JPS60244158A - Digital demodulator of facsimile signal - Google Patents

Abstract

PURPOSE:To reproduce a received picture with high quality by using a COSTAS type digital phase locked loop to obtain a desired picture signal applied to the synchronizing detection and suppressing sufficiently the carrier wave without deteriorating the high frequency component of a demodulated picture signal. CONSTITUTION:The AM signal supplied through an A/D converter 11 of a demodulator is sampled by the sampling signal sent from a terminal 2 and converted into a complex form by a digital phase branching filter 12 after digital conversion. This complex signal is supplied to a digital synchronizing detector 13 containing a COSTAS type digital phase locked loop DPLL. A desired picture signal underwent the synchronizing detection is obtained at the read number part of the detector 13 and also outputted in the form of the carrier wave leakage in case a DC component is superposed on the input signal. Then the output of the detector 13 is supplied to a digital filter part 14 having desired characteristics to eliminate the power supply frequency component and the DC component. Thus, it is possible to output a reproduced picture signal with high quality to a data bus 10.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to an AM demodulation device that extracts an image signal from an AM-modulated analog facsimile signal.

(Prior art) Conventionally, this type of demodulation device performs AM demodulation using envelope detection, etc., and uses a low-pass filter to suppress leakage of the carrier signal, or uses a balanced modulator to convert the input AM signal into a balanced modulator. A method was used in which AM demodulation was performed after converting the frequency to a high frequency using a mixer such as the above and a band-pass type F-wave device.

FIG. 1 is a block diagram for demodulating a facsimile image signal from a conventional geostationary meteorological satellite. As shown in the spectrum of Fig. 2 Tal, the image signal received from the satellite and demodulated has a frequency of ±1 on the 2.4kHz subcarrier f1.
．． A signal including a 7 kHz frequency range is supplied from input terminal 1. This input signal is passed through buffer 3 to 15
．． It is mixed with the output of the 4 kHz oscillator 4 in the mixer 5, and a 17.8 kHz component is extracted by the bandpass p-wave generator 6 (component A in Fig. 2 (b)), which is square-law detected by the detector 7 and low-pass. The signal is demodulated into a baseband signal through an Oki waver 8. This signal is further converted into a digital signal by an A/D converter 9 and output to a path line 10 for data processing.

Since this facsimile image is a 16-gradation signal, the band-pass F wave filter 6 has an attenuation amount C of 30 dB or more outside the band, and a 15.4 k
A steep roll-off characteristic with attenuation of 20 dB or more at Hz is required.

This conventional method requires an F-wave generator with steep cutoff characteristics in order to sufficiently suppress carrier signal leakage, and as a result, ringing occurs in the demodulated image signal when the image signal level fluctuates sharply, causing echoes in the image. There were some shortcomings that appeared. If the high frequency components of the demodulated image signal are cut off in order to reduce the ringing of the demodulated image signal, there is a drawback that the resulting image has poor rise characteristics.

(“Objective of the Invention”) The object of the present invention is to eliminate these drawbacks, reduce ringing without damaging the high frequency components of the demodulated image signal, sufficiently suppress the carrier wave, and provide a high-quality received image without causing echo. An object of the present invention is to provide a digital demodulation device for facsimile signals that can reproduce facsimile signals.

(Structure of the Invention) The facsimile signal digital demodulation device of the present invention includes an A/p conversion unit that converts a demodulated R*hM facsimile signal into a digital signal, and a conversion output of the A/p conversion unit that converts the demodulated R*hM facsimile signal into a digital signal. A digital phase demultiplexer that separates the phase and outputs a real part and an imaginary part; a digital synchronous detector that frequency-shifts each output of the digital phase demultiplexer using a digital phase-locked loop and outputs it as an image signal; It is configured to include a digital filter section that removes power frequency components and DC components from the output of the synchronous detection section.

(Example) Next, the present invention will be explained in detail with reference to the drawings.

FIG. 3 is a block diagram of an embodiment of the present invention.
The demodulator is shown. In the figure, 1 is an input terminal, 10 is a data bus, and 11 is an analog/digital (A/
D) Converter, 12 is a digital 90° phase demultiplexer, 13
14 indicates a digital synchronous detection section, and 14 indicates a digital filter section. Further, FIG. 4 111) to td) are frequency spectrum diagrams illustrating the operation of FIG. 3.

In this embodiment, the spectrum of the AM signal input from the input terminal 1 has a maximum image signal frequency of 1.7 kHz (1.68 kHz) and a carrier wave 2.
This is a 4kHz AM modulated signal, and the analog-digital converter 11 converts the frequency 4fs (
For example, it is sampled with a sampling signal of 15.4k)Iz), converted to A/1), becomes a digital signal, and is input to the digital 90° phase demultiplexer 12, as shown in FIG. 4(b).
It is complexized as shown in . The complex signal is input to the digital synchronous detection section 13 and subjected to a frequency shift by a C08TAS DPLL (Costas type digital phase locked loop) to convert it to a baseband signal, that is, an image signal (FIG. 4(C)).

