Captioning For Digital Cinema Packages

Luke Kreger4 min read

Theaters are striving to become more accessible to everyone. This means that filmmakers will often be required to provide caption files for their films. Those caption files must meet certain requirements that are determined by SMPTE (the organization that creates all the standards for cinema).

Moviegoer watching a film with captions shown on a seat-mounted closed caption device

Create a source caption file

The first step in creating caption files is to generate a source caption file in .srt format. You can do this by exporting your caption track in editing software, like Premiere Pro, or use an online service to automatically generate your caption text. Our favorite service for generating caption text is Rev.com.

When creating the caption file, keep the SMPTE caption limits in mind. A track that breaks them puts your DCP at risk of being rejected by the cinema server. These are the limits from SMPTE RDD 52, the specification theater servers check against:

Characters per line, closed captions: 32 maximum, spaces included
Lines per caption: 3 maximum
Characters per line, on-screen subtitles: 52 recommended, 79 maximum
How long each caption stays up: At least 15 frames
Gap between captions: At least 2 frames
First caption: At least 4 seconds into the film
Closed caption file size: 256 KB per reel. You don't need to manage this. We split your track into reels that meet it.

The two problems we see most are too many characters per line and too many lines per caption. If we build your DCP, we re-break your lines to fit these limits and never change the words.

After generating the .srt file, it’s essential to view it in a text editor to ensure it adheres to the SMPTE requirements before proceeding to create a timed text file for DCP. Double check that your caption track falls within the character and line limits. 

Convert to the correct frame rate

If your film has a frame rate of 23.97 or 29.97, then you will need to do a frame rate conversion. DCP does not accept fractional frame rates, so you’ll need to convert them to a DCP compliant frame rate like 24 or 30 fps. Tools like MacCaptions or Subtitle Edit can help with the frame rate conversion. If this conversion is not done, it can lead to the captions drifting out of sync during playback.  

Stay under the size limit

A closed caption file can't be larger than 256 KB per reel. Density matters more than length here: a dialogue-heavy 18-minute stretch can bust the limit on its own. A feature with captions is usually split into reels of about 20 minutes, so it ends up with several reels depending on its length.

If we build your DCP, you don't need to manage this. Send us one .srt for the whole film and we split it into reels that meet the limit. If you're building the DCP yourself, keep the caption font simple too. The font file travels with the timed text, and the spec caps it at 10 MB.

Black-and-white film reels mounted on a projector

Choose how your captions appear on screen

Before making your Digital Cinema Package (DCP), you will also need to decide how you want your captions to be displayed. Typically, closed captions use a periphery device to display the captions to each individual audience member, while open captions are burned into the film. These both have their own trade offs. Burning in the captions allows you more flexibility with the font you want used for the captions and avoids having to adhere to SMPTE guidelines. The down side of OCAPs is that the captions are obscuring the view of the picture. Closed captions allow for display flexibility, but you will need to go through the process of creating SMPTE compliant timed text files. 

Consider versioning your DCP

You will also need to consider whether to include the captions in your original version (OV) or add them as a version files (VF) to an already existing DCP. These have their own trade offs as well. A VF with captions must be paired with the OV for playback because a VF is just a supplemental package and needs the OV to reference the picture and sound. A DCP with captions in the OV will not need any supplemental files to display the captions properly. If you plan to distribute your DCP to other countries, you may want to use version files to output multiple languages. Version files are a great way to add captions and subtitles to an already existing DCP without having to rewrap the DCP.

Be aware of software limitations

Finally, it’s essential to consider software limitations when creating SMPTE timed text files. Some software may have limitations when creating DCPs with captions. For example, Resolve does not have the capabilities to create DCPs with closed captions. In such cases, it may be necessary to use a different software or service to create the files. 

Accessibility is the future

Accessibility requirements are on the rise in the film industry. Many festivals and venues require that filmmakers adhere to these new accessibility standards. Although the captioning process is time intensive, accessible films are more universally accepted and inclusive. If you’re feeling overwhelmed, Cinematiq can help create cinema compliant timed text files for your DCP. Reach out today and make your film more accessible for your audience.

Need A DCP With Captions?

Send us your .srt and we'll build the timed text track for $150 on a short or $350 on a feature, inside our standard 5 business days turnaround. No caption file yet? We write English captions from scratch for $4 per minute of runtime. Other languages are quoted per film.

See the caption rules and what to send on our digital cinema package page.