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AD1846JP

Low Cost Parallel-Port 16-Bit SoundPort Stereo Codec

Analog Devices 

AD1846

(Continued from page 1)

AD1846

CS

A1

A0

WR

RD

DATA7:0

DBDIR

DBEN

PDRQ

CDRQ

PDAK

CDAK

INT

ADDRESS

DECODE

18

8

7

8

4

DIR 2

G

4

5

B

A

AEN

SA19:2

SA1

SA0

IOWC

IORC

DATA7:0

ISA BUS

DRQ<X>

DRQ<Y>

DAK<X>

DAK<Y>

IRQ<Z>

Figure 1. Interface to ISA Bus

The pair of 16-bit outputs from the ADCs is available over a

byte-wide bidirectional interface that also supports 16-bit digital

input to the DACs and control information. The AD1846 can

accept and generate 16-bit twos-complement PCM linear digital

data, 8-bit unsigned magnitude PCM linear data, and 8-bit

µ-law or A-law companded digital data.

The ∑∆ DACs are preceded by a digital interpolation filter. An

attenuator provides independent user volume control over each

DAC channel. Nyquist images and shaped quantization noise

are removed from the DACs’ analog stereo output by on-chip

switched-capacitor and continuous-time filters. Two stereo pairs

of auxiliary line-level inputs can also be mixed in the analog do-

main with the DAC output.

AUDIO FUNCTIONAL DESCRIPTION

This section overviews the functionality of the AD1846 and is

intended as a general introduction to the capabilities of the de-

vice. As much as possible, detailed reference information has

been placed in “Control Registers” and other sections. The user

is not expected to refer repeatedly to this section.

Analog Inputs

The AD1846 SoundPort Stereo Codec accepts stereo line-level

and mic-level inputs. LINE, MIC, and AUX1 inputs and post-

mixed DAC output analog stereo signals are multiplexed to the

internal programmable gain amplifier (PGA) stage.

The PGA following the input multiplexer allows independent

selectable gains for each channel from 0 to 22.5 dB in +1.5 dB

steps. The Codec can operate either in a global stereo mode or

in a global mono mode with left channel inputs appearing at

both channel outputs.

Analog Mixing

AUX1 and AUX2 analog stereo signals can be mixed in the ana-

log domain with the DAC output. Each channel of each auxil-

iary analog input can be independently gained/attenuated from

+12 dB to –34.5 dB in –1.5 dB steps or completely muted. The

post mixed DAC output is available on OUT externally and as

an input to the ADCs.

Even if the AD1846 is not playing back data from its DACs, the

analog mix function can still be active.

Analog-to-Digital Datapath

The AD1846 ∑∆ ADCs incorporate a fourth order modulator.

A single pole of passive filtering is all that is required for anti-

aliasing the analog input because of the ADC’s high 64 times

oversampling ratio. The ADCs include linear phase digital deci-

mation filters that low-pass filter the input to 0.4 ϫ FS. (“FS’’ is

the word rate or “sampling frequency”). ADC input overrange

conditions will cause bits to be set that can be read.

Each channel of the mic inputs can be amplified digitally by

+18 dB to compensate for the voltage swing differences between

line levels and typical condenser microphone levels. This +18

dB digital gain is enabled with the same control bits (LMGE

and RMGE) as the +20 dB analog gain in the AD1848.

Digital-to-Analog Datapath

The ∑∆ DACs contain a programmable attenuator and a low-

pass digital interpolation filter. The anti-imaging interpolation

filter oversamples by 64 and digitally filters the higher frequency

images. The attenuator allows independent control of each

DAC channel from 0 dB to –94.5 dB in 1.5 dB steps plus full

mute. The DACs’ ∑∆ noise shapers also oversample by 64 and

convert the signal to a single bit stream. The DAC outputs are

then filtered in the analog domain by a combination of switched-

capacitor and continuous-time filters. They remove the very

high frequency components of the DAC bitstream output. No

external components are required. Phase linearity at the analog

output is achieved by internally compensating for the group

delay variation of the analog output filters.

Changes in DAC output attenuation take effect only on zero

crossings, thereby eliminating “zipper” noise. Each channel has

its own independent zero-crossing detector and attenuator

change control circuitry. A timer guarantees that requested vol-

ume changes will occur even in the absence of an input signal

that changes sign. The time-out period is 8 milliseconds at a

48 kHz sampling rate and 48 milliseconds at an 8 kHz sampling

rate. (Time out [ms] ≈ 384/FS [kHz].)

Digital Mixing

Stereo digital output from the ADCs can be mixed digitally with

the input to the DACs. Digital output from the ADCs going out

of the data port is unaffected by the digital mix. Along the digi-

tal mix datapath, the 16-bit linear output from the ADCs is

attenuated by an amount specified with control bits. Both chan-

nels of the monitor data are attenuated by the same amount.

(Note that internally the AD1846 always works with 16-bit

PCM linear data, digital mixing included; format conversions

take place at the input and output.)

REV. A

–9–

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