Question about interlacing in 1080i - Displays half the lines or refreshes it?

 

New member
Username: Kungfutse

Richardson, TX United States

Post Number: 1
Registered: Nov-05
I have a question about how interlaced lines are displayed on screen, especially with 1080i.

Everysince the Xbox 360 came out, there's been a lot of questions about HD from all the gaming forums (IGN, Gamespot, TeamXbox, etc.). Not surprisingly, one of the most common questions is "Should I set my Xbox 360 to 720p or 1080i?".

One of the things that's bothered me about these 720p vs 1080i arguments is that a lot of people say that 1080i only displays 540 lines at one time. Meaning you only see half the image in one field.

I always thought that it "refreshes 540 lines" every field. The words "display" and "refresh" have different meanings. Here's what I mean:

Display 540 lines:

This implies that there are only 540 lines at one time. That means in the first field, all the odd lines are shown. That means there are no even lines. In the second field, the even lines are shown, but no odd lines. They're missing. Then in the 3rd field, the odd lines are shown, but the even lines are gone. That is what "displays only 540 lines" implies. Only 540 lines are displayed, the other 540 are gone or are black.

Refreshes 540 lines:
This is what I was led to believe. All 1080 lines are shown, but only half are being refreshed. So in the first field, the odd lines are shown. In the second field, the even lines are shown, but the odd lines are still showing the first field. In the 3rd field, the odd lines refresh, but the even lines from field #2 are still there. In other words, whenever an odd field refreshes, the previous even field is still there, and vice-versa.

So as you can see, changing from "display" to "refresh" changes the meaning a lot.

So here are my question about 1080i:

1) Which is it? Are 540 lines only being displayed in one field, switching from odd to even? Or does it just simply refresh from odd to even?

2) Also, how is NTSC interlacing handled? Are all 525 lines displayed at one time (refreshing only the odds or evens), or just 262 lines at once (meaning blanks in between)?

3) And finally, is there a difference between the way CRTs HDTVs handle interlacing and the way LCDs and DLPs handle it?
 

Silver Member
Username: Fyi

Dallas, Texas

Post Number: 454
Registered: May-05
You can Google this subject and find your answers.

Just remember that a set will display what it was designed to display and nothing else. Most fixed pixel displays are 720P by design. They will scale or convert any input signal to 720P, period. The new 1080P sets will scale any input signal to 1080P, period. The "P" means progressive scan. That means all lines are displayed/refreshed in every frame.

The definition you launched into is flawed. There is no difference between refreshed and displayed. All 1080i lines are not present in the same field. There are odd line fields and even line fields. Fields are displayed sequentialy, therefore causing the interlaced operation.

None of this matters unless your set is an HD CRT that displays 1080i. There are not many out there.

Most likely, you have a 720P HD display. Set the game box to 720P. If you have purchased one of the new 1080P HD models then set the game box to 1080i and the set will convert the scanning method to progressive for you.

About your questions on 1080i:

1) Even lines are in one field and odd lines are in the next. They are never all in the same field.

2 NTSC uses odd and even fields. The 525 lines are never displayed at once.

3) Yes, CRT's can display interlacing. Fixed pixel displays such as LCD, DLP, and LCOS convert interlaced signals to their designed progressive display format, which is 720P or on some new models, 1080P.

Hope this helps!
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