Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!news.cs.indiana.edu!bobmon@iuvax.cs.indiana.edu
From: bobmon@iuvax.cs.indiana.edu (RAMontante)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: What is "pitch" on a CRT
Message-ID: <1991Feb7.205602.27208@news.cs.indiana.edu>
Date: 8 Feb 91 01:55:43 GMT
References: <1991Feb6.234713.17376@odetics.com> <1991Feb7.171036.14822@grape.ecs.clarkson.edu> <1991Feb7.200222.19663@magnus.ircc.ohio-state.edu>
Organization: 'twixt Dali and Dada
Lines: 35

smsmith@hpuxa.ircc.ohio-state.edu (Stephen M. Smith):
| I'm no expert on this, but I think two things are being confused here.
| "Pixels" (1024x768 for example) are NOT the same thing as
| "dot pitch."  You can disply 300x200, 640x480, 800x600, etc. at
| different times on a multisync monitor, but your "dot pitch" will
| always remain the SAME.  In other words, a "pixel" has to do with
| the resolution (1024 *pixels* x 768 *pixels*), but the dot pitch
| has to do with the holes in the mesh which are just behind the tube
| screen on your monitor.  The best dot pitch available is 25 "dpi"
| (Sony Trinitron 14" tube).  This means that the tube is physically
| constructed with a "mesh" which has "25 holes in the mesh for every
| inch across."  (I think that's the way it's figured, or could mean
| that each hole is .25mm in size.)  In any case, the more "holes"
	[ ... ]

Close.  The pitch (typically .28mm) is the spacing between discrete "dots"
on the screen.  Note that in a color tube a "dot" is composed of *three*
phosphor patches, a Red, a Green, and a Blue one.  Many of the best tubes
(maybe all, these days) use a perforated mask around these phosphor
triplets, which keeps the electron beam from stimulating more than one set
of phosphors at a time.  (To put it another way, the mask makes sure that
the dots are less than .28mm in diameter, so they don't overlap.)

At .28mm/dot, a horizontal line of 1024 dots is 287mm long.  This fits
well with the active area of a 13" (diagonal) CRT tube, so is nicely
suited to a 1024x768 sVGA image.

Consider a .38mm dot pitch:  a 1024-dot line needs a screen 389 millimeters
wide, while 287mm holds only 755 dots.  So if you tried to display a
1024-pixel-wide image, either 1) 35% of the image would overrun the edge of
the screen, 2) the 1024 image pixels have to be mapped over only 755 screen
dots, resulting in blurriness and loss of image quality, or 3) images
greater than 755 pixels width must be rejected.  On the other hand, a
horizontal line on a 19" (diagonal) monitor is about 390mm, so a .38mm dot
pitch on a 19" monitor is fine for images up to 1024 pixels width.