JPS6322098B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency modulation signal transmission and reception system.

The purpose of this is to detect the distortion that occurs when an FM modulated signal passes through a bandpass filter, and compress the maximum frequency deviation to obtain high-quality reproduced sound with little distortion. The purpose is to provide a signal transmission method.

A system for transmitting and receiving a multi-channel program source after FM modulation is configured as shown in FIG. 1, for example. In the system shown in FIG. 1, program sources applied to input terminals 11a to 11c are modulated by carriers of different frequencies by modulators 13a to 13c, mixed by a mixer 15, and transmitted. On the receiving side, the FM modulated signal that has passed through the distributor 16 is demodulated by demodulators 14a to 14c,
The program source is reproduced from the output terminals 12a to 12c.

Band-pass filters having center frequencies a to c as shown in FIG. 2 are inserted at the output sides of the modulators 13a to 13c and the input sides of the demodulators 14a to 14c shown in FIG. 1, respectively. There is.

Bandpass filters (hereinafter referred to as BPFs) inserted on the output side of the modulators 13a to 13c are intended to prevent interference from occurring due to the influence on the sideband of each carrier. ~
The BPF inserted at the input side of 14c prevents carriers and sidebands of other frequencies from being mixed into each demodulator.

In the modulator shown in Figure 1, when the frequency shift is large, the widened sideband
It is larger than the passband of BPF shown in the figure,
Generation of amplitude modulation components or loss of sidebands occurs. When such an FM modulated signal is demodulated by a demodulator, the audio signals obtained at the output terminals 12a to 12c will generate distortion and shock noise, degrading the sound quality.

The present invention eliminates the drawbacks of the conventional system, and its configuration is shown in FIG.

In Fig. 3, A is the transmitting side and B is the receiving side. 5, a mixer 7. Also, on the receiving side B, the distributor 8, BP
F4', FM demodulator 9, expander 10, BEF6',
It consists of an AM detector 5' and an audio signal output terminal 11.

BPF4, 4' and BEF6 used in Figure 3,
The characteristics of 6' are as shown in FIG. That is,
The center frequency ₀ is the same, but the bandwidth is ( ₁ − ₂ ) for BPF, while BEF is ( ₁ ′− ₂ ′
)
It is narrower than BPF.

Next, the operation of the configuration shown in FIG. 3 will be explained with reference to FIG. Now, as shown in Figure 5a, a signal whose amplitude up to t ₁ is V ₂ and whose amplitude between t ₁ and t ₂ is V ₁ is,
When applied to audio signal input terminal 1, the audio signal passes through compressor 2 to FM modulator 3.
Modulated by The signal modulated by the FM modulator 3 is applied to a BEF 6 and a mixer 7 through a BPF 4. As shown in Figure 4, the frequency change range of this added signal is ₁ to ₂ , and ₁ ' to 2.
The range is wider than ₂ ′.

That is, if the FM modulation output frequency becomes lower when the amplitude of the input signal a becomes (-), then when the input signal a becomes more than a certain level (-),
The frequency of the FM modulation output becomes lower than ₁ , but since the variation range is ₁ to ₂ as described above, frequencies lower than ₁ are not FM modulated. Conversely, when the amplitude of the input signal is on the (+) side, the FM modulation output frequency becomes high, so when the input signal a becomes above a certain level (+), the frequency of the FM modulation output becomes higher than ₂ . However, since the variation range is ₁ to ₂ , frequencies higher than ₂ are not FM-modulated, so the signal shown in FIG. 5b is output from the BPF 4. B.
The output signal of P.F4 is input to BEF6,
Now, the range in which the input signal a exceeds the change range ₁ to ₂ is detected.

Here, if the BPF 4 is an ideal filter, all signals exceeding the change range ₁ to ₂ are cut off, so that the output of the BPF 4 does not require signals containing frequency components of ₁ or less and ₂ or more.

However, in a BPF that can actually be used, there is a frequency difference between the filter cutoff frequency (-3 dB) and the cutoff frequency due to limitations such as the number of filter stages.

That is, between the cutoff frequency and the shearing frequency, the shearing characteristic has a constant slope with respect to frequency. Therefore, frequency components exceeding the change range ₁ to ₂ will also appear at the output terminal of the BPF 6.

Now, if compressor 2 were to operate instantaneously, the output of BPF4 should be the output of the waveform shown in Figure 5e, but in reality, a time delay occurs mainly in BPF4 and BPF6, so compressor 2 The operation generates a time delay with respect to the input signal a, and an output as shown in FIG. 5b is obtained at the output terminal of the BPF 4.

Now, when the amplitude of input signal a is on the (-) side, the output amplitude of BEF6 becomes large at frequencies below ₁ ' as shown in Figure 4, and when the amplitude of input signal a is on the (+) side, , the output amplitude of BEF6 becomes large at frequencies of ₂ ' or more as shown in FIG. Therefore, an AM modulated signal as shown in FIG. 5c is obtained at the output terminal of the BEF 6.

