KR820001532B1 - Circuitry for a facsimile reciever for regeneration of carried for an amplitude modulated suppressed carrier signal - Google Patents

Abstract

The circuit includes a phase comparator(10) for switching device(20) a low-pass filter(30), a voltage controlled oscillator(VCO)(40), a limiter(50) and a divider(60). The comparator, when connected to the low-pass filter by the switching device, the VCO and a feedback connection(70) from an output of the VCO to an input of the comparator provide a phase lock loop(PLL) circuit. The switching device(20) provides a sample data mode of operation for the PLL.

Description

A facsimile receiver circuit for amplitude modulation suppressed carrier signal reproduction

1 is a schematic diagram of a circuit for practicing the present invention.

FIG. 2 is a schematic diagram of a partially modified circuit of FIG.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for reproducing a carrier signal from a signal including a suppressed carrier signal amplitude modulated on an unmodulated carrier having a constant period in a facsimile receiver, and more particularly, to a circuit for reproducing a carrier signal using a phase locked loop. will be.

Conventional facsimile receivers use an amplitude modulated residual side band (AMVSB) system, which signals are used to synchronously demodulate the amplitude modulated signal, including the net carrier signal used at the receiver for reproduction.

Such net carrier signals do not contain useful information data. It is desirable that the net carrier be suppressed because commercial telephone lines used to interconnect the facsimile devices limit the overall power level of the signal that can be transmitted. Suppressed carrier systems provide a greater signal to noise ratio (SNB) than non-suppressed carrier systems. However, the removal of the net carrier requires a suitable method of reproducing the carrier signal to synchronously demodulate the AMVSB suppressed carrier signal received at the facsimile receiver.

The standard is adopted to ensure compatibility with other manufacturers of facsimile devices when operating in an amplitude modulated mode that includes an AMVSB suppressed carrier system. The standard provides for an initial synchronization time period (approximately 6 seconds) during which a full carrier is transmitted, during which the full carrier is periodically interrupted to remove the carrier for a short phase period (10 m seconds) once per scan line.

The initiation of this short interruption is controlled in the drum type facsimile by the phase or "start" position of the delivery drum. This short time ruler is a phase period and can be used by the receiving facsimile device to synchronize the position of the drum and compare the device with the delivery drum. Once synchronization has been established, delivery begins scanning the document and sending the scanned information through the AMVSB suppression and return system. In such a system, unmodulated carriers appear during short-term phase periods, which continue to appear periodically during the scan of the white region portion of the document and during the delivery process.

The carrier signal required for the receiver for demodulation of the AMVSB suppressed carrier signal is obtained from a full waveform that rectifies the received signal and supplies a double frequency signal synthesized in a phase locked loop (PLL). While this technique can be performed in practice, there is no guarantee that a carrier always exists during document transmission, so there can be no carrier in the black copy. Each phase period has been known to include a full carrier but in practice appears only on each scan line for a short time. Known PLL systems will drift and jitter when the signal is fixed. Also, any part of the carrier provided in the received signal according to the back area of the document being scanned is not sufficiently serialized while providing a basis for reproduction of the carrier without the above-mentioned problems.

The present invention is for reproducing a carrier from an amplitude modulated suppressed carrier signal such as an AMVSB suppressed carrier signal used in a facsimile system while avoiding phase jitter or drift problems that are eliminated when using the prior art PLL system as described above.

The present invention relates to a circuit of a facsimile receiver when operated in an amplitude modulated suppressed carrier mode and includes a PLL to which a full-wave rectified receive signal is applied. The PLL includes a phase comparator to receive the rectified signal, a lowpass filter, and a voltage adjust oscillator (VCO). The VCO supplies a comparator with a square wave signal as the second input and a triangular wave signal that provides the necessary 90 ° phase shift function. The divider has a carrier frequency and contains a circuit that responds to a triangular wave signal after limiting the signal supply whose wave is 180 ° out of phase with the original phase. The circuit includes a switching device coupled between a low pass filter and a comparator arranged to limit the time when the circuit operates as a Peruuf (PLL).

The loop can be used for the modulation of a carrier modulated during the emergence of an unmodulated carrier while the information signal is being received or a time slot is provided each time the drum or comparator of the facsimile receiver reaches its "start" or phase position. It is closed in response to the appearance. In addition, the operation of the filter is automatically modified to provide a larger frequency band for the PLL so that the output from the comparator indicates that the PLL frequency is different from the desired fixed frequency.