The frequency shift residue of this circuit is detected and suppressed as a phase error by the DPLL. In this way, a desired synchronously detected image signal is obtained in the real part of the C08TAS loop. However, if a DC component is superimposed on the input DSB-AM signal, this will ultimately be output as a 2.4 kHz carrier leak. Similarly, the leakage of 50Hz commercial power supply is 2.35kHz.

It is output as a 2.45kHz component. In order to prevent this, the C08TAS loop real part signal is
The signal is passed through a digital filter section 14 having characteristics as shown in FIG. The roll-off of this digital filter section (DLPF) may be gradual, and is realized by linear phase FIR. The output signal of this digital filter section 44 is connected to the system data bus 10 and subjected to data processing.

In this way, in this embodiment, the entire AM demodulation section can be digitalized, and this can be constructed using a one-chip signal processor μPD7720 (manufactured by NEC Corporation). The construction of each part using this signal processor is shown in FIGS. 5 to 7. In the figure, 31.36 is an adder, 32.34 is a multiplier, 33.37 is a unit delay element, and 35 is a sine-cosine (THCM) conversion table.

FIG. 5 is a circuit diagram of the digital phase demultiplexer 120. The signal from the input terminal 21 is sent to two adders 31 through coefficients (α1~
σ4. β□~β4) multiplier 32, two delay elements 33
A 7th-order phase demultiplexer is formed by the combination of
The output of is extracted.

FIG. 6 is a circuit diagram of the digital phase synchronization section 130. Input the real part ■ and imaginary part (I) from input terminals 24 and 25,
With four multipliers 32 and two adders 31,
(2) θ and (2) θ-R are formed, the former is output from the output terminal 26, and the latter is used for a complete quadratic PLL.

That is, by multiplying α and β, the transformation table 35
The above calculation is performed using .

FIG. 7 is a circuit diagram of the digital filter section 14. The signal from the input terminal 27 is sent to a 16-stage unit delay element 37 (z
-γ”), multipliers 32 of H1 to H□6, and adder 31
16th order phase linear FIR (Finite I
The signal is outputted from the output terminal 28 via a digital filter based on the NPULAE Re5ponse.

(Effects of the Invention) As explained above, according to the present invention, %AM demodulation can be performed without using a filter having steeply continuous characteristics, and high frequency components of the image signal can be blocked. It is possible to reduce ringing that occurs when there is a sharp level change in the image signal, and to obtain a high-quality image without echoes.

In addition, in this embodiment, it is also possible to configure the digital 90° phase demultiplexing section, digital synchronous AM detection section, and digital filter section with a single-chip arithmetic unit, and it is also possible to connect it to the system bus. Therefore, it has the advantage of being able to reduce costs compared to conventional circuits.

[Brief explanation of the drawing]

Fig. 1 is a diagram of a conventional AM demodulation section for facsimile image signals, Fig. 2 (1), (bl is a spectrum diagram and attenuation characteristic diagram in Fig. 1, and Fig. 3 is an embodiment of the present invention). Figures 4 (al to (d) are spectrum diagrams explaining the operation of Figure 3, Figures 5 to 7 are spectrum diagrams of Figure 3)
FIG. 2 is a block diagram of the digital phase demultiplexing section, digital synchronous detection section, and digital filter section shown in the figure. In the figure, 1
, 2... Input terminal, 3... Buffer,
4...--Oscillator, 5...Mixer, 6...
...Bandpass F wave detector, 7...Detector, 8...
...Low pass F wave device, 9.11...A/D
Converter, 1o... Data bus, 12...
・Digital phase demultiplexing section, 13...Digital synchronous detection section, 14...Digital filter section, 21,
24, 25.27... Input terminal, 22, 23°26, 28... Output terminal, 31.36...
... Adder, 3434 ... Multiplier, 33.3
7...delay element, 35...=Ih1ax conversion table. Rod 1 time ±7.7 Jft ￥-2fl Y3 top-Kaya 4@

Claims (1)

[Claims] an A/D converter that converts the demodulated AM facsimile signal into a digital signal; and a digital phase demultiplexer that digitally demultiplexes the conversion output of the A/D converter and outputs a real part and an imaginary part. a digital synchronous detection section that frequency-shifts each output of the digital phase demultiplexing section using a digital phase-locked loop and outputs it as an image signal; and a digital synchronous detection section that removes power frequency components and DC components from the output of this synchronous detection section. A digital demodulation device for facsimile signals, including a digital filter section.