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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Image Format Sizes
Message-ID: <17660049@hpfcdj.HP.COM>
Date: 19 Mar 90 19:39:16 GMT
References: <1990Mar16.205807.5971@cs.rochester.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 35

>I have two concerns.  First, the frame rate should be high enough so that
>there is no observable flicker.  Second, the frame rate should not be close
>enough to existing power line frequences to cause the screen to beat with
>flourescent lighting.  Which rates have which problems?

Two problems here; first, increasing the frame rate of the transmitted image
will eat up bandwidth like nobody's business, and for no particularly good
reason.  If you're really worried about getting a flicker-free image, then
the best way to go about it while still having reasonable TV channels (the
RF spectrum is *not* in infinite resource), then I'd suggest broadcasting
at 24 or 30 frames/second (there are some good reasons for picking 24), and
putting the received image into a frame store.  You can then *display* the
image at your choice of rates - 72 Hz sounds like a good number.

The second problem has to do with a common misunderstanding - "beating" with
fluorescent lighting is *not* a primary cause of CRT flicker (for that matter,
how many home viewers use fluorescent lighting in the TV viewing area?).  CRT
flicker is basically a function only of refresh rate, brightness, and the
ambient lighting *level*.  "Beating" with the lights is a secondary problem,
occuring due to a perceived change in the contrast ratio of the screen as the
reflected light changes.  It's usually not a big deal, and in most situations
displays will appear to flicker worse with the local lights OFF - due to the
fact that the apparent brightness of the displayed image increases.  (Also,
consider the fact that many workstations in the U.S. are now using refresh
rates in excess of 60 Hz - the nominal line frequency - and have *better*
flicker performance than their 60 Hz cousins.

The best compromise, IMHO, between bandwidth and flicker would require either
a sufficiently cheap full-color frame store or some other technique which would
allow broadcast at 24 frames/sec. and display at 72.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers%hpfcla@hplabs.hp.com               |  sentient life-form on this planet.