JPS5829907B2 - Carrier extraction method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a carrier extraction method for extracting a carrier for demodulation using a phase synchronized signal having the same frequency as the carrier in a facsimile receiving apparatus that demodulates a received signal by coherent detection.

In facsimile, image signals are transmitted by amplitude modulation, and envelope detection has conventionally been generally used as a demodulation method on the receiving side.

However, when VSB transmission or multilevel transmission is used to increase the speed, envelope detection cannot provide satisfactory image quality, so synchronous detection is being adopted instead.

As is well known, synchronous detection requires a demodulation carrier that is synchronized with the carrier.

In order to always synchronize this demodulation carrier with the image signal carrier, a method can be considered in which a certain amount or more of the carrier component is always leaked when transmitting the image signal from the transmitting side, but this is not preferable because it causes deterioration of the S/N. do not have.

Therefore, in facsimile transmission, it is desirable to transmit the carrier only during the white signal period or the black signal period of the image signal, and to transmit the carrier during the other periods without leaking the carrier at all.

FIG. 1 is a diagram for explaining this type of facsimile transmission system, where a is a binary image signal, b is an amplitude modulation waveform of the image signal a, and the phase of the carrier is constant.

Further, C is a waveform obtained by converting a into three values, and d is a carrier suppression amplitude modulation waveform of the image signal of C, and the carrier phases are alternately shifted by π.

The former method is called a binary AM method, and the latter method is called a ternary AM method (also referred to as a two-phase AMPM method).

In both figures, the portion where the carrier is sent out corresponds to the white signal, and the portion where the carrier is not sent out corresponds to the black signal.

This is because a normal image signal has many white signal components, which makes it easier to extract demodulation carriers.

However, the black signal may continue for a long time in some places, and the carrier is not sent out for a long time in such places, so the demodulation carrier generation circuit runs by itself without being synchronized with the carrier.

Therefore, when the white signal, that is, the carrier, arrives again, the phase of the demodulating carrier may be significantly shifted from the carrier phase, and in such a case, the waveform of the rising part of the demodulated white signal will be distorted. , which causes deterioration of image quality.

By the way, the carrier extraction circuit for demodulation generally has a PLL (Phase Lock Loc) as shown in FIG.
op) is used.

That is, the carrier extracted from the received signal by the phase comparator 1 is compared with the output of the voltage controlled oscillator 3, an output voltage corresponding to the phase difference between the two signals is extracted, and this is applied to the low frequency film 2.
It is applied as a control voltage to the oscillator 3 via the oscillator 3 to obtain a carrier for demodulation at the output of the oscillator 3.

Here, the low-pass filter 2 is often composed of a simple CR filter, but if its time constant T is made small, the PL
The loop bandwidth of L becomes wider, making it easier for the oscillator 3 to phase-lock, but when the human-powered carrier disappears, it quickly returns to the free-running frequency.

Conversely, if the time constant T is increased, the loop bandwidth becomes narrower and it becomes difficult to achieve phase lock, but even if the input carrier disappears, it is difficult to return to the free-running frequency.

However, in reality, the input carrier may not arrive for a long period of time, so a PLL is desired to have characteristics such that the response is quick when phase locking is achieved, but phase shift is less likely to occur when the manual carrier runs out.

Now, in facsimile transmission, in addition to the image signal, a phase synchronization signal is normally sent periodically for each scanning line, and generally the same signal as the black signal is used as this phase synchronization signal.

However, the same signal as the carrier can also be used as this phase synchronization signal.

For example, a signal that is the same as the white signal with guards of the same signal as the black signal shown in Figure 3a (carrier output state with no carrier state before and after it, or an intermediate part as shown in the same figure). A carrier or the like whose phase is reversed by 180° is used as a phase synchronization signal.

In this way, the phase synchronization signal can be used as a reference signal for extracting carriers for demodulation.

The present invention has been made with attention to such points, and in a facsimile machine that intermittently transmits a phase synchronized signal consisting of the same signal as the image signal carrier at a constant cycle, the receiving side uses a PLL for extracting carriers for demodulation. By changing the control voltage of the voltage controlled oscillator according to the phase of the carrier frequency component in the received signal only during the phase synchronization signal period and keeping it almost constant during the image signal period, demodulation is possible even when the image signal carrier is interrupted for a long time. The purpose of the present invention is to provide a method for extracting a carrier for demodulation, which allows reproduction of a high-quality image without phase shift of the carrier for demodulation.

The present invention will be described in detail below with reference to FIGS. 4 to 7.

FIG. 4 is an explanatory diagram of the configuration of a facsimile receiving apparatus showing an embodiment of the present invention.

A received signal transmitted from a facsimile transmitter is input to the input terminal 11 .

This received signal is inserted at a fixed period, for example, every scanning line, in addition to the image signal carrier obtained by amplitude modulating the carrier with a binary or ternary facsimile image signal as shown in FIG. 5a. It includes a phase synchronization signal that is the same signal as the image signal carrier, that is, a signal that has the same frequency as the image signal carrier and is synchronized therewith.

This received signal is guided to the synchronous detection circuit 12, and
It is also led to the front circuit 13.

The pre-circuit 13 is a circuit that extracts the image signal carrier frequency component or its twice frequency component from the received signal.

When the modulation method of the image signal carrier is binary set width modulation,
A filter is used that balances the upper sideband and lower sideband and extracts the image signal carrier frequency component.

In addition, in the case of ternary AM (two-phase AM-PM), an additional two
A high-pass filter is used that extracts a frequency component twice that of the image signal carrier from the output of the multiplication circuit and the square circuit.

