What’s the problem with playing 4K DCPs on 2K projectors?

Inserts of two terrier dogs, one at a higher bitrate than the other
Adapted from a Creative Commons photograph courtesy of Hilarmont

Here at theDCPmaster.com we work with a lot of indie films and independent filmmakers. We’re seeing a big increase lately in 4K masters, even on low-budget films. That means we’re having a lot of discussions about the differences between 4K and 2K DCPs.

We’ve written about resolution before – particularly why you don’t need to “blow up” a 2K DCP. But we also mention in that article that when you play a 4K DCP on a 2K projector it can be lower quality than a normal 2K DCP. Some filmmakers wanted more information about that, so we figured it was worth a full answer.

Bitrate vs Resolution

It all comes down to the relationship between bitrate and resolution. Bitrate is the amount of bits (data) that can be processed per unit of time. It’s how much information is given. Resolution refers only to the dimensions of an image. Many people think more resolution guarantees a higher quality, but this isn’t always true. The two pictures of the dogs at the top of this post are about the same size (resolution), but one clearly has more detail than the other.

An explanation I often use is to think about bitrate like “number of words”. The more words you can use to describe when writing about something, the more detail you can provide:

dcp-master-bitrate1

Resolution, on the other hand, is like the size of the sheet of paper you can write on. It gives you more space to write, but that only matters if you can use enough words to fill it. Many words on a small sheet of paper will still be a more detailed description than a few words on a big sheet of paper:

dcp-master-bitrate2

So to judge “quality” you need to look at resolution and bitrate together.

Bitrate limits of DCP’s

Most Digital Cinema Packages have a bitrate limit of 250 Mb/s (mega bits per second). It doesn’t matter if a DCP is 4K or 2K. 250 Mb/s is the most that theatre servers and projectors can process (our article on DCP file size talks about the few exceptions).

So let’s assume we have a 4K and 2K DCP of the same films at “the highest possible bitrate” – so they are both at 250Mb/s:

dcp-master-bitrate3

2K Projectors can play back 4K DCPs they just ignore parts of each image that they can’t project. There’s no rescaling or averaging, some of the information is just ignored, something like this:

dcp-master-bitrate4Please note the graph isn’t necessarily accurate in terms of how *much* information may be discarded, it’s going to be completely dependant on the individual frames, and encoder(s) used – but *some* amount of information above the 2K “core image” is going to be thrown out. If a 4K frame is encoded utilizing the entire maximum available bitrate, the 2K image preserved in a 4K feature file is going to be at a lower bitrate than it would be if the film was encoded at 2K.

So given same target bitrates, a 4K feature played on a 2K projector could have quality loss over a 2K DCP encoded at the same bitrate.

How much does this bitrate decrease affect a film?

If this is likely to be the case, or how much a theoretical decrease changes “overall quality” will vary on a film by film basis (and may be a non issue). In practice films are never recorded at maximum bitrate for their whole duration. DCPs use advanced compression so that each shot of a film only uses as much data as it needs until the change in “quality” is not perceptible (or master compression may happen during the post-production work-flow which makes high bitrates unnecessary).

But because 2K projectors are still the vast majority of all cinema screens worldwide, many filmmakers and distributors with 4K masters choose to make separate 4K and 2K DCPs. They can even both be put on the same drive, so each theatre can choose the version best suited to their particular venue. Other filmmakers are happy only making one version of the DCP (either 4K or 2K) to limit costs and depending on their particular specific film or exhibition plans.

Helping guide filmmakers through the options for their DCP resolutions is only one way a good DCP facility can help make sure films look their best on the big screen. If you think the team at theDCPmaster.com can help you – don’t hesitate to get in touch!’

Do you have questions to help size up the confusing world of Digital Cinema? Don’t worry Ask The DCP Master is here to help. Check out our most common questions, or ask your own.

Are you an independent filmmaker, distributor, or festival looking for professional, indie-friendly, DCP services? Visit TheDCPMaster.com to see what we can do for you!