Browse Prior Art Database

TRACKING AUTOMATIC GAIN CONTROL FOR AM FACSIMILE TERMINALS

IP.com Disclosure Number: IPCOM000024378D
Original Publication Date: 1980-Jun-30
Included in the Prior Art Database: 2004-Apr-02
Document File: 2 page(s) / 108K

Publishing Venue

Xerox Disclosure Journal

Abstract

An AM facsimile terminal, which may or will be operated in a receive mode, is equipped with a tracking automatic gain control (AGC) circuit to ensure that the video baseband signal is referenced to substantially the same level in the receiving terminal as in a remote transmitting terminal. Course adjustments to the reference level for the received video baseband signal are made in a course adjustment loop 11 while the terminal is being initialized for operation in a receive mode, and finer adjustments are thereafter periodically made in a fine adjustment loop 12 during the backstroke intervals, which separate successive scan lines of video data.

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XEROX DISCLOSURE JOURNAL

TRACKING AUTOMATIC GAIN CONTROL FOR AM FACSIMILE TERMINALS
Otto Sperber

Proposed Classification
U.S. C1. 358/280
Int. C1. H04n 1/40

i 1- .

EQU 1 L

  I ZER AND ANALOG

-MULT I P L

I E R

f"

-BASEBAND

OUTPUT

An AM facsimile terminal, which may or will be operated in a receive mode, is equipped with a tracking automatic gain control (AGC) circuit to ensure that the video baseband signal is referenced to substantially the same level in the receiving terminal as in a remote transmitting terminal. Course adjustments to the reference level for the received video baseband signal are made in a course adjustment loop 11 while the terminal is being initialized for operation in a receive mode, and finer adjustments are thereafter periodically made in a fine adjustment loop 12 during the backstroke intervals, which separate successive scan lines of video data.

At the outset of an incoming facsimile transmission a carrier detect signal is applied to a logic circuit 13. That causes the logic circuit 13 to incrementally increase the attenuation provided by a course attenuator 14 within the course adjustment loop 11 at the local clock rate and in predetermined steps of, say, 7 dB/step. The incoming passband signal is routed through the attenuator 14 and then through an amplifier 15 to an equalizer and demodulator 16. A portion of the

Volume 5 Number 3 May/June 1980 299

PASSBAND - --+

t 17.

REC TI FIE R

CARRIER

DETECT

CLOCK

DECODER

CIRCUIT

CIRCUIT

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TRACKING AUTOMATIC GAIN CONTROL FOR AM FACSIMILE TERMINALS (Con t Id)

passband signal is diverted from the equalizer and demodulator 16 to a full wave rectifier 17 to provide a DC signal level for comparison in a comparator 18 against a predetermined reference voltage supplied by a reference source 19. Thus...