Why you want a 120Hz LCD TV


In talking with Alan the other day it became obvious that we should have a write up about why 120Hz LCD HDTVs are a big deal and why they should be your TVs of choice.

Read on for the details

A quick refresher on the common HDTV formats:

  • 720p is 1280×720 progressive scan with 60 frames per second [720p/60]
  • 1080i is 1920×1080 interlaced and is normally found in 60 interlaced fields per second (this is effectively 30 frames per second, with an exception we’ll cover below) [1080i/60]
  • 1080p is 1920×1080 progressive scan, which can be 24 frames per second, 30 frames per second or 60 frames per second. 1080p/24 is the most common format today, it is the native frame rate of filmed material and is how both HD DVD and Blu-ray store filmed content. 1080p/30 is also a possible rate from video material targeted at Blu-ray and HD DVD. 1080p/60 content doesn’t really exist aside from HTPC output or perhaps some next generation console games. However 1080p/60 is the most common output format for Blu-ray and HD DVD players currently (to match the 60Hz rate of common HDTVs).

A problem of math:
You’ll notice that most of these are divisible evenly into 60Hz, which is the power grid rate of North America, and not surprisingly, the refresh rate of most flat panel TVs. The big issue here is that 24 FPS doesn’t work with 60Hz evenly, and 24 FPS is the film industry standard, so it’s rather important. How it was handled in the DVD era was to code a 3:2 cadence of information into the MPEG stream, so that every 3rd field was repeated to make 24 FPS ‘fit’ into the 30 FPS of the NTSC standard. This is also called "Telecine." The process of undoing this to reconstruct the proper 24 FPS material is called "Inverse Telecine" — also known as "Reverse 3:2 Pulldown" or "3:2 Pulldown Removal."

Video processors today do a pretty good job of locking on the 3:2 cadence and reversing it, but the fact remains that there is repeated data. This repeated data causes what is known as "telecine judder" the extra data causes a slight hitching, most obvious during slow, steady camera movements which appears as if the image jerks a bit.

Blu-ray and HD DVD players still have 60Hz issues. Despite being 24 FPS native on the optical disc, the output of most next gen players is 1080p/60. This means that the 24 FPS material on the disc has to be fudged for 60Hz just like the other formats (a doubling of the 3:2 cadence used for 30 fps — a 6:4 cadenace, is used for 60hz). This is changing thanks to displays that accept 1080p/24 natively.

The 120Hz fix:
Wouldn’t it be great if we could skip all this 24 into 30 junk? The answer is to find a number that is divisible by all the frame rates and is fast enough to not cause flicker. The answer is 120Hz! 120÷5=24, 120÷2=60, 120÷4=30. With 120Hz 3:2 judder can be a thing of the past… with the right sources and TV with proper 5:5 pulldown (24fps x5) support. 

New equipment is needed:
Unfortunately you’ll need a new display and sources that offer 24 FPS natively.

120Hz displays are starting to become more common, nearly every vendor of LCD TVs has announced upcoming product lines with 120Hz support. Plasma vendors don’t seem to be ready with 120Hz processing just yet, it may also not really be needed for Plasma vendors since they don’t have the lag LCD has, 72Hz seems to be more then fine for plasma HDTVs, Pioneer has models out now that accept 1080p/24 and display it at 72Hz (3:3 pulldown).

You’ll need a video source that can output the film natively too. There are two ways to get high definition 24 FPS data.

The idea of a "pure" mode is something still new to most DVD player manufacturers and isn’t really where you’ll see this kind of feature. Where the 24 FPS native output matters most is in the new high definition optical formats: HD DVD and Blu-ray. These two formats were made for videophiles, as such they have native 24 FPS video in their specifications (DVD doesn’t). Every Blu-ray or HD DVD disc of a film is stored in 1080p/24. Unlike DVD, the 3:2 cadence isn’t set in the video stream, it is extra data that can be ignored. The video circutry of the player is where the output is prepared for compatibility with your display. The video on the disc is basically the film digitized for optical disc media. Thus, in some players it is possible to allow native 24 FPS output. Currently only the Sony and Pioneer Blu-ray players have this option. Toshiba is supposed to add 1080p/24 native output to their 2nd generation high-end HD DVD player soon. Supposedly the PlayStation 3 has an upcoming firmware update that will add 1080p/24 output as well. 1080p/24 native output from HD DVD or Blu-ray is the only way to get totally unprocessed films in the home today.

A second source of 24 FPS material is the exception I noted back in the overview of the HDTV formats under 1080i. 1080i/60 can make use of telecine and transmit 24 FPS content with 3:2 cadences added. A good video processor will use inverse telecine and get the 24 FPS data back out. This source is of course not technically pure 24 FPS data any more because it has been processed to work in 60 fields. However, this is the next best source of 24 FPS data and is very common, as most prime time drama shows are shot at 24 fps and then transmitted as 1080i/60. This source type is entirely dependant on your HDTV having a good video processor that knows how to handle this type of 1080i/60 stream.

Videophiles rejoyce!
As you can see the abilty to view films the way they were intended without being tweaked to fit the NTSC or PAL concept of video standards is coming, and coming very soon. Keep a look out for more 120Hz processing HDTVs as we get further into 2007, nearly every major vendor of flat panel TVs has 120Hz models planned.

Resources around the web: 

For an incredible explanation with images of the whole 1080i vs. 1080p and 1080i/60 as a carrier for 1080p/24 check out Secrets of Home Theater and High Fidelity’s article High Definition 1080p TV: Why You Should Be Concerned. Part 3 is especially noteable as it explains what I talk about under the second source of 1080p/24 material.

This is a nice list of displays that have been confirmed by actual reviews to support display at a multiple of 24 FPS: