WP24 STREAMING MEDIA SPOTLIGHT SERIES JUNE 2017 SPONSORED CONTENT
Constant Bitrate vs. Variable Bitrate
Constant bitrate (CBR) encoding does just what the
name describes—it encodes the video at a more or less
consistent bitrate for the duration of the live stream or
entire video file. There will still be some variation, because
different frame types will have different data rates,
and because buffers on clients allow a small amount of
variability in the bitstream rate during playback. Variable
bitrate (VBR) encoding, on the other hand, means the video
bitrate can vary greatly over the video’s duration (Figure 5).
A VBR encode spreads the bits around, using more bits to
encode complex parts of content (like action or high motion
scenes) and fewer bits for simpler, more static shots. In
either CBR or VBR encoding, the bitrate target is followed
as closely as possible resulting in outputs that are very
VBR-encoded video is going to look much better than CBR-
encoded video. One other key factor to consider with VBR is
that the encoder needs to be configured so that the highest
data rate isn’t so high that the viewing device can’t process
the stream or file.
Quality-based encoding takes VBR one step further by
basing the differences in bitrate on the desired image quality,
which can be defined by objective measurements, subjective
analysis, or a combination of the t wo. AWS Elemental’s encoders
employ sophisticated algorithms to consistently deliver the
highest-quality picture at the lowest possible bitrate.
AWS Elemental Encoders
As you may have guessed, AWS Elemental offers two types of
encoders: real-time and file-based. AWS Elemental Live (Figure
6) allows content publishers to take media files, camera outputs,
or live source feeds and encode those feeds at a constant
rate. Then, the encoder will pass those encoded streams on
to a content origin product like AWS Elemental Delta, which
packages them and delivers them to viewers in real time.
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Figure 5. Constant bitrate (CBR) encoding encodes
a video at a relatively consistent bitrate, while
variable bitrate (VBR) encoding encodes different
frames of the video at different bitrates depending
on the complexity of the image.