有了这些天我们拥有的所有优秀的硬件，似乎我们应该享受高质量的观看无论什么，但如果不是这样呢？今天的超级用户问答旨在为困惑的读者澄清问题。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

图片由lge（Flickr）提供。

问题

超级用户读者alkamid想知道为什么HDMI-DVI和VGA在质量上有明显的区别：

I have a Dell U2312HM monitor connected to a Dell Latitude E7440 laptop. When I connect them via laptop -> HDMI cable -> HDMI-DVI adaptor -> monitor (the monitor does not have a HDMI socket), the image is much sharper than if I connect via laptop -> miniDisplayPort-VGA adaptor -> VGA cable -> monitor.

The difference is difficult to capture with a camera, but see my attempt at it below. I tried playing with the brightness, contrast, and sharpness settings, but I cannot get the same image quality. The resolution is 1920*1080 with Ubuntu 14.04 as my operating system.

VGA:

HDMI:

Why is the quality different? Is it intrinsic to these standards? Could I have a faulty VGA cable or mDP-VGA adaptor?

为什么两者在质量上有差别？

答案

超级用户贡献者Mate Juhasz、youngwt和Jarrod Christman为我们提供了答案。首先，伙计朱哈兹：

VGA is the only ****og signal among the ones mentioned above, so it is already an explanation for the difference. Using the adapter can further worsen the quality.

Some further reading: HDMI vs. DisplayPort vs. DVI vs. VGA

然后是youngwt的回答：

Assuming that brightness, contrast, and sharpness are the same in both cases, there could be two other reas*** why the text is sharper with HDMI-DVI.

The first has already been stated, VGA is ****og, so it will need to go through an ****og to digital conversion inside the monitor. This will theoretically degrade image quality.

Second, assuming that you are using Windows, there is a technique called ClearType (developed by Microsoft) which improves the appearance of text by manipulating the sub pixels of an LCD monitor. VGA was developed with CRT monitors in mind and the notion of a sub pixel is not the same. Because of the requirement for ClearType to use an LCD screen and the fact that the VGA standard does not tell the host the specificati*** of the display, ClearType would be disabled with a VGA connection.

I remember hearing about ClearType from one its creators on a podcast for This().Developers().Life() IIRC, but this Wikipedia article also supports my theory. Also HDMI is backward compatible with DVI and DVI supports Electronic Display Identification (EDID).

Jarrod Christman最后的回答是：

The others make some good points, but the main reason is an obvious clock and phase mi**atch. VGA is ****og and subject to interference and mi**atch of the ****og sending and receiving sides. Normally one would use a pattern like this:

Clock and Phase

Then adjust the clock and phase of the monitor to get the best match and the sharpest picture. However, since it is ****og, these adjustments may shift over time, and thus you should ideally just use a digital signal.

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