Contrast/Picture Set-Up


 Settings for contrast (white levels) and brightness (black levels) are critical to the proper viewing of a picture on a video monitor or television. The blacker the blacks and the whiter the whites the better the contrast ratio. If misadjusted, the picture looks too washed out or bright objects smear or bloom. The best balance is achieved by careful analysis of your monitor's performance by providing controlled signals representing the "whitest white" and "blackest black". Always keep in mind that the room environment will affect these settings. Making adjustments in a sunny room will result in a brighter setting than if adjustments are made in the evening. So it's wise to make adjustments under the viewing conditions likely to exist when you watch the movie. Motion pictures are mastered in a room that's moderately dark. White levels (contrast) are adjusted to 35-foot lamberts using a light meter and a signal very much like what's provided by THX Optimizer. We don't expect you to use a light meter, but the THX Optimizer test has been designed to deliver results very close to what's achieved during mastering.

Video is measured on expensive test equipment during the post production process. The scale on a waveform monitor is graduated in IRE units (Institute of Radio Engineers, the group that set this measurement standard). For DVD component video, the useful picture content of the video signal is measured as a percentage, where 0% represents black and 100% is white. With VHS, Broadcast and the composite video output of your DVD player, the black level is elevated to 7.5% IRE. Normally, the picture intensity is clipped, or not allowed to exceed 100% IRE and little if any information transitions above this level.

The Contrast/Picture adjustment procedure provides a box with four distinct shades of white in eight boxes surrounded by black.  If the Contrast/Picture control is set to high, you can lose the distinction between the various white boxes and can result in picture bleeding, which is the oversaturation of the picture tube. This can be seen as a solarization or blooming of the picture highlights (see Figure 1). This can also be noticed on the right side of the white block as it transitions to black due to the way the picture tube is scanned. The brightest white boxes will represent the highest video level in the feature. The goal here is to adjust your Contrast/Picture control on your television/monitor to a point where the large white area is bright but all four shades of white are still visible. At the same time, insure that the transitions from black to white and white to black have no smearing.

As your picture tube ages, this will be one of the more difficult adjustments to make. The light level produced by the picture tube decreases with age, making it necessary to drive the picture tube harder. The harder the picture tube is driven, the less likely the beam scanning the front of the screen remains sharply focused. If you notice any of the mentioned defects, it doesn't mean you need a new monitor or television. It may only require a visit from your local television service agency to make some adjustments.

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设置对比度(白电平)和亮度(黑电平)对显示器或电视的画质是很关键的。黑白要有适当的对比。 如果失调,画面会看起来暗淡,明亮的物体看起来会太晃眼。要仔细分析你的显示器的"最亮点"和"最暗点"才可以找到最佳的平衡点。要注意环境也会影响这些设置。在光线充足的房间里调整会比在夜晚的调整亮许多。因此聪明的方法就是在你通常看电影的条件下做调节。电影制片通常在一个适度黑暗的房间里进行。用光度计和优化信号把对比度(白电平)调节到 35-foot lamberts。如果不用到光度计,THX Optimizer测试设计也可以使你达到最接近制片时的设置。

在后期制作时,视频信号都在昂贵的测试设备上测试过。测试用示波器上的单位是IRE(Institute of Radio Engineers,以建立这些测试标准的工作组命名)。对DVD的视频信号,最有用的测试图形是一个矩形,这个矩形包含100%亮度的纯白视频信号和0%亮度的纯黑视频信号。用DVD播放机输出VHS,电视广播,复合视频时,黑电平提高到7.5% IRE。为了转换信号,通常图象亮度也削减了,通常比100% IRE低一些。

对比度调节程序提供了一个环绕了黑边的白色区域,这个区域里有8个不同亮度的白色块,分4个亮度层次。如果对比度开的过高,就分辨不出不同的亮度层次,或者显象管会过饱和,图象就会有溢出的现象。在图象对比强烈的地方有溢出和负像(参见附图 1)。按照显象管的运作模式,在对比度高的右边黑白交界的地方会出现这种情况。最亮的白色块就是最高的视频电平,调节的目标就是尽可能的调亮画面,同时白色块中可以分出4个层次,并保证黑白交界的地方不出现干扰。


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