Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2001-10-12
2003-11-25
Rao, Andy (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C375S240020
Reexamination Certificate
active
06654418
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention related to a quantizer for use in image coding.
It is known to scale discrete cosine transformation coefficients in video coding applications to conserve bandwidth. Known systems either scale by a small constant, such as divide by 8, or scale by a linear scaling factor that is twice a quantization parameter (2×Q
p
). Scaling by the small constant does not achieve significant bandwidth savings. Scaling by the 2*Q
p
linear scaling function achieves significant bit savings, but results in poor image quality at lower and mid-level Q
p
values particularly in the chrominance video signals. Accordingly, there is a need in the art. for a quantizer characterized by a scaling function that achieves good signal quality, and achieves bit rate savings for all values of Q
p
particularly for chrominance.
An encoder and decoder must use the same quantization parameter to encode and decode video information intelligibly. Known systems report changes to the quantization parameter with codes that cause changes in Q
p
with a uniform step size, regardless of the value of Q
p
.
Experience teaches that, at low values of Q
p
, changes in Q
p
are relatively small. However, for large values of Q
p
, changes in Q
p
values are relatively large. Systems that allocate additional bits to report the larger Q
p
changes waste bandwidth at the lower Q
p
values where the large changes do not occur. However, systems that limit the number of bits available to coding Q
p
changes may become saturated if larger changes must be coded. Accordingly, there is a need in the art for a quantizer that reports both large and small changes in Q
p
with a minimum number of bits.
SUMMARY OF THE INVENTION
The disadvantages of the art are alleviated to a great extent by a quantizer that applies a non-linear scaling function based on the quantization parameter. A different scaling function applies for luminance data than chrominance data. Both scaling functions at low Q
p
values approximate constant scaling functions. At large Q
p
values, the luminance scaling function approximates a 2*Q
p
scaling function and the chrominance scaling function approximates a 1*Q
p
scaling function. The quantizer may include a non-linear scaling function for AC coefficients.
The present invention may include a way to update values of Q
p
. Changes in Q
p
are reported in a fixed length code, but each code means different things based on the previous values of Q
p
. If the previous Q
p
value is large, the code represents a larger change than if the previous value of Q
p
were small.
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Haskell Barin Geoffry
Puri Atul
Schmidt Robert Louis
AT&T Corp.
Rao Andy
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