Newsgroups: comp.graphics
Path: utzoo!utgpu!watserv1!watcgl!imax!dave
From: dave@imax.com (Dave Martindale)
Subject: Re: 3/4" -vs- S-VHS, etc.
Message-ID: <1990Nov23.052059.28481@imax.com>
Keywords: NTSC, video, videotape, recording
Organization: Imax Systems Corporation, Oakville Canada
References: <14556@smoke.brl.mil>
Date: Fri, 23 Nov 90 05:20:59 GMT

In article <14556@smoke.brl.mil> mike@brl.mil writes:

[ lots of useful and interesting stuff about video recording.  However,
there were a few errors I'd like to correct.]

>Regular 3/4" has about 260 lines of resolution
>(e.g. about 520 pixels/scanline), while SP-format 3/4" has about
>360 lines, or about 720 pixels/scanline.

Video resolution is given in lines, not line pairs.  2 lines == 1 line pair
== 2 pixels.  So a vertical resolution of 485 lines is really just
485 pixels.  Horizontal resolution is a bit odd, since it is usually
specified as "lines per picture height" rather than "lines per picture
width".  Because of the 4:3 aspect ratio, a horizontal resolution figure
of "330 lines per picture height" is really "440 lines per picture width",
or 440 pixels, or 220 line pairs.

Even when you see resolution quoted as "330 lines" with no other qualifiers,
they really mean 440 pixels H resolution.  This is because vertical resolution
should always be the same with a properly-interlaced TV, so nobody
quotes it, while horizontal resolution depends on bandwidth which does vary.
And because H resolution is quoted in terms of picture height, not picture
width, even when only H resolution is being given.

This is all a bit weird for those of us used to thinking in pixels.

>The NTSC format is
>bandwidth limited by law to have no more than 720 pixels/scanline,
>so this is as good as it gets in real NTSC.

An NTSC signal is limited to 4.2 MHz bandwidth when broadcast.  This is
equivalent to just about "330 lines" or 440 pixels (horizontal).
More pixels than that doesn't gain you any resolution if it's going to
be broadcast.  Some frame buffers have 512 because it's a convenient
number (power of 2), some use 640-650 because it's a convenient number
(square pixels), but they won't give any more resolution when broadcast.

720 pixels is what is used by D-1 digital recorders; that's just what
the width comes out to with a 13.5 MHz sampling rate.  This probably
allows them to have about a 6 MHz bandwidth, but anything beyond 4.2 will
be lost on broadcast (but is useful for multi-generation processing).

All of the above figures are for luminance bandwidth only.  Colour
bandwidth is always less.  D-1 digital recorders give half as much
bandwidth for the two colour components as the luminance gets,
and everything else is worse.  The NTSC standard itself specifies
about 500 kHz bandwidth for the Q colour channel, which is equivalent
to about 53 pixels horizontally.  Yes, yechh.  That's why colours
smear well beyond the boundaries of the pixels that are supposed to
be coloured.  The I channel gets somewhat more than twice that, but
most (or all) consumer TV sets discard it, limiting both I and Q
to 500 kHz.

	Dave Martindale