Path: utzoo!utgpu!watmath!att!tut.cis.ohio-state.edu!unmvax!gatech!prism!loligo!pepke
From: pepke@loligo.cc.fsu.edu (Eric Pepke)
Newsgroups: comp.sys.mac
Subject: Re: Help Me! Mac II -> NTSC (repost)
Keywords: video,rgb,II
Message-ID: <257@loligo.cc.fsu.edu>
Date: 17 Aug 89 13:06:43 GMT
References: <475@mit-amt.MEDIA.MIT.EDU> <604@brazos.Rice.edu> <7386@microsoft.UUCP>
Reply-To: pepke@loligo.UUCP (Eric Pepke)
Organization: Supercomputer Computations Research Institute
Lines: 63

In article <7386@microsoft.UUCP> georgeh@microsoft.UUCP (George Hu) writes:
>
>Yes, the MacII -> NTSC is quite real, although it is far from perfect.
>I have built the cable, and successfully gotten it to work.
>I tried it both with a multiscanning monitor that accepts NTSC 
>and a regular VCR.  On both, I had the following problems:
>
>1)  It is too large for the screen, so the menu bar and other edges are cut off.
>(This was mentioned in another MacWeek article)
>2)  Everything is quite blurry -- regular text is out of the question.
>3)  You see double images.
>4)  The image is in B&W only.  (It's supposed to be that way) 

It's soapbox time for what little I know about video:

1) is due to the overscanning of TV's.  NTSC has 525 lines per frame.  Some 
are stolen for vertical retrace, and the remainder scan an area that is a bit
larger than the visible area of the tube.  This was decided upon a long time
ago, because the high frequencies involved in retracing caused bad artifacts
near the edge of the screen.

2) is due to a couple of factors.  Your cables may have bad impedence and
cause some ringing, but even if the cables were perfect, there would still
be some problems.  NTSC can only guarantee about 300 pixels per line for
gray scale, fewer for color depending on the color.  Remember that NTSC
is a massive kludge which had to make signals compatible with already 
assigned bandwidth and TV's that were built in the forties.  The bandwidth
never used to be a problem because the analog devices that were used to 
produce the signal (tubes) were inherently slow and had inherent low-pass
filtering.  Now we have transistors and current feedback loops and stuff,
and the frequencies are much higher.  Studios now use things called comb
filters to smooth out the signals a bit before broadcast.  Many of the
better industrial RGB to NTSC converters also have comb filters.

Another problem, flicker, is due to the fact that NTSC is interlaced, so
alternate lines are refreshed 1/30 sec. apart.  If one line is black and
the next line is white, the eye sees flicker.  Again, in normal video
this is seldom a problem, as home-quality vidicon-based cameras can only
resolve about 200 lines, and although studios usually use higher-quality 
image orthicon cameras, they do filtering.

You can effectively do the filtering in software, but it takes a lot of
work.  This is a good application for fuzzy fonts.

As an aside, the reason that TV's scan at the same rate as the line frequency
is not because they derive their scan rate from the line, as some
people believe.  Back a long time ago, it was impossible completely to 
keep the line frequency from getting through the power supply and causing
problems.  If the frame frequency is close to the line frequency, one might
see bands slowly crawling up or down the screen, while if it is different,
one might see bands quickly flickering across the screen.  The former is
less distracting.

By the way, has anybody tried Apple's hack running the color signals 
through an RGB-NTSC filter?

Eric Pepke                                     INTERNET: pepke@gw.scri.fsu.edu
Supercomputer Computations Research Institute  MFENET:   pepke@fsu
Florida State University                       SPAN:     scri::pepke
Tallahassee, FL 32306-4052                     BITNET:   pepke@fsu

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