QMCoder
T
he arithmetic coders used in JPEG, JPEG
2000 and JBIG are called QMcoder. It
handles only binary strings or input and
it is designed for simplicity and speed. It uses
approximation for multiplication operation,
fixedprecision
integer
arithmetic
with
renormalization of the probability interval from
time to time.
The main idea of the QMcoder is to classify
the input bit as
M
ore
P
robable
S
ymbol (
MPS)
and
L
ess
P
robable
S
ymbol (
LPS)
. Before the
next bit is input, the QMcoder uses a statistical
model (using a context, typically a two
dimensional context of black and white pixel in
an image) to predict which one of the bits (0 or
1) will be the
MPS
. If the predicted
MPS
bit
does not match with the actual bit, then the
QMcoder will classify this as
LPS
; otherwise,
it will continue to be classified as
MPS
. The
output of the coder is simply a stream of
MPS
or
LPS
, which are assigned probability values
dynamically to be used by the simplified
arithmetic coder. The decoder has only the
knowledge of whether the next predicted bit is
MPS
or
LPS
. It uses the same statistical model
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 Spring '11
 Mukherjee
 MPS, High – Low, Symbol (MPS)

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