Datasheet

OBSOLETE
LM4550
SNAS032F –SEPTEMBER 2001–REVISED APRIL 2013 www.ti.com
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 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. 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 selection 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 (AV
SS
) as the
signal reference. Either of the two mono 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 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 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 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 General
Purpose register, 20h, and is a fixed depth implementation. The 3D Control register, 22h, is therefore not
programmable (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.
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