JPS6232644B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a frequency identification method for identifying a signal that has been amplitude modulated (hereinafter referred to as AM) on one of two nearby frequencies, such as a control signal in television audio multiplex broadcasting. It is related to the device.

The configuration of a conventional device of this type is shown in Figure 1. In the figure, 1 is an input terminal for an AM input signal modulated by one of two nearby frequencies, and 2 is connected to input terminal 1. Limiter 3 is a frequency divider connected to limiter 2. 4 is a frequency mixer to which the above AM input signal and the output signal from the frequency divider 2 are applied, and 5 is a frequency mixer for extracting the difference component between the output signal from the above frequency divider 2 and the modulation frequency of the AM input signal. 1 is a low-pass filter, 6 is a second low-pass filter, 7 is a phase comparator, and 8 is an output terminal.

Next, the operation of the above configuration will be explained.

Now, assuming that the carrier frequency of the AM input signal is c and the modulation frequency is ₁ or ₂ , the reference signal m is obtained by passing this AM input signal through the limiter 2 and the frequency divider 3. In this case, the reference signal m is expressed as follows.

m=c/N (however, N is the frequency division ratio of the frequency divider)
(1) Here, the AM input signal and the reference signal m are passed through the frequency mixer 4, and then only the difference component between the two signal frequencies is extracted by the first low-pass filter 5. Therefore, the signal after passing through the first low-pass filter 5 is | ₁ −m| or | ₂ −
The signal has a frequency component of m|.

This signal is passed through a second low-pass filter 6 having characteristics as shown in FIG. In other words, the cutoff frequency o is | ₁ −m| and ₂ −
Due to the filter 6 between m|, | ₁ −
A phase rotation is caused between m| and | ₂ -m|. Therefore, by applying the signals before and after the second low-pass filter 6 to the phase comparator 7,
It is possible to identify whether the modulation frequency is ₁ or ₂ .

Taking actual television audio multiplex broadcasting as an example, the carrier frequency c is 55.125 kHz, and the modulation frequency is 982.5 Hz ( ₁ ) for stereo broadcasting and 922.5 Hz ( ₂ ) for dual audio broadcasting. Here, the division ratio N is
61, the reference signal m is 903.7Hz. Therefore ₁ -m and ₂ -m are respectively
78.8Hz and 18.8Hz. (If the frequency division ratio N is 55, the reference signal m will be 1002.3Hz | ₁ −
m | ≒ 19.8 Hz, | ₂ − m | = 79.8 Hz, which is the reverse. ) Here, in the first low-pass filter 5, 100
Set to extract components below ~200Hz.
When the frequency division ratio N = 61, the thus obtained signal of 78.8Hz for stereo broadcasting and 18.8Hz for dual audio broadcasting is to be identified, and the cutoff frequency o of the second low-pass filter 6 is selected to be 40 to 50Hz. Therefore, sufficient identification is possible by passing it through the phase comparator 7. Since the second low-pass filter 6 requires only phase rotation, it is obvious that a similar effect can be obtained by using a high-pass filter.

However, in the conventional device, as shown in _FIG .
Not only does phase rotation occur on the m side, but also a drop in level occurs, so the S/N ratio deteriorates due to weak electric fields, etc.
There is a possibility that it will not be possible to clearly distinguish this from the AM input signal.

This invention was made in order to eliminate the above-mentioned conventional drawbacks, and by changing the frequency division ratio of the frequency divider, not only phase rotation is performed at the time of identification, but also
It is an object of the present invention to provide a frequency identification device that can suppress the level drop and eliminate the possibility of malfunction in a weak electric field.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 is a block diagram showing an example of the frequency identification device of the present invention, and the same parts as in FIG. 30 is the division ratio
The first frequency divider 300 that can be changed to N ₁ or N ₂ is a second frequency divider connected to the second low-pass filter 6 via the waveform shaping circuit 9; The frequency division ratio switching circuit 10 is operated by the output of the frequency divider 300 and the output of the phase comparator 7 to control the frequency division ratio of the first frequency divider 30. Reference numeral 11 denotes an output discrimination circuit which outputs an identification signal depending on the state of each output of the frequency division ratio switching circuit 10 and the phase comparator 7.

Next, the operation of the above configuration will be explained.

Let us assume that the carrier frequency of the AM input signal input to input terminal 1 is c, and the modulation frequency is ₁ or ₂ . The operation of each part from the input terminal 1 to the phase comparator 7 is the same as that in FIG. When is set to o, it is necessary to have the following relationship.

| ₁ −c/N ₁ |<o<| ₂ −c/N ₁ |
...(2) | ₂ -c/N ₂ |<o<| ₁ -c/N ₂ |
...(3) In other words, when the division ratio is N ₁ , the difference component of the signal at frequency ₁ is, and when the division ratio is N ₂ , the frequency
The difference component between _{the two} signals is passed through the second low-pass filter 6.
so that it falls within the passband of As a result, signals having the same phase and approximately the same level are input to the phase comparator 7. The frequency division ratio switching circuit 10 is latched in that state when an in-phase signal is input to the phase comparator 7. Furthermore, when a signal with a different phase, such as a reverse phase signal, is input to the phase comparator 7, the output frequency of the second low-pass filter 6 is used to generate a repetition frequency that is ₃ times the division ratio N of the second frequency divider 300. 1st
The frequency division ratio of the frequency divider 30 is switched to N ₁ or N ₂ . In other words, the repeated switching operation to the division ratio N ₁ or N ₂ is intermittent until a signal within the passband of the second low-pass filter 6 is obtained and the phase comparator 7 detects the same phase. What is important here is that since the second filter 6 is a low-pass filter, falling within the passband means that the repetition frequency becomes low. Therefore, in the case of an in-phase signal, the detection period of the phase comparator 7 becomes longer.

