JPH0441640Y2 - - Google Patents

Description

[Detailed description of the invention] [Field of industrial application] The invention relates to an FM receiver for audio broadcasting, and in particular
The present invention relates to an FM receiver having a noise removal circuit that removes components near a zero cross level according to the electric field strength of an FM received signal to reduce noise and enable distortion-free demodulation.

[Prior Art] When receiving FM broadcast waves, in places where the electric field strength is low, noise increases in the FM demodulated signal, which deteriorates the S/N ratio and makes it difficult to listen to the signal. To prevent this, so-called
Several methods have been proposed to improve the signal-to-noise ratio. For example, one method is to provide the output of the de-emphasis circuit shown in Fig. 2b with a circuit that removes components in the amplitude range of ±△/2 near the zero level of the received signal, as shown in Fig. 6a. , the signal distribution shown in FIG. 6b is obtained. In this method, low-level noise N is removed from the signal S,
It is known that the signal-to-noise ratio is improved.

[Problem to be solved by the invention] However, in this method, the most important zero cross component in speech recognition, that is, -△/2 ~
The disadvantage is that components in the amplitude range of Δ/2 are removed, making it difficult to hear the received audio signal.

The purpose of this invention is to provide a noise reduction circuit in the demodulation circuit of an FM broadcast wave receiver, and to improve the SN ratio of the overall received signal without impairing the zero cross section, which is essential for recognizing the received voice. Our goal is to provide an FM receiver that allows you to:

[Means for solving the problem] In order to achieve the above purpose, the FM according to the present invention
The receiver includes an FM detection means for receiving a transmitted signal with high frequency components emphasized by a pre-emphasis means on the transmitting side and performing FM detection on the received signal to obtain a detected signal; an electric field intensity detection means for detecting the intensity and outputting a detection signal according to the intensity; and an electric field intensity detection means for inputting the detection signal and removing a component near the substantially zero cross level of the detection signal based on the detection signal to generate a noise reduction signal. A noise reduction means for outputting the signal; and a de-emphasis means for reducing the high frequency component of the noise reduction signal by the amount emphasized by the high frequency component of the received signal by the pre-emphasis means and returning it to the original state, and giving a phase difference and outputting the high frequency component. The present invention is characterized in that after removing components near the zero cross level according to the electric field strength, a noise reduction signal is obtained from the de-emphasis means to remove distortion of the near components.

In the above-mentioned FM receiver, in particular, the noise reduction means comprises a polygonal characteristic element whose input/output characteristics are approximately polygonal, and the polygonal linear operating point of the element is made dependent on the electric field strength based on the detection signal. The control may be configured to remove components near the substantially zero cross level.

[Operation] A component near the substantially zero cross level of the FM detection signal is removed according to the received electric field strength, and a noise reduction signal is output. The high frequency component of this noise reduction signal is reduced by the de-emphasis means by the amount emphasized by the pre-emphasis means on the transmission side and restored to its original state, and is outputted with a phase difference.

Distortion occurs in the vicinity of the zero cross due to the removal of the neighboring components, but since the phase is shifted by de-emphasis as described above, the zero cross component is preserved and its distortion position can be removed from the vicinity of the zero cross.

[Example] The present invention will be further explained below with reference to the drawings.

As is well known, in FM broadcast transmitters, an emphasis circuit is installed on the transmitter side to emphasize the high frequency components of the signal S, in order to suppress the influence of noise distributed in proportion to the frequency (triangular noise) that occurs in FM reception. A de-emphasis circuit is provided on the receiver side to reduce high frequency components.

FIG. 2a shows an example of the pre-emphasis circuit, and FIG. 2b shows an example of the de-emphasis circuit. Figure 2a
, R>R ₀ . When the circuit shown in FIG. 2b is used for de-emphasis, the relationship between the input e _i and the output e _p becomes equation (1).

e _p = 1/1 + jωCRe _i ...(1) If the relative values of e _p and e _i are illustrated as vectors, the third
The output e _p is much larger than the input e _i in phase θ=tan ^-1
It is rotating only by ωCR.

It is assumed that a signal of signal component S+noise component N is added to the input of such de-emphasis.
This is shown in Figure 4a. When the ±Δ/2 components close to the zero level are removed before the de-emphasis and then added to the de-emphasis circuit, the signal S undergoes phase rotation θ as described above and becomes as shown in FIG. 4b. The solid line waveform in Figure 4b is ± from the waveform in Figure 4a.
The Δ/2 component is removed, and the broken line waveform is the de-emphasis output. The above operation corresponds to the removal of the zero cross component ±Δ/2, but since the phase is rotated by the de-emphasis circuit 7, the zero cross component of the audio signal input to the FM transmitter is preserved. In other words, on the transmitting side, the audio signal is phase-rotated by the pre-emphasis circuit shown in Figure 2a,
On the receiving side, components near zero level are removed while the phase is rotated, and the de-emphasis circuit returns the phase to its original state.

FIG. 1 is a block diagram showing _the configuration of an embodiment of a noise reduction circuit based on _the operating principle _of the present invention described above. ₁ is a broken line characteristic element 2, 3, △
The control unit 45 and power supplies E ₁ and E ₂ are composed of an amplifier 6.
7 is a de-emphasis circuit consisting of R ₇ and R ₇ ; 8
9 represents its output, and 9 represents an electric field strength detection circuit.

