LM4549B datasheet(16/28 Pages) NSC

16 / 28 page

Functional Description

GENERAL

The LM4549B 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

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 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

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; each are

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

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 at 48 kHz,

bypassing the the SRC logic even if an SRC rate other than

48 kHz is selected.

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

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.

LM4549B Datasheet(PDF) 16 Page - National Semiconductor (TI)

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