Xref: utzoo comp.graphics:14501 comp.sys.next:9800
Newsgroups: comp.graphics,comp.sys.next
Path: utzoo!censor!geac!alias!imax!dave
From: dave@imax.com (Dave Martindale)
Subject: Re: Next machine as animation platform
Message-ID: <1990Nov17.154611.7252@imax.com>
Keywords: NeXTdimension, JPEG
Organization: Imax Systems Corporation, Oakville Canada
References: <2463@dover.sps.mot.com> <4072@amc-gw.amc.com> <1990Nov15.115234.4438@nntp-server.caltech.edu>
Distribution: usa
Date: Sat, 17 Nov 90 15:46:11 GMT

In article <1990Nov15.115234.4438@nntp-server.caltech.edu> madler@piglet.caltech.edu (Mark Adler) writes:
>
>On another note, the standard for digital TV (used by digital VTR's) is
>720x243 (before interlacing, giving about 720x484 since there are two
>empty lines).  I wonder why NeXT is using 640x480?  I hope they weren't
>influenced by (shudder) Mac's or PC's.

There are two types of digital VTR's, D-1 and D-2.  D-1 uses a 13.5 MHz
sampling frequency, while D-2 uses 4 times the subcarrier frequency.
Both of these frequencies were chosen for convenience in digitizing
and reconstructing an analog signal without regard to the actual horizontal
pixel count - 720 in one case, something else in the other.  The vertical
pixel count is fixed by the number of active scan lines in the picture.
Both of these standards give non-square pixels.

On the other hand, most frame buffers that work with video signals want
to provide square pixels, so they use whatever pixel clock rate is
necessary to give square pixels given the fixed vertical resolution.
The difference between 480 and 484 is a few lines at the very top and
bottom of the screen that are never visible to an ordinary viewer anyway.
The digital VTR's, of course, are expected to record all of the relevant
parts of the video signal (more than 484, perhaps all 525 lines) while
the frame buffer need provide only the visible portion.