The key benefit of capped CRF is that it’s a
single-pass technology; most other technologies
are two-pass plus at least one analysis pass.
This accounts for the 98 saves.
It wouldn’t be baseball without highlights
and bloopers, and Figure 1 shows them both for
Capped CRF. On the left is Capped CRF’s single
home run, where a boost in data rate increased
the PSNR values by greater than 1% in five of
seven rungs. To explain, the Data Rate column
compares the control and capped CRF output
and shows the change. The PSNR Percent column tracks the change in PSNR percentage,
while the PSNR dB column tracks the absolute
change in PSNR. The VMAF column tracks the
absolute difference in the VMAF score between
the control and capped CRF output.
The blooper on the right is a bitrate graph
(in the Bitrate Viewer application) showing the
output of a synthetic test clip that mixes 30 seconds of talking head with 30 seconds of ballet.
The concern are the swings in data rate, which
can result in frequent ladder switches and
stoppages (see my article “Bitrate Control and
QoE—CBR is Better” at go2sm.com/slcbitrate).
With capped CRF, you have no control over data
rate swings within the file, which is a significant
QoE concern for me. As a counterpoint, OVP JW
Player uses capped CRF for its per-title encodes,
and obviously the group wouldn’t if it caused significant issues.
Capella Systems’ SABL is a feature of the
company’s Cambria FTC encoder, which I tested
in a review you can read at go2sm.com/capella.
SABL assesses video complexity by running a
CRF encode of the target clip, and it allows the
user to set the duration of the measurement pe-
riod. If set to a short duration, like 10 seconds,
the most complex 10 seconds in the clip deter-
mine the data rate of the entire clip. If set to a
longer duration, say, 2 minutes, then a short,
complex section won’t impact the overall data
rate that much, but might look a bit degraded
after the encode.
Besides setting the measurement duration,
you also set how the data rate of the CRF encode, or complexity value, impacts the original encoding ladder. This is shown in Figure
2, where you can see that a value of more than
7000Kbps directs the encoder to shift the data
rate of each rung in the encoding ladder by
50%. At the other end of the spectrum, Cambria will adjust the data rate of the encoding
ladder downward by 50% if the complexity value is less than 2000Kbps.
The script shown in Figure 2 is completely
customizable, and it includes the ability to adjust video resolution based on the complexity
value and even to cut rungs below a certain
data rate. Unfortunately, I didn’t test either of
these capabilities because I didn’t know it was
possible, and didn’t think to ask until I analyzed
the Brightcove results. By that point, it was too
late to retest.
highlight on the
left, blooper on
ladder according to