Technically, CAE considers four factors when
producing its standard encoding ladder:
1. The properties of the content
2. The distribution of user devices (connected
TVs, PCs, smartphones, tablets, etc.)
3. The properties of user devices
4. The constraints specific to video codecs,
like encoding profiles
To evaluate content, the CAE profile gen-
erator runs several “probe” encodes over the
video, measuring quality with SSIM and oth-
er metrics, and then assigns a mathematical
model to the video.
Interestingly, within the context of Brightcove’s core OVP business, the two middle parameters mean that the devices that you distribute to and their effective bandwidth will
impact your encoding ladder. So, if you’re targeting Android phones over 3G connections in
a developing country, you’ll get one ladder; if
targeting 4G in Scandinavia, you’ll get another.
As shown in Table 1, CAE offers the broadest feature set of the four new technologies, with
the ability to add or cut rungs from the encoding ladder, change output resolutions, and apply post-encode quality checks. CAE is also the
most configurable. When defining the parameters of the dynamic output, you can specify the
minimum and maximum number of renditions,
min/max resolution, max frame rate, key frame
rate, min/max bitrate, max first rendition bitrate, min/max SSIM for the quality check, and
the H.264 profiles for the encoded files. Then you
can also specify all of the normal encoding parameters like bitrate control technique, b-frame
and reference frame parameters, and the like.
CAE is a complex work in process, and several of the issues discovered during our testing
have already been resolved. So I’ll focus on the
positives of Brightcove’s approach and reserve
a full consideration of strengths and weaknesses for when the product ships.
The best results were shown by the Tutorial
file, a PowerPoint-based video that has been deployed by corporations around the globe. The
original encoding ladder is shown as Table 1;
the CAE ladder is on the left in Figure 3, along
with how the values changed from the control
to the CAE output.
Because the underlying content was so simple,
CAE encoded this clip in only three rungs, with
the 1080p rung so compact that viewers down to
the fifth rung could view it at a lower data rate
than the original file in the control ladder (which
was 900Kbps). Despite dropping the 1080p data
rate by more than 534%, CAE retained a PSNR
value of over 45, and the video proved crisp and
artifact-free. Benefits in lower rungs were even
greater, with PSNR values boosted by as much
as 41% and VMAF scores that would translate
to vastly superior QoE. Overall, Brightcove also
shaved 33 rungs from the 13 encoding ladders
that it completed, accounting for the 33 saves.
Brightcove produced 14 videos, but kicked
one out because some of the rungs could not
be produced at the quality level specified in the
script. In practice, you would reencode at a lower quality level, but we didn’t have time to get
that done for this article.
From a commercial perspective, Brightcove
expects to offer the functionality as an add-on
service for its Video Cloud customers, but pricing is still TBD. At press time, Brightcove hadn’t
decided whether to offer a standalone version
of the service for Zencoder customers.
One of the many
home runs hit