24 frame projection / progessive tv -

[blazingoat]

Okay, it's my understanding that when projected in a theatre, 24fps film is actually projected @ 48fps by doubling each frame. This is because 24fps is below the threshold of presistance of vision. - the image would strobe if you eye only got 24 pictures per second. is this correct? - This is the same reason why NTSC tv uses interlacing, to prevent "strobing" right?

So, when you're watching a 24p DVD played back on say a plasma tv with a progressive scan dvd player, how is this issue overcome?

am I making ANY sense?

 

[khmuse]

Okay, it's my understanding that when projected in a theatre, 24fps film is actually projected @ 48fps by doubling each frame. This is because 24fps is below the threshold of presistance of vision. - the image would strobe if you eye only got 24 pictures per second. is this correct? - This is the same reason why NTSC tv uses interlacing, to prevent "strobing" right?

So, when you're watching a 24p DVD played back on say a plasma tv with a progressive scan dvd player, how is this issue overcome?

am I making ANY sense?

You are correct about the methodology that is used for theatrical film playback, each film frame is flashed twice using a rotary "gate" (similar to a motion picture cameras gate) that is synchronized to the pawls that move the film.

I design high performance audio and video electronics so this is a very well understood system for me, but since I am an EE I am not always the best individual to explain these types of things in an easily comprehensible way. If I fail to fully explain this, please feel free to address specific questions and I will be happy to follow up with a more verbose reply. Here goes:

In the case of a progressive scan TV, there is an additional step being performed in the playback chain, namely, "line doubling". All DVDs are based upon interlaced signals, even those that originate as progressive sources (like film, 24p, etc). What occurs is in the playback chain (either in the DVD player or within the TV/monitor) is that the interlaced footage is effectively upsampled to 60 (actually 59.97) progressive frames per second, instead of 30 (29.97) interlaced ones. This is done by an interpolation algorithm that estimates what is missing between two fields. The effect is to increase the effective sample rate and hence decrease any flicker which would otherwise occur from the material being below the optical persistence rate. DVDs also provide what is know as a cadence flag which helps the deinterlacer (line doubler) determine which fields belong to which original frames (very much like the 24pa frame cadence). The deinterlacer then generates 48 distinct progressive frames but conveys them as 60 individual frames per second using a frame repeat pattern. Some deinterlacers ignore the cadence flag on the disc and instead employee a cadence detection routine.


If the original material is interlaced, then the deinterlacer takes each field and interpolates it to a progressive frame by using a motion predictive approach, this gives you twice as many frames however fully 50% of the information is based upon the estimation of what occurred between the 240 lines (1/2 or 1 field of a 480 line system).

One aspect of deinterlaced video processing systems that is not commonly understood is the fact that they have a built in latency (delay). The video that enters the device is delayed by up to several frames before it exits the processor. This can lead to a video and audio sync issue as the audio now leads the video. Since the speed of light is considerably faster than the speed of sound it can cause some users to notice the delay. More advanced deinterlacers now also employee and audio delay (in the digital domain) that corrects for their video latency.

 

[Barry_Green]

Okay, it's my understanding that when projected in a theatre, 24fps film is actually projected @ 48fps by doubling each frame. This is because 24fps is below the threshold of presistance of vision.

Sort of. Film is projected at 24 frames per second, because that's all there is on the reel, is 24 frames for each second. Each film frame gets flashed two or three times, but it's the same frame. So you're seeing 24 frames per second, at a 72fps refresh rate (assuming a three-bladed shutter).

- the image would strobe if you eye only got 24 pictures per second. is this correct?

The image does strobe, and you do only get 24 pictures per second. What the multi-bladed projector does is even out the flicker in the brighter parts of the image.

This is the same reason why NTSC tv uses interlacing, to prevent "strobing" right?

Again, sort of. Interlacing is also a form of analog compression, a way to get twice as many motion samples per second at the same bandwidth.

 

[blazingoat]

wow. thanks for the info. I had to read those posts over a few times.

 

[Rich Lee]

when they shoot rear projection, like in the natural born killers titles or kill bill driving scenes, they project 24fps to sync up to the film camera...the strobing is very noticeable to the eye.