Descrambling device in CATV system

Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry

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325476, 358121, H04N 716

Patent

active

041457161

ABSTRACT:
A descrambler system for a CATV television signal having IF audio, video and synchronizing components that is scrambled by a first amplitude modulation of the IF audio component and a second amplitude modulation of the combined audio, visual and synchronizing components by a key signal having a frequency that is an integral multiple of a 15.75 KHz horizontal synchronizing frequency includes a demodulator responsive to the scrambled signal, a key signal detecting circuit coupled to derive the key signal from the audio portion of the demodulator output and a dynamic limiter circuit coupled to remove noise from the key signal and provide a noise free sine wave key signal to the demodulator. The dynamic limiter circuit is an inexpensive circuit providing the signal processing characteristics of a much more expensive narrow band filter. It includes a current limiter circuit driven by a current source in proportion to the magnitude of the scrambled signal to provide a square wave signal at the key signal frequency that is proportional in magnitude to the scrambled signal and a simple filter that converts the square wave signal to a sine wave key signal that is used by the demodulator circuit with the exact shape, frequency and amplitude characteristics for noise free unscrambling of the scrambled CATV signal.

REFERENCES:
patent: 2709218 (1955-05-01), Gabrilovitch
patent: 3081376 (1963-03-01), Loughlin et al.
patent: 3544904 (1970-12-01), Eness
patent: 3729576 (1973-04-01), Court
patent: 4024575 (1977-05-01), Harney et al.

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