cs4237b Cirrus Logic, Inc., cs4237b Datasheet - Page 84

• Manufacturer Part Number: cs4237b
• Description: Crystalclear Advanced Audio System With 3d Sound
• Manufacturer: Cirrus Logic, Inc.

8-BIT COMPANDED
The 8-bit companded formats (A-Law and
Law) come from the telephone industry.
is the standard for the United States/Japan while
A-Law is used in Europe. Companded audio al-
lows either 64 dB or 72 dB of dynamic range
using only 8-bits per sample. This is accom-
plished using a non-linear companding transfer
function which assigns more digital codes to
lower amplitude analog signals with the sacrifice
of precision on higher amplitude signals. The -
Law and A-Law formats of the WSS Codec
conform to the CCITT G.711 specifications. Fig-
ure 23 illustrates the transfer function for both
A- and -Law. Please refer to the standards men-
tioned above for an exact definition.
ADPCM COMPRESSION/DECOMPRESSION
In MODE 2 and 3, the WSS Codec also con-
tains Adaptive Differential Pulse Code
Modulation (ADPCM) for improved perform-
ance and compression ratios over
A-Law. The ADPCM format is compliant with
the IMA standard and provides a 4-to-1 com-
pression ratio (i.e. 4 bits are saved for each
16-bit sample captured). For more information
84
Figure 22. Linear Transfer Functions
-Law or
-Law
-
on the specifics of the format, contact the IMA
at [phone]. Figures 18 and 19 illustrate
the ADPCM data flow.
The ADPCM format is unique with respect to
the FIFO depth and the DMA Base register
value. The ADPCM format fills the FIFOs com-
pletely (64 bytes); therefore, the FIFOs hold 64
stereo samples and 128 mono samples. When
samples are being transferred using DMA, the
DMA request stays active for four bytes, similar
to the 16-bit stereo data mode. In PIO mode, the
Status register (R2) indicates which of the four
bytes is being transferred.
When CEN is 0 (capture disabled), the ADPCM
block’s accumulator and step size are cleared.
When CEN is enabled, the ADPCM block will
start converting. Care should be taken to insure
that the "overrun" condition never occurs, other-
wise the data may not be constructed properly
upon playback. If pausing the capture sequence
is desired, the ADPCM Capture Freeze bit (ACF,
I23) should be set. When this bit is set, the
ADPCM algorithm will continue to operate until
a complete word (4 bytes) is written to the FIFO.
Then the ADPCM’s block accumulator and step
size will be frozen. The software must continue
A-Law: 2Ah
u-Law:
+FS
-FS
0
00h
Figure 23. Companded Transfer Functions
15h
3Fh
DIGITAL CODE
55h/D5h
7Fh/FFh
95h
BFh
CS4237B
DS213PP4
AAh
80h