EN5990 Encoder for MPEG-4 Part 10
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5.7
What Parameter Values Should be Used to Optimise Video
Quality?
The parameters discussed in
Chapter 4
are the most important parameters for the
operation of the encoder and these parameters should be set initially. However
there are other parameters that can be used to further optimise the video quality.
Video quality can be optimised for two mutually exclusive scenarios:
!
Viewing by human subjects called subjective quality testing: where it is
important that the coding artefacts are not annoying.
!
Analysis by computer called objective quality testing: where the coded video
must be as close as possible to the original video source.
The recommended parameter values are shown in
Table 5.1: Recommended Values for the Different Video Parameters
Parameter Objective
Subjective
Noise reduction
Off
Adaptive 1
Bandwidth
Sharp to medium
Medium to soft
Adaptive QP
Off
On
5.8
Why are the Audio and Video Presentation Time stamps so
Different?
In some configurations, the presentation time stamps associated with audio and
video frames that are located at a similar time in the Transport Stream can differ
by several seconds. This causes problems for the decoder if either the video or
audio time stamp is used as a clock reference.
This difference in time stamp is valid as the decoder is required to have a much
larger video buffer than audio buffer. For most audio standards such MPEG-2
AAC, the buffer is less than 1 second while the H.264/AVC video buffer for SD
applications is 10 Mbit/s. To obtain the optimal video quality overall, the encoder
varies the number of bits allocated to each frame dependent on a large number of
parameters. This means that for a 2.0 Mbit/s video rate, the presentation
time stamp of the video frame can vary from the system clock to 5 seconds ahead
of the system clock. The difference associated with audio will always be less than
1 second so there could be up to 4 seconds difference between the audio and
video time stamps. An example is shown in
