GB2246676A

GB2246676A - Stereo/mono fm tuner with noise suppression

Info

Publication number: GB2246676A
Application number: GB9107414A
Authority: GB
Inventors: Toru Ohashi
Original assignee: Pioneer Electronic Corp
Current assignee: Pioneer Corp
Priority date: 1990-07-05
Filing date: 1991-04-09
Publication date: 1992-02-05
Anticipated expiration: 2011-04-09
Also published as: DE4111709A1; GB9107414D0; GB2246676B; DE4111709C2; JPH0429244U

Description

cjE, e,'7 (S FM TUNER_ This invention relates to an improvement of an FM

tuner which performs its original high fidelity signal reception when the strength of the electric field is high, and which, as the antenna input level decreases, perf orms an automatic separation function of smoothly switching a stereophonic signal reception over to a monaural signal reception, an automatic high cut function of decreasing high frequency noises, and a soft mute function of decreasing a reproduction output; i.e., so-called automatic reception control functions (hereinafter referred merely as "ARC functions", when applicable).

When an FM radio wave is received, its electric field strength is greatly affected, for instance, by the concerned geographical features, buildings and the distance between the FM broadcast station and the FM signal receiving point. In order to overcome this difficulty, the FM tuner has the above-described ARC functions (cf. Japanese Utility Model Examined Publication No. Sho. 59-31077).

The technical concept disclosed by the Japanese Utility Model Examined Publication No. Sho. 59-31077 will be described with reference to Fig. 3.

In Fig. 3, reference numeral 1 designates an antenna; 2, a front end; 3, an IF amplifier; 4, a detector; 5, a multiplexer; 1 - 1 1 6, an S meter detector for detecting the level of the output signal of the IF amplifier 3; 7, a separation control circuit which, when it becomes impossible to perform the stereophonic signal reception because the output level of the S meter detector 6 decreases slightly, operates to switch the operation mode of the multiplexer 5 to the monaural mode; 8, a high cut circuit which, when the output level of the S meter detector 6 is further decreased, operates to high-cut the output of the multiplexer 5; and 9, a soft mute circuit which, when the 10 output level of the S meter detector 6 is further decreased, operates to decrease the output level of the IF amplifier 3.

In the FM tuner thus organized, as the antenna input level decreases, the automatic separation function, the automatic high cut function and the soft mute function are continuously is performed, so that the signal is reproduced with high fidelity while the noises are effectively reduced.

On the other hand, in the case of the above- described ARC functions, the system is greatly fluctuated in total gain. In order to absorb the fluctuation, it is necessary to provide a number of adjusting points therein.

The number of adjusting points may be decreased by the following method: As shown in Fig. 3, a gain adjusting IF amplifier 31, which is variable enough to absorb the total gain f luctuation is connected between the front end 2 and the IF amplifier 3.

A 1 1 1 i 1 1 1 1 i i i i i i 1 In practice, the gain adjusting IF amplifier 31, needs a range of variation of at least 10 dB. Therefore, an amplifier comprising a field-effect transistor or a transistor cannot satisfy the requirement completely. Accordingly, the circuit is unavoidably intricate, and high in manufacturing cost.

On the other hand, in a recent FM tuner, the IF amplifier 3, the detector 4 and the multiplexer 5 are, in general, provided as integrated circuits which are different chips. Therefore, the chips are non-uniform in fluctuation.

Accordingly, even if the total gain is adjusted, it is impossible to completely eliminate the fluctuation.

is Accordingly, an object of this invention is to eliminate the abovedescribed difficulties accompanying a conventional PM tuner. More specifically, an object of the invention is to provide an PM tuner in which various control circuits controlled by the S meter output and the S meter output voltage are low in manufacturing cost and can be adjusted at only one point.

The foregoing object has been achieved by the provision of an PM tuner according to the invention. The specific feature of the PM tuner resides in that a level detection is carried out to detect the IF output, and an adjustable DC offset voltage is added to the level detection output, to provide a DC 3 offset output, which is utilized for controlling the ARC functions.

In the accompanying drawings:- Fig. 1 is a block diagram showing one embodiment of an FM -5 tuner according to this invention; Fig. 2 is a graphical representation indicating S meter outputs with respect to input levels in an offset voltage adder; and Fig. 3 is a block diagram showing a conventional FM tuner.

