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US 4,731,838 AGranted
AGC circuit used in a CATV receiver
AssigneeHitachi, Ltd.
InventorsYoshio Amemiya, Masanori Oguino
Life of this patent
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Abstract
An AGC circuit used in a subscriber's receiver of a CATV system in which the television signal is scramble-modulated so that only subscribers are allowed to view toll programs, wherein the detected video signal is conducted through a low-pass filter to extract the vertical sync signal, a keying pulse is produced in response to the vertical sync signal, and the AGC operation is performed by the keying pulse in accordance with the magnitude of the vertical sync signal.
Claims13
An automatic gain control (AGC) circuit used in a subscriber's receiver of a cable television system comprising:
- means including a reception amplifier circuit for receiving a television signal which is scramble-modulated except for the vertical blanking period and for amplifying the received signal;
- a detector circuit for detecting the amplified video signal from said reception amplifier circuit;
- a AGC signal generating circuit including means for extracting vertical sync pulses the vertical blanking period of the detected video signal from said detector circuit and for producing an AGC signal depending on the magnitude of of the extracted vertical sync pulses of the vertical blanking period;
- and
- a circuit for controlling the gain of said reception amplifier circuit according to the AGC signal provided by said AGC signal generating circuit.
2 more elements
An automatic gain control (AGC) circuit used in a subscriber's receiver of a cable television system comprising:
- means including a reception amplifier circuit for receiving a television signal which is scramble-modulated except for the vertical blanking period and for amplifying the received signal;
- a detector circuit for detecting the amplified video signal from said reception amplifier circuit;
- a AGC signal generating circuit including means for extracting a predetermined signal portion of the vertical blanking period of the detected video signal from said detector circuit and for producing an AGC signal depending on the magnitude of the extracted signal portion of the vertical blanking period;
- and
- a circuit for controlling the gain of said reception amplifier circuit according to the AGC signal provided by said AGC signal generating circuit;
- wherein said AGC signal generating circuit comprises means for generating a keying pulse having a predetermined pulse width in response to the detected video signal from said detector circuit, an AGC signal detection output circuit for extracting a predetermined portion of the vertical sync signal in the detected video signal by using the keying pulse and producing an AGC signal depending on the magnitude of the extracted portion of the vertical sync signal, a vertical sync separation means for suppressing a scramble-modulated portion of the detected video signal from said detector circuit so as to separate the vertical sync signal, and means for generating the keying pulse in response to the separated vertical sync signal;
- and
- wherein said keying pulse generating means comprises a monostable multivibrator which produces a keying pulse having a predetermined pulse width in response to the vertical sync signal provided by said vertical sync separation means.
5 more elements
An automatic gain control (AGC) circuit used in a subscriber's receiver of a cable television system comprising:
- means including a reception amplifier circuit for receiving a television signal which is scramble-modulated except for the vertical blanking period and for amplifying the received signal;
- a detector circuit for detecting the amplified video signal from said reception amplifier circuit;
- a AGC signal generating circuit including means for extracting a predetermined signal portion of the vertical blanking period of the detected video signal from said detector circuit and for producing an AGC signal depending on the magnitude of the extracted signal portion of the vertical blanking period;
- and
- a circuit for controlling the gain of said reception amplifier circuit according to the AGC signal provided by said AGC signal generating circuit;
- wherein said AGC signal generating circuit comprises means for generating a keying pulse having a predetermined pulse width in response to the detected video signal from said detector circuit, and an AGC signal detection output circuit for extracting a predetermined portion of the vertical sync signal in the detected video signal by using the keying pulse and producing an AGC signal depending on the magnitude of the extracted portion of the vertical sync signal;
- and
- wherein said AGC signal detection output circuit comprises means for generating a reference signal, a first current circuit for providing a predetermined current in accordance with the reference signal, a second current circuit for providing a current in accordance with the reference signal, a second current circuit for providing a current in accordance with the video signal from said detector circuit, means for supplying a power voltage to said first and second current circuits only during the active period of the keying pulse provided by said keying pulse generating means, a current mirror circuit connected to said second current circuit so as to provide a current equal in quantity to the current flowing in said first current circuit, a capacitor which is charged or discharged depending on the difference between currents flowing in said second current circuit and said current mirror circuit, and means for producing an AGC signal in accordance with a voltage across said capacitor.
5 more elements
Dependencies are inferred from the claim text.