ADS Technologies USBAV-708 INSTANT VIDEOMPX, Руководство пользователя

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Note that what the encoder views to be a scene-change does not necessarily
correspond with what humans actually perceive to be a scene-change—no
matter what value you configure the scene-change threshold to be there is no
guarantee that the encoder will key-frame on an actual scene change, just as
there is no guarantee that the encoder will not key-frame elsewhere.
The scene-change threshold is respected by the encoder only when Fastest
or Standard Performance/Quality modes are selected.
Key-frames (intra-frames) are useful throughout a video for a number of
reasons. Where too few blocks can be predicted from one frame to another it
can be more economical to key-frame rather than use one of the predicted
frame types. More frequent key-frames can reduce the seek recovery time in
media players when the viewer attempts to skip to a specific point in the
video. Due to the nature of predicted frames a media player must decode
every frame from the nearest past key-frame until the seek target in order to
resume playback at the desired frame.
Reducing the scene-change threshold will cause an increase in the
proportion of key-frames to predicted frames throughout the video. Key-
frames are the most expensive in terms of bit-spend of all frame types and
using too many key-frames will starve the encoder of free bits causing the
rate control to return higher quantizers, in turn reducing the overall quality of
the video.
Raising the scene-change threshold will cause a decrease in the proportion
of key-frames to predicted frames throughout the video. Fewer key-frames
translate to more free bits causing the rate control to return lower quantizers,
in turn increasing the overall quality of the video. However, too few key-
frames will lead to long seek times when the viewer attempts to skip to a
specific point in the video and increase the recovery time should the video
stream become damaged (e.g. when in transit over a broadcast network).
The default scene-change threshold is 50%. It is recommended the scene-
change threshold not be lower than 40% or higher than 60% for most
content.
Scene-change threshold:
-sc <Threshold>
Threshold is the percentage of intra-blocks required to trigger scene-change
detection.