JPS63177631A - Fm stereo receiver - Google Patents

Abstract

PURPOSE:To improve S/N at the time of noise, and also, to give no sense of incongruity to a user, by switching to a stereo state or a pseudo-stereo state in accordance with whether a multi-pass noise exists or not. CONSTITUTION:A stereo demodulating circuit 16 receives a stereo decoding signal SCMP outputted from a noise eliminating circuit 15, and outputs stereo signals SL, SR of both L and R channels. On the other hand, a pseudo-stereo circuit 17 receives the stereo decoding signal SCMP, and outputs pseudo-stereo signals SL', SR' of both the L and R channels. A switching circuit 18 outputs the stereo signals SL, SR in case when a multipath noise is generated, and outputs the pseudo-stereo signals SL', SR' in case when the multi-path noise is generated. In such a way, the S/N at the time of the noise can be improved. Also, no sense of incongruity is given to a user.

Description

[Detailed Description of the Invention] <Industrial Application> The present invention relates to an FM stereo receiver, and is particularly concerned with improving the S/N ratio when the noise level is large even in a weak electric field or in a strong electric field. Regarding the FM stereo receiver, which could be improved.

<Prior art> In an FM stereo receiver, by changing from stereo to monaural during weak electric fields where the noise level increases,
It is known that the S/N ratio can be improved in a weak electric field.

For this reason, in the past, the received electric field strength was monitored, and when the received electric field strength fell below a predetermined threshold level, the system switched from stereo to monaural, and when it exceeded the threshold level, it switched from monaural to stereo. I have to.

<Problems to be Solved by the Invention> As described above, conventional FM stereo receivers switch between stereo and monaural at a predetermined threshold level. For this reason, if the received electric field strength fluctuates around the threshold level, it will become stereo at the fluctuation period,
It may become monaural. As a result, the sound becomes tremolo, and the switching is performed in a switching manner, which gives a strange feeling to the listener (user).

From the above, the purpose of the present invention is to improve S
To provide an FM stereo receiver that can improve the /N ratio and does not give a user a sense of discomfort.

Means for Solving the Problems> FIG. 1 is a block diagram of an FM stereo receiver according to the present invention.

1) is an antenna, 12 is a front end, 13 is an intermediate frequency amplifier, 14 is an FM detector, 15 is a noise removal circuit, 1
6 is a stereo demodulation circuit, 17 is a pseudo stereo circuit, 18
21 is a switching circuit, and 21 is a multipath noise detection circuit.

Effects> The stereo demodulation circuit 16 receives the stereo decoded signals Sc, .lp which are the outputs of the noise removal circuit 15.

Outputs stereo signals SL, S, and monaural signal S0 for both R channels. Further, the pseudo stereo circuit 17 receives a stereo composite signal S. MP is input and pseudo stereo signals SL' and SR' of both L and R channels are output.

The switching circuit 18 outputs stereo signals SL, S when multipath noise is not occurring, and outputs pseudo stereo signals SL when multipath noise is occurring.
'p SF+' is output.

<Embodiment> FIG. 1 is a block diagram of a vehicle-mounted FM stereo receiver according to the present invention.

In the figure, 1) is an antenna, 12 is a front end, 13 is an intermediate frequency amplifier, 14 is an FM detector, 15 is a noise removal circuit that removes pulse noise (such as ignition noise) contained in the FM detection output, and 16 is the stereo decoded signal S output from the noise removal circuit. MP is input L, R
A stereo demodulation circuit outputs stereo signals SL and S of both channels, and 17 is a stereo composite signal SC.
Pseudo stereo signal by inputting MP 3. /,3Pl/
This is a pseudo-stereo circuit that outputs .

The pseudo-stereo circuit 17 is connected to the amplifier 1 as shown in FIG.
7a, a comb-shaped filter 17b having the frequency characteristics shown in FIG. 3, and a differential circuit 17C that outputs the difference between the comb-shaped filter output and the amplifier output. The L-channel pseudo signal SL' is combined, and the output of the differential circuit 17C is made into the R-channel pseudo signals 3 and 7, so that a stereo feeling can be obtained compared to monaural signals.

Returning to FIG. 1, 18 is a switching circuit, and 21 is a multipath noise detection circuit. This multipath noise detection circuit 2
1 receives the output from the FM detector 14 or the noise detection circuit 15 and extracts and outputs multipath noise components of 100 KHz or more; 2; an Ipass filter 21a; a noise amplifier 21b; It has a converting detector (low-pass filter) 21c and a time constant circuit 21d that inverts the level of the switching signal SW1 when the detected level of multipath noise continues to be above or below a predetermined level for a predetermined time T1 or more. There is. Note that the predetermined time T1 is determined by the time constant of the time constant circuit 21d.

