STAC9704 Datasheet, PDF (14/49 Pages) Hynix Semiconductor – Multimedia Audio Codec for AC97

SigmaTel, Inc.
STAC9704/7
3.1.1 AC-link Audio Output Frame (SDATA_OUT)
The audio output frame data streams correspond to the multiplexed bundles of all digital output data
targeting the STAC9704/7 DAC inputs, and control registers. Each audio output frame supports up to
12 20-bit outgoing data time slots. Slot 0 is a special reserved time slot containing 16 bits that are used
for AC-link protocol infrastructure.
Within slot 0, the first bit is a global bit (SDATA_OUT slot 0, bit 15) which flags the validity for the
entire audio frame. If the “Valid Frame” bit is a 1, this indicates that the current audio frame contains
at least one slot time of valid data. The next 12 bit positions sampled by the STAC9704/7 indicate
which of the corresponding 12 times slots contain valid data. In this way data streams of differing
sample rates can be transmitted across AC-link at its fixed 48kHz audio frame rate. The following
diagram illustrates the time slot based AC-link protocol.
Figure 6. AC-link Audio Output Frame
Tag Phase
Data Phase
20.8 uS (48 kHZ)
SYNC
BIT_CLK
12.288 MHz
SDATA_OUT
valid
Frame
slot1
slot2
slot(12) "0" "0" "0" 19
"0"
End of previous audio frame
Time Slot "Valid" Bits
("1" = time slot contains valid PCM data)
Slot 1
19
"0" 19
"0"
Slot 2
Slot 3
19
"0"
Slot 12
A new audio output frame begins with a low to high transition of SYNC. SYNC is synchronous to the
rising edge of BIT_CLK. On the immediately following falling edge of BIT_CLK, the STAC9704/7
samples the assertion of SYNC. This following edge marks the time when both sides of AC-link are
aware of the start of a new audio frame. On the next rising edge of BIT_CLK, the AC’97 controller
transitions SDATA_OUT into the first bit position of slot 0 (Valid Frame bit). Each new bit position is
presented to AC-link on a rising edge of BIT_CLK, and subsequently sampled by the STAC9704/7 on
the following falling edge of BIT_CLK. This sequence ensures that data transitions, and subsequent
sample points for both incoming and outgoing data streams are time aligned.
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10/02/98