HD-SD Conversion
Issues for Converting between High Definition and Standard Definition
- High Definition Compared to Standard Definition
- Aspect Ratio
- Common Aspect Ratios
- Adapting the Ratio
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High Definition Compared to Standard Definition
(Measured in Pixels)
1. Standard Definition (SDTV) – DTV: 480i (720 x 480, interlaced)
2. Enhanced Definition (EDTV) – DVD:
- 480p (720 x 480, progressive)
- 480p-wide (854 x 480, progressive)
3. High Definition (HDTV) – DTV, HD Discs:
- 720p (1280 x 720, progressive)
- 1080i (1920 x 1080, interlaced)
- 1080p (1920 x 1080, progressive)
4. High Def DVD – Blu-Ray only
Aspect Ratio
The term Aspect Ratio refers to the width of a picture (or screen) in relation to its height.
Ratios are expressed in the form “width x height”.
For example, a 4:3 ratio means the picture is 4 units wide by 3 units high. It can also be written 4×3.
Note that the actual physical size of the picture is irrelevant.
Aspect Ratio refers only to the relationship between width and height.
16 x 9 High Definition Image
Letterboxed for 4 x 3
Common Aspect Ratios
There are three common aspect ratios:
4:3
This is the standard television format used throughout the second half of the 20th Century. Sometimes referred to as 12×9.
16:9
This format has gained acceptance as the new standard for widescreen TV, DVD and high-definition video
21:9 (Cinemascope)
A very wide screen format used for theatrical release movies.
Adapting the Ratio
Letterbox
Letterboxing is a technique used to display widescreen content on a traditional 4:3 screen such as those used by old-style television sets.
The technique is very simple — the picture frame is reduced in size until it fits the screen, leaving black bars above and below the picture.
The effect is like looking through the slot of a letterbox, hence the name.
Pixel Issues
The information of a 4:3 or 16:9 aspect ratio is imbedded within the formatting of the program.
A device that does not correct for aspect will either display an image that is stretched out too wide, or narrowed and elongated.