While the stability or loop is of no importance to the response, the filter operates when a periodic carrier appears while the information signal is received for untuning. In this case, the switching device operates to apply the output of the comparator to the filter so that any large error response indicated by the output of the phase comparator is not effective, but it compensates for any leakage current present in the filter and keeps any small error constant. Genie responds for a long time. Small modifications to the switching device and filter are necessary to reduce this phenomenon.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

1 illustrates a schematic diagram of a circuit according to the invention consisting of a phase comparator 10, a switch device 20, a low pass filter 30, a VCO 40, a limiter 50 and a divider 60. will be. When the comparator 10 is connected to the low pass filter 30 and the VCO 40 through the switch device 20, the output of the VCO 40 is connected to the input terminal of the phase comparator 10 through the feedback connecting line 70. An applied phase locked loop is provided. The switching device 20 provides an operating mode of sample data for the phase locked loop. A bandwidth correction circuit 80 is connected to both terminals of the resistor 31 of the low pass filter 30.

The basic phase locked loop includes a phase comparator, a low pass filter, and a VCO, the output of which is fed back to the input terminal of the comparator 10.

The principle of operation of this phase locked loop of FIG. 1 is as follows.

When the signal does not appear at the input terminal 11 of the comparator 10, the voltage applied to the VCO 40 in the low pass filter 30 is proportional to the free running bias voltage for the VCO 40. To zero.

VCO 40 is adjusted to operate at a “free running” frequency in accordance with the provided free running bias voltage. When an input signal is applied to the input terminal 11 of the comparator 10, the comparator 10 is connected to the output terminal 12 of the comparator 10 according to the phase and frequency difference from the feedback connection line at the VCO 40 to the error voltage ( error voltage). This error signal is filtered by the low pass filter 30 and applied to the control input terminal 45 of the VCO 40 to thereby reduce the frequency difference between the signal of the VCO 40 and the input signal of the comparator 40. The frequency of 40 is changed.

If the frequencies of the two signals are approximately equal, the feedback of the PLL causes the VCO 40 to be synchronized or "locked" to the incoming signal. Once fixed, the frequency of the VCO 40 limits the frequency of the VCO 40 to produce the output from the low pass filter 30 to shift the frequency of the input signal of the comparator 10 at the free running frequency. Same as the input signal except for the phase difference. The system's own locking function allows the PLL to track the frequency of change in the input signal once the PLL is locked.

As described above, if the output of the VCO 40 is relatively independent of the phase jitter and the drift, the PLL operates only when the input signal continues to appear in the comparator 10.

The modulation of the facsimile system is the amplitude modulation of the residual sideband filter with the coded data, which has no net DC and the net carrier signal does not appear in the modulated signal. Used for a short time for each line scan.

For example, the reference for one transmission rate is to send an unmodulated carrier for about 8 msec every 166 msec. If the facsimile signal is a rectified first field wave before being applied to the comparator 10, the basic PLL will be used to reproduce the carrier at the receiver.

As described above, this is done, but phase jitter and drifting appear at the output of the VCO because the loop must be corrected when no unmodulated carrier is received.

This problem is eliminated by using the switching device 20, where the circuit of FIG. 1 is non-modulated while a time slot is provided in response to the drum of the facsimile receiver reaching "starting" or reaching a phase position. This switching device 20 limits the time when it is connected to function as a Peruvian PLL by selectively connecting the filter 30 to operate according to the appearance of the carrier. Unmodulated carriers appear in the information signal when the back regions of a document are scanned at the facsimile transmitter.

The switching device 20 includes an amplitude detector 21 connected as shown at the input terminal 11 of the comparator 10 to respond to the period of the unmodulated carrier of the received signals. The amplitude detector 21 may also be connected to respond to a received signal before rectifying.

The switching device 20 comprises a first switch 22 connected to the output terminal of the amplitude detector 21 and to the output terminal of the comparator 10 for being controlled by the output of the amplitude detector 21. The first switch 22 is schematically shown to show only a function, which uses an electronic switch of a field effect transistor (F.E.T.).

The full width detector 21 generates a control signal for operating the switch 22 in response to the appearance of the unmodulated carrier period among the received signals, and outputs a signal at the output terminal of the comparator 10 to the first input of the filter 30. It is applied to the terminal 33 and the second switch 23 of the switch 20.

