Functional Description
GENERAL
The LM4550 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 in-
puts 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 AD-
Cs 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. Adjustments
are in 1.5 dB steps over a gain range of 0 dB to +22.5 dB and
both channels mute together (D15). Input selection for the
ADC is through the Record Select Mux controlled from the
Record Select register, 1Ah, together with microphone selec-
tion controlled by the MS 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 (AVSS) as the signal reference. Either of the
two mono microphone inputs can be muxed to a pro-
grammable boost amplifier before selection for either channel
of the ADC. The Microphone Mux is controlled by the Micro-
phone 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 selected 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 microphone input is
mixed equally into left and right stereo channels but stereo
mixing is orthogonal, i.e. left channels are only mixed with
other left channels and right with right. The left and right am-
plitudes 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 available
for processing by the National 3D Sound circuitry. If MIX2 is
selected, the DAC output will bypass the 3D processing. 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 enhancements. National
3D Sound circuitry is enabled by the 3D bit (D13) in the Gen-
eral Purpose register, 20h, and is a fixed depth implementa-
tion. The 3D Control register, 22h, is therefore not pro-
grammable (read-only). The 3D Sound circuitry 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 Headphone output (HP_OUT) and the
Line output (LINE_OUT) and can also be selected as the input
to the ADC at the Record Select Mux. In addition, the two
channels of Stereo Mix are summed to form a mono signal
(Mono Mix) also selectable at the Record Select Mux as an
input to either channel of the ADC.
STEREO OUTPUTS
The output volume from LINE_OUT and HP_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 Vol-
ume (02h) and Headphone 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 sig-
nals 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 (LP-
BK 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 > 1 µs. It is a complete reset. All registers and 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 > 1 µs and the codec AC Link digital interface is in pow-
erdown (PR4 = 1, Powerdown Control / Status register, 26h).
It is used to clear PR4 and power up the AC Link digital in-
terface but otherwise does not change the contents of any
registers nor reset 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 accordingly but
does not reset any other internal circuits.
www.national.com 18
100972 Version 6 Revision 3 Print Date/Time: 2011/09/23 10:22:49
LM4550