Converting 29.97i for OTT is Often More Difficult Than Expected

Guest post written by Ernie Sanchez, VP & COO, Cinnafilm

29.97i is a wonderful container for broadcast television because it works even though it allows for different video essence frame rates to be placed back-to-back in the same project! But converting 29.97i for OTT is the gift that keeps on giving and can be a challenge beyond expectations.

In the first post in this guest post series we covered broken cadence. Here we dive into the difficulties of converting 29.97i for OTT and provide insights into how to overcome them.

Editors are faced with the challenge of telling the story, and oftentimes they are working with disparate frame rates and scan types because that is what they were handed to work with. But luckily, and unfortunately, the 29.97i timeline can accept just about anything – as long as the right patterns are applied to each video essence so that everything is “normalized” to 29.97i. We’ll call this situation a “Frankenstein” 29.97i.

Why converting 29.97i for OTT is necessary

For broadcast playback, Frankenstein 29.97i files work just fine. But once the initial broadcast playback is complete, the world of OTT and On-Demand take over and the file needs to be in a progressive state for those platforms. Without having a roadmap that outlines how the editor created Frankenstein, converting 29.97i for OTT or On-Demand delivery with multiple applied patterns to a progressive state can be a significant challenge.

What makes creating that progressive-only version difficult is the original video essence must be extracted from each patterned section and then converted to a common frame rate in a manner that does not cause pauses or skips. Pauses and skips will take viewers out of the story because it is highly distracting when motion is not smooth. Before we go into too much detail about how to achieve the goal of creating a smooth progressive version from a Frankenstein broadcast asset, I think it is important to explain what I mean by “video essence.”

Video essence is simply the original frame or field rate the images were captured at by the camera. Here is a list of common rates:

  • 60 frames/s
  • 59.94 fields/s
  • 59.94 frames/s
  • 50 fields/s
  • 50 frames/s
  • 30 frames/s
  • 29.97 frames/s
  • 25 frames/s
  • 24 frames/s
  • 23.976 frames/s

Frames/s are a progressive capture rate, and fields/s are an interlaced capture rate. Field rate is unique in that a 59.94 fields/s capture is always placed in a 29.97FPS container (50 fields 25 frames is the European equivalent). There are two fields for every interlaced frame – an upper and lower field. So while 59.94 images are being captured, it takes two fields for each frame. For this article, FPS will refer to frame rate, and Field Rate will always be spelled out.

The difficulty with converting 29.97i for OTT

Ok – back to why Frankenstein can create a headache for OTT distribution. 29.97i is unique because it allows great flexibility when it comes to video essence and there are three video essences that can be placed in the container, all in the same file.

Think about an interview you have seen where they are talking to an actor or director and the images have a stark, video look. Then they cut to a scene from a movie, and that segment has a decidedly different, filmic look. Then they take some other behind-the-scenes footage taken with a smartphone that looks different than the previous two. In this example:

  • The interview part was probably captured at 59.94 fields
  • The movie segment was certainly captured at 23.976 or 24 FPS
  • The behind-the-scenes footage was probably captured at 29.97 progressive FPS

The reason the filmic footage can be seen with the 59.94 fields is because of something called “pulldown” patterns. The most common of these patterns is called “telecine.” Telecine is when you take four progressive 23.976 frames (A, B, C, D) and you pack those four progressive frames into an interlaced 29.97i container using the A/A, B/B, B/C, C/D, D/D pattern (upper/lower field) – thus giving you five frames from four.

The reason the 29.97 progressive frames can be seen with the 59.94 fields and 23.976 is because 29.97 progressive can just be placed in the 29.97 interlaced containers. When this is done, it is called 29.97i PsF (progressive segmented frame). Many cameras provide the option to output 29.97p or 29.97i PsF, and they provide this so it can accommodate both a progressive and interlaced editing timeline.

So when converting 29.97i for OTT in this file example, a simple deinterlace (and there is NOTHING simple about a proper deinterlace) is not going to work. Well, you can, but it will not be a very good result. Telecine has three progressive frames for every five, and deinterlacing a progressive frame will cause resolution loss. If the 29.97 PsF sections are deinterlaced, resolution loss will occur as well in those sections. So for those that care about quality, a blanket deinterlace is out of the question.

Correct solution for converting 29.97i for OTT

The correct solution is to identify the video essence of each scene of Frankenstein and treat each video essence differently/optimally. Is it 59.94 fields? Is it 23.976 with telecine? Or is it 29.97 PsF?

Tachyon’s intelligent frame analysis does just that! Think of it as the automated solution that has the “editor’s roadmap” of what they did when they created Frankenstein. Of course, we don’t have the EDL, but Tachyon’s analysis engine identifies every segment that is 59.94 fields, 29.97i PSF, or 23.976 that has a pulldown pattern and extracts the original video essence automatically.

Once the original video essence is in hand, PROPER conversion to any other frame rate, both progressive and interlaced, is possible. Having a tool like Tachyon that can maintain the original look and feel of the 29.97i broadcast is essential for repurposing billions of hours of content accurately. Tachyon is a tool that is trusted more than any other image processing technology because it respects the original essences and stays true to the original artistic intent.

If content repurposing/versioning has been problematic in your organization especially where converting 29.97i for OTT and on-demand, we look forward to talking about how Tachyon in Vantage can provide the immediate solution you have been looking for.

More Information

For more information about the integration of Cinnafilm Tachyon and Dark Energy with Telestream Vantage, you can read the solution brief posted on the Telestream website.

For more information about Cinnafilm Tachyon, you can go to the Cinnafilm website.

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