Audio Codec '97

AC ‘97 Component Specification Revision 2.3 Rev 1.0
71
• DA conversion rate may match the S/PDIF transmitter rate, or be independent
• SRC implementations differ
• Controller implementations differ
Given the complexity, there needs to be agreement on minimum required concurrency support at 48 kHz, and a
simple mechanism that enables the Driver to easily determine whether other S/PDIF configurations are supported
(the SPCV bit is described above).
There are two principal sources of audio data to consider:
• “Mixed” data originating from the OS mixer service. This data is typically stereo PCM format and passed
on AC-link slots 3&4. Requirement 5.10.1.1 below ensures 48 kHz OS mixer output on AC-link slots 3&4
can be routed to both DAC and S/PDIF transmitter. Option 5.10.2.1 below expands support to 8-48 kHz
OS mixer output.
• “Pass-thru” data originating from an “independent S/PDIF source”. This data can be PCM, AC-3**,
MPEG, DTS, or other format, and is typically passed on AC-link slots 7&8 in 2-ch systems, AC-link slots
6&9 in 4-ch systems, and AC-link slots 10&11 in 6-ch systems. The sample rate of “independent S/PDIF
source” data passed across AC-link must match the programmed SPSR (typically 48 kHz). Requirements
5.10.1.21 through 5.10.1.4 ensure availability of independent AC-link slot allocation for “pass-thru” data.
5.10.1 Required concurrency support for S/PDIF transmission (48 kHz operation)
The AC ‘97 architecture is optimized for 48 kHz operation, at which rate the AC-link signaling protocol reduces to
one sample request per frame. Restricting the mixed PCM and independent S/PDIF source data to 48 kHz is a
simple way to ensure the greatest number of S/PDIF concurrency options, but requires the OS mixer to perform high
quality SRC up-sampling of all PCM data to 48 kHz. Additional SRC capability may be needed if the independent
S/PDIF source supports non 48 kHz PCM data.
The following are the minimum concurrency requirements for S/PDIF enabled AC ‘97 2.3 compliant Codecs.
5.10.1.1 Simultaneous DAC playback and S/PDIF transmission of a single 2-ch 48 kHz PCM stream
All S/PDIF capable Primary Codecs must be configurable to accept 48 kHz PCM data on AC-link slots 3&4 for DA
conversion and concurrent S/PDIF transmission. SPCV must indicate valid configuration when programmed as
follows:
• Register 2Ch controlling AC-link slots 3&4 DAC sample rates set to “BB80h” (48kHz)
• Register 3Ah, field SPSR[1,0] controlling S/PDIF sample rate set to “10” (48kHz)
• Register 2Ah, field SPSA[1,0] controlling S/PDIF AC-link slot assignment set to “00” (slots 3&4)
This requirement ensures basic support for OS mixer 48 kHz PCM output to DAC and S/PDIF transmitter.
5.10.1.2 Simultaneous DAC playback of a 2-ch 48 kHz PCM stream and S/PDIF transmission of an
independent 48 kHz PCM or encoded multichannel stream
2-ch S/PDIF capable Primary Codec must accept 48 kHz PCM data on AC-link slots 3&4 for DA conversion and
independent 48 kHz PCM or encoded multichannel data on AC-link slots 7&8 for concurrent S/PDIF transmission.
SPCV must indicate valid configuration when programmed as follows:
• Register 2Ch controlling AC-link slots 3&4 DAC sample rates set to “BB80h” (48kHz)
• Register 3Ah, field SPSR[1,0] controlling S/PDIF sample rate set to “10” (48kHz)
• Register 2Ah, field SPSA[1,0] controlling S/PDIF AC-link slot assignment set to “01” (slots 7&8)
4-ch S/PDIF capable Primary Codec must accept 48 kHz PCM data on AC-link slots 3&4 and 7&8 for DA
conversion and independent 48 kHz PCM or encoded multichannel data on slot 6&9 for concurrent S/PDIF
transmission. SPCV must indicate valid configuration when programmed as follows:
• Registers 2Ch and 2Eh controlling AC-link slots 3&4 and 7&8 DAC sample rates set to “BB80h” (48kHz)
• Register 3Ah, field SPSR[1,0] controlling S/PDIF sample rate set to “10” (48kHz)
• Register 2Ah, field SPSA[1,0] controlling S/PDIF AC-link slot assignment set to “10” (slots 6&9)
These requirements ensure that 2- or 4-ch Codecs w/ S/PDIF can support 2- or 4-ch OS mixer 48 kHz PCM output