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Path: utzoo!mnetor!seismo!ut-sally!utah-cs!utah-gr!pwa-b!philabs!micomvax!onfcanim!dave
From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: comp.sys.amiga
Subject: Re: Vsprite stuff
Message-ID: <15294@onfcanim.UUCP>
Date: Mon, 27-Apr-87 13:05:33 EDT
Article-I.D.: onfcanim.15294
Posted: Mon Apr 27 13:05:33 1987
Date-Received: Mon, 4-May-87 01:45:31 EDT
References: <283@mcdsun.UUCP> <431@applix.UUCP> <1484@cadovax.UUCP> <1485@cadovax.UUCP>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 39

In article <1485@cadovax.UUCP> keithd@cadovax.UUCP (Keith Doyle) writes:
>
>How do you determine what 'correct' video centering is for video recording?
>The Amiga Preferences centering control allows you to adjust for individual
>monitors and TVs, but where is the right place for VCR's and television
>broadcasting?

You check it with an oscilloscope.

The best thing is to get a hold of a book on television engineering; it
will tell you exactly where all the portions of the video signal should
start and end, and correct voltage levels, for video that meets RS-170A
(NTSC) specs.

Briefly, the simplest method of centering the image is to draw a white
box around the outside of the screen.  Make the line 2 pixels wide so
it appears in both fields of an interlaced raster.

Then look at the resulting signal at the output of the NTSC encoder
with an oscilloscope.  With the timebase set up to view 1-2 horizontal
lines, the first pixel of a line should appear at 9.4 microseconds
after the beginning of horizontal sync, and the last pixel of the line
should occur 1.5 microseconds before horizontal sync.  All timing
measurement are made relative to the point at which the negative-going
(leading) edge of horizontal sync reaches its half-amplitude point, 140
mV below blanking level.

If the "active" portion of one scanline does not take exactly 52.66
microseconds, you will have to either increase or decrease the figures
above (9.4 and 1.5) by the same amount in order to maintain centering
of the image within the area allocated for it.

Then switch the oscilloscope to a slower sweep rate to display the
vertical sync and blanking interval.  There should be 17-21 (nominal
20) lines that are blanked between the last scanline at the bottom of
one field and the first scanline at the top of the next field.
There should be exactly three blanked lines between the last visible
scanline and the leading edge of the vertical sync pulse (in one field
this is 3 whole lines, in the other it is 2 whole and 2 va.Ue atE(Ri