Eyre Films Blog

Why Vengeance Trail is So Progressive: The Pain of Converting Analogue Filmmaking to Digital!

(Day 14 Counting Down to Memphis Premiere on April 23!)

Yesterday, I output Vengeance Trail at 24 fps (frames per second) to do a test screening. I played it from Vimeo on my 55" TV and it worked seamlessly. All I had to do was change the Sequence Settings from 25 fps to 24 fps.

YOU DIGITAL NATIVES HAVE IT SO EASY! This is not our origin story! Vengeance Trail was shot on Standard Def interlaced. Do you younguns even know what that means?

Instead of posting a quick update yesterday, I wanted to geek out with a teaching moment about how hard it was back in my day, LOL! Here's yesterday's belated blog...

Ian Max

TLDR: A primer on fields vs. frames. NERD OUT ALERT!

Films are edited at 24 fps and a frame is a frame. Film projectors actually double project each frame twice, to get the strobe into the persistence of vision range of humans. Think about the mechanics of advancing the film without tearing the sprocket holes, holding it locked while a shutter flashes twice, then advancing the frame again accurately. Now think about aligning two of these cameras and projectors for stereo 3D film projection. It's no wonder 3D needed digital capture and projection to work without giving people headaches. The vibrations!

CRT televisions required a raster to draw the frame twice, skipping lines, so a top (Odd) field and a bottom (Even) field. Who knows why they chose 30 fps back in the BW days? Oh, I remember, because NTSC is based on American AC electricity which comes into your house at 60Hz. So, cameras actually record at 60 fields per second and playback at 60 fields per second, but combining two fields into an interlaced frame becomes 30 fps. And don't get me started on changing the frame rate to 29.97 fps in order to squeeze in the color information. It took me years to realize that Drop Frame and Non Drop Frame were just different ways of counting the frames, the frame rate wasn't actually changing.

Nowadays, your digital monitor or television doesn't care if the video playback frame rate is 12, 15, 29.97, 30, 60, 120, 240, etc. It's so easy!

When we shot Vengeance Trail, DP Stephen McCurry chose a PAL prosumer camera, the Canon XL-1S because PAL is based on 50Hz AC electricity and its frame rate is 25 fps, which is pretty close to film's 24 fps and therefore once transferred to DVD would have a more filmic look.

How does 24 fps or 25 fps become 29.97 fps? I'm glad you asked. It's called The 3:2 Pulldown. 

So you see, in NTSC, frames 3 and 4 would have motion blur. 

Studio movies just repeat every 4th frame. 

Frame advance through a DVD and you'll see the pattern, every 4th frame is played twice, so 24 fps becomes 30 (ish - from now on I'm lumping 29.97 fps into 30 fps)). This method of repeating every 4th frame is a bit jittery. Since SD footage is actually captured at 60i

This time-saving factoid didn't apply to use, because we intended to burn NTSC DVDs! 

"PAL and SECAM television systems run at 25 frames per second. They are close enough to the film frame rate that 3:2 pulldown isn't required. One film frame is transferred to one video frame. The slight speed discrepancy makes a movie slightly shorter when it is transferred to video, but the speed variation is so slight it will not be noticed in viewing.

There is a method to convert 25 frames to 30 using a slightly different 3:2 pulldown to convert every 5 frames into 6 frames. In PAL, frames 4 and 5 (out of 6) would have motion blur. Unfortunately, Stephen converted all the footage before editing, and so locked in all sorts of motion blur. See a previous post about recapturing all the footage.

Don't forget about compression artifacts. If there's movement between two fields, it shows up as blur in a frame. And if you use JPEG compression on a frame with movement between each 60i field, the interlaced stripes become magnified.

Now you see why I sighed in relief when Topaz Labs added the de-interlace function to their Video Enhance A.I. blowup software? I don't know how they create good looking progressive frames, but I'm glad I don't have to ignore a field or blow up motion blur artifacts, and then reinterlace them to create a progressive frame.

You can definitely get into the nerd-weeds on JPEG compression.

And compression leads to generation loss, like when the first photocopiers would add dust to each copy and after a while a copy looked like it had black dandruff specs everywhere. Remember that? LOL. Remember when you recorded TV onto VHS tapes so you could watch a movie later? And if you copied a VHS, the edges would get blurry and colorful. Some weird people in Los Angeles have nostalgia for this. 

Again, you digital natives have no idea how good you have it. Even coding has become visual and intuitive; no more peeking and poking computer memory with machine code. Lucky! Ah, I've forgotten my roots. I think it's true, that I've forgotten more than my high schoolers have learned.

Out of curiosity, I exported the movie at 24 fps and 25 fps. Same timeline length of 01:23:17. Same exact file size. All I changed was the Sequence frame rate. But when I open both windows in QuickTime and hit play at the same time, there's no noticeable delay or difference over time. Then I frame advanced and 24 frames in the first equals 25 frames in the second. I don't need to find that dropped frame, but the sound track scaled perfectly. Now, to see if it matters to the theater when I deliver the DCP.

See you tomorrow with the post I started writing yesterday.

Ian Max