Video Resolutions for Beginners
Continuing our series of Beginner's Guides for HTPC and Home Theater, this guide will take a closer look at Video Resolutions--what it means, why's it important and how to make your picture as good as possible for your particular environment.
What does 1080p/720p/1080i/480p mean?
The numbers 1080, 720 and 480 refer to the lines of vertical resolution in the picture as illustrated in the following diagram:
Note that each line is drawn to the screen beginning at the left. If there are 1080 lines of vertical resolution then there are 1080 lines drawn from the left side of the screen to the right side with line 1 located at the very top of the screen and line 1080 at the very bottom.. The higher the number of lines there are, the greater the amount of detail there is in the picture.
What about the “i” and “p”?
There are two ways to express each frame of video, interlaced and progressive. The simplest to understand is progressive. Progressive simply means that each line of the video frame is drawn to the screen one after the other so line one is first, line two is second, etc.
Interlaced essentially means that each frame of video is divided into two halves or “fields”. Each field is half of a video frame with the odd field expressing line one, line three, line five, etc. and the even field expressing line two, line four, line six, etc. The following diagram illustrated the video fields:
Why isn’t there just one way, either interlaced or progressive?
Interlaced format was created back in the early days of television. The reason interlaced is used is because it is a form of compression so not as much bandwidth is needed to express video information. The compression is not without cost. Primarily, interlacing results in less vertical resolution with moving video scenes. It also can cause errors in the video frame known as artifacts. For these reasons, progressive is often a preferred format if given a choice.
Progressive is especially preferred for fast action programming like sports which is why you will find a network such as ESPN today uses 720p for its broadcasts. Other networks that focus more on drama programs and slower moving content such as NBC use 1080i for its broadcasts to benefit from the higher resolution it brings with still and slower moving images.
What is the difference between HD and SD?
High-definition (HD) are resolutions 720p and above whereas standard-definition (SD) resolutions are those lower than 720p.
What is 16:9, 4:3, 1.78:1, 1.33:1, etc.?
These expressions are mathematical representations of aspect ratios. In the days before HDTVs, the common TV aspect ratio was 4:3 (4/3 = 1.33). The old NTSC TV standard defined 480i with a display resolution using a vertical resolution of 480 and a horizontal resolution of 640. 640/480 can be reduced to 4/3 or 4:3 as it is commonly written.
Today, broadcasters use a widescreen aspect ratio of 16:9. So 1080i and 1080p have a vertical resolution of 1080 and a horizontal resolution of 1920; 720p has a vertical resolution of 720 and a horizontal resolution of 1280. 1920/1080 = 1280/720 = 16/9 = 1.78.
You might notice that sometimes, you see black bars on the screen. If they are on the side of the content, the picture is said to be pillarboxed and if they are on the top and bottom of the content, the picture is said to be letterboxed. The reason the black bars appear is because the original aspect ratio (OAR) of the content is different than the display’s aspect ratio. The black bars are there to preserve the OAR so that you can see the picture as it was intended to be viewed without distortion or cropping.
So what resolution TV do I need?
In today’s TV market, the choice of resolution is generally between 720p and 1080p with 720p displays offering a lower price. If the display receives a format that is not in its native resolution, the image is scaled to meet the native resolution. There are times when 720p displays are all that is required and this is due to the eye’s ability to resolve each line of resolution from a given distance. You can calculate this distance roughly by the following equations:
720p: 2.35 x diagonal screen size = optimal seating distance
1080p: 1.57 x diagonal screen size = optimal seating distance
If your seating distance is greater than the calculated seating distance for 720p, then the 1080p capable display will offer little additional benefit when just comparing the displays strictly on their capability to render a given resolution. It should be noted that the above calculations for optimal seating distance are based on the eye’s ability to fully resolve each vertical line of resolution.
One more word regarding 720p. Often, 720p displays have native resolutions other then 1280 x 720. Common resolutions for 720p displays are 1366 x 768 and 1024 x 768. These situations are not so ideal as it means that the content must be scaled to fit those resolutions. They can also present more configuration headaches for HTPC users depending on how the HTPC is being setup and connected to the display.