3: Recording options
MPEG4000-D4 Hardware Reference Manual
© 2011 Advanced Micro Peripherals
Document version A.01
3
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Frame type encoding
The MPEG4000-D4 supports two frame encoding schemes within the MPEG-4
requirements to provide flexibility and optimize encoding efficiency and storage
space. The default is I/P encoding, where both I and P frames are involved in the
encoding. An alternative is I-only encoding. With I/P encoding, the I interval, which
is defined as the number of P-frames between I-frames can be set to any number
between 1 and 255.
Video Bit Rate control
The Video Bit Rate can be set separately for each channel of compression (and
hence storage efficiency) by using one of the three bit rate control schemes –
Variable Bit Rate (VBR), Constant Bit Rate (CBR) and Hybrid Bit Rate (HBR).
With VBR, the Quantization value (default of 5) can be set between 1 and 31 to
optimize efficiency for a particular application (with 1 being highest and 31 being
lowest quality). This mode strives to keep the quality constant and will change
the bit rate accordingly.
The default bit rate for CBR is 1Mbit/sec. However, the actual bit rate may change
by +/-10% around the set figure until the system ‘settles down’.
CBR therefore does not guarantee that the bit rate in practice will be exactly as
set – the MPEG4000-D4 simply does its best to achieve the set CBR figure. The
real life bit rate when in CBR mode will depend on the incoming video, motion
and other environmental factors.
With HBR, the MPEG4000-D4 is given an upper and lower bit rate limit and will
attempt to avoid abrupt changes in bit rate by changing the picture quality
gradually only when the bit rate limits are exceeded.
It may be worth finding out experimentally the optimum CBR setting for a
particular application environment in order to make the best use of CBR
configuration.
