Closed Captioning in Wowza Media Server 3.5
The introduction of caption transmuxing simplifies deployment by minimizing the number of source files required, though the process is slightly different for live vs. on-demand
At press time for my article on closed captions, I received word that Wowza was releasing closed caption support in Wowza Media Server 3.5. I spoke with Scott Kellicker, chief architect of the Wowza Media Server, about the closed captioned functionality.
He shared that caption-related functionality would be different for live and VOD files. For live, the server could accept captions embedded in the input stream, either CEA-608 captions or onTextData events; or could inject onTextData captions into the live stream using a Wowza API. The onTextData events would be transmuxed for live output to Apple HLS (via embedded CEA-608 captions) or the onTextData for RTMP and Adobe HDS delivery. Silverlight is not supported for live streams at this time.
For video on-demand (VOD), the server can accept 3GPP data tracks embedded in MP4 files or separate TTML caption files. From this data, the server can output CEA-608 data for HLS output, or onTextData events for either RTMP or HTTP-based Dynamic Streaming. The server can pass through embedded CEA-608 caption data to iOS devices, but can't transmux it for delivery to other formats. Silverlight is not yet supported for VOD files either.
Consultant Robert Reinhardt of VideoRx.com beta tested the caption-related features of Wowza Media Server 3.5 and can attest to how caption transmuxing simplifies the caption deployment process. During 2012, before the release of the 3.5 version, Reinhardt helped his client the Government of British Columbia create a microsite with adaptive streaming videos that supported 11 different languages with closed captions. In order to serve caption-enabled video to iOS devices, separate VOD content was encoded and delivered outside of their Wowza infrastructure. Now, with Wowza Media Server 3.5, they can simplify their encoding and delivery processes.
Though Wowza Media Server 3.0 supported audio and video transmuxing, it didn’t transmux captions, so Reinhardt had to create separate groups of adaptive files for Flash and mobile playback. In addition, because Wowza Media Server didn’t natively support captions during delivery to iOS devices, he had to create 11 additional files, one for each language, that were sent via progressive download when an iOS client clicked the closed caption button, ending the adaptive streaming experience. With version 3.5, Reinhardt could upload a single version of the adaptive group, which WMS 3.5 will transmux as necessary for Flash and iOS delivery.
Whether you're required by law to offer closed captions or not, there's good reason to add them to your online video workflow. Here's how.