Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!asuvax!mcdphx!mcdchg!ddsw1!corpane!sparks
From: sparks@corpane.UUCP (John Sparks)
Newsgroups: comp.graphics
Subject: Re: Frame rate (was: Re: HDTV and ATV Glossary (TN32))
Summary: Interlace flicker
Message-ID: <979@corpane.UUCP>
Date: 25 Aug 89 19:15:13 GMT
References: <13130@well.UUCP> <17400006@hpfcdj.HP.COM>
Organization: Corpane Industries, Inc., Louisville Ky
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<2434@dciem.dciem.dnd.ca>
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In article <2434@dciem.dciem.dnd.ca> king@dretor.dciem.dnd.ca (Stephen King)
writes:
>In article <17400006@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes:
>> [...]     <flicker> is MUCH  worse in an
>>interlaced display over a non-interlaced display of the same size and
>>brightness at the same *frame* rate.  [...]
>                         ^^^^^
>This is a new one on me, Bob; how can 30Hz (frame rate) non-interlaced be
>any better than 30Hz (frame rate) interlaced video, such as NTSC? Surely
>both will have the same horizontal scan rate and will refresh lines at the
>same rate, so why is there this difference? I think you have the relationship
>backwards: the interlaced display will flicker LESS than the non-interlaced
>one AT THE SAME FRAME RATE. (all other things being equal) If what you
>state were true, there would be no need for interlace, n'est-ce pas? 

Well, Interlace lets you get twice the information on the screen (at least in
the case of my amiga). The flicker is not because of the frame rate, but
because of the *difference* in the information presented in each field (1/2
frame). 

An interlaced 30Hz 1K x 1K screen is probably delivering 1K x 500 pixels each
1/60th of a second. The monitor draws the first 500 lines then shifts down
slightly  and draws the second 500 lines in the next 1/60th/sec to make a 30Hz
frame with 1000 lines. This will flicker as your eye will notice the
differences between one field and the other, especially in pixels that are lit
in one field but not the other.

A non-interlaced display at 30Hz must display all 1000 lines each frame, there
are no 'fields'. The fields are the frames. since each frame has the same
information being displayed there will be less flicker. The better solution
would be to make the monitor 60Hz non interlaced.

The advantage of using interlace is that it lets you get twice the information
on equipment that is only qualified for 1/2 the informational amount. In other
words, it lets you save money by using cheaper monitors. 

-- 
John Sparks   |  {rutgers|uunet}!ukma!corpane!sparks | D.I.S.K. 24hrs 1200bps
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You are in a maze of twisty little passages, all alike.