Native 1080p Material: A Hidden Reality

The amazing thing is that the whole 1080i vs. 1080p argument is more than just analogous to the 480i NTSC vs. 480p Progressive Scan DVD of yesteryear, so the groundwork for an understanding is already in place. The principles and math are all the same, only now at a higher resolution. If one understands why Progressive Scan DVD players even exist, then you should already be able to understand how and why 1080p not only exists, but is already ubiquitous.

We said that 1080p is the entire 1920 x 1080 raster sampled and/or displayed at one time. No fields. Just full, 1920 x 1080 frames. No jaggies. No line twitter. Just perfect pictures. The question is, at what temporal resolution? If it were captured with the same 60 Hz temporal resolution of 1080i60, it would indeed be well beyond the scope of today's HDTV transmission system as well as the new HD disc formats.

1080p exists today as a 24 frame-per-second format. The shorthand for this format is 1080p24. But if there is no medium to carry 1080p24 why should we care? We care for the same reason we cared about 480p Progressive Scan DVD: Because a p24 signal can be perfectly "folded" into an i60 carrier(2).

Most of the HDTV material you could tune into tonight falls into one of two categories: either the material was shot with a digital camera at 1080p24, or it was shot on 35mm film and transferred to this very same 1080p24 digital format. With the exception of some sports and some other "live" shows, everything from sitcoms to dramas, and of course all movies, fall into this 1080p24 realm.

So how do we get our hands on this 1080p24 if the TV signals and discs are all 1080i60?

To find out we need to understand the transfer of 1080p24 to 1080i60 (which incidentally follows the exact same principal used to convert 24fps movies to yesteryear's i60 NTSC TV system for decades).

Lets consider a sequence of 4 frames.  The first frame of the p24 source gets "cut" into two fields, the odds and the evens again.  Each field contains exactly half of the original frame.  But we can't carry on like that because we'd end up with 48 fields every second, not 60.  For this reason we simply "double up" on one field every other frame.

In other words, the second frame of our sequence is still cut into two fields, but we repeat its first field.  The third frame is cut into two fields, as is the fourth, but again we repeat its first field.  So we end up with a 2-3 pattern in the fields.

The photo below illustrates 2-3 telecine. Click on it to see a larger version.

Field B3 is a duplicate of B1, and D3 is a duplicate of D1.

So we get 10 fields from 4 original frames, or 60 fields from 24 frames every second.

To "reconstitute" the 1080p24 source, it is a relatively simple matter of weaving together the fields which came from each frame (and discarding the redundant ones).

The photo below illustrates reverse 2-3 telecine. Click on it to see a larger version.

It's that simple, with one little caveat: digital displays like LCDs and plasmas don't "operate" at 24 Hz.  They refresh the image on their face at a rate of 60 Hz.  So now that we have 1080p24 "reconstituted" as it were, we need to convert it to 1080p60.  To do that, we use the same 2-3 cadence.  That is, we show the first frame twice, the second we show three times, the third we show twice, and the fourth is shown three times.  So from 24 frames each second, we get 60.

Some of you who are more conversant with the whole progressive scan DVD realm are probably already balking at this, citing the trouble we ran into there with regards to putting 24p on DVD and the bumps in the road with getting it back out.  Those problems are fortunately for the most part a relic of the past.  The issue in the DVD era was that films were first transferred to interlaced video.  Often they were manipulated or even edited in that format on equipment, oblivious to the 2-3 cadence within, which would then break that cadence.  We would then feed that potentially imperfect interlaced signal to a DVD video encoder which had to "detect" the film cadence within, more often than not with less than perfect results.

In this HD Digital era we are either shooting 1080p24 digital or we are transferring film to 1080p24.  There is no interlaced intermediary.  When it comes time to convert it to 1080i60 for transmission or storage on disc, we are feeding a perfect digital p24 stream to the encoder which turns out a 1080i60 signal with, for all intents and purposes, a "perfect" 1080p24 buried within.  All it takes is correct video processing at our end (the high definition DVD player and/or display) to realize it.

⁽²⁾ You might have heard of a slight variation on this, known as Progressive Split Frame.  It was a way for Sony to get their legacy gear, such as D5 tape, to handle 1080p24.  It is essentially a 1080i48 signal which is carrying a 1080p24 source, consisting of a simple 2-2 cadence (as opposed to the 3-2 cadence being carried by 1080i60).  While there have been a couple of projectors able to handle this format natively, it is at this time of little concern to consumers.