AN2121 Freescale Semiconductor / Motorola, AN2121 Datasheet - Page 20

• Manufacturer Part Number: AN2121
• Description: JPEG2000 Arithmetic Encoding on StarCore SC140
• Manufacturer: Freescale Semiconductor / Motorola

Background Theory
The decoder checks the bit following any $FF byte. If the bit is set, the decoder knows that a carry has
occurred.
2.4.6
In the binary arithmetic coding scheme described here, both the interval (A) and the code word (C) are
altered whenever the encoder receives an MPS, which is most of the time. This is not of significant concern
in hardware because the updates to A and C can be done in parallel. However, in a software
implementation it is not generally desirable to perform more operations on the more probable path. A good
discussion of this can be found in [6]. One remedy to this situation is to have C point to the right-hand end
of the current interval instead of the left-hand end, as shown in Figure 7.
In this configuration, the code word is updated by subtracting P
comparison, the hardware-preferred method for updating C as described in the FDIS [4] is also shown (i.e.,
C points to the left side of the interval and is only updated when an MPS is received). By definition, an
MPS occurs more often than an LPS, so the software version updates the code word less often than the
hardware version because it only does so when an LPS is received. The resulting code words from the
hardware and software conventions always result in code words that differ from one another by the current
interval. It should also be noted that the process of subtraction for the software convention can result in a
borrow propagation, as opposed to a carry propagation when using the hardware convention. The FDIS for
JPEG2000 makes allowances for software implementations which are discussed in annex J of [4].
Note however that the StarCore processor makes it possible for a software implementation to update A and
C in parallel, just as in hardware. This is why the example provided in this application note follows the
hardware convention for updating A and C.
16
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In hardware version,
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LPS
Software Versus Custom Hardware Implementation
Figure 7. Codeword (C) Changes: Software vs. Hardware Implementation
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JPEG2000 Arithmetic Encoding on the StarCore SC140
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Freescale Semiconductor, Inc.
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For More Information On This Product,
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