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Technical Reference Guide
5.8.4 AUDIO CODEC
The audio codec provides pulse code modulation (PCM) coding and decoding of audio
information as well as the selection and/or mixing of analog channels. As shown in Figure 5-13,
analog audio from a microphone, tape, or CD can be selected and, if to be recorded (saved) onto a
disk drive, routed through an analog-to-digital converter (ADC). The resulting left and right PCM
record data are muxed into a time-division-multiplexed (TDM) data stream (SD IN signal) that is
routed to the audio controller. Playback (PB) audio takes the reverse path from the audio
controller to the audio codec as SD OUT data and is decoded and processed by the digital-to-
analog converter (DAC). The codec supports simultaneous record and playback of stereo (left
and right) audio. The Sample Rate Generator may be set for sampling frequencies up to 48 KHz.
Analog audio may then be routed through 3D stereo enhancement processor or bypassed to the
output selector (SEL). The integrated analog mixer provides the computer control-console
functionality handling multiple audio inputs.
Audio
Format
ADC
PB
Data (R)
PB
Data (L)
DAC
DAC
Rec
Data (R)
ADC
Rec
Data (L)
S
E
L
(R)
(L)
(R)
(R)
(L)
(L)
3D Proc.
3D Proc.
SW
PB
Gain
PB
Gain
Rec
Gain
Σ/Mixer
CD In (R)
CD In (L)
Line In (R)
Line In (L)
Mic In
S
e
l
e
c
t
o
r
SD Out
SD IN
AC97
Link
I/F
Sample
Rate
Gen.
Right
Audio
Rec
Gain
Analog
Output
Circuits
Left
Audio
Audio
Controller
Figure 5-12. AD1885 Audio Codec Functional Block Diagram
All inputs and outputs are two-channel stereo except for the microphone input, which is inputted
as a single-channel but mixed internally onto both left and right channels. The microphone input
is the default active input. All block functions are controlled through index-addressed registers of
the codec.
Compaq Evo and Workstation Personal Computers
Featuring the Intel Pentium 4 Processor
Second Edition - January 2003
5-29