Optimizing Streaming Video
Delivery With Quality-Defined
Variable Bitrate Control
QVBR rate control helps content providers ensure consistent high video quality and save costs
Meeting video quality requirements is an ongoing challenge
for content providers. Information contained in encoded video is
inherently variable: different content and scenes carry different
amounts of information in each video frame.
At the same time, inefficient encoding
processes can add substantially to the costs
of video processing and distribution, through
unnecessary network bandwidth usage and
investments in storage capacity.
Ideally, bitrate would vary over time to
match changing complexity and maintain
consistent video quality, while making optimal
use of storage and network resources. Until
now, content providers have been limited
to encoding techniques that require costly
tradeoffs between efficiency, quality, and cost.
Video encoders that rely on a fixed or
constant bitrate (CBR) present a number
of challenges. CBR can often use more bits
than are necessary to meet video quality
requirements. Yet for some scenes or frames
of video, the established encoding rate may
not provide enough bits to maintain quality.
Different types of content require different
bitrate choices, and CBR requires operators to
make an imprecise, best-guess estimate.
Variable bitrate (VBR) encoding addresses
some of the challenges of CBR while presenting
other barriers to efficiency. Most significantly,
when encoding video at a variable bitrate, the
encoder uses as many bits as required to achieve an average bitrate,
which can exceed the number of bits needed to maintain video
quality for certain scenes or content. This means different encoding
Figure 2. Variable bitrate encoding uses as many bits as needed to reach a predefined average bitrate, even when fewer bits are
required for to maintain video quality.
Figure 1. For less complex video content, constant bitrate encoding uses more bits than needed to meet quality requirements; for more
complex video, CBR impairs quality by failing to provide sufficient bits.