EN5990 Encoder for MPEG-4 Part 10
16
5 Frequently
Asked
Questions
5.1
How to Define the Bitrate?
There are several bitrates involved with the operation of the Encoder:
!
Transport System (TS) bitrate: this is defined in the
Output/Mux Menu
and
defines output rate of the TS packets
!
The bitrates for the output of each underlying module including video and
audio.
The recommended method to set the bitrate:
!
Set the Transport Stream bitrate: this is set first as it is normally defined by an
external parameter such as bandwidth of the DSL network or the bandwidth
of the satellite transponder.
!
Set the audio bitrate: choose the value that corresponds to the minimum
acceptable broadcast quality. For MPEG-1 Layer II, this is typically 192 kbit/s
for a stereo pair. For MPEG-2 AAC, this is typically 96 kbit/s.
!
Set the video bitrate to be the maximum allowed. The Menu system restricts
the video bitrate so that the total bitrate of all of the modules does not exceed
the Transport Stream bitrate.
The encoder implements a compression mode called “Capped VBR” where VBR
denotes Variable bitrate. This means that the rate control for the video will ensure
that the short-term (measured over the period of the video decoder buffer) video
bitrate will not exceed the value defined by the operator. The video encoder does
not add any padding to the video stream and so it will not deliver constant bitrate
(CBR).
5.2
Video Quality Versus Bitrate Versus Channel Switching
The implementation of H.264/AVC algorithm used for the EN5990 has been
optimised to produce broadcast quality video at very low bitrates. However there
are bitrates below which the algorithm cannot maintain broadcast quality. It should
be remembered that the cut-off bitrates involved depend on the content being
encoded. For example, sports content requires more bits to encode than the image
of a newsreader to achieve the same visual quality.
However, in broadcast systems, the time taken to switch between two television
channels needs to be kept below a maximum value. The two requirements, quality
and switch time, work against each other in that setting parameters to improve
video quality will generally increase channel-switching time. The values of the
following parameters can be modified to achieve different compromises between
video quality, bitrate and channel switching:
