有了這些天我們擁有的所有優秀的硬體，似乎我們應該享受高質量的觀看無論什麼，但如果不是這樣呢？今天的超級使用者問答旨在為困惑的讀者澄清問題。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

圖片由lge（Flickr）提供。

問題

超級使用者讀者alkamid想知道為什麼HDMI-DVI和VGA在質量上有明顯的區別：

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?

為什麼兩者在質量上有差別？

答案

超級使用者貢獻者Mate Juhasz、youngwt和Jarrod Christman為我們提供了答案。首先，夥計朱哈茲：

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

然後是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最後的回答是：

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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