JPS59154824A - Radio receiver - Google Patents

Abstract

PURPOSE:To reproduce a transmission picture signal of facsimile broadcast such as synoptic chart with fidelity and to improve the transmission speed by supplying an intermediate frequency signal to a PLL so as to obtain a frequency shift demodulating output. CONSTITUTION:When a broadcast signal frequency-converted into an IF band is applied to an IF band FS (frequency shift) demodulator 20, a phase comparator 20a compares the phase and frequency of the IF band FS signal input with those of an output of a VCO20c and an FS demodulating output is obtained from a low pass filter 20b. In this case, the delay time in the demodulated output of the PLL20 at a transient point of time is practically negligible even to the sampling period of a printer, the problem of delay of white/black signal changing point of a reproduced picture to the transmitted original picture is eliminated and a minute detailed part is reproduced with fidelity. Since the reference frequency of the PLL is increased, the circuit copes quickly with the change in white and black picture signals.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a radio receiver capable of receiving frequency-shifted signals such as facsimile broadcasts of weather maps.

BACKGROUND ART AND PROBLEMS Generally, as one of facsimile transmission methods, a frequency shift (hereinafter referred to as FS) method has been proposed in which the carrier wave frequency is changed in a rectangular manner depending on whether the image signal is black or white. This FS
This method is a special example of frequency modulation, and is effective in transmission lines with a lot of noise and level fluctuations, and is used in shortwave weather facsimile broadcasts and the like.

In weather facsimile broadcasting, a wireless carrier wave is single-sideband amplitude modulated using a signal subjected to FS in the voice band. That is, as shown in FIG. 1, the audio bandpass filter FS signal has a white level of the image signal at 2300 Hz and a black level at 1500 Hz.

Further, the transmission speed is expressed as the reciprocal of the time required to send a unit element of an image signal (hereinafter referred to as pixel transmission time) tp, and in this case, it is 500 baud. Therefore, ip=2ms.

Conventionally, K, which receives weather facsimile broadcasts, has used a shortwave radio receiver having a normal superheterodyne configuration as shown in FIG. That is, in Figure 2, (
1) indicates a receiving antenna, and a high frequency signal induced in this antenna (1) is selectively amplified by a high frequency amplifier (2) and added to a mixer (3).

(4) represents a local oscillator, the output of which is mixed with a high frequency signal in a mixer (3). The output of the mixer (3) is an intermediate frequency (hereinafter referred to as IP) signal, which is supplied to a detector (6) via an IF amplifier (5). (7) is a beat frequency oscillator, which is used when receiving AI radio waves (only the carrier wave is intermittent), which is widely used in short wave communication. The output of the detector (6) is the audio frequency amplifier (8)
The signal is then led to the monitor speaker terminal (9).

General shortwave communication is received in this step.

When the receiver shown in Fig. 2 is tuned to the weather facsimile IJ broadcast, the high frequency FS signal is transmitted to the IF@F via the same route as described above.
Converted to S signal and further audio frequency (hereinafter referred to as AF)
The signal becomes a band FS signal and is supplied to the AF band FS demodulator (11) via the AF amplifier (8).

The output of this FS demodulator sum is a black and white image signal, which is supplied from a demodulation output terminal (111) to a printer (not shown) to reproduce a weather map.

However, as shown in FIG. 1, the 2300 Hz white level corresponding signal and the 1500 Hz black level corresponding signal can be separated and demodulated using a normal band F wave generator or the like. However, at such frequencies, the size of the capacitor that constitutes the F-wave device becomes large, making it difficult to downsize the F-wave device and, by extension, to downsize the receiver.

Therefore, phase-locked loops (phase 1 locked), which have been miniaturized using semiconductor IC technology,
oop.

As shown in Fig. 2, the AF band FS
Radio receivers that are used as demodulators have already appeared.

This PLL includes a phase comparator (10a), a low-frequency F-wave unit (ta)
b) and a voltage controlled oscillator (hereinafter referred to as VCO) (10c)
), a frequency modulated signal can be input to the phase comparator (10a), and a demodulated output can be obtained from the low-frequency F wave generator (10b).