One preferred embodiment of this invention is shown in Fig. 1, in which those parts which have been previously described with reference to Fig. 3 are'designated by the same reference numerals.

An FM tuner according to the invention is different from the abovedescribed conventional tuner (Fig. 3) in that the level of the electric field strength of the received signal is detected by detecting the output signal of the IF amplifier 3 with an IF level detector 11, whose output is applied to an offset voltage adder 12 with a gain adjusting resistor 12a in order to provide an offset output which is utilized for controlling the ARC functions including various functions.

The operation of the FM tuner thus organized will be described.

- 4 i 1 i 1 1 1 i i 1 1 1 i is The output of the IF amplifier 3 is detected by the IF level detector 11, to provide a DC output (S meter output). In the of f set voltage adder 12, the DC output of the IF level detector 11 is offset by adjusting the variable resistor 12a.

More specifically, as shown in Fig. 2, when the DC output (S meter output) is high, it is decreased by adjusting the variable resistor 12a as indicated by the arrow A; and when the DC output is low, it is increased as indicated by the arrow B; that is, the DC output is maintained constant by adjusting the variable resistor 12a. Thus, in the integrated circuits, the total gain fluctuation of the system is adjusted.

The output of the offset voltage adder 12 is utilized for controlling the ARC functions such as the automatic separation function, the automatic high cut function, and the soft mute function. Thus, even if the system total gain is fluctuated, the control can be achieved positively by only one adjustment operation.

As was described above, in the PM tuner of the invention, the IF level output proportional to the electric field strength is not directly used, and instead the ARC functions are controlled using the offset voltage adjusted at one point. Therefore, it is unnecessary for the PM tuner to employ an intricate expensive gain adjustable amplifier. Accordingly, the PM tuner can be provided at a low manufacturing cost, and can be applied to systems which utilize the S meter output in such a manner that the offset voltage is utilized for 1 1 temperature correction or switching the sensitivity in automatic broadcast selection.

- 6 z i 1 i 1 i i 1 1 1 i i 1 i i f 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Claims

CLAIMS 1 1. An FM tuner comprising: an amplifier for amplifying an input signal; output means coupled to receive the amplified input signal, said output means operable for outputting a stereophonic signal in response to a first control signal, and operable f or outputting a monaural signal in response to a second control signal; a separation control circuit, coupled to said output means, for supplying one of said f irst and second control signals; a high cut circuit, coupled to said output means, said high cut circuit operable for decreasing high frequency noise in an output of said output mean's; a soft mute circuit operable for decreasing an output level of said amplifier; a level detector for detecting the output level of said amplifier; and an offset voltage adder operable for adding an adjustable offset voltage to an output of said level detector, said separation control, high cut and soft mute circuits being controlled in accordance with an output of said offset voltage adder.

1 1 1

2 1 2 1 4 5 1 2 3 1 2 3 2. The FM tuner as def ined in claim 1, wherein said 2 amplifier is an intermediate frequency amplifier.

3. The FM tuner as def ined in claim 1, wherein said 2 output means comprises a multiplexer.

4. The FM tuner as def ined in claim 2, wherein said level detector is an intermediate frequency level detector.

The FM tuner as def ined in claim 4, wherein said level detector provides a DC output.

6. The PM tuner as defined in claim 5 further comprising a variable resistor coupled to said offset voltage adder, and wherein said DC output of said level detector is adjusted by said offset voltage adder in accordance with a resistance of said variable resistor.

7. The FM tuner as def ined in claim 6, wherein. said variable resistor is adjusted such that the output of said offset voltage adder is maintained constant.

8. The FM tuner as defined in claim 6, wherein each of said separation control, high cut and soft mute circuits is an integrated circuit.

9. An FM tuner substantially as described with reference to Figures 1 and 2 of the accompanying drawings.

Published 1992 at The Patent Office, Concept House. Cardiff Road. Newport, Gwent NP9 1RH. Further copies may be obtained from Sales Branch, Unit 6. Nine Mile Point, Cwmfelinfach. Cross Keys. Newport, NP1 7HZ. Printed by Multiplex techniques ltd. St Mary Crky. Kent.

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