[opendtv] Re: The Math on Screen sizes
- From: "John Willkie" <johnwillkie@xxxxxxxxxx>
- To: "John Shutt" <shuttj@xxxxxxxxx>, <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 7 Jun 2004 18:19:48 -0700
I believe that you have described ONLY ONE way that sets create images.
What you speak of as being a "color CRT" is just one method of doing color
on a CRT. Your first paragraph indicates that you are describing all color
CRTs, which is patently not true.
What you describe as being an artifact (actually you don't call it that) has
not to do with the width of the screen, but the depth of the electron gun
firing position's) versus the screen width. I've always heard electron gun
geometry described that way.
It's another reason (4:3 scaling of 16:9 images is another) dot/pixel pitch
is not consistent across screens. However, dot pitch across the entire scan
line will be consistent with any digital set I buy. Period.
Then, you start to hint that you are familiar with the truth: "Even though
the image is made up of dots in the case of a color triad shadow mask CRT,
or stripes in the case of an inline shadow mask CRT" ...
This belies your first paragraph, which purports to talk of all color CRTs.
And, I wasn't speaking of intensity. Do you believe that the position of
pixels, triads or slots is random across the screen, AND that they have no
two or three dimensional characteristics?
Me, I take that their positions are fixed, that the intensity of each varies
according to source material, and that the positions have geometric
relationships.
And, even if everything you said is true, it still won't permit a 45 inch
(diagonal measure) 4:3 "HDTV" set to display 1280 distinct pixels across the
screen. Let alone 1920. And, 1280 pixels across the screen would permit
only 640 TVL across the screen. What's the theoretical limit for NTSC? 440
lines? So, at best, such a set could do NTSC with 50% better resolution.
(440 + 220)
John Willkie
-----Original Message-----
From: John Shutt [mailto:shuttj@xxxxxxxxx]
Sent: Monday, June 07, 2004 4:21 PM
To: opendtv@xxxxxxxxxxxxx; Johh Willkie
Subject: Re: [opendtv] Re: The Math on Screen sizes
----- Original Message -----
From: "John Willkie" <johnwillkie@xxxxxxxxxx>
> Hmmh.. were those old computer displays monochrome (one dot per pixel) or
> color (three dots per color)? When you mention dot pitch, were you
talking
> about pixel pitch or dot pitch?
In a color CRT, there is only one hole or slot in the shadow mask for every
three color triad or stripe. The red, blue, and green guns approach the
shadow mask at slightly different angles, so that each gun only "sees" it's
own color phosphors. Dot pitch is simply the spacing between the holes or
slits in the shadow mask.
If you want to get really picky about CRT resolution, look at the color
stripe spacing at the extreme left and right edges versus the center of a
16x9 CRT. Due to the angle of the beam at the extreme edges, the spacing
between color triads or strips are greater than in the middle.
Even though the image is made up of dots in the case of a color triad shadow
mask CRT, or stripes in the case of an inline shadow mask CRT, there is no
relationship between one color triad and one 'pixel.' Each of the three
color guns are modulated in intensity by an analog amplitude signal, and it
is the beam spot size and frequency response of the amplifiers that
determines how many triads are illuminated per 'pixel.' Likewise, counting
the number of color stripes across the face of an inline gun CRT will tell
you the theoritical maximum number of 'pixels' that can be reproduced, but
the actual number will be smaller due to beam spot size and video amp
response.
In fact, with a computer display, given two CRT monitors with identical
resolution the one with the higher 'dot pitch' will give the crisper
display, precisely because there is more than a single color triad
illuminated by one computer 'pixel.'
In a monocrome CRT or in a three CRT projector, there is no shadow mask.
Without a shadow mask or color phosphor triad to rely upon, the idea of a
CRT having a 'pixel' falls apart altogether. It is simply the upper
frequency limit of the video amplifiers' ability to turn the beam on and off
that determines resolution.
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