JPH0348709B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a pilot AGC method for video signal transmission that is not affected by cross-modulation distortion between a video signal and a pilot carrier.

(Background Art) Figure 1 shows a block diagram of the conventional pilot AGC system. In Figure 1, VIDEO IN is a video signal input, BA1 is a video input amplification circuit, OSC is a pilot carrier generation circuit, MIX is a mixing circuit that mixes the video signal and pilot carrier,
DR is the transmission output circuit, L is the transmission line, AMP is the receiving side input amplifier, AGC is the gain control circuit, LPF1 is the low-pass filter that removes the pilot carrier, BA2 is the video signal output circuit, VIDEO OUT
is the video signal output, BPF is the bandpass filter for extracting the pilot carrier, DET
is a detector that detects the pilot carrier,
LPF2 is configured as a low-pass filter that smoothes the detector output, and DA is configured as a comparison amplifier circuit that compares and amplifies the low-pass filter LPF2 output and a reference potential.Automatic gain control AGC operation is inserted on the transmitting side. This is done by detecting and smoothing the pilot carrier on the receiving side, comparing it with a reference potential, and controlling the gain control circuit so that the pilot level is always constant.

Here, from the mixing circuit MIX that mixes the video signal and the pilot carrier to the low pass filter LPF1 that removes the pilot carrier,
If nonlinearity exists in DR, L, AMP, and AGC,
The video signal and pilot carrier undergo cross modulation.

Therefore, it has the disadvantage of causing beat disturbances due to cross-modulation.

(Problem to be solved by the invention) The present invention is characterized in that a pilot carrier is inserted intermittently in synchronization with a video signal (synchronization signal period) so that the video signal and the pilot carrier do not exist at the same time. is the removal of cross modulation between the video signal and the pilot carrier,
This will be explained in detail below.

(Structure and operation of the invention) Figure 2 shows an embodiment of the present invention.
IN is video signal input, BA1 is video input amplification circuit,
CLP1 is a clamp circuit (DC regeneration circuit), MIX is a mixing circuit, DR is an LED drive circuit, LED is a light emitting diode, OPT.OUT is an optical output, SS is a horizontal synchronizing signal separation circuit, BO is a blocking oscillation circuit, PG1
is a clamp pulse generation circuit, OSC is a pilot carrier generation circuit, ASW is an analog switch, PG2 is an analog switch control pulse generation circuit, OF is an optical fiber cable, and OPT.
IN is optical input, APD is light receiving diode (avalanche photo diode), AMP is light receiving amplifier, LPE1 is low pass filter, CLP2 is clamp circuit (DC regeneration circuit), BA2 is video signal output circuit, VIDEO OUT is video signal Output, BPF
is an extraction filter for extracting the pilot signal, and is composed of a bandpass filter or a highpass filter. DET is a pilot carrier detector;
LPF2 is a low-pass filter, DA is a comparison amplifier circuit, DC/DC is a DC/DC converter, LPF3 is a low-pass filter, and PG3 is a clamp pulse generation circuit. Here, the frequency of the pilot carrier generation circuit OSC is set outside the band of the video signal, the free-running frequency of the blocking oscillator circuit BO is set at a frequency slightly lower than the horizontal synchronization frequency, and the cutoff of the low-pass filter LPF1 is set. The frequency is set to pass the video signal and block the pilot carrier, and the cutoff frequency of the low-pass filter LPF2 is set sufficiently low compared to the horizontal synchronization signal frequency.
The cutoff frequency of filter LPF3 is higher than the horizontal synchronization signal frequency and lower than the pilot carrier (can reproduce the horizontal synchronization pulse).
It is assumed that this has been set.

FIG. 3 is an explanatory diagram of the operation of FIG. 2, and the numbers 1 to 19 in FIG. 3 correspond to those in FIG. In FIG. 3, the diagram on the left is a waveform diagram during normal operation, and the diagram on the right is a diagram when video input on the transmitting side is cut off. Under normal conditions, a pilot carrier is inserted into the horizontal synchronization signal portion of the input video signal on the transmitting side, and the pilot carrier is detected and smoothed on the receiving side, and then sent to the comparison amplifier circuit.
The DA compares and amplifies the reference potential, and then DC/DC
The converter converts it to high voltage and the light receiving diode
It is added as a bias to APD and performs AGC operation. The gain-controlled video signal 17 has DC fluctuations and low-frequency distortion improved by the clamp circuit CLP2. The clamp pulse 16 is generated by applying the detection output 14 to a clamp pulse generation circuit PG3 via a low-pass filter LPF3. When the video input on the transmitting side is cut off, the synchronization separation circuit output 4 disappears, so the blocking oscillation circuit BD oscillates at a free-running frequency close to the horizontal synchronization frequency and sends out the pilot carrier as in normal times. Therefore, the receiving side AGC loop APD, AMP, BPF, DET, LPF
2. DA and DC/DC operate as they normally would, and the AGC loop can operate instantly no matter when a video signal is input.

