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20-Bits PCM Support
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AC ’97 Programmer’s Reference Manual 43
5 20-Bits PCM Support
The ICH5 AC ’97 controller provides support for 16- or 20-bit PCM out. Software can determine
if 20-bit samples are supported in the controller by reading the sample capabilities bits in
GLOB_STA registers as follows: MBBAR + 30h: Global Status Registers.
Table 23. Sample Capabilities
Bit Description
23:22 Sample Capabilities: Indicates the capability to support greater than 16-bit audio.
00 = 16-bit Audio only supported (ICH1, ICH2, and ICH3 value)
01 = 16 and 20-bit Audio supported (ICH5 value)
10 = Reserved (for 24-bit audio support)
11 = Reserved
After determination of the controller capabilities, software can enable 20 –bits formats by
programming the GLOB_CNT register as follows: MBBAR + 2Ch: Global Control Registers.
Table 24. PCM Out Mode Selector
Bit Description
23:22
PCM Out Mode (POM): Enables the PCM out channel to use 16 or 20-bits. This does not affect
microphone, microphone2, PCM In, PCM2 In, or S/PDIF DMA. When greater than 16-bits audio is
used, the data structures are aligned as 32-bits per sample, with the upper
order bits representing
the data, and the lower
order bits as don’t care.
00 = 16 bit audio (default)
01 = 20 bit audio
10 = 24-bit audio (not supported. If set, indeterminate behavior will result.
11 = Reserved. If set, indeterminate behavior will result.
Note: Software must ensure that the PCM out DMA engine is stop and a new descriptor initialized
before changing the value of PCM Out mode. These bits must not be modified when the PCM Out
engine is running.