As is clear from Figure 2, the PLL is connected to the input signal and CO
Phase comparison (and frequency comparison) with the output is performed, but when a PLL is used to demodulate the AF band FS signal mentioned above, the 2300 Hz white level corresponding signal becomes 4.6 Hz within the 2 mS pixel transmission time. Cycle included, 15008!
The black level corresponding signal includes only three cycles. Usually, the free oscillation frequency of ■CO is chosen close to the input signal frequency, and the circuit employs a square wave oscillator, such as a multivibrator or a blocking oscillator. Therefore, ■For example, the rising edge of the CO output square wave is compared with the input signal, but the number of comparisons within the 2 mS pixel transmission time is three times for the black level corresponding signal, and for the white level corresponding signal, the input signal Depending on the phase relationship between the output and the VCO output, the number of times is 4 or 5. Generally, at the point in time when the frequency of the FS signal changes in a rectangular manner (hereinafter referred to as the transition point 5), the phase difference between the input signal and the vCO output ranges from 0 degrees to 360 degrees.
It is random and not constant within a range of degrees. Therefore, at the above-mentioned transient point, the rise of the output waveform of the PLL demodulator (for the signal corresponding to the white level) is a maximum of 435 μs-2/4.6
mS delay, falling edge (for black level compatible signal) maximum 667
This results in a delay of μs=2/3ms. Moreover, the delay time td at each transition point is I-random and not constant.

Current weather facsimile broadcasting printers sample the output signal of the FS demodulator at a rate of about 600 μs/point and print it on recording paper. Since the above-mentioned delay time td is comparable to the sampling period of the printer, the printer may notice that the first sampling point immediately after the transition point is, for example, black when it should be black, but it is just before the transition point, for example, it is white. In addition, there is a delay in the point at which the reproduced image becomes black-and-white compared to the transmitted original image. Moreover, this delay occurs randomly at each change point and is difficult to correct, making it impossible to faithfully reproduce particularly fine parts of the original image.

Further, in AF band PLL demodulation, since the reference frequency of the PLL is low, there is also a drawback that it is not possible to quickly follow changes in black and white of the image signal.

Purpose of the Invention In view of these points, it is an object of the present invention to make it possible to faithfully reproduce a sending image signal when demodulating an FS signal, and to quickly follow changes in black and white of an image signal. be.

SUMMARY OF THE INVENTION The present invention provides a radio receiver with a super heterogeneous four-dyne configuration, in which an IP multiplied signal is supplied to the PLL to obtain an FS demodulated output, and the transmitted image signal can be reproduced faithfully and at high speed. It is designed to be fully compatible with transmission.

Embodiment Hereinafter, one embodiment of the radio receiver of the present invention will be described with reference to FIG. In FIG. 3, parts corresponding to those in FIG. 2 are designated by the same reference numerals, and redundant explanation thereof will be omitted.

In Fig. 3, ■ is an IF band FS repeater;
It is fed with the output signal of the amplifier (5). In this case, the IF band FS demodulator (20) has a PLL configuration similar to that illustrated in FIG.
20b) and (20c) are (10a) and (10b) respectively
and (10c). Also, at this time, V CO'
(20c) free oscillation frequency is selected for the IF band, etc.
Required changes have been made. The rest of the structure is the same as that shown in FIG.

Since this example is configured as described above, when the facsimile broadcast signal frequency-converted to the IP band is applied to the IF band FS demodulator (A), the phase comparator (20a)
), the phase and frequency of the IF band FS signal input and the output of the V CO (20c) are compared, and the low frequency f-wave generator (20c) is compared.
The FS demodulation output can be obtained from b).

In this case, since the IF band FS @ is around 455 kHz, approximately 9
10 cycles are also included. Therefore, the PLL input signal and V CO (20
Even if the phase difference with the output of (c) is nearly 360°, the delay time td of the PLL demodulated output at the transition point is only 2.2 μS/9111 ms K at most.

Even for a printer's sampling period of about 600 μs, the delay time is virtually negligible.

Therefore, the problem of the delay in the black-and-white transition point of the reproduced image with respect to the transmitted original image, as in the case of the above-mentioned AF band PLL demodulation, is solved, and by using the receiver of this embodiment, it is possible to faithfully reproduce especially fine parts.

Furthermore, in the IF/1 PLL demodulation, since the base single frequency of the PLL is high, it is possible to quickly follow changes in black and white of the image signal.

The above-described IF band FS demodulator can be used not only for weather facsimile broadcasts but also for wireless facsimiles transmitted at higher speeds.

Effects of the Invention As detailed above, according to the present invention,
Since demodulation is performed using the LL, it is possible to faithfully reproduce the transmitted image signal of facsimile broadcasting, and the remarkable effects of being able to sufficiently cope with high-speed transmission can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining an FS signal, FIG. 2 is a block diagram showing an example of a conventional radio receiver, and FIG. 3 is a block diagram showing an embodiment of the radio receiver of the present invention. (5) is an IF amplifier, (6) is a detector, (8) is an AP amplifier, aα and ■ are FS demodulators, respectively, (tOa) and (20a) are phase comparators, respectively, (10c) and (20C) are each voltage controlled oscillators.

Claims (1)

[Claims] A radio receiver having a superheterodyne configuration, characterized in that an intermediate frequency signal is supplied to a PLL to obtain a frequency shift demodulated output.