|Understanding Video Terms|
|Home Theater Feature Articles Video Related Articles|
|Written by Mike Levy|
|Thursday, 01 September 2005|
Page 2 of 3
ore Advanced Terms
• Throw Distance is the ratio of the width or diagonal of a screen to the distance a particular projector must be placed from that screen. It is essential to know the throw distance of a projector before you purchase one, so that you can determine if and where it will fit into your theater.
• Keystone is the angling of the sides of the image, so that one end is narrower or wider than the other. You adjust a keystone control on a projector until the sides are parallel to each other.
• Gamma is the control of gray scale light values. A true gamma is linear, but most display devices have several gammas available to allow the user to adjust the dynamics of the image.
• Video Screen: The screen is presently used as the generic name for where the image is displayed. It originally meant the white surface onto which a movie was projected.
• Gray Screen: Gray screens are used to increase the black level on digitally projected images. These are popular with some DLP and LCD projectors.
• Roll-Down Screen: A roll-down screen is automated to unfurl on command. Roll-down screens definitely add “wow” factor in any home theater.
• Perforated Screen: The movie industry standard is a perforated screen through which the sound passes. The holes are small enough to allow most of the light to be reflected, but large enough to allow most of the sound through. Perf screens are not too common in home theater applications, unless the system very large in scale.
• Fixed Screen is a permanently installed screen.
• Painted Screen is a screen that is actually painted on a wall surface or board that is suitable for video reproduction. This is the most cost-effective screen.
• Masking is the use of black to block out the screen at its edges.
• Auto-Masking is when the masking for different aspect ratios is programmed into a display unit so it can be done automatically.
• Aspect Ratio is the ratio of the width to the height of any screen. 4:3 is the standard for traditional TV, which is squarer. 16x9 is the most common, more rectangular image found in movie and home theaters.
• Widescreen refers to any screen with an aspect ratio greater than 4x3.
• 4x3 is more square shape of traditional NTSC televisions. 4:3 is being quickly replaced by HDTVs, but unfortunately, much of the content created today is still in 4:3. This requires many HDTV users to zoom or squeeze their picture to fit their more rectangular 16x9 TVs.
• 16x9 is the HDTV aspect ratio common for movies. There are other aspect ratios that are used but are normally slight variations of the 16x9 format. These more obscure aspect ratios show up as black lines over and under the image.
• Anamorphic Screen: This is a set-up available on DVD sources where, rather than losing the detail available in the 4x3 image when viewing a 16x9 movie, the movie is put on the DVD using all of the vertical detail. The image is then squeezed down to 16x9 by the display device.
• Moire (or Moray) Pattern is a pattern of lines and colors caused by defects in decoding small details.
• Dot Crawl is when a small detail on an image moves between two pixels, jumping from one to the other, rather than moving smoothly. This is caused by an image that is small enough relative to the pixel density of the display device to jog between pixels.
• Rainbow Effect is a video artifact that looks like a rainbow, caused by defects in the analog decoding of the color information.
• Convergence is the bringing together of the red, green and blue images to create full color at any point on the screen. The convergence is correct when a white dot shows no hint of color anywhere on the screen.
• Blooming is caused by poorly controlled power supplies on CRT display devices. The effect looks like bright images expanding in size.
• Fluorescent Colors are oversaturated colors that are caused by poorly set display devices or nonstandard primary colors.
• Hot Spots are areas on the screen that are brighter than the rest of the screen.