Closed Caption and Subtitle Formats for the Digital Distribution of TV and Film
In April, we held a webinar with Netflix and the Entertainment Merchants Association on the best practices for captioning the digital distribution of TV and film. Sean Bersell, the VP of Public Affairs at the Entertainment Merchants Association, and Dae Kim, Video Engineer at Netflix, discussed (among other things) the best practices for closed caption formats and the issues involved with converting broadcast captions to Internet-compatible caption formats.
Converting Closed Captions for Internet Delivery
In order for broadcast video to be delivered over the Internet with closed captions, the closed caption file must be converted from the CEA-608 protocol (line 21) used for TV closed captions to a format suitable for Internet delivery. From there, the captions must be edited.
Conversion and editing can be done manually, from scratch, or by using software to extract and reformat the data (which is the preferred method). Even with software, however, conversion can be challenging, especially when it involves a broadcast closed caption file in a legacy format.
It’s preferable to have the closed caption file delivered in a format that is relatively easy to extract and reformat. It is also desirable and often necessary to preserve the original caption presentation.
Best Practices for Closed Caption (and Subtitle) Formats
While there are many different caption (and subtitle) formats, the EMA Closed Captions Working Group has proposed best practices based on two preferred caption file formats. The first is the closed caption format SCC (Scenarist Closed Caption). That’s the de facto standard for the conveyance of CEA-608 or CEA-708 data. The working group believes that SCC files are acceptable and provide a lot of flexibility.
The second option identified is SMPTE Timed Text. SMPTE stands for the Society of Motion Picture and Television Engineering, who developed this file format (SMPTE-TT). Under the law there is a safe harbor if the content provider provides compliant captions in SMPTE-TT. Content providers like to use SMPTE-TT because of this compliance. A lot of times the distributors will come back and ask for SCC files, and then the providers will end up sending in both formats so they can make sure they are compliant while also pleasing their distributors.
Because SMPTE-TT has a lot of options, there are best practices to follow for SMPTE-TT delivery. They are:
- SMPTE-TT must be constrained by a defined profile.
- The use of other features defined in SMPTE-TT is not precluded.
- Additional or alternative preferences should be clearly communicated.
Best practice for caption file format is the one issue that the EMA working group has not yet come to a full conclusion about. As of now, SCC and SMPTE-TT are their suggestions.
However, there are new caption formats emerging, such as WebVTT and EBU-TT, which will be standardized soon. The FCC has also mandated that online video distributors must comply with the advanced closed captioning standard CEA-708. With a new baseline for captioning of 708, the only viable caption option that is an official standard now is TTML. Once it is standardized, WebVTT will also support 708. It is important to note that most caption files are still being created with 608 standards and most people have not yet seen 708 in action.
We encourage you to read the white paper to learn further best practices for caption and subtitle formats.