FIG. 4 shows the levels of the signal S and noise N with respect to the antenna input level (received electric field strength). The weaker the electric field, the thicker the noise level (becomes) and the higher the detection level of multipath noise.

Now, there is no multipath noise, that is, the detection level of multipath noise is at the first threshold level.
In the following cases, the switching signal SW1 is at a high level. Therefore, in this case, the switching circuit 18
, S, is output.

However, if multipath noise occurs and the detection level of the multipath noise continues for 18 hours or more and reaches the threshold level and reaches 1 or more, the switching signal SWI becomes low level. As a result, switching @ approximately 18 is the pseudo stereo circuit 1
Pseudo-stereo signal input from 7 SL' p
Output S,'. The reason for outputting a pseudo-stereo signal is that the S/N ratio becomes poor due to the occurrence of multipath noise. Outputting a pseudo-stereo signal created using monaural signals improves the S/N ratio, and You can get a stereo feeling and the strange feeling will fade.

On the other hand, when the detection level of multipath noise becomes lower than the threshold level L for a predetermined period or more in a pseudo-stereo state, the switching signal SW1 becomes low level, so the switching circuit 18 outputs the stereo signals SL and SF1. do.

The above is a case of a stereo state and a pseudo-stereo state, but a monaural state may also be considered.

FIG. 5 is a block diagram of another FM stereo receiver of the present invention considering a monaural state. 5 differs from FIG. 1 in that (i) the (L+a) monaural signal S5 is input from the stereo demodulation circuit 16 to the switching circuit 18; (ii)
The difference is that a time constant circuit 21e is newly provided, and a second switching signal SW2 is outputted from the circuit to the switching circuit 18.

The time constant circuit 21e outputs the second switching signal 5W20 level when the detection level of the multipath noise continues to be 1 or more for a predetermined time and becomes equal to or lower than the second threshold level L2 (L, <L,). "1" → "0" or rOJ →
Reverse to "1".

When the detection level of multipath noise is L, L, <
In the states of L, L, ≦L, <L, a stereo state or a pseudo-stereo state occurs based on the same criteria as in the case of FIG.

However, when the multipath noise detection level LM continues to be equal to or higher than L2 for a predetermined period of time or more, the second switching signal SW2 becomes low level. As a result, the switching circuit 18 outputs the monaural signal SN to both the L and R channels regardless of the switching signal SWI.

In this way, when the received electric field strength becomes considerably weak and multipath noise occurs frequently, the S/N ratio can be improved by switching to monaural mode for easier listening. There is no sense of strangeness because a pseudo-stereo state is involved.

Although the signal input to the pseudo-stereo circuit 17 in FIGS. 1 and 5 is taken from the output of the noise removal circuit 15, it may also be taken from the output of the FM detector 14. <Effects of the Invention> According to the present invention, the S/N ratio during noise can be improved by changing from a stereo state to a pseudo-stereo state, or from a pseudo-stereo state to a stereo state, as appropriate depending on the presence or absence of multipath noise. However, since a pseudo-stereo state is interposed between the stereo state and the monaural state, it is possible to enjoy good FM stereo listening without any discomfort.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an FM stereo receiver according to the present invention, Fig. 2 is a block diagram of a pseudo-stereo circuit, Fig. 3 is a frequency characteristic diagram of a comb filter in the pseudo-stereo circuit, and 4rIA is an antenna input level. FIG. 5 is a block diagram of another FM stereo receiver according to the present invention. 1)... Antenna, Book 12 - Front end, 16... Stereo demodulation circuit, 17... Pseudo stereo circuit, 18... Switching circuit, 21... Multipath noise detection circuit Patent applicant Alpine Co., Ltd. Agent Patent attorney Chiki Saito Figure 2 f7 Figure 3 Figure 4

Claims (3)

[Claims] (1) A multipath noise detection circuit, a stereo demodulation circuit that outputs stereo signals S_L and S_R of both L and R channels, and a stereo demodulation circuit that receives the stereo composite signal S_C_M_P and generates pseudo stereo signals S_L' and S_R of both L and R channels. A pseudo-stereo circuit that outputs stereo signals S_L and S_R when no multipath noise occurs, and a switch that outputs pseudo-stereo signals S_L' and S_R' when multipath noise occurs. FM stereo receiver equipped with a circuit. (2) The multipath noise detection circuit notifies the switching circuit of the occurrence of multipath noise when the detection level of the multipath noise is equal to or higher than a first threshold level L_1. FM described in section 1)
stereo receiver. (3) The switching circuit outputs the monaural signal S_M when the detection level of multipath noise is higher than the second threshold level L_2 which is equal to or higher than the threshold level L_1. FM stereo receiver according to item (2).