The output of the pre-circuit 13 is applied to the phase comparator 14 together with the output of the voltage controlled oscillator 16.
An output voltage corresponding to the phase difference between both signals is extracted from the output voltage.

The output of the phase comparator 14 is applied to the oscillator 16 as a control voltage via a low-pass filter 15.

That is, the phase comparator 14, low-pass filter 15, and voltage-controlled oscillator 16 form a PLL, and the oscillator 16 is controlled in synchronization with the output of the front circuit 13.

Note that the low-pass filter 15 is configured so that its bandwidth can be varied by external control.

In this case, the resistance value of R of the CR constituting the low-pass filter 15 is variable by electrical control.

The time constant may be made variable.

The output of the voltage controlled oscillator 16 is led to a post circuit 17.

This post circuit 17 has a built-in fixed phase shifter so that its output is in phase with the image signal carrier and the synchronization signal.
In the case of ternary set width modulation, the output frequency of the oscillator 16 (
It has a built-in frequency dividing circuit that divides the frequency (twice the frequency of the image signal carrier) into 1/2, that is, the same frequency as the image signal carrier.

The output of the post circuit 17 is the output of the synchronous detection circuit 12.
The output of the detection circuit 12 is sent to an output terminal 19 via a low-pass filter 18.

As a result, a demodulated baseband signal is obtained at the output terminal 19, so in the case of binary AM modulation, this becomes the facsimile reception image signal as it is, and in the case of ternary width modulation, this signal is further subjected to full-wave rectification. By doing this, a facsimile reception image signal is obtained.

The output of the low-pass filter 18 is further led to a phase synchronization signal detection circuit 20, and the phase synchronization signal detected by this circuit 20 is taken out to an output terminal 21 and is supplied to a control circuit 22.

As shown in FIG. 5, this control circuit 22 is a circuit that generates a rectangular wave output only during the phase synchronization signal period, and this output is supplied as a control signal to the low-pass filter 15 in the PLL.

As a result, the low-pass filter 15 is controlled so that the bandwidth is wide, that is, the time constant is small, only during the period of the phase synchronization signal, and it is controlled so that the bandwidth is narrow, that is, the time constant is large, during the period of the image signal. .

That is, as shown in FIG. 5C, the output voltage of the low-pass filter 15 does not change significantly during the period when no image signal carrier exists (for example, during the black signal period), and remains almost constant during the image signal period. Retained.

Therefore, even if, for example, a black signal continues for a long time during the image signal period, the phase lock of the PLL is difficult to lose, and even if the phase lock is lost, the phase lock state is quickly established in the next synchronization signal period. A phase shift in the voltage controlled oscillator 16 is less likely to occur.

Therefore, since the demodulation carrier can always be synchronized with the image signal carrier, stable synchronous detection can be performed and a high quality reproduced image can be obtained.

FIG. 6 is a diagram showing another embodiment of the present invention. The difference from the previous embodiment is that a sample and hold circuit 23 is newly provided on the output side of the low-pass filter 15 of the PLL, and the phase is synchronized by the output of the control circuit 22. Low-pass filter 1 in the signal period
The point is that the output voltage of No. 5 is sampled and held.

In this case, the low-pass filter 15 may always have a somewhat wide bandwidth, and its output waveform will be as shown in FIG. 7A for the received signal waveform in FIG. Then, the voltage immediately becomes zero.

FIG. 7C shows the output waveform of the control circuit 22. During the output period of this signal (during high levels M and A in the figure), the sample and hold circuit 23 samples the waveform shown in FIG. When held until the period, the output waveform of the sample and hold circuit 23 becomes as shown in FIG. 7d, and is maintained at a constant value during the image signal period.

Therefore, if this output is applied as a control voltage to the voltage controlled oscillator 16, the oscillator 16 will continue to oscillate at a constant frequency during the image signal period, and no phase shift will occur.

As described above, according to the present invention, even if a period without an image signal carrier continues for a long time, phase shift of the demodulation carrier can be prevented, and a reproduced image with good image quality can be obtained.

[Brief explanation of the drawing]

Figure 1 is a diagram showing a facsimile image signal waveform and modulation waveform to explain the binary AM modulation method and the three-value AM modulation method, Figure 2 is a diagram showing the basic configuration of a PLL circuit, and Figure 3 is a diagram showing the facsimile image signal waveform and modulation waveform to explain the binary AM modulation method and the three-value AM modulation method. FIG. 4 is a configuration diagram of a facsimile receiving apparatus for explaining an embodiment of the present invention, FIG. 5 is a waveform diagram of each part for explaining the operation of FIG. 4, FIG. 6 is a block diagram of a facsimile receiving apparatus for explaining another embodiment of the present invention, and FIG. 7 is a waveform diagram of each part for explaining the operation of FIG. 6.

Claims (1)

[Claims] 1 In a facsimile receiving device that receives an image signal carrier amplitude-modulated by a facsimile image signal and a phase synchronization signal containing a signal having the same phase as this carrier, and performs synchronous detection of the received signal using a demodulation carrier to obtain a demodulated output. , a circuit for extracting a carrier frequency component or a frequency component twice the carrier frequency component from the received signal, and comparing the phase between the output of this circuit and the output of the voltage controlled oscillator for extracting the carrier for demodulation, and calculating the phase difference between them. a circuit that supplies a corresponding voltage to the voltage controlled oscillator as a control voltage; A carrier extraction method characterized by comprising a circuit that maintains a signal period substantially constant.