What's the frame rate of progressive scan?

[NicolasB]

I thought I knew the answer to this question, but a few posts recently have been making me doubt myself.

What is the frame rate of a progressive scan signal?

For example, let's say a PAL DVD player is correctly deinterlacing film material. Obviously there are only 25 pictures per second on the disc, but does it actually output it as 25 pictures per second, or is each one doubled up so that there are 50 entire frames per second being sent to the display?

And what happens with 720p high definition? Is that actually 50 (or 60) distinct 1280x720 pictures per second, or is it actually only 25 (or 30) pictures?

[welwynnick]

Hmmm, yes.

There's different ways of thinking about this, but forgetting NTSC and 30Hz for a moment:

If a progressive output was 25 fps, not 50 fps, it would only be sending the same amount of information as an interlaced output; just in a different line order. We know that progressive has twice the data rate (that's why we had interlaced in the first place!).

So if an interlaced frame is written at 25 fps by writing 50 interlaced fields per sec, then a progressive frame is written twice as fast - 50 fps. In this case the data rate is twice as fast: 31.2k lines/sec instead of 15.6k lines.

Thinking about HD, 720 is expressed as 720/50p, and 1080 as 1080/25i. The data rate for each is similar, because 1080 has about double the pixels but half the frame rate - you takes your pick ( I chose de-interlacing 1080i! )

[Nick_UK]

Yes, you are completely correct, and congratulations on knowing the difference between a frame and a field (rare on here) !

Interlacing was used in the early days of television to allow a (what was then) a high definition picture which was economical on bandwidth, but which did not suffer from bad flicker problems associated with low frame rates.