Mixed cadence, a broadcaster’s view
Mixed cadence, a broadcaster’s view
Sadly, nothing in life is perfect and this is as applicable in the broadcast sector as anywhere else. Despite the best laid plans, what starts out as something good will over time fall foul to simple time and commercial pressure:
As soon as compromises are made, the potential for problems – either immediate or further down the workflow – is created.
The media value chain is long and the lifetime of media can also be long. In today’s busy world, there just isn’t the time or money to keep material in its native format throughout a broadcast facility. The consequence is that a US broadcaster or media company will normalize all of its content to a standard Mezzanine form on ingest. Commonly this will be a format like XDCAM HD or AVCIntra or DNxHD or ProRes, but in nearly all cases the frame rate will be 29.97.
How deeply seated is the mixed cadence problem?
Providing that the only thing that happens next is simply the playout of the media, then all is well. Standard, reliable, known ways of inserting a 2:3 cadence are in common use in ingest devices, transcoders etc. and those devices, by and large, are quite good at it. Life, however, is never that simple. Many processes are required before the content airs, depending on the material and the territory. Whereas a few years ago, these processes may have been done on a live video stream, there is a trend for these processes to happen in the editing software.
- Edit for duration
- Transcode on the way into and / or out of an edit platform
- Overlay a logo
- Edit for censorship
- Insert black segments / slugs for adverts
- Squeeze & tease
- New credit roll
- Transcode on the way into an archive
- Transcode on the way out of an archive (maybe as a partial restore)
So we now have a sensibly made movie that has become a video sequence with a 2:3 cadence where the chance that 2:2 cadence video edits, effects and overlays corrupting the underlying 2:3 cadence has dramatically increased. In short, your perfect film has turned into a video nasty.
Commercial pressures lead to rushed decisions
Again, if all you’re going to do is play it out at 30/1.001 fps then any problems are generally invisible. Life, today, is getting more complicated and there is huge commercial pressure to quickly and automatically push that content on a multi-platform distribution system, or move the content to another territory for re-broadcast.
The lifecycle of the media becomes more erratic now. Sometimes there will be an attempt to recover the original 24fps material, often there will be scaling to change resolution, sometimes there will be scaling to change aspect ratio, sometimes there will be a standards conversion to a new frame rate, sometimes there will be further editing. This is where the problems really start. Compression algorithms work best when content is predictable. A regular 2:3 sequence should be easier to compress than a 2:3 sequence with breaks, hiccups and video overlays.
Removing a 2:3 sequence is trivial if the sequence is regular. If, however, the sequence is irregular with video overlays on portions of the content and with video inserts, then getting back to the “original” 24fps becomes very difficult.
Failing to handle the cadence correctly has a knock-on effect on every downstream process because once upstream problems have been “stamped into” the content by a compression stage, then removing is more and more difficult.
Mixed cadence – keep the solution simple
Fixing cadence issues assumes that you know what problem you want to fix. There are some very complex, and costly, cadence correction solutions on the market but you don’t need, or want, something that is too complicated. In an ideal world, your operators need a solution with a GUI that is intuitive and easy to operate.
Today, more flexible post-production workflows make this mixed cadence challenge is a more common occurrence. You need a solution that fixes the business problem and delivers your perfect film back from the video nasty.
At AmberFin, we have looked at the various techniques involved in removing cadence problems and in changing frame rates in general. We have developed a method of adaptively switching between different conversion mechanisms, which provides the ability for user control of conversion policy on a file by file basis as well as the ability to review automated decisions within a QC environment.
To learn more about this really neat, business focused mixed cadence solution you can download our free White Paper.
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