In this embodiment, the pilot carrier is inserted in the horizontal synchronizing signal section (sync chip), but the effect is the same even if it is inserted in the back pouch or front pouch where there is no video signal.

(Effects of the Invention) As explained above, in the embodiment, the video signal and the pilot carrier do not exist at the same time, so nonlinearity occurs in the light emitting diode LED, the light receiving diode APD, the LED drive circuit, and the light receiving amplifier AMP. This has the advantage of not causing beat disturbances due to cross-modulation even if there is a problem.

In addition, since the pilot carrier is inserted in synchronization with the horizontal synchronization signal, the pilot carrier is inserted on the receiving side.
By using a carrier detector, the clamp pulse 16 for DC reproduction can be reliably reproduced.
That is, it has the advantage of being less susceptible to APL fluctuations and noise of the video signal. Furthermore, since it has a blocking oscillation circuit, it can be used even when the video signal is cut off.
AGC operation is performed.

The present invention has the advantage that even if there is nonlinearity in the transmission system, there is no beat disturbance due to cross modulation, and it can be used for transmitting video signals by direct optical intensity modulation. The pilot carrier inserted on the transmitting side can be used for AGC monitoring other lines and generating clamp pulses.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional pilot AGC system, FIG. 2 is a block diagram of an embodiment of the present invention, and FIG. 3 is an explanatory diagram of the operation of FIG. 2. VIDEO IN...Video signal input, BA1...Video input amplifier circuit, OSC...Pilot carrier generation circuit, MIX...Mixing circuit, DR...Transmission output circuit/LED
Drive circuit, L...transmission line, AMP...receiving side input amplifier/light receiving amplifier, AGC...gain control circuit, LPF1,
LPF2, LPF3...Low pass filter, BA2...
Video signal output circuit, VIDEO OUT...Video signal output, BPF...Band pass filter, DET...Detector, DA...Comparison amplifier circuit, CLP1, CLP2...Clamp circuit (DC regeneration circuit), LED...Light emitting diode, OPT.OUT …Optical output, SS…Horizontal synchronization signal separation circuit, BO…Blocking oscillation circuit, PG1,
PG2, PG3...Pulse generation circuit, ASW...Analog switch, OF...Optical fiber cable,
OPT.IN…Optical input, APD…Photodetector diode (avalanche photo diode), DC/DC…
DC/DC converter.

Claims (1)

[Scope of Claims] 1. On the transmitting side of the video signal transmission system, a pilot signal is inserted only in the horizontal synchronization signal period, and on the receiving side of the video signal transmission system, gain control is performed according to the reception level of the pilot signal. In the pilot AGC method, on the transmitting side, there is an input terminal VIDEO IN that receives the video signal including the synchronization signal, a horizontal synchronization signal separation circuit SS that extracts the synchronization signal from the video signal, and a trigger that is triggered by the extracted horizontal synchronization signal. a blocking oscillation circuit BO whose free-running frequency is slightly lower than the horizontal synchronization frequency, and a pilot carrier generation circuit OSC which generates a frequency outside the video signal band.
means PG2 for switching the output of the generating circuit in synchronization with the output of the blocking oscillation circuit BO;
The ASW is equipped with a mixing circuit MIX that mixes the pilot signal and video signal output from the means and inserts the pilot signal into the horizontal synchronization period, and means that sends the output to the transmission line. A gain control means APD, an extraction filter BPF that extracts the pilot signal from the received signal, a detector DET that detects the output thereof, a low pass filter LPF2 that smoothes the output of the detector, and compares the output level with a reference potential. a comparison amplifier circuit DA that controls the gain of the gain control means APD according to the difference between the two;
and a low-pass filter LPF1 that extracts only the video signal and synchronization signal from the output of the gain control means,
an output terminal VIDEO OUT which is connected to the output via the clamp circuit CLP2 and provides video output;
Clamp circuit according to the output of the detector DET
means LPF3 for applying a clamp pulse to CLP2;
A synchronous pilot AGC system characterized by being configured with PG3. 2 Extraction filter that extracts the pilot signal
The synchronous pilot AGC system according to claim 1, wherein the BPF is a bandpass filter. 3. The synchronous pilot AGC system according to claim 1, wherein the extraction filter for extracting the pilot signal when the frequency of the pilot signal is higher than the frequency of the video signal is a high-pass filter.