The problem with High Frame Rate (HFR) films

December 04, 2014

Today I read an article on The Technium on why High Frame Rate (HFR) films don’t look like traditional films, and why some people don’t like it. The article contains excerpts from a conversation with John Knoll, who gives a great explanation in layman’s terms. Via Coudal:

Imagine you had the lucky privilege to be invited by Peter Jackson onto the set of the Hobbit. You were standing right off to the side while they filmed Bilbo Baggins in his cute hobbit home. Standing there on the set you would notice the incredibly harsh lighting pouring down on Bilbo’s figure. It would be obviously fake. And you would see the makeup on Bilbo’s in the harsh light. The text-book reason filmmakers add makeup to actors and then light them brightly is that film is not as sensitive as the human eye, so these aids compensated for the film’s deficiencies of being insensitive to low light and needing the extra contrast provided by makeup. These fakeries were added to “correct” film so it seemed more like we saw. But now that 48HFR and hi-definition video mimic our eyes better, it’s like we are standing on the set, and we suddenly notice the artifice of the previously needed aids. When we view the video in “standard” format, the lighting correctly compensates, but when we see it in high frame rate, we see the artifice of the lighting as if we were standing there on the set.

That makes a lot of sense, but it only explains why HFR movies look different, not why they feel different. Image quality is one thing, but motion is a separate problem, and the article doesn’t negate the fact that, for most people, the extremely fluid motion typically associated with HFR movies feels extremely awkward, placing the entire viewing experience right into the uncanny valley. Typically, that fluid motion effect is explained away as a side effect of HFR films being more realistic, but I certainly wouldn’t describe them as such. Life, after all, isn’t viewed at 48 fps.

The Hobbit - The Battle of the Five Armies

Hyper-realistic Middle Earth is not so fun, after all

I also take issue with this statement:

I told Knoll that these complaints about the sterility of the new digital format reminded me of the arguments against CD music albums. Digital was “too clear” “too clinical” not “warm and fuzzy enough” according to audiophiles. CDs missed the musical ambiance, the painterly soul of a song. The critics were not going to buy CDs and the labels would have to pry their beloved analog vinyl albums from their dead hands. Of course, for average music fans, the clear hiss-free quality of CDs were soon perceived as much superior, particularly as the “frame” rate of the digital sampling increased past the point of most ear’s perception. “That’s exactly what it is like, ” exclaimed Knoll. HFR is the CD of movies right now.

I believe the comparison between digital movies and digital audio vs. their analog counterparts is mostly fair, but there’s quite a bit more to it than that, and it’s not all rosy. HFR movies are not a new digital format as that paragraph seems to suggest, they just have a higher frame rate. To use their own analogy, in digital music processing a higher sampling rate does not increase playback fidelity beyond a certain point. In fact, a too-high sampling rate — above 96 kHz — may actually degrade the listening experience due to a phenomenon known as “intermodulation distortion”. In a very interesting article, Justin Colletti explains how this works:

192kHz digital music files offer no benefits. They’re not quite neutral either; practical fidelity is slightly worse. The ultrasonics are a liability during playback.

This runs counter to many initial intuitions regarding super-sonic sampling rates – my own included. But the evidence is there. Since analog circuits are almost never linear at super-high frequencies, they can and will introduce a special type of distortion called intermodulation distortion.

This means that two super-sonic frequencies that cannot be heard, say 22 kHz and 32 kHz, can create an intermodulation distortion down in the audible range, in this case at the “difference frequency” of 10kHz. This is a real danger whenever super-sonic frequencies are not filtered out.

There’s nothing strange or non-factual about that. It’s not a matter of opinion or personal preference, either: it’s just basic signal theory. I’m not sure how it relates to digital video processing, but I wouldn’t be surprised if the effect was similar: there’s probably an optimal playback frame rate for the human eye, and exceeding it may bring unwanted side effects such as the uncanny motion we’ve seen in the HFR versions of the Hobbit movies.

There’s certainly more work to be done in this area, and these are early days. The movie industry may eventually find a higher-than-24 fps frame rate to be the optimal compromise between image quality and motion realism, but judging by the looks of it, 48 fps isn’t it. And if I had to bet, I wouldn’t place my money on going higher.