The switch device 20 consisting of a switch 22 and a second switch 23 which is identical to the switch 22 includes a phase pulse generator 24 connected to control the operation of the switch 23. The generator 24 provides a signal in response to a position signal provided to the receiver at each time the phase or "start" position of the fax receiver drum is detected. These position signals are used when the transmitter and receiver drums are initially synchronized and continue to occur at the receiver following synchronization of the transmitter and receiver. The signal from generator 24 occurs for a short time during which an unmodulated carrier is sent by the fax receiver for each line. Detection of the drum position can be performed according to a known method.

The signal from the generator 24 drives the switch 23 to apply the output of the switch 22 to the second input terminal 32 of the low pass filter 30.

The switch device 20 configured as described above has a line when the received AMVSB signal contains a period of an unmodulated carrier, i.e., when the facsimile transmitter scans the back region of the document for at least the above period and the phase pulse period is each line. When provided during the scan, the thrash signal from the comparator 10 has a function of providing the input terminal 33 and the switch 23 of the filter 30. The switch device 20 also functions to connect the output terminal of the comparator 10 and the input terminal 32 of the filter 30 when the timeslot provided by the phase pulse generator 24 operates the switch 23. Has If the sending and receiving devices of the facsimile system are properly synchronized, the time slot provided by the generator 24 occurs when the carrier is transmitted for the short period of time required for each line scan.

The carriers provided to the facsimile receiver are selected synchronously by the PLL because the error signal according to the frequency and the phase value is different between the signal derived from the carrier and the signal received by the comparator 10 at the VCO 40. This is because it is transmitted to the input terminal 32 of the filter 30 for a short time according to the scan line.

Filter 30 includes an operational amplifier 34 having a non-inverting input terminal connected to ground. This inverting input terminal is connected to the input terminals 32 and 33 of the filter through the angular resistor 31 and the resistor 35. The resistor 35 has a resistance 8-10 times larger than the resistor 31. The filter 30 includes a resistor 36 and a capacitor 37 connected in series with the output terminal and the input terminal of the operational amplifier 34. When synchronous sample ether signals are applied to the input terminal 32, the filter 30 is selected with a very narrow frequency range in which the PLL can be fixed as an incoming signal.

For example, if the AMVSB suppressed carrier is used in a facsimile device where 2100 Hz is selected as the reference frequency of the carrier, the range of 2100 Hz ± 100 Hz may be acceptable. This corresponds to a capture range of 4200 HZ ± 200 Hz as indicated by the PLL. This filter component is also selected and the filtered error signal responds at a much slower rate than the sample rate. The filter provides an error signal with high frequency characteristics that satisfies the 2/1 larger ratio.

In response to the operation of the switch 22, a double of the carrier base error signal provided to the filter 30 is applied to the input terminal 33 of the filter 30 so that the resistor 35 is connected in parallel with the resistor 35. The operation of the filter 30 is determined instead of the resistor 31. Since the resistor 35 is much larger than the resistor 31, the filtered error signal only receives an error signal that responds more slowly than the filtered error signal obtained when the error signal is applied to the filter input terminals 32 and 33. When received at 33 is provided. This causes the filter 30 to respond to an error signal provided to the filter 30 in response to the operation of the switch 22, which occurs between the popularity of the error signal provided on the synchronous coupling base and is therefore insignificant. However, it provides a modification of the behavior of the limited VCO. When the amplitude of the error signal from the comparator 10 applied to the input terminal 32 is preferably very large, it may take time when this occurs to affect the increase in the bandwidth of the PLL that facilitates the capture process.

This can be solved by increasing the bandwidth of the filter 30. The bandwidth correction circuit 80 connected to both ends of the resistor 31 is provided for this purpose and includes a resistor 81 to be connected in series with two parallels but includes diodes 82 and 83 with different polarities. Thus, the diode 82 is energized when the error is large enough in one direction and the diode 83 is energized when the error is sufficiently large in the opposite direction. The energization of diode 82 or diode 83 causes resistor 81 in parallel with resistor 31 to change the operation of filter 30 in a preferred manner.

VCO 40 includes a function generator 41, which is a 566-type function used in international semiconductor regions to which resistance bacteria including resistors 42, 43, and 44 have been added. Generator. Resistors 42 and 43 connected in series between voltage V and ground supply the voltage to the function generator 41, which is adjusted to pre-run at twice the carrier frequency of 4280 Hz. Resistors 42, 43, 44 are selected to provide a total VCO gain of about 100 Hz / volt.

The function generator 41 supplies two output signals to the VCO. One output is a square wave signal applied to the input terminal of the comparator 10 through the line 70, and the other output is a trigonal wave signal in phase with the square wave. . The triangular wave output signal is applied to the limiting circuit 50 consisting of the coupling capacitor 51, the biasing resistor 52 and the limiting amplifier 53 via the line 90.

The output of the limiter 53 is a square wave 90 ° shifted from the square wave signal applied to the feedback line 70. Since the square wave signal thus transferred is twice the frequency of the desired carrier signal frequency, the output of the limiting amplifier 53 is applied to the divider 60 so that the frequency is divided into two. For example, divider 60 may be a flip-flop circuit. The output of the divider 60 is a signal having a carrier signal frequency and shifted by 180 degrees with the anti-hollow signal phase. The signal is applied to the output of the divider 60, which can be used to demodulate the AMVSB signal received at the fax receiver.

As described above, the use of the error signal applied to the filter 30 according to the operation of the switch 22 limits the modification of the operation of the VCO 40 but is not important for stability or loop response. It is an advantage of the present invention that no "untuned" provided by the error signal is required.

In this case, the connection between resistor 35 or switch 22 and resistor 35 can be eliminated and the arrangement of FIG. 2 can be used. 2 is a modification of the circuit of FIG. 1, in which the input terminal 32 is used for the filter 30 of FIG. 2, and the switch 22 or the switch 23 used only applies an error signal to the filter 30. Used to control. In the circuit arrangement of FIG. 2, the amplitude detector 21 and the phase pulse generator 24 are connected to the AND circuit 25 so that the AND circuit 25 receives signals from the detector 21 and the phase pulse generator 24. FIG. Only when the switch 23 is operated. That is, the signal by the facsimile receiver contains an unmodulated carrier period and the phase pulse generator 24 provides a time slot in response to the detection of the receiver drum reaching the phase or "start" position.

It is apparent in the present invention that the switch device 20 is connected to the PLL and another arrangement can provide the error signal application function described in the switch device 20.

Comparator 10, which has not been described in detail, has taken Hanner from a known PLL of the prior art. The comparator 10 shown in FIG. 1 includes a limiter 17 connected to the input terminal 11 to provide a logic signal to one output terminal of the exclusion OR circuit 13. The other input terminal of the exclusive OR circuit is connected to the return line 70 at the VCO. The output terminal of the exclusive OR circuit is connected to the amplifier 14 through a filter that includes a resistor 15 and a capacitor 16. The filter removes unnecessary 8400 Hz frequency components, prevents the amplifier from saturation and provides AC to the return path. Suppress execution. The time constant of the filter is chosen to be much less than the constant of the PLL, which does not affect the overall loop response or stability.

While the above description has been described in connection with carrier reproduction for AMVSB carrier suppressed signals by using a facsimile system, the present invention also applies to other amplified modulated suppressed carrier signals used in facsimile systems. The signal used in the facsimile system includes a suppressed carrier signal that is amplitude modulated in the period of the unmodulated carrier, each interval being synchronized with the false signal generated by the facsimile receiver at the time the receiver reaches each phase position.

Claims (1)

A circuit for facsimile receiver for reproducing a carrier signal from a full-wave rectified signal obtained from a received signal comprising an amplitude modulated suppressed carrier signal and a short unmodulated carrier, comprising: a first input coupled to receive a full-wave rectified signal A comparator 10 having a second input terminal connected to receive a signal of the function generator 41 through a terminal and a feedback connection signal 70, a low pass filter 30, and an output of the low pass filter 30. A phase locked loop comprising a VCO 40 connected to a terminal and a second input terminal of the comparator 10 and having a free run frequency twice the carrier signal frequency; a received signal for detecting a period of an unmodulated carrier wave; Is connected to the facsimile device to respond to and provide a time slot for each position signal, and one of the short intervals of the unmodulated carrier switches during the time slot. A switch device 20 connected to the phase locked loop for connecting the output terminal of the comparator 10 to the filter device 30 when detected by the device; one of the signals appearing at the output terminal of the VCO 40; A facsimile receiver, characterized in that consisting of a divider (60) connected to the output terminal of the VCO (40) to supply a signal for grinding the / 2 frequency.

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