Silicon Laboratories SiM3U1xx Bedienungsanleitung PDF herunterladen (Seite 10)

AN726

10 Rev. 0.1

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.

Figure 7. µLaw Algorithm Table

Figure 8 shows the plot of the 14-bit signed inputs versus the 8-bit compressed output.

Figure 8. µLaw Algorithm Plot

00000001ABCDX

0000001ABCDXX

000001ABCDXXX

00001ABCDXXXX

0001ABCDXXXXX

001ABCDXXXXXX

01ABCDXXXXXXX

1 ABCDXXXXXXXX

000ABCD

001ABCD

010ABCD

011ABCD

100ABCD

101ABCD

110ABCD

111ABCD

14-bit Signed Input Data 8-bit µLaw Encoded Data

Input Data

Compressed

Output

-8000 -4000

-6000 -2000 80004000 60002000

100

-50

-100

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 27 28

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Silicon Laboratories SiM3U1xx Bedienungsanleitung Seite 10