AN726 データシートの表示（PDF） - Silicon Laboratories
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AN726 Datasheet PDF : 28 Pages
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AN726
The example in Figure 6 assumes a original 12-bit signed data value of 1931 and shows how the average energy
increases. In this particular example, the data remains the same for each EPCA overflow for simplicity, but this will
not necessarily be the case in the operation of the algorithm.
5.2.3. µLaw (or muLaw)
The µLaw companding algorithm has long been used in telephony and other voice applications. This algorithm
takes advantage of the behavior of the human ear by reserving most of the compression bins for low volume levels
where the ear is most sensitive. Using this algorithm, the Class-D firmware can store more data in flash with no
noticeable degradation in audio quality.
This algorithm takes a 14-bit signed number and adds 32 to the magnitude, which ensures that a 1 occurs in bits 5–
12 of the value. This means the valid input range is –8160 to 8159. This value is then converted to an 8-bit
compressed result as shown in Figure 7, where S is the sign bit. Finally, the 8-bit value is complemented.
14-bit Signed Input Data
S 0 0 0 0 0 0 0 1ABCDX
S 0 0 0 0 0 0 1ABCDXX
S 0 0 0 0 0 1ABCDXXX
S 0 0 0 0 1ABCDXXXX
S 0 0 0 1ABCDXXXXX
S 0 0 1ABCDXXXXXX
S 0 1ABCDXXXXXXX
S 1ABCDXXXXXXXX
8-bit µLaw Encoded Data
S 0 0 0ABCD
S 0 0 1ABCD
S 0 1 0ABCD
S 0 1 1ABCD
S 1 0 0ABCD
S 1 0 1ABCD
S 1 1 0ABCD
S 1 1 1ABCD
Figure 7. µLaw Algorithm Table
Figure 8 shows the plot of the 14-bit signed inputs versus the 8-bit compressed output.
100
50
Input Data
-8000 -6000 -4000 -2000 2000 4000 6000 8000
-50
-100
Compressed
Output
Figure 8. µLaw Algorithm Plot
10
Rev. 0.1
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