Difference between raster & image
- Status
1 - 6 of 6 Posts
Joined
·
4,801 Posts
Guys!
Raster is all the points of the tube face where the electron beam is directed by the sweep systems; there is a set of circuits, and magnetic windings on the tube neck, for vertical sweep (typically 60hz for video) and a second set of circuits and windings for horizontal sweep (15khz and up, depending on the projectors' sweep range and the signal currently being displayed). The actual image is the portion of the raster where the beam is, or could be, on enough to light up the phosphors. To see actual raster, one could raise the brightness or G2 to a higher than normal level, this will deliver some beam to the entire raster and may even make retrace lines visible. Scenes at night may not light up the image all the way to every edge. Interlaced NTSC video may have gaps between the scan lines where the phosphor of the tube face does not get lit, thus explaining the popularity of scalers and HTPCs that take NTSC video and draw the image employing a sufficient density of scan lines to make the gaps between scan lines disappear. Anamorphic and HDTV signals are displayed with shorter raster height than 4:3 material, so the top 20% and bottom 20% of the phosphor is not employed.
Raster is all the points of the tube face where the electron beam is directed by the sweep systems; there is a set of circuits, and magnetic windings on the tube neck, for vertical sweep (typically 60hz for video) and a second set of circuits and windings for horizontal sweep (15khz and up, depending on the projectors' sweep range and the signal currently being displayed). The actual image is the portion of the raster where the beam is, or could be, on enough to light up the phosphors. To see actual raster, one could raise the brightness or G2 to a higher than normal level, this will deliver some beam to the entire raster and may even make retrace lines visible. Scenes at night may not light up the image all the way to every edge. Interlaced NTSC video may have gaps between the scan lines where the phosphor of the tube face does not get lit, thus explaining the popularity of scalers and HTPCs that take NTSC video and draw the image employing a sufficient density of scan lines to make the gaps between scan lines disappear. Anamorphic and HDTV signals are displayed with shorter raster height than 4:3 material, so the top 20% and bottom 20% of the phosphor is not employed.
Joined
·
7,138 Posts
This is a confusing subject for most people at first, but an important one to understand. Here's an alternative explanation:
The raster is not the image, but instead the 'path' the electron beam follows from left to right, top to bottom in order to paint an image on the screen:
http://www.atomica.com/content2/img/CDE/RASTSCAN.GIF
The electron beam can be turned on during the black portions of the image above to paint an image. (The image is limited to the raster area - you can't have an image that's outside the raster area - that makes no sense).
CRT's use this method whereby electrons are shot onto the phosphor coating on the tube face a line at a time from left to right starting at the top-left corner (black lines). At the end of the line, the beam is turned off and moved back to the left and down one line, which is known as the horizontal retrace (flyback - red lines). When the bottom-right corner is reached, a vertical retrace (flyback - blue line) returns the gun to the top-left corner. In a TV signal, this is known as the vertical blanking interval.
As Tim stated, if you want to see the raster in order to move it or size it (this will also move the visible image), you can crank down the contrast (white level) close to zero and crank up the brightness (black level) while using an all black test pattern.
Some hints: Often the raster is somewhat 'unstable' after the retrace (red & blue lines). This can result in the left side of the image being wavy near the top left of the image So often people will shift the image to use the right-most area of rasterto allow some space on the left of the raster for the signal to stabilize first.
Clear as mud?
Kal
The raster is not the image, but instead the 'path' the electron beam follows from left to right, top to bottom in order to paint an image on the screen:
http://www.atomica.com/content2/img/CDE/RASTSCAN.GIF
The electron beam can be turned on during the black portions of the image above to paint an image. (The image is limited to the raster area - you can't have an image that's outside the raster area - that makes no sense).
CRT's use this method whereby electrons are shot onto the phosphor coating on the tube face a line at a time from left to right starting at the top-left corner (black lines). At the end of the line, the beam is turned off and moved back to the left and down one line, which is known as the horizontal retrace (flyback - red lines). When the bottom-right corner is reached, a vertical retrace (flyback - blue line) returns the gun to the top-left corner. In a TV signal, this is known as the vertical blanking interval.
As Tim stated, if you want to see the raster in order to move it or size it (this will also move the visible image), you can crank down the contrast (white level) close to zero and crank up the brightness (black level) while using an all black test pattern.
Some hints: Often the raster is somewhat 'unstable' after the retrace (red & blue lines). This can result in the left side of the image being wavy near the top left of the image So often people will shift the image to use the right-most area of rasterto allow some space on the left of the raster for the signal to stabilize first.
Clear as mud?
Kal
Joined
·
16,014 Posts
A related question...
How close to the edge of the phosphor face can I have the image and still be safe? Can it be right up to the edge but not overscanning? Or should it be set back from the edge and by how much?
Bob Wood
How close to the edge of the phosphor face can I have the image and still be safe? Can it be right up to the edge but not overscanning? Or should it be set back from the edge and by how much?
Bob Wood
Joined
·
274 Posts
Hey Guys,
To answer Bob's question, the consensus on Barcos seems to be 1/4" from the tube border for maximum phosphor use. I run mine at about 3/16" to 1/8", and accept the slight geometric distortion. What you choose to do will be governed by how brave you feel! Important to note here is you should use electronic blanking as insurance against the image being drawn outside the forbidden area (too close to the side of the tube), as rupture of the glass CRT can result.
Hope this helps...
Darrin
Kal, did you try out the [email protected] setting for your HTPC? What did you think?
Cheers!
To answer Bob's question, the consensus on Barcos seems to be 1/4" from the tube border for maximum phosphor use. I run mine at about 3/16" to 1/8", and accept the slight geometric distortion. What you choose to do will be governed by how brave you feel! Important to note here is you should use electronic blanking as insurance against the image being drawn outside the forbidden area (too close to the side of the tube), as rupture of the glass CRT can result.
Hope this helps...
Darrin
Kal, did you try out the [email protected] setting for your HTPC? What did you think?
Cheers!
Joined
·
7,138 Posts
Quote:
Nope! Never tried it! I remember mentioning that I would... just haven't had the chance. Maybe I'll try it real quick with the green gun only (so I don't have to reconverge) and see... I'm curious about how you mentioned it was much brighter then 720p!
Too many DVD's left to still watch and today's not helping either, what with the release of some of my favourites: Sopranos season 2, Star Trek: The Motion Picture, Doctor Zhivago, Empire of the Sun, etc!
Kal
| Originally posted by DarrinLinders Kal, did you try out the [email protected] setting for your HTPC? What did you think? |
Too many DVD's left to still watch and today's not helping either, what with the release of some of my favourites: Sopranos season 2, Star Trek: The Motion Picture, Doctor Zhivago, Empire of the Sun, etc!
Kal
1 - 6 of 6 Posts
- Status
Join the discussion