Taking the case of television audio multiplex broadcasting as an example, the situation is as follows.

The carrier frequency c is 55.125kHz, the modulation frequency ₁ during stereo broadcasting is 982.5Hz, and the modulation frequency ₂ during dual audio broadcasting is 922.5Hz. Here N ₁ = 55.
Assuming N ₂ = 61, the reference signal m is 1002.3Hz (m ₁ ) when the frequency division ratio is N ₁ , and 903.7Hz (m 1 ) when the frequency division ratio is N ₂ .
m ₂ ).

Therefore, the difference component during stereo broadcasting | ₁ −
m| becomes 19.8 Hz when the frequency division ratio is _N1 and becomes 78.8 Hz when the frequency division ratio is _N2 . Conversely, the difference component during dual audio broadcast |
₂ -m| is 79.8Hz when the frequency division ratio is N ₁ , and when the frequency division ratio is N ₂
When , it becomes 18.8Hz. Therefore, when the cutoff frequency o of the second low-pass filter 6 is set to 40 to 50 Hz, the passband of the second low-pass filter 6 is set to a frequency division ratio of N ₁ during stereo broadcasting and a frequency division ratio of N ₂ during dual audio broadcasting. Go inside.

Here, when the above-mentioned difference component exists within the pass band, the frequency division ratio switching circuit 10 latches, so the frequency division ratio is
Stereo broadcasting and dual audio broadcasting can be distinguished depending on whether it is N ₁ or N ₂ .

When the difference component is outside the passband, the frequency division ratio switching circuit 9 changes the frequency division ratio from _N1 to _N2 or from _N2 to _N1 . When the difference component frequency after this switching operation is within the passband, if the phase comparator 7 detects the same phase within a time period of N3 _times 20Hz, the frequency divider switching circuit 9 latches and maintains that state. . Also, if the difference component is not within the passband even after switching operation,
The division of N ₁ and N ₂ is repeated with a period of N ₃ . In other words, _N3 is the time required for latch and is determined by the responsiveness of the phase comparator 7.

FIG. 4 shows an embodiment of the invention. Here, the first frequency divider is a frequency divider 30a with a frequency division ratio of N ₁ and a frequency division ratio of
The frequency mixer 4 corresponds to one of the _N2 frequency dividers 30b, and the frequency mixer 4 corresponds to one of the frequency dividers 30a.
a and the other frequency mixer 4b, and the first low-pass filter is also connected to one of the frequency mixers 4a, and the other frequency mixer. 4b and a first low-pass filter 5b connected to the low-pass filter 5b. Either one of the outputs of the two series of first low-pass filters 5a, 5b is sent to the second low-pass filter 6 via the switching circuit 12 that receives the output of the frequency division ratio switching circuit 10.
and the phase comparator 7. While the above embodiment directly changes the frequency division ratio, this embodiment switches the output of signals having different frequency division ratios, and can achieve the same effects as the above embodiment.

As described above, the present invention has a simple configuration in which the frequency division ratio of the first frequency divider that divides the AM input signal is variably controlled from the output of the second frequency divider and the output from the phase comparator. This ensures that it always operates at an appropriate level,
Thus, it is possible to provide a frequency identification device that can prevent malfunctions in weak electric fields.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional frequency identification circuit.
Figure 2 is a characteristic diagram of the low-pass filter of the conventional device.
FIG. 3 is a block diagram showing an example of a frequency identification circuit according to the invention, and FIG. 4 is a block diagram of another embodiment of the invention. 4, 4a, 4b...frequency mixer, 5, 5a,
5b... First low pass filter, 6... Second low or high pass filter, 7... Phase comparator, 10... Frequency division switching circuit, 30, 30a, 30
b...first frequency divider, 300...second frequency divider.
In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A first frequency divider that divides the carrier wave in an amplitude modulated signal modulated by one of two frequencies close to each other, and a signal frequency-divided by the first frequency divider and the amplitude modulated signal. a first low-pass filter that extracts only the difference component between the frequency-divided signal frequency and the modulation frequency of the amplitude modulation signal of the output signal from the frequency mixer; a phase comparator that receives the output of the first low-pass filter and the output passed through the second low- or high-pass filter and identifies the phase difference between the two; and a phase comparator that separates the output of the second low- or high-pass filter. A second frequency divider that rotates the frequency, and a signal frequency-divided by the second frequency divider and the output of the phase ratio comparator are input to switch and control the frequency division ratio of the first frequency divider. A frequency identification device comprising a frequency division ratio switching circuit.