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Correction of Phase Distortion in a Television Receiver When Used for Character Display Purposes

IP.com Disclosure Number: IPCOM000083220D
Original Publication Date: 1975-Apr-01
Included in the Prior Art Database: 2005-Mar-01
Document File: 3 page(s) / 76K

Publishing Venue

IBM

Related People

Nasser, MH: AUTHOR [+2]

Abstract

A method of correcting phase distortion in a domestic TV receiver, when used for character display purposes, is described. Certain characters such as letters H,E etc. when displayed on an "unmodified" domestic television receiver, appear as being slightly `etched' away where the vertical and horizontal bars meet. A correction circuit incorporated externally in the modulator circuit `stretches' the horizontal line whenever it meets the vertical bar, and thus eliminates this etching phenomenon which is designated `pulling effect'.

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Correction of Phase Distortion in a Television Receiver When Used for Character Display Purposes

A method of correcting phase distortion in a domestic TV receiver, when used for character display purposes, is described. Certain characters such as letters H,E etc. when displayed on an "unmodified" domestic television receiver, appear as being slightly `etched' away where the vertical and horizontal bars meet. A correction circuit incorporated externally in the modulator circuit `stretches' the horizontal line whenever it meets the vertical bar, and thus eliminates this etching phenomenon which is designated `pulling effect'.

A serial-to-parallel shift register in conjunction with combinational logic, detects the presence of both leading and trailing edges of a horizontal bar pattern in the video signal. An edge triggered flip-flop in conjunction with a fourphase clocking scheme alters the phase of the video signal, which is used to drive the RF modulator. A phase compensated video signal is used to correct for phase distortion at the receiver, and thus allow for improved characters on the standard TV receivers when they are used as terminals.

When a vertical bar or bars meet a horizontal line to form a character font, a phenomenon designated "pulling effect" is observed on an unmodified domestic television receiver. This effect is readily noticeable on the cathode-ray tube (CRT) cathode when character fonts such as H,E etc. are displayed. Figs. 1a and b show typical examples of such waveforms.

A method of compensating this shift is now described. The video signal is fed serially into a 4-bit shift register, as shown in Fig. 2. The clock signal phi 1 is used to transfer the input video to the parallel outputs of the shift register. These out...