Bitrates

[lg.richard]

Hi all,

as I'm a new user here, let me introduce myself.
I'm Richard from Brazil. I used to be an editor as a student, but things led me to film festivals, from 35mm print traffic to video projection. I used to work at small festival playing files from a tvix box, later I started working as some kind of QC and print traffic in bigger festivals, that's where i met DCPs.

I'm just starting creating dcps, mostly to small advertising in festivals and some old films from low quality files or dvds.

In the time i worked with the Tivx the files were limited to 40 mbps, not that that bad i used to think, because blurays were about 25mbps. Most of the times I had to convert something (the player wont play avi, prores or 2K) I had 2 options: keeping the low bitrate from original file or reducing it to less then 40mbps.

As DCPs bitrates are sugested to be at 250 mbps maximun I use to trust the 100 mbps or 150mbps DOM sugests as default, most of the times it's 5 or 10 times more then the original files.

But today I had 2 situations that made me come here to ask if there's some kind of logic i should follow to choose bitrats in DOM: a 173 mbps full hd prores (that i just repeated the number in DOM) and a 38mbps 4K feature. I had to encode both 2k and 4k dcps from the prores (as it takes too long here to proccess the 4k one i did the 2k just in case the 4k ingest fails), and choose the same bitrate, generating files with the same size. That's what made me think if the 4k shouldn't be larger or if it doesn't make any sense as the source is already so compressed that I'm just generating useless aditional data in both 2k and 4k.

Can anyone light my path? sorry for the long text, im not a native english speaker! Hope DCPs get bigger for me and I can join this community

[Carsten]

It is indeed so that J2K is highly efficient retaining high detail. That's why 4k DCPs usually don't need twice or four times the data rate.

In general, ProRes and DCP share similar compression technology, so, it is indeed a useful rule of thumb to seek similar data rates. And with similar, I'd say, choose J2K somewhat higher than ProRes. That of cause is no mathematical relation. Just an educated guess.
Now that rule of thumb doesn't apply to interframe codecs like MP4. They work very differently and more data efficient. Even if a 4K HVEC or MP4 has only 38MBIt/s, you should probably use 150-200MBit/s for the DCP.

That said - as I know that J2K is very good, I don't want to think about it again and again. So I set my default compression to 200MBit/s and never think about it. As in my everyday work, I don't have to care about the DCP filesizes I create. I think, anything between 150 and 200MBit/s is better than usually needed.

Sometimes we need to reach a specific DCP file size, in order e.g. to fit a specific media capacity, in that case, it may be useful to reason a bit longer.

Keep in mind - other than resolution, frame rate DOES matter, so, the moment you go beyond 25 fps, you probably should use a higher data rate.

[lg.richard]

Thank you! Ill keep that in mind

[IoannisSyrogiannis]

Since this is a discussion where one can hardly get hard facts, I will share my view on it:
More and more, and the less one wants to get a headache the more, people make DCPs with "more is better" in mind.
I will get the image bitrate to 230Mb/s and what do I have to worry about? Not that the bitrate will be too low, certainly.
Getting that warning when validating with DCP-o-matic that one or more frames exceed the maximum bitrate is no surprise.

So, I have seen 2K Scope DCPs running on 235Mb/s more often than I would like.

And the reason that I write that I don't like those, is that -unless those DCPs have too much high contrast static imaging, like a test pattern or similar- there is no visual benefit to go over 75Mb/s on 2K. And that is not me talking, that is people who were actively involved on setting standards.
In addition to that overzealous rising of the bitrate, you have the results of it. That is, more storage needed, more time to transfer (between screens, between a drive and a screen server, through internet, etc.), less screenings you can prepare on a single screen, if you run a festival (where screenings are close to unique, especially if you work with different venues or screens).

For 235Mb/s DCPs, you don't need an example, you will find a lot. I will give you -though- an example of a 4K trilogy that we had to screen on the cinema I work with a couple of months ago. (Almost to the day.) They were multi-reel, from a well known distribution company, and I was very curious, so I run clair-meta probe (a program that does DCP checking, look it up if interested, it's free and I could make it run on Windows through that windows-on-linux feature) to check the bitrate on those. Here is what I found:
Movie 1 of the trilogy:
Reel#5, lasting 18:54 had an average of 230,6 Mb/s, with a peak of 237.38. On other reels, averages were not higher than 121Mb/s.
Movie 2 of the trilogy:
The only reel that was higher than 107Mb/s was the credits' one. Most of them averaged lower than 100Mb/s.
Movie 3 of the trilogy:
The higher bitrate was on the credits' reel, 192 Mb/s. No other reel averaged more than 118Mb/s.

So, my two-cents (or centavos): If you don't have anything to lose, you may set the default to 200Mb/s and forget about it.
If the storage, distribution time, internal transfer delays make a difference, start low (yes, as low as 75Mb/s) and go up depending on frame rate and resolution, keeping in mind that 2xResolution doesn't ask for 2x bitrate.
DCP-o-matic does not have what -say- EasyDCP describes as "JPEG2000 Visual Weightings", but I believe one can be both be sensible and have good results by setting the bitrate without "more is better" in mind.
Take it or leave it. Either way, there is not going to be a big difference on what the trend is.