Technical Discussion: Frame Rates


#1

In my experimentation with deinterlacing I have noticed that the output of certain video filters produce a double frame rate. Instead of the NTSC/ATSC 29.97 fps the output is 59.94 fps. However, I can play these files normally in a video player such as VLC.

I became interested in television perhaps 50 years ago and, although I never worked in that industry, for several years I worked as a volunteer editing video tapes for public access cable tv. We used 3/4 inch U-Matic cartridge tape. Which leads to my question…

In my head, even though technically trained, I think video identified as double the normal frame rate ought to play twice as fast! Questions:

  1. Why does this not happen?
  2. Since it seems to not happen in our modern digital domain, then why don’t we produce all destination video in double frame rate since it would likely have smoother motion and fewer observable artifacts?

Bonus question: Some of our modern TVs have circuitry that boosts display fps to 120 and even 240 fps. Why not just push the envelope and do those frame rates?

-=Ken=-


Reduce Noise - replace smartblur with hqdn3d or nlmeans
#2

Video filters cannot change the frame rate. You are probably misreading something - like clip properties vs. Master properties. You can have a 59.94 fps clip in a 29.97 fps project.

video identified as double the normal frame rate ought to play twice as fast

Frames are dropped or repeated as-needed to adapt to the project frame rate. In my example above, every other frame is dropped from the clip.

why don’t we produce all destination video in double frame rate since it would likely have smoother motion

Because it creates bigger files that need more bandwidth. Because some people do not like the look of high frame rate for certain types of video, typically movies - film look.

Why not just push the envelope and do those frame rates?

Many of those TVs or the media players playing to them do not handle these high frame rates. The TVs do interpolation, and many people including myself do not like those effect and turn this off. The high refresh rates can also have something to do with how a 24 fps source is played to give a more film look.


#3

Hi Dan. Sorry - I was referencing preprocessing before adding clips to Shotcut. Brian suggested using FFmpeg to deinterlace my old analog video so I can retain the full vertical resolution. FFmpeg has deinterlace “filters” that have options to write a frame for each field, yielding a double frame rate. Since they play at normal speed I am curious how this can be so.

-=Ken=-


#4

“fps” is ambiguous. It could be frames per second or fields per second.

In the pre-digital era, video was transmitted as 60 interlaced fields per second. Each field consisted of alternate scan lines, i.e. lines 1, 3, 5, 7, etc. The next field contained the opposite complement of scan lines, i.e. lines 2, 4, 6, 8, etc. It took two fields to make up a complete frame. The field rate was 60 fields per second. The frame rate was 30 frames per second.

In modern digital video, progressive scan is the opposite of interlaced. All scan lines are sent during each period rather than alternate scan lines every other period.

Scroll down to see a table of typical frame/field rates used in U.S. broadcast.

Remember that TV broadcasters are allotted 6 MHz of RF spectrum in the U.S. and that’s it. Nowadays they also squeeze in digital subchannels, carrying standard-definition programming of vintage shows.


#5

Hi Chris. Thanks for weighing in. I have another topic going - trying to get my old analog video deinterlaced without losing the vertical resolution. All of the video is from the same source, digitized the same way, and some looks pretty good but others have very obvious combing that so far have defied my attempts to remove it while retaining the maximum possible vertical resolution.

Since the FFmpeg deinterlacing filters I have tried so far have an option to write a frame for every field then perhaps I could simply mass convert my source files to 59.97 frames per second and edit it in Shotcut that way. My destination would be a DVD but maybe the Export in that case would be cleaner and more detailed?

-=Ken=-


#6

Interlace was a neat trick the RCA engineers came up with in the 1930’s to increase vertical resolution (number of scan lines) by tricking the human eye’s persistence of vision. As you can see, sending alternate scan lines every other field cuts the required bandwidth in half.


#7

Hi again Chris. I am very familiar with television technology “conceptually” but certain “practical” issues still elude me. If I ran a videotape past the heads twice as fast the video would be playing twice as fast. But not so for digital playback. How come?


#8

To make a long story short, digital is much more complex than analog videotape. There needs to be time for the digital bits to be processed, and it needs to be done in conjunction with the software that will process those bits.

This gives you an idea of how simple analog was compared to digital.


#9

Ken -

Why can’t you leave your video interlaced?

As an exercise I have added interlace to video shot at 60p (60 frames progressive). It looked fine to me. I tried this because broadcast wants either 1080 interlaced at 59.94 fields/29.97 frames per second, or 720p at 29.97 frames per second. BTW those are the same frame/field rates we had in analog.

In the U.S., ABC and Fox have standardized on 720p. CBS, NBC and PBS have standardized on 1080i.


#10

Thanks again for the reply. As I mentioned in an earlier post there are some portions of my video that defy de-interlacing. Some looks good, others look like viewing the scenes through a screen door - all with the very same origination. Perhaps you might take a look at my other topic Technical Discussion: Interlacing Revisited
Download the two photos there and enlarge them to full screen and you will see what I mean.

In 2007 I bought an HDV camcorder which records in 1080i and I never have a problem with the interlacing. It always looks good.So, not an issue there.


#11

I don’t know. If the original is Hi-8 it might be a case of garbage in - garbage out.


#12

Hi Ken -

You should visit videohelp.com. They have a section devoted entirely to restoration:

https://forum.videohelp.com/forums/41-Restoration


#13

Chris, I really do appreciate any help I can get. But I am not a neophyte when it comes to video processing. I have hung out on Videohelp and Doom9 forums. I have followed the writings of lordsmurf on Videohelp and owner of digitfaq.com for years. I used Avisynth and VirtualDub some ten years ago. It’s just that some video is problematic and defies correction.

More modern video sources seem to have solved these problems and you don’t have to fuss with them so much. Old analog video has often presented problems and there are endless discussions on those and other forums. Some people play with this stuff for years before they develop something that works for a particular tough problem. I put off this project for years because there seemed to be no solution to my particular problem. However, there are new deinterlacing filters that some people are raving about. I was hoping that present-day editors would have incorporated these newer solutions. It appears that is simply not the case.

My home video processed by Shotcut actually looks pretty good. Video of family members in the house look great. But outdoor video, under certain circumstances, seems to exceed the ability of those older systems to hide the inherent flaws.

-=Ken=-


closed #14

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