Information Delivered by the Various Digital TV Formats
|Format Name||Pixels/Second||Uncompressed Bits/Second||Compressed Bits/Second*||Required Compression Ratio**||Remarks|
|1080/60p||124,416,000||1,990,656,000||18,000,000||111:1||DVD HD format|
|1080/30i||62,208,000||995,328,000||18,000,000||55:1||HDTV as defined|
|1080/30i||44,712,000||715,392,000||18,000,000||40:1||HDTV as practiced***|
*This is the fixed size of the Congressionally allotted digital TV channel, and why we argue that one should measure visual quality AFTER COMPRESSION to appreciate what the American consumer will see (as opposed to what is shown to Congress or in a studio, which is uncompressed). No matter how many bits are crammed into the channel, only 18,000,000 get through per second. See DTV Compression.
**What this chart doesn't show is that interlaced video is about twice as hard to compress as progressive. Compression is required to fit the information content of an HD signal into the relatively tiny broadcast channels provided.
***1080/30i is defined to be 1080 lines of 1920 pixels each delivered every 1/30th of a second (540 of them at a time), but it is implemented by its practitioners as 1035 lines of 1440 pixels each. Even Joel Brinkley of the New York Times - strongly in the Interlacers camp - uses these numbers. Of course, the format should be called 1035/30i in this case (or, by the renaming proposal, 518/60i).
Rule of Thumb
To convert an interlaced format to its perceptual progressive equivalent:
N/30i = .6N/60p
For example, 1080/30i = .6*1080/60p = 648/60p. For an accurate indication, compare this to 720/60p.
For example, 480/30i = .6*480/60p = 288/60p. For an accurate indication, compare this to 480/60p.
The formula above is well-known and acknowledged by the Interlacers too (SMPTE quote).
This doesn't mean that every frame of an interlaced display flickers. It means that there are many cases where it does - particularly if there are horizontal lines, like the edges of floors of a stadium, or as in text. In progressive scan systems, this problem simply does not exist! In fact, that is exactly why progressive scan was chosen for computer terminals - so that text would not flicker. You'd be surprised at how many horizontal lines there are in reality.