JPH0426569B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an FM stereo receiver, and particularly to an FM stereo receiver that can improve the S/N ratio in a weak electric field.

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

For this reason, conventional methods have been to monitor the received electric field strength, switch from stereo to monaural when the received electric field strength falls below a predetermined threshold level, and switch from monaural to stereo when it exceeds the threshold level. That's what I do.

<Problems to be Solved by the Invention> As described above, conventional FM stereo receivers switch between stereo and monaural at a predetermined threshold level. Therefore, when the received electric field strength fluctuates around the threshold level, the signal becomes stereo or monaural depending on the period of variation. 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, an object of the present invention is to provide an FM stereo receiver that can improve the S/N ratio in a weak electric field 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.

11 is the antenna, 12 is the front end, 13
is an intermediate frequency amplifier, 14 is an FM detector, 15 is a noise removal circuit, 16 is a stereo demodulation circuit, 17 is a pseudo-stereo circuit, 18 is a switching circuit, 19 is a level detection circuit, 20 is a time constant circuit, and 21 is a multipath This is a noise detection circuit.

<Operation> The stereo demodulation circuit 16 receives the stereo composite signal S _CMP , which is the output of the noise removal circuit 15, and outputs L,
Outputs stereo signals S _L and S _R of both R channels and a monaural signal S _M. Moreover, the pseudo stereo circuit 17
receives the stereo composite signal S _CMP and outputs pseudo stereo signals S _L ′ and S _R ′ of both L and R channels.

The switching circuit 18 outputs the stereo signals S _L and S _R when the received electric field strength detected by the level detection circuit 19 is equal to or higher than a predetermined threshold level E _H , and when the received electric field strength exceeds a predetermined threshold level E H . Pseudo-stereo signals S _L ′, S _R ′ when the current level is below E _H
Output.

Note that the received electric field strength is at the threshold level.
Even if it is higher than E _H , the S/N ratio can be improved by outputting the pseudo stereo signals S _L ′ and S _R ′ when multipath noise is occurring.

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

In the figure, 11 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 (ignition noise, etc.) contained in the FM detection output, and 16 is a noise removal circuit that receives the stereo decoded signal S _CMP output from the noise removal circuit and outputs both L and R channels. A stereo demodulation circuit 17 outputs stereo signals _SL and _SR , respectively, and a pseudo-stereo circuit 17 receives the stereo composite signal _SCMP and outputs pseudo-stereo signals _SL ' and _SR '.

The pseudo stereo circuit 17, as shown in FIG.
An amplifier 17a, a comb filter 17b having the frequency characteristics shown in FIG. 3, and a differential circuit 17 that outputs the difference between the comb filter output and the amplifier output.
c, and the output of the comb-shaped filter 17b is L.
− channel pseudo signal S _L ′, differential circuit 17
The output of signal c is used as the R-channel pseudo signal S _R ', and a stereo effect can be obtained compared to monaural.

Returning to FIG. 1, 18 is a switching circuit, and 19 is connected to an intermediate frequency amplifier 13, which outputs a level meter signal LMO having a value corresponding to the received electric field strength (antenna input level) E as shown in FIG. The level meter (level detection circuit) 20 is a switching signal when the level meter signal LMO remains above or below a predetermined threshold level L _H (corresponding to received electric field strength E _H ) for a predetermined time T ₁ or more. This is a time constant circuit that inverts the level of SW1 from "1" to "0" or from "0" to "1". In addition, the predetermined time
T ₁ is determined by the time constant of the time constant circuit 20.

21 is a multipath noise detection circuit. This multipath noise detection circuit 21 includes an FM detector 1
4 or inputs the output from the noise detection circuit 15 and extracts multipath noise components of 100KHz or more.
The high-pass filter 21a that outputs, the noise amplifier 21b, and the detector (low-pass filter) 21c that converts the multipath noise signal into direct current, and the multipath noise that continues for a predetermined time _T2 or more and exceeds or below a predetermined level. It has a time constant circuit 21d that inverts the level of the time switching signal SW2. Note that the predetermined time _T2 is determined by the time constant of the time constant circuit 21d.

Now, when the received electric field strength E is a strong electric field equal to or higher than the threshold level E _H (approximately 40 dBμ), the first switching signal SW1 is at a high level. Furthermore, when there is no multipath noise and the multipath noise level is below a predetermined level, the second switching signal SW2 is activated.
is at a high level. Therefore, in this case, the switching circuit 18 outputs the stereo signals S _L and S _R.

However, even in the case of a strong electric field, if multipath noise occurs and the level of the multipath noise exceeds a predetermined value, the second switching signal SW2 becomes low level. As a result, the switching circuit 18 receives the pseudo stereo signal input from the pseudo stereo circuit 17.
Output S _L ′ and S _R ′. The reason for outputting a pseudo-stereo signal is that the S/N ratio deteriorates due to the occurrence of multipath noise, and if a pseudo-stereo signal created using monaural is output, the S/N ratio will deteriorate.
The ratio is improved and a stereo feeling can be obtained.

On the other hand, the received electric field strength E continues for a predetermined period of time or more.
When the electric field becomes lower than _EH , that is, when the weak electric field state continues for a predetermined time _T1 or more, the first switching signal SW1, which is the output of the time constant circuit 20, is inverted to low level.
When the first switching signal SW1 becomes low level, the switching circuit 18 outputs the pseudo stereo signals S _L ′, S _R ′ regardless of the second switching signal SW2. At this time, if a monaural signal is output as in the conventional case, tremolo may occur, and the switching may change from stereo to monaural, or from monaural to stereo, resulting in a strange feeling.However, as in the present invention, a pseudo-stereo signal If you output , tremolo will not occur and the strange feeling will be reduced.

