Video Compression with Intra/Inter
Mode Switching and a Dual Frame Buffer
Standard video coders often
use the immediate past frame as a reference frame
with motion compensation for video encoding. In
our research, we have used dual reference frame
motion compensation in the context of high bandwidth
to low bandwidth switching such as from an Ethernet
connection to a GPRS system. The implementation
is based on MPEG-4. Simulation results show that
there is a significant gain in the PSNR for relatively
static video sequences.
To evaluate the effectiveness
of the dual frame buffer technique, we simulated
it by modifying the standard MPEG-4 coder. We
allocated additional memory for the long-term
frame. An extra bit is transmitted per inter coded
MB to inform the decoder which frame it referenced.
The intra refresh period was set to 100. Lowering
the intra refresh period enhanced the performance
of the dual frame encoder, but frequent intra
refresh results in higher bit rates, which would
exceed the bit rates available for a GPRS system.
As inputs, we used the News, Container and the
Foreman sequences. To investigate the effects
of switching to different low bandwidth networks,
we simulated switching from 1 Mbps to low bandwidth
networks ranging from 10 kbps (GPRS) to 150 kbps
(1xRTT CDMA). Each sequence was encoded employing
our dual frame buffer coder as well as by a conventional
MPEG-4 coder for comparison.
We found that retaining the
high quality frame to be used as the long term
past frame for the dual frame encoder results
in better video quality as quantified by the PSNR
of the decoded sequence at a small cost in memory
to retain the dual reference frame.
In this sub-project, we explored
end-to-end loss differentiation algorithms (LDAs)
for use with congestion-sensitive video transport
protocols for networks with either backbone or
last-hop wireless links. As our basic video transport
protocol, we used UDP in conjunction with a congestion
control mechanism extended with an LDA. For congestion
control, we used the TCPFriendly Rate Control
(TFRC) algorithm. We extended TFRC to use an LDA
when a connection uses at least one wireless link
in the path between the sender and receiver. We
then evaluated various LDAs under different wireless
network topologies, competing traffic, and fairness
scenarios to determine their effectiveness. In
addition to evaluating LDAs derived from previous
work, we also proposed and evaluated a new LDA,
ZigZag, and a hybrid LDA, ZBS that selects among
base LDAs depending upon observed network conditions.
We evaluated these LDAs via
simulation, and found that no single base algorithm
performs well across all topologies and competition.
However, the hybrid algorithm performed well across
topologies and competition, and in some cases
exceeded the performance of the best base LDA
for a given scenario. All of the LDAs were reasonably
fair when competing with TCP, and their fairness
among flows using the same LDA depended on the
network topology. In general, ZigZag
and the hybrid algorithm were the fairest among
all LDAs.