In Figure 5c, the envelope frequency is twice as high as in Figure 5b, but this is because the input to BEF6 is already an FM signal whose center frequency is ₀ as shown in Figure 4. However, frequencies exceeding ₁ ′ and ₂ ′
When signals of ₁ ″, ₂ ″ ( ₁ ″＜ ₁ ′, ₂ ″＞ ₂ ′) are added,
The output of BEF6 at this time is shown in Figure 4, BEF
The level (E ₁ ) is greater than the maximum level possible between ₁ and ₂ of 6. This level (E ₁ ) is
This occurs both in the case of ₁ ″ and in the case of ₂ ″. Therefore, the voltage generated corresponding to ₁ '' or ₂ '' becomes a signal with twice the envelope by passing through the so-called tank circuit in the BEF6.

The output signal of the BEF 6 obtained as described above is input to the AM detector 5. The AM detector 5 detects the AM modulated signal to obtain a signal as shown in FIG. 5d, which is input to the compressor 2.

The compressor 2 _is configured as _shown in _{FIG .} When applied to control terminal 2-1 of , the (+) side of the input signal applied to the IN terminal,
The (-) side is compressed by signals with frequency components outside the range of ₁ to ₂ , respectively.

That is, the amplitude of input signal a is compressed.

Therefore, from the OUT terminal, ₁ to ₂ during FM modulation.
A compressed signal is obtained such that only signal components within the range of .

Therefore, an FM signal as shown in FIG. 5e is applied to the mixer 7 and sent to the receiving side.

On the receiving side, the FM signal sent from the transmitting side is input to the distributor 8, selected by the BPF 4', demodulated by the FM demodulator 9, and an audio signal is extracted and applied to the expander 10.

On the other hand, the output signal of the BPF 4 is input to the FM demodulator 9 as well as the BEF 6'. The configurations of BEF6' and AM detector 5' are the same as those on the transmitting side, and when a signal as shown in Figure 5e is applied to BEF6', the signal shown in Figure 5c is the same as that on the transmitting side. The signal shown is obtained.

When the signal shown in FIG. 5c is obtained at the output of the BEF 6', the signal shown in FIG. 5d is obtained through the AM detector 5', and the signal shown in FIG. 5d is applied to the expander 10. In the expander 10, 1 in Fig. 6b
By adding it to the 0-1 terminal, the amplitude of the FM demodulated audio signal is expanded, and the input terminal 1 is
You get the same audio signal that was added to the .

The FM signal transmission system of the present invention has the above configuration, and according to the present invention, the following effects can be obtained.

(1) Since the control signal uses a signal synchronized with the audio signal, only the peak part of the overdeviation can be compressed and expanded instantly.

(2) By performing compression and expansion, it is possible to reduce noise in the transmission system when the signal level is small.
Further, this compression and expansion operation can be performed over the entire frequency band of the audio signal.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional FM signal transmission system, Figure 2 is a characteristic diagram of a bandpass filter used in the same system, and Figure 3 is an FM signal transmission system according to an embodiment of the present invention.
A block diagram of the signal transmission system, FIG. 4 is a characteristic diagram of a band-elimination filter and a band-pass filter used in the same system, and FIGS.
6a and 6b are electrical circuit diagrams of a compressor and an expander, respectively, used in the system of the present invention. 1...Input terminal, 2...Compressor, 3...FM modulator, 4, 4'...Band pass filter, 5, 5'...
...AM detector, 6,6'...Band elimination filter,
7... Mixer, 8... Distributor, 9... FM demodulator, 10... Expander, 11... Output terminal.

Claims (1)

[Claims] 1 a compressor that compresses the amplitude of an audio signal;
an FM modulator that performs FM modulation on the output of the compressor;
The first step is to control the frequency band of the FM modulated signal.
a bandpass filter having a bandwidth narrower than the bandwidth of the first bandpass filter;
a first band-removal filter that removes a predetermined band of the output signal of the band-pass filter; and a first band-removal filter that converts the output of the first band-removal filter into a control signal.
an AM detector, and a transmitter configured to operate the compressor with the control signal to compress the amplitude of the audio signal output to the FM modulator; a second bandpass filter having the same configuration as the first bandpass filter, which limits the frequency band of the signal; and demodulating the output signal of the second bandpass filter.
an FM demodulator, an expander that expands the FM demodulated signal, and a first band remover that has a narrower bandwidth than the second bandpass filter and removes a predetermined band of the FM signal. a second band-removal filter having the same configuration as the filter; and a second band-removal filter that converts the output of the second band-removal filter into an expansion control signal.
An FM signal transmission system comprising: an AM detector; and a receiver that operates the expander according to the expansion control signal to expand the amplitude of an audio signal after FM demodulation.