When the received electric field strength exceeds E _H for a predetermined period of time or more in a pseudo-stereo state, the first switching signal is activated.
After SW1 becomes high level, stereo signals _SL , _SR or pseudo stereo signals _SL ', _SR ' are output depending on the level of multipath noise.

In the above, we have explained the case where the stereo state and pseudo-stereo state are set appropriately based on the received field strength and multipath noise, but it is also possible to switch between the stereo state and pseudo-stereo state based only on the received field strength without considering multipath noise. It can also be configured to do so. In this case, when the received electric field strength shown in FIG. 4 is greater than or equal to E _H , the stereo state is entered, and when it is below, the pseudo stereo state is entered.

Moreover, although the above is a case of a stereo state and a pseudo-stereo state, 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+R) monaural signal S _M is input from the stereo demodulation circuit 16 to the switching circuit 18, and (ii) a time constant circuit 22 is newly provided. , the third switching signal SW3 is output from the circuit to the switching circuit 18.

The time constant circuit 22 is configured such that the level meter signal LMO continues for a predetermined time period _T3 or more to reach a second threshold level L _L (corresponding to the received electric field strength E _L , L _L < L _H , see FIG. 6). or below, the level of the third switching signal SW3 is inverted.

When the level of the level meter signal LMO is L _M , in the state of L _M > L _H , L _H ≧L _M > _L
A stereo state or a pseudo-stereo state is achieved using the same criteria as in the case shown in the figure.

However, if the received electric field strength E becomes less than E _L for more than a predetermined time, in other words, if the level L _M of the level meter signal LMO becomes less than L _L for more than a predetermined time, the third switching signal SW3 goes low. level. As a result, the switching circuit 18 receives the switching signal SW.
1, outputs monaural signal S _M to both L and R channels regardless of SW2.

In this way, when the received electric field strength becomes considerably weak, the noise level increases, so the signal is changed to a monaural state in which the S/N ratio can be improved to make it easier to listen to. In this case, there is no direct transition from stereo to monaural, but a pseudo-stereo state intervenes, so tremolo does not occur and there is no sense of discomfort.

In FIGS. 1 and 5, the signal input to the pseudo-stereo circuit 17 is taken from the output of the noise removal circuit 15, but it may also be taken from the output of the FM detector 14.

<Effects of the Invention> According to the present invention, since the stereo state is changed from a stereo state to a pseudo-stereo state, or from a pseudo-stereo state to a stereo state, according to the received electric field strength, tremolo of the sound does not occur, and moreover, it is possible to switch from a stereo state to a pseudo-stereo state according to the received electric field strength. Since there is no change from audio to monaural, you can enjoy good FM stereo listening without any discomfort.

Furthermore, according to the present invention, even in the case of a strong electric field, if multipath noise occurs, a pseudo-stereo state is created, so the S/N ratio can be improved even when the noise level is large, and good listening can be achieved. be able to.

Furthermore, according to the present invention, the stereo state, pseudo-stereo state, and monaural state are set depending on the received electric field strength, and the pseudo-stereo state is interposed between the stereo and monaural states, so there is no tremolo, and there is no strange feeling. , furthermore, the S/N in weak electric field
ratio can be improved.

[Brief explanation of drawings]

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-shaped filter in the pseudo-stereo circuit, and Fig. 4 is a stereo state. 5 is a block diagram of another FM stereo receiver according to the present invention, and FIG. 6 is a level meter output characteristic diagram illustrating switching levels between stereo, pseudo-stereo, and monaural states. FIG. 3 is a level meter output characteristic diagram for explaining. 11...Antenna, 12...Front end, 16
...stereo demodulation circuit, 17...pseudo stereo circuit,
18... switching circuit, 19... level detection circuit, 20,
22... Time constant circuit, 21... Multipath noise detection circuit.

Claims (1)

[Claims] 1. A level detection circuit that detects received electric field strength;
A stereo demodulation circuit outputs stereo signals S _L and S _R of both L and R channels, and a pseudo demodulation circuit receives the stereo composite signal S _CMP and outputs pseudo stereo signals S _L ′ and S _R ′ of both L and R channels. Stereo circuit and received field strength at a predetermined threshold level
Outputs stereo signals S _L and S _R when E _H or higher,
An FM stereo receiver comprising a switching circuit that outputs pseudo stereo signals S _L ′ and S _R ′ when the threshold level is below the threshold level. 2 The received electric field strength is at the threshold level.
_{E _ _}
Claim 1 is characterized in that it outputs
FM stereo receiver as described in section. 3. A level detection circuit for detecting received electric field strength;
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 pseudo stereo signal S _L ′ of both L and R channels that receives the stereo composite signal S _CMP . A pseudo-stereo circuit that outputs S _R ′ and a received electric field strength at a predetermined threshold level.
Stereo signals S _L and S _R are output when the received field strength is above the predetermined threshold level E H and no multipath noise occurs _, and multipath noise occurs even if the received field strength is above the predetermined threshold level E _H. An FM stereo receiver comprising a switching circuit that outputs pseudo stereo signals S _L ′ and S _R ′ when the received electric field strength is equal to or lower than a threshold level E _H . 4 The received electric field strength is at the threshold level.
4. The FM stereo receiver according to claim 3, wherein the switching circuit outputs a monaural signal _SM when the threshold level E L is lower than a second threshold level E _L which is equal to or lower than E _H.