Interlace for broadcast

Hi everyone. Just produced my first TV commercial and saw it on air for the first time today. Bit of a shock as it looked completely different than how it looked on my computer. I'm not talking about the color, Im talking about the motion. The motion looked overly smooth, it was like watching the Hobbit in 48fps for the first time - just odd.

The ad was filmed on a Lumix GH4, at 4K, 1080 and 96fps, with the shutter speed set to double whatever the frame rate was. Edited in a 1080 25p timeline, exported as a 1080 25i (top first) ProRes as requested by the network. From what i can gather this is a standard workflow so I'm not sure why the motion looks so smooth. The surrounding commercials looked like the normal, familiar motion that we are all used to.

Any ideas? Is it the GH4 just not matching the high end cameras that filmed the other ads? Although they must use similar settings and workflow so surely motion would be the same? Have I done something wrong with the interlacing? Maybe I should be filming at a higher shutter rate?

Any tips on your broadcast workflow would be great.

Shoot 25i, edit 25i, deliver 25i. You did a frame rate conversion, and I'm attributing that to what you saw. I get odd effects when I shoot 120fps and edit/export to more normal frame rates.

Usually when we talk about "smooth" motion we mean the motion looks good, as opposed to "jerky" or "choppy". So I wonder what you exactly mean by "overly smooth".

If motion does not look perfect this often has to do with frame rates. The golden rule is: always shoot at the frame rate of your delivery format, except for retiming purposes (fast or slow motion).

So if you don't need to change the timing of your footage there is no benefit at all in shooting at a different frame rate than your delivery format. Furthermore, 96fps is absolutely bad for conforming to a 25fps timeline because there is no way you can divide 96 by 25 and keep integer frames. This could pretty much explain the motion issues.

You may not have noticed the issues on your computer monitor because computer monitors don't show you the real quality of your video. If you work for broadcast it is imperative that you work with a calibrated video monitor hooked to your edit system. This is the only way you can see how the video will look on television sets.

- Ronny

Yes, computer monitors do not work the same as TV screens. Can't use a compute monitor as a broadcast monitor. If you don't have a broadcast monitor, always play it back on a TV somewhere to test it out first.

BenB - filming 25i not an option as need slowmo. Agree about the need for a TV screen.

Ronny - By overly smooth I mean it looks like its playing at 50fps, like the unusual motion "The Hobbit' had that a lot of people hated. It's missing that slight jerkiness that I should have got from filming and editing in a progressive environment. It must have something to do with the interlacing of the final export. Maybe I'm not doing it right but I can't see where I'm going wrong.

For "in camera" slow motion shots you should shoot at a higher frame rate that can be divided by the editing frame rate. For editing@25 fps that would be 50, 100, 1000 etc... This will also make your high-speed footage look natural when played at normal speed. If you don't need slow-motion it is always better not to shoot at a higher frame rate.

96 fps is only used for creating in-camera slow motion in a 24p timeline. When you play 96fps video at normal speed (i.e. using rate conform) in a 25fps timeline it will not produce natural motion. That's where you went wrong from the start.

Exporting a progressive project as interlaced ("Field Rendering") does not affect motion as it only renders out the existing full frames as separate fields. 90% of the programs we edit are shot progressive and edited progressive. But many of these programs will also be exported as interlaced, depending on the delivery specifications of the television station that will be airing the programs. It's a very common process that does not affect the picture. The opposite (de-interlacing interlaced video for progressive output) does produce motion artifacts because the full frames have to be created artificially by doubling or interpolating the half fields.

- Ronny

I don't know of Ronny's last post answers the question already, but I had a thought. Native 25i does look like 50fps because each half frame is a 50th of a second offset from the last. When you convert something from a progressive source, generally you get the same frame offset by 50th if a second, so it's sort of pseudo progressive.

Is it possible that as the source was 96fps when you exported a 25i file it used the extra frame info to make a natural interlaced movie giving you that smooth look? What you could try is exporting a 25p master file and then converting that to interlaced so the encoder doesn't have any extra frames to work with. It's a guess but might be worth a go.

Hey Seamus,

Great thought, but I'm afraid this won't work. When you drop a 94fps clip on a 25fps timeline and you don't select "Automatic Speed" the footage will get conformed to the project frame rate, which means FCP X will remove any redundant frames to fit the original frame rate in the Project's frame rate. Those frames are gone and you cannot get them back when exporting the project.

You can test this: drop a 50fps clip with fast motion on a 25fps timeline and manually Retime the clip to 50%. Although the original video has 50 frames/sec the retiming will not be perfectly smooth because you only have 25 frames left on the timeline and FCP X will artificially double those 25 frames to achieve the 50 frames you need to get the 50% slow motion, it will not use the 50 original frames.

The problem here is that you cannot divide 94fps by 25. When you drop 94fps footage in a 25fps project FCP X will drop redundant frames, but as it cannot divide 94 by 25 this will create slightly unnatural motion. You may not notice this on a common computer display because of the odd refresh rates, but a video monitor will show this.

- Ronny

That makes sense, but wouldn't you expect that to cause a certain 'juddering' rather than the smoother motion described?

I've just run my own tests on some footage in a 25i timeline. The first is the 50p running on a 25i timeline, the second is the same shot exported as 25p and then put into the 25i timeline. As you can see the first one looks like 'native interlaced' footage while the second looks like progressive. This would surely better account for the smooth motion seen?

Thanks for your discussion guys. There has been a lot of focus on the whole 96fps thing. To be honest it looks fine and that's not really my issue - its all of the footage, the 25p and 50p that look odd after being edited in a 25p timeline and exported as 25i.

The general theme I am picking up is to export a 25p master first and then convert to 25i. I'm still not sure why this would give different results to than exporting straight from a 25p timeline though.

The general theme I am picking up is to export a 25p master first and then convert to 25i.

To be honest, that's what I thought you had done. How have you exported your 25p project to 25i? Using File > Send to Compressor?

- Ronny

Yep, that's right and then created the required presents in compressor then exported. I figured creating a master file from FCPX and then running through Compressor again is just adding another unnecessary compression.

Aha, this could in deed explain the issues. And in this case Seamus is correct.

When you send an FCP X project to Compressor, Compressor will disregard any render files or rate conforms on the timeline and it will recompress every frame on the timeline based on the format and frame rate of the original media that the clips on the timeline are referencing to. Which means it will also take the original frame rate of these clips and re-conform or retime it to the export frame rate.

When you export your 25p project as a Master File and you convert the Master File to 25i using Compressor, Compressor will just take each frame from the file and no re-conforming or retiming will be done. So yes, this can make a difference in the final file.

Send to Compressor was useful in the DV era when people wanted to export a project from a DV timeline to a different format such as DVD without exporting a DV master first and recompressing this already highly compressed master once again. With today's high-quality mastering formats such as ProRes 422 and higher, Send to Compressor is not really useful anymore. In many cases it's even better not to use it. Tom Wolsky has written an excellent overview of Compressor 4 and how to use it best with FCP X: www.fcpxbook.com/articles/compressor1/

- Ronny