n s the most expensive to encode, and
would have swelled total encoding
requirements to 31 files, with po-
tentially 17 more required for HDR.
During the session, these requirements generated significant
discussions among the attendees,
many of which had been producing
HLS presentations for years. Most
stated that they provided one or two
trick play files, with few providing at
all resolutions, and most pointing to
the I-frames in existing files rather than encoding separate, I-frame-only files. Producers will
have to make their own cost/benefit analysis to
decide upon the optimal approach for them.
8. Should I Use Apple’s Suggestions Verbatim?
Sometime during the last revision or two of
the Authoring Specification, Apple addressed
per-title encoding implementations, stating that
“The above bitrates are initial encoding targets
for typical content delivered via HLS. We recommend you evaluate them against your specific content and encoding workflow then adjust
accordingly.” So Apple isn’t dictating a fixed encoding ladder.
Beyond data rates, if you study Apple’s ladder, you’ll note that it uses essentially the same
resolutions for HEVC and H.264 for all rungs
below 2K. At the preconference session, one of
the more technically savvy attendees suggested
that Apple’s ladder should have completely different rungs for HEVC to account for the codec’s
greater efficiency with high-resolution videos.
This led to the analysis presented in an article
entitled, “Apple Got It Wrong: Encoding Specs
for HEVC in HLS” ( go2sm.com/applewrong).
Long story short, the article proposes that the
optimal ladder for HEVC would eliminate several lower resolution rungs, and push higher resolution rungs lower in the ladder. This is shown
in Table 5, which shows Apple’s suggested ladder on the left and a more optimal ladder on the
right (customized for the animated movie Sintel), along with VMAF scores rating the quality
of both alternatives. For optimal QoE, you’ll get
better results with the Should Be ladder, rather
than the Was ladder designated by Apple.
9. What Are My Live Options?
Live options are nascent but rapidly becom-
10. What Does the Spec Say
ing available, and the presentation handout lists
encoders from Bitmovin, Elemental, Harmonic,
and Hybrik, as well as transcoding solutions from
Wowza and Nimble Streamer. For developer-
level producers, Multicore Ware, MainConcept,
and Beamr all have SDKs, and the handout de-
tails how to produce output using FFmpeg and
About High Dynamic Range (HDR)?
The Authoring Specification states that HDR
video must be encoded as either HDR10 or Dol-byVision, and that HDR encoded streams should
be provided at all resolutions. If you provide HDR
content, you should also provide SDR content for
the main video files and trick play files, as well as
H.264 content, boosting the stream count to potentially dozens of individual files.
Note that Apple doesn’t yet provide an example file with HDR, leaving several questions un-answered, such as whether the required H.264
content can also serve as the SDR content, or
whether producers should also supply separate
HEVC-encoded SDR streams (and trick play
files). I’m guessing that Apple will always supply
the most expansive (and expensive) way to meet
the requirements stated in the Authoring Specification, leaving developers to choose their own
configuration based on cost and the desired QoE.
It’s early days for HEVC in HLS, and the topic and technology will be fast moving. Hopefully, these questions and answers have helped
you off to a quick start.
Jan Ozer ( firstname.lastname@example.org) is a streaming
media producer and consultant, a frequent contributor to industry
magazines and websites on streaming-related topics, and the
author of Video Encoding by the Numbers. He blogs frequently at
Comments? Email us at email@example.com, or check
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ladder on the