JPS63301686A - Picture signal receiving circuit - Google Patents

Abstract

PURPOSE:To prevent the deadlock of an AGC loop when an excessive optical signal is inputted at the time of the input of a silence signal by providing a pilot extracting preamplifier on the output of a photodetector and executing an AGC operation to the photodetector and an automatic gain control amplifier (AGC) by the extracted pilot signal. CONSTITUTION:The pilot signal extracting preamplifier 3 for extracting and amplifying the pilot signal superimposed on the output of the photodetector 1 is provided to execute the AGC by the extracted pilot signal. A pilot signal level is lower by 10dB-20dB than the picture signal level of a main signal. Further, the pilot signal has a frequency completely separate from the picture signal. Thereby, even at the time of inputting the excessive optical signal from the state of the absence of an optical input, the signal is not dissipated even though the output of the pilot signal extracting preamplifier 3 is distorted but the pilot signal can be constantly detected and the AGC operation is maintained in this case to prevent the deadlock of the AGC loop.

Description

[Detailed Description of the Invention] [Summary] A light-receiving element having a variable amplification factor and an AGC amplifier fitting are provided. A pilot extraction preamplifier is provided at the output of the light-receiving element in an image signal receiving circuit of the AGC system.
By performing AGC operation on the light receiving element and the AGC amplifier using the extracted nine pilot signals, deadlock of the AGC loop in the case where an excessive optical signal is input even when no signal is input is prevented.

[Industrial application field]

The present invention relates to a circuit for receiving image signals, and more particularly to an image signal receiving circuit in an optical analog image transmission system using a pilot AGC method.

In optical analog image transmission systems, a method is used in which a pilot signal is superimposed on a transmission signal, and the level of this pilot signal is detected to perform automatic gain control (AGC).

In this case, there is a need for an image signal receiving circuit that can operate the AGC function normally even when an excessively large optical signal is input during photothermal manual operation.

[Conventional technology]

FIG. 5 shows a block diagram of a conventional pilot AGC system receiving circuit.

An input optical signal is received by a light receiving element 1 consisting of an APD and converted into an electrical signal. This electrical signal is amplified in a preamplifier 2, then subjected to AGC control in an AGC amplifier 3, and further amplified to a required output level in an output amplifier 4 to generate an output signal.

A part of the output of AGC amplifier 3 is passed through a bandpass filter (B
PF ) 5 to extract the pilot signal. Then, the wave detector (DET) 6 detects the wave and its level, and the AGC controller 7 performs gain control on the AGC amplifier 5 according to the required characteristics based on the pilot signal level, and the DC/DC converter 4
By applying a control current to the light receiving element 1 through the light receiving element 1 and controlling its amplification factor, AGC is controlled as a whole in the receiving circuit.

FIG. 6 shows the power signals of each part during normal operation in the circuit of FIG. 5. (a) shows the output of the preamplifier 2, (6) shows the output of the bandpass filter 5, It is shown that even if the amplitude of the image signal at the output of the bandpass filter 5 varies, the level of the pilot signal at the output of the bandpass filter 5 remains constant.

[Problem that the invention seeks to solve]

In an AGC receiving circuit such as the one shown in FIG. 5, the amplification factor of the light receiving probe 1 consisting of an APD is generally at its maximum during photothermal manual power.

In this state, if an excessive optical signal is input, the power signals of each part in the next case will be as shown in FIG. In Fig. 7, (,) shows the output of the preamplifier 2, and (b) shows the output of the bandpass filter 5.6, as shown in (α), saturation occurs at the peak of the image signal and the pilot The signal component disappears, thus creating a period in which no pilot signal is present at the output of the bandpass filter 5, as shown in (6), and this period becomes longer as the input level becomes excessive.

In this way, when no pilot signal appears to exist, the AGC is controlled to the highest gain state, so the pilot signal is no longer output from the pan/bass filter 5 (F), and the AGC loop becomes deadlocked. The problem is that a situation may arise where recovery is impossible.

[Ninth way to solve the problem]

The present invention is an attempt to solve the problems of the conventional technology, and has a basic configuration as shown in FIG. The light is received by the light receiving cable 1 and the output signal t″AGCAGC
Amplify the light receiving element 1 according to the level of the pilot signal.
and an image signal receiving circuit that performs AGC for AGC Ang 2, including a pilot extraction preamplifier 5,
This is a simple method that performs AGC using extracted pilot signals.

The pilot extraction preamplifier 6 extracts and amplifies the pilot signal superimposed on the output of the light receiving element 1.
Ru.

[Effect]

The transmission band of the image signal is generally as shown in FIG. 26, and the pilot signal level is 10 dJ to 2 rJ dB lower than the image signal level of the main signal. Furthermore, the pilot signal is sufficiently separated in frequency from the image signal. The pilot extraction amplifier 3 extracts a pilot signal from the output of the light receiving element, amplifies it, and outputs it.

