Image Processing Reference
file. Allowing a larger buffer may automatically flip an internal switch so the content is
encoded as a variable bit rate (VBR) file. You would probably prefer to explicitly control
this, but unless you are running a broadcast installation, it's probably not worth worrying
Provided you have access to this level of control, the variable buffer verifier (VBV) dynam-
ically controls the buffer size for the video frame. This is also related to VBR and CBR
When deciding the setting for GOP structures, remember that 15 frames is
the maximum for PAL and 18 is the maximum for NTSC if you are encoding
with MPEG-2 for DVD publication.
Use VBR when streaming content on your intranet and on the Internet. Buffers can
be set to store between 5 seconds and a minute of video for Internet and high-bandwidth
Use CBR bit rate settings when you are streaming to networks with a low band-
width, such as mobile-phone networks running GPRS protocols. For these low-bandwidth
networks, a CBR setting with a small buffer that is under 5 seconds is optimal.
In circumstances where the bit rate changes and you get peaks due to the content
containing scene dissolves or periods of high motion, bit starving is provided by 2-pass
compression. This is no use for video conferencing, which must have 1-pass compression
and zero latency.
Automatic Frame Dropping
Popwire Compression Master and Popwire's other encoding tools provide a feature
called Frame Skip Probability. This comes into play if the bit rate cannot be sustained.
The value you define in this parameter sets the priority for frames that can be skipped
during coding in order to maintain the desired bit rate. When you are coding footage for
use on platforms such as GPRS or 3G mobiles, this is helpful because of the extreme
bandwidth limitations. It is also beneficial for content that is streamed via a dialup
Popwire encoding software allows key frames to be set in four different modes:
Natural Key Frames Only: The codec decides when to insert a key frame. This
mode would be used when a scene change is detected, for example.
Natural Key Frames With Distance: This works similarly to the Natural Key Frames
selection but forces a key frame periodically whether a scene change is detected or