Transcript
>> Welcome, my name is Eryk
Vershen, I'm am AVFoundation
engineer at Apple.
This is session 515 which is
about HLS authoring and the HLS
tools.
HLS is an abbreviation for HTTP
Live Streaming.
HLS is the way you stream
content to all Apple devices.
I'm going to be talking about
the document HLS Authoring
Specification for Apple Devices
and the tools we distribute for
creating and validating HLS
content.
The Authoring Specification and
other important documents about
HLS can be found at this URL
developer.apple.com/streaming.
The Authoring Specification is
Apple's advice for authoring HLS
content.
We wanted the HLS specification,
the Internet draft, to include
only absolutely necessary
requirements.
The Authoring Specification, on
the other hand, includes
requirements that are specific
to Apple's players, as well as
things that, while not
absolutely required, are part of
best practice.
The big news this year is we've
added support for HEVC.
Here are the limits on what is
playable.
Main 10 Profile, Level 5.0, High
Tier.
Now that we support HEVC content
we've added bit rate guidelines
for that as well.
You should treat those
recommendations as very
preliminary.
HEVC encoders are still
evolving, we expect to see
improvements over time so we may
revise our guidelines in the
future.
You should also remember that
the bit rate recommendations in
the Authoring Specification are
guidelines, they are not cast in
stone.
We are aware that all content is
different, a bit rate that works
for a video of two people
talking is not likely to work
for a sporting event.
And you may have different
constraints on the number of bit
rate variants that you can
provide.
So, use the bit rate guidelines
as a starting point.
Having HEVC means that you can
have both H.264 and HEVC content
in the same master playlist.
So, we've come up with some best
practices.
First and most important, always
use the CODECS attribute.
You need this so that older
players don't try to play the
HEVC content.
Second and almost as important,
always include some H.264
content.
You need this so older players
will have something that they
can play.
Now the player will switch
between H.264 and HEVC.
However, for two variants with
similar bit rates the player
will typically favor the HEVC
content.
While H.264 content could be in
FMP4 files for maximum
compatibility keep your H.264 in
transport streams.
The last point I want to make is
that these same rules apply to
so-called trick play content,
that is I-frame-only content.
Along with adding HEVC support
we also added support for a
second subtitle format, IMSC.
If you want more detail on our
support for IMSC1 you should
look at this year's Advances in
HTTP Live Streaming talk.
IMSC1 is based on TTML, the
timed text markup language and
it's compatible with standards
from the EBU and SMPTE.
Let's talk about best practices
for subtitle content.
We support the text profile of
IMSC and note that IMSC content
must be in fragmented MP4 files,
you can't use plaintext files.
WebVTT is the same as it has
been, text files typically
having an extension of .VTT or
.webVTT.
Remember that you can mix up
IMSC and webVTT with H.264 HEVC
content however you want,
they're completely independent.
You should always use the IMSC1
CODECS value.
Including that value ensures
that older clients will ignore
the variant.
However, for maximum backward
compatibilities don't put in the
value for the webVTT content.
I want to change gears and talk
about the HLS tools and the
changes we've made there.
The tools are available on the
developer site and you can find
them through the same URL I
mentioned earlier,
developer.apple.com/streaming.
The tools fall into three rough
categories, tools for segmenting
video and audio, tools for
validating content and making
validation reports and finally,
tools for creating subtitles,
iv3 metadata and master
playlists.
Now the validator does not do
low-level bitstream checks, you
should be using third-party
tools if you think you may have
issues with your encoders.
Let's look at some of the
specific changes we made in the
tools.
Well firstly, we've added the
ability for MediaStreamValidator
to check local files.
You no longer have to put an
HTTP server to validate your
streams.
If you have direct file access
you can check them that way and
you can use relative paths,
absolute paths or file URLs.
However, I should note that you
can't mix file URLs and HTTP,
that is if the master playlist
is a file path then the media
playlist it references can't be
HTTP URLs.
Of course, we added support for
HEVC into the tools.
We've also made a lot of general
improvements to the tools,
principally the
MediaStreamValidator and HLS
report.
We check more things about the
stream.
There are too many to list, but
for example, we're now
validating the CODEC usage.
We have better handling for
errors on the stream during
validation.
And we made the ordering of
variants renditions in the
reports be more consistent.
We've added several new options,
I want to call out just three of
them.
First, the description option on
MediaStreamValidator.
The URLs for a stream don't
necessarily make clear what the
content is.
This option allows you to add
some text describing the stream.
For example, such and such show,
season two, episode five.
The text will appear at the top
of the report near the URL.
For HLS report, we've added some
new optional columns.
Rather than continue the way we
had previously with separate
options for each column, instead
we've added a columns option
that takes a list of column and
identifiers.
The set of available column
identifiers can be found using
the Help option or in the
[inaudible] page.
The old id and pl options that
gave you data id values and
playlist bit rates are still
supported, but are deprecated in
favor of column identifiers.
And the column identifiers for
those are just id and pl.
Last year we added hdcp-level
support, but we forgot to tell
you about the hdcp-level option
on MediaFileSegmenter.
This allows you to set the
hdcp-level.
The hdcp-level is included in
the Plist that gets generated by
MediaFileSegmenter.
And you can pass that Plist to
the variant playlist creator to
get the hdcp-level attribute in
the generated master playlist.
Here's a quick look of a sample
report.
Notice the second line, that's
from the description option that
we passed in
MediaStreamValidator.
And notice that we've now added
a CODECS column so you can see
which variants are H.264 and
HEVC.
Now we use the AVC, the
alternate name of H.264 so that
we'll have more visible
separation between the two CODEC
types.
As part of the changes in the
ordering of variants and
renditions, we now keep audio,
closed caption and subtitle
renditions separate.
To summarize, remember that URL,
developer.apple.com/streaming,
everything you need to know is
pointed to from that page.
Besides the various documents it
also has links to sample
streams, to the tools download,
to past videos about HLS, the
information about FairPlay
Streaming and so on.
I encourage you all to read the
HLS Authoring Specification for
Apple Devices and the HLS spec.
For more information go to the
webpage for this talk,
developer.apple.com/WWDC17/515.
We have some related video
sessions, Advances in HTTP Live
Streaming.
This has more discussion of HEVC
and IMSC1, as well as some other
interesting topics.
And Error Handling Best
Practices for HTTP Live
Streaming.
That will help you understand
what your server needs to do to
be a good HLS server.
You can watch either of these
videos via the web or in the
WWDC app.
I hope you found the talk
useful, thanks for watching.