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CMAF also simplifies delivering closed captions for multiple
languages, which has always been a complex challenge. By way
of background, Apple uses the text-based Web Video Text Tracks
( WebV TT) standard for closed captioning in HLS (Figure 4) and
will likely continue to use it going for ward.
DASH can use WebV T T, but also uses a standard called
Timed Text Markup Language Profiles for Internet Media
Subtitles and Captions, or IMSC1, which allows for not only text
but also images, as required for many Asian and Middle Eastern
languages or non-Latin based languages. (Figure 5). For this
reason, CMAF supports both WebV TT and IMSC- 1 formats.
For DRM, CMAF supports Common Encryption (CENC)
which can incorporate multiple DRMs into a single package.
However, CMAF also supports two incompatible encryption
modes, AES-128 Counter (C TR) and AES-128 Cipher Block
Chaining CBC (CBC). When CMAF originally launched,
Apple’s DRM FairPlay only supported CBC, while PlayReady,
Widevine, and many other DRMs only supported C TR, which
prevented a single encrypted file package from playing on
both Apple and non-Apple platforms.
Since then, Google has incorporated CBC into
Widevine, while Microsoft has promised that PlayReady 4.0,
which is scheduled for release in 2018, will also support
CBC. Does this mean that a single encrypted set of files can
play on all relevant endpoints? Unfortunately, not in the
Figure 4. Apple’s HLS uses the WebV TT standard for closed captioning.
Figure 5. DASH uses the IMSC- 1 standard for closed captioning.