Browse Prior Art Database

Prevention of Feedback in Video Switches

IP.com Disclosure Number: IPCOM000109779D
Original Publication Date: 1992-Sep-01
Included in the Prior Art Database: 2005-Mar-24
Document File: 2 page(s) / 63K

Publishing Venue

IBM

Related People

Haigh, DC: AUTHOR [+3]

Abstract

A design is disclosed which provides a low cost alternative to unidirectional buffer stages in video switches. Operational amplifiers conventionally used to prevent feedback oscillations between video input streams are effectively superseded by resistor circuitry.

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Prevention of Feedback in Video Switches

       A design is disclosed which provides a low cost
alternative to unidirectional buffer stages in video switches.
Operational amplifiers conventionally used to prevent feedback
oscillations between video input streams are effectively superseded
by resistor circuitry.

      One means used in prior art for implementing the switching
between two analogue video streams is shown in Fig. 1.  Normally the
first stream passes through the analogue switch to the op-amp, but
when the signal level of the first stream is below a certain
threshold, as detected by the level detector block, the second stream
is switched through instead.

      This method suffers from the disadvantage that if both switches
are on together, as may happen when switch 2 is turning on and switch
1 is turning off, the stream 2 signal can pass through the stream 1
switch and alter the signal level seen by the level detector.  The
detector decides that stream 1 should be switched through instead, so
now the stream 1 level is not corrupted by stream 2, the cycle
repeats and oscillation results.

      A solution to this problem is to put a unity gain op-amp as a
buffer to the input of switch 1, so that the stream 2 signal cannot
get to the level detector.  This is shown in Fig. 2.  Unfortunately
the Fig. 2 solution suffers from the disadvantage that the added
op-amp is an additional source of noise which may be added to stream
1.  Also, it is expe...