lm4549a National Semiconductor Corporation, lm4549a Datasheet - Page 16

• Manufacturer Part Number: lm4549a
• Description: Ac ?97 Rev 2.1 Multi-channel Audio Codec With Sample Rate Conversion And National 3d Sound
• Manufacturer: National Semiconductor Corporation

www.national.com
Functional Description
GENERAL
The LM4549A codec can mix, process and convert among
analog (stereo and mono) and digital (AC Link format) inputs
and outputs. There are four stereo and four mono analog
inputs and two stereo and one mono analog outputs. A single
codec supports data streaming on two input and two output
channels of the AC Link digital interface simultaneously.
ADC INPUTS AND OUTPUTS
All four of the stereo analog inputs and three of the mono
analog inputs can be selected for conversion by the 18-bit
stereo ADC. Digital output from the left and right channel
ADCs is always located in AC Link Input Frame slots 3 and
4 respectively. Input level to either ADC channel can be
muted or adjusted from the Record Gain register, 1Ch. Ad-
justments are in 1.5 dB steps over a gain range of 0 dB to
+22.5 dB and both channels mute together (mute bit D15).
Input selection for the ADC is through the Record Select Mux
controlled from the Record Select register, 1Ah, together
with microphone selection controlled by the MS bit (bit D8) in
the General Purpose register, 20h. One of the stereo inputs,
CD_IN, uses a quasi-differential 3-pin interface where both
stereo channel inputs are referenced to the third pin,
CD_GND. CD_GND should be AC coupled to the source
ground and provides common-mode feedback to cancel
ground noise. It is not a DC ground. The other three stereo
inputs, LINE_IN, AUX and VIDEO are 2-pin interfaces,
single-ended for each stereo channel, with analog ground
(AV
microphone inputs can be muxed to a programmable boost
amplifier before selection for either channel of the ADC. The
Microphone Mux is controlled by the Microphone Selection
(MS) bit (D8) in the General Purpose register (20h) and the
20 dB programmable boost is enabled by the 20dB bit (D6)
in register 0Eh. The mono PHONE input may also be se-
lected for either ADC channel.
ANALOG MIXING: MIX1
Five analog inputs are available for mixing at the stereo
mixer, MIX1 – all four stereo and one mono, namely the
microphone input selected by MS (D8, reg 20h). Digital input
to the codec can be directed to either MIX1 or to MIX2 after
conversion by the 18-bit stereo DAC and level adjustment by
the PCM Out Volume control register (18h). Each input to
MIX1 may be muted or level adjusted using the appropriate
Mixer Input Volume Register: Mic Volume (0Eh), Line_In
Volume (10h), CD Volume (12h), Video Volume (14h), Aux
Volume (16h) and PCM Out Volume (18h). The mono micro-
phone input is mixed equally into left and right stereo chan-
nels but stereo mixing is orthogonal, i.e. left channels are
only mixed with other left channels and right with right. The
left and right amplitudes of any stereo input may be adjusted
independently however mute for a stereo input acts on both
left and right channels.
DAC MIXING AND 3D PROCESSING
Control of routing the DAC output to MIX1 or MIX2 is by the
POP bit (D15) in the General Purpose register, 20h. If MIX1
is selected (default, POP=0) then the DAC output is avail-
able for processing by the National 3D Sound circuitry. If
MIX2 is selected, the DAC output will bypass the 3D pro-
cessing. This allows analog inputs to be enhanced by the
analog 3D Sound circuitry prior to mixing with digital audio.
The digital audio may then use alternative digital 3D en-
hancements. National 3D Sound circuitry is enabled by the
SS
) as the signal reference. Either of the two mono
16
3D bit (D13) in the General Purpose register, 20h, and is a
fixed depth implementation. The 3D Control register, 22h, is
therefore not programmable (read-only). The 3D Sound cir-
cuitry defaults to disabled after reset.
ANALOG MIXING: MIX2
MIX2 combines the output of MIX1 (Stereo Mix 3D) with the
two mono analog inputs, PHONE and PC_BEEP, these each
level-adjusted by the input control registers Phone Volume
(0Ch) and PC_Beep Volume (0Ah) respectively. If selected
by the POP bit (D15, reg 20h), the DAC output is also
summed into MIX2.
STEREO MIX
The output of MIX2 is the signal, Stereo Mix. Stereo Mix is
used to drive both the Line output (LINE_OUT) and the Line
Level output (LNLVL_OUT) and can also be selected as the
input to the ADC by the Record Select Mux. In addition, the
two channels of Stereo Mix are summed to form a mono
signal (Mono Mix) also selectable by the Record Select Mux
as an input to either channel of the ADC.
STEREO OUTPUTS
The output volume from LINE_OUT and LNLVL_OUT can be
muted or adjusted by 0 dB to 45 dB in nominal 3 dB steps
under the control of the output volume registers Master
Volume (02h) and Line Level Volume (04h) respectively. As
with the input volume registers, adjustments to the levels of
the two stereo channels can be made independently but
both left and right channels share a mute bit (D15).
MONO OUTPUT
The mono output (MONO_OUT) is driven by one of two
signals selected by the MIX bit (D9) in the General Purpose
register, 20h. The signal selected by default (MIX = 0) is the
mono summation of the two channels of Stereo Mix 3D, the
stereo output of the mixer MIX1. Setting the control bit MIX =
1, selects a microphone input, MIC1 or MIC2. The choice of
microphone is controlled by the Microphone Select (MS) bit
(D8) also in the General Purpose register, 20h.
ANALOG LOOPTHROUGH AND DIGITAL LOOPBACK
Analog Loopthrough refers to an all-analog signal path from
an analog input through the mixers to an analog output.
Digital Loopback refers to a mixed-mode analog and digital
signal path from an analog input through the ADC, looped-
back (LPBK bit – D7, 20h) through the DAC and mixers to an
analog output.
RESETS
COLD RESET is performed when RESET# (pin 11) is pulled
low for
internal circuits are reset to their default state. It is the only
reset which clears the ATE and Vendor test modes.
WARM RESET is performed when SYNC (pin 10) is held
high for
powerdown (PR4 = 1, Powerdown Control / Status register,
26h). It is used to clear PR4 and power up the AC Link digital
interface but otherwise does not change the contents of any
internal circuitry.
REGISTER RESET is performed when any value is written
to the RESET register, 00h. It resets all registers to their
default state and will modify circuit configurations accord-
ingly but does not reset any other internal circuits.
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1 µs and the codec AC Link digital interface is in
1 µs. It is a complete reset. All registers and