Functional Description
GENERAL
The LM4546B codec can mix, process and convert among analog (stereo and mono) and digital (AC Link format) inputs and outputs.
There are two stereo and four mono analog inputs and one 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
Both 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. The stereo input, 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 stereo input, LINE_IN, is a 2-pin interface, single-ended for each stereo channel
with analog ground (AVSS) as the signals' 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 other selectable mono input, coupled directly to the Record Select Mux, is PHONE.
ANALOG MIXING: MIX1
Three analog inputs are available for mixing at the stereo mixer, MIX1 – both 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) 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; each are 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 the Line output (LINE_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 OUTPUT
The output volume from LINE_OUT can be muted or adjusted by 0 dB to 45 dB in nominal 3 dB steps under the control of the
Master Volume register, 02h. 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. This is an 18 bit digital loopback, bypassing the SRC logic, at a 48 kHz rate, even
if another sample rate conversion is selected.
LM4546B
16 Copyright © 1999-2012, Texas Instruments Incorporated