demodulation circuit can be prevented with an extremely simple configuration, so there is no need to select MPX circuit elements. There are advantages.

The circuit example shown in FIG. 1 is merely an example, and various modifications can be made without being limited thereto.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing the state of occurrence of switching noise, and Fig. 3 is a circuit diagram showing an embodiment of the present invention.
FIG. 3 is a diagram showing a state in which switching noise is eliminated by the circuit shown in the figure. Explanation of symbols of main parts, 1... MPX demodulation circuit, 2, 3... Left and right channel signal output lines,
4...Stereo/monaural mode switching control circuit,
FETQ ₂ ...Switching transistor.

Claims (1)

[Scope of utility model registration request] In response to a stereo reception mode command signal, the left and right channel signals are separated and derived from the received signal to left and right channel signal output lines, respectively, and in response to a monaural reception mode command signal, the sum of the left and right channel signals is outputted to the left and right channel signal output lines. A noise canceling device for a stereo receiver, comprising a circuit for deriving a signal and an active element connected between the left and right channel signal output lines, wherein the active element is quickly turned on in response to the monaural reception mode command signal. 1. A noise canceling device comprising control means for quickly making the active element conductive in response to the stereo reception mode command signal and then gradually increasing the impedance of the active element.