The receiving circuit detects the level of this pilot signal and includes a light receiving element having a variable amplification factor according to the detection result,
AG controls the AGC AG that amplifies the received light output signal.
Perform C action.

In this case, even if an excessive optical signal is input due to the state of photothermal power, the output of the pilot signal extraction Daria/G will be distorted, but the signal will not disappear and the pilot signal will always be detected. Can be done. Therefore, even in this case, the AGC operation is maintained and the AGC loop is not deadlocked.

〔Example〕

FIG. 3 shows one embodiment of the present invention, and FIG.
The same parts as in the figure are indicated by the same numbers, and 9 is a pilot extraction preamplifier.

In FIG. 3, light receiving elements 1. Preamplifier 2゜AGC amplifier 31 Output Angle 4 converts an optical signal into an electrical signal, amplifies and outputs the obtained electrical signal, and a detector <Dr:
T) 6, AGC controller 7. AGC control for the receiving circuit by the DC/DC converter 8 based on the pilot signal level can be performed in the same manner as in the case of FIG.

The pilot extraction preamplifier 9 extracts a pilot signal from the output of the light receiving element 1, amplifies it, and outputs it. The detector 6 detects this output and detects its level, and according to this level, the AGC controller 7 adjusts the AGC output.
The required IZ) AGC operation is achieved by controlling the gain for the optical input signal 5 and controlling the amplification factor for the light receiving element IK via the DC/DC converter 8.

FIG. 4 illustrates the detailed configuration of the preamplifier 2 and the pilot extraction preamplifier 9 in FIG. ag3.

In the same figure, QFI and OF2 are operational amplifiers a, n, .
R2 is a resistance, L is an inductance, and C is a capacitor. The preamplifier 2 is configured as a normal inverting amplifier, but the pilot extraction preamplifier 9 is configured as an operational amplifier O.
Inductance L between one input terminal and output terminal of F2
1. It is shown that it has a parallel resonant circuit for the pilot signal frequency consisting of a capacitor C and is configured as a filtering amplifier that selectively amplifies the pilot signal component.

In this way, in the receiving circuit of the present invention, the entire pilot signal is extracted from the output of the light receiving element 1 via the pilot signal extraction preamplifier 9. Therefore, even if an excessive amount of the original signal is input from the unmanned state of the optical signal, and saturation occurs where the pilot signal disappears at the output of the preamplifier 2, the pilot signal output at the output of the pilot signal extraction preamplifier 9 is Even if it is distorted, it will not disappear.

Therefore, in the receiving circuit of the present invention, even if an excessive optical signal is input from an unattended optical signal state, the AGC loop will not deadlock and the AGC operation will not stop.

〔Effect of the invention〕

As explained above, according to the present invention, the pilot AGC
By using this method, the pilot signal output does not disappear due to saturation in the part where the pilot signal is extracted in the next image signal receiving circuit, so that excessive input is applied from a no-signal state and the result is 7'C. successfully pilot λ even if
GC operation can be performed. In addition, the circuit configuration in that case is simple, and the receiving circuit can be made into a hut.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the basic configuration of the present invention, Fig. 2 is a diagram showing the transmission band of the image signal, Fig. 3 is a diagram showing an embodiment of the present invention, and Fig. 4 is a preamplifier and pilot extraction. Figure 5 is a block diagram showing a conventional pilot AGC image signal receiving circuit; Figure 6 is a diagram showing the signals of each part of the circuit in Figure 51!5 during normal operation. , The seventh cause is shown in a diagram showing the signals of various parts in the circuit of FIG. 5 when an excessive input occurs. 1... Photodetector 2... Preamplifier 5... AGC amplifier 4°°° Output amplifier 5... Band pass filter (BPF) 6... Detector (DET) 7... AGC controller 8 ...DC/DC converter 9...Preamplifier for pilot signal extraction (, lFl
, OF2...Operational amplifier R+*R2...Resistor L...Inductance C...Capacitor Figure 1 shows the principle configuration of the present invention Figure 1 Figure shows the transmission band of frequency image signal Figure 2 Book Figure 3 shows an embodiment of the invention Figure 7 Figure 7 shows an example of the configuration of a reamplifier and a pilot extraction dahlia/no 4 rA

Claims (1)

[Claims] An input optical signal on which a pilot signal is superimposed is received by a light receiving element (1) having a variable amplification factor, an output signal is amplified by an AGC amplifier (2), and the received light signal is adjusted according to the level of the pilot signal. In the image signal receiving circuit that performs AGC on the element (1) and the AGC amplifier (2), a pilot extraction preamplifier (3) extracts and amplifies the pilot signal superimposed on the output of the light receiving element (1).
An image signal receiving circuit, comprising: performing the AGC using the extracted pilot signal.