The broken line characteristic elements 2 and 3 which apply the FM detection output 1 to the broken line characteristic elements 2 and 3 are set to △ near the 0 level by controlling the operating point of the broken line characteristic elements by the △ controllers 4 and 5.
Remove the amplitude region of . In addition, the above C ₁ , R ₁ , C ₂ ,
C ₃ is used to prevent the DC operation of the elements 2 and 3 from being influenced by the operating point of the amplifier 6 or the detection output of the terminal 1. FIG. 5 is an explanatory diagram of Δ control, and FIG. 5a shows a state where Δ is increased, which is actually a case where the received electric field is weak and the noise level is high. The bias is controlled by the Δ control units 4 and 5 so that no output is produced unless the amplitude applied to the polygonal characteristic elements 2 and 3 is large. b is a case where Δ is set to a small level, and c is a state where Δ=0, that is, the nonlinear characteristic does not operate and all the FM detection output is transmitted to the amplifier 6.

The outputs of the linear characteristic elements 2 and 3 are summed and amplified by an amplifier 6, and then sent to a de-emphasis circuit 7 as an output 8.

Incidentally, when the received electric field is strong, it is also possible to manually bring the polygonal characteristic elements 2 and 3 into the conductive state, that is, into the state shown in FIG. 5c. In addition, the output of the electric field strength detection circuit 9 shown in Fig. 1 automatically controls the △ control units 4 and 5, and when the received electric field is weak, the range of △ is increased as shown in Fig. 5 a, and the received electric field If Δ is strong, it may be automatically controlled to make Δ zero, as shown in FIG. 5c.

Note that FIG. 1 shows an example applied to monaural FM reception. In stereo reception, after FM detection, stereo demodulation is performed to generate left ear signal E _L and right ear signal E _R.
and then the de-emphasis circuit connects E _R and E _L
Separately provided for. In the case of stereo, to apply the present invention, a noise reduction circuit may be provided before each de-emphasis circuit for E _R and E _L.

According to the circuit shown in Figure 1, even if the noise is reduced by removing the component near the zero cross, the phase is rotated by de-emphasis, so the zero cross component is preserved, and moreover, as shown below, the zero cross component is Distortion is also removed.

That is, in FIG. 4b, the output waveform from which components near zero crosses have been removed by the noise reduction circuit is as follows:
As shown by the solid line, distortion is expressed in the vicinity of the zero cross a. Therefore, if this signal is listened to as it is, it will interfere with the recognition of the received voice and become difficult to listen to.

Therefore, in the embodiment of the present invention, FIGS.
As shown in the figure, the integral operation of the de-emphasis circuit 7 is further utilized to shift the phase θ as shown by the broken line to remove distortion in the vicinity of the zero cross a.

In the de-emphasis circuit 7, due to the integration operation, the distortion portion a near the zero cross of the solid line waveform has a lower frequency component than the portion other than the vicinity of the zero cross, so that the phase shift θ changes. As a result, distortion is eliminated in the vicinity of the zero cross as indicated by the broken line waveform b.

However, as shown in FIG. 4b, the position of the distortion a
At approximately the same time position as , a broken line waveform c is obtained which has been integrated according to the change in the distorted portion.

As described above, there is no distortion at all in the vicinity of the zero cross, which has components important for speech recognition, such as the broken line waveform, and even if distortion occurs in areas other than the vicinity of the zero cross, speech recognition is achieved and it is not noticeable to the auditory sense. It can be eliminated.

[Effects of the invention] As explained above, noise increases in the audio output due to weak electric field reception, but according to the invention, by removing components near the zero cross level just before de-emphasis, no audio signals and In addition to removing the noise in the near state, the de-emphasis circuit then rotates the phase to eliminate distortion and improve the overall SN ratio without damaging the components near the zero cross level of the audio signal, which is important for speech recognition.
This has the advantage of making it easier to hear.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a noise reduction circuit according to the present invention, FIG. 2 is a circuit diagram showing an example of a de-emphasis-de-emphasis circuit, and FIG. 3 is a diagram illustrating phase rotation by the de-emphasis circuit of FIG. FIG. 4 is a waveform diagram when ±Δ/2 is removed, FIG. 5 is a diagram illustrating the operation of the Δ control section, and FIG. 6 is a waveform diagram illustrating the conventional noise component removal method. 1...FM detection output, 2, 3...broken line characteristic element, 4, 5...△control section, 6...amplifier, 7...
De-emphasis circuit, 8...output, 9...electric field strength detection circuit.

Claims (1)

[Claims for Utility Model Registration] (1) FM detection means for receiving a transmitted signal with high frequency components emphasized by pre-emphasis means on the transmitting side and performing FM detection on the received signal to obtain a detected signal. , field strength detection means for detecting received field strength from the received signal and outputting a detection signal according to the strength; inputting the detected signal and detecting a component near the substantially zero cross level of the detected signal based on the detected signal; noise reduction means for removing the noise reduction signal and outputting a noise reduction signal; and reducing the high frequency component of the noise reduction signal by the amount that the high frequency component of the received signal was emphasized by the pre-emphasis means and returning it to the original state, giving a phase difference and outputting it. and a de-emphasis means for removing the component near the zero cross level according to the electric field strength, and then obtaining a noise reduction signal from the de-emphasis means to remove distortion of the near component. Machine. (2) The noise reduction means is composed of a polygonal characteristic element whose input/output characteristics are approximately polygonal, and based on the detection signal, the polygonal linear operating point of the element is adjusted to the approximately zero cross point depending on the electric field strength. 2. The FM receiver according to claim 1, wherein the FM receiver is controlled to remove components near the level.