CDB6420 Просмотр технического описания (PDF) - Cirrus Logic

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CDB6420

Full-Duplex Speakerphone Chip

Cirrus Logic 

CS6420

OVERVIEW

The CS6420 is a full-duplex speakerphone chip for

use in hands-free communications with telephony

quality audio. Common applications include

speakerphones, inexpensive video-conferencing,

and cellular phone car kits. The CS6420 requires

very few external components and allows system

control through a microcontroller interface.

Hands-free communication through a microphone

and speaker typically results in acoustic feedback

or howling because the loop gain of the system ex-

ceeds unity by the time audio amplitudes are ad-

justed to a reasonable level. The solution to the

howling problem has typically been half-duplex,

where either the transmit or the receive channel is

active, never both at the same time. This prevents

the howling, but diminishes the overall communi-

cation quality by clipping words and forcing the

talker at each end to wait for the talker at the other

end to stop speaking.

Full-duplex conversation, where both transmit and

receive channels are active simultaneously, is the

conversation quality we enjoy when using hand-

sets. Full-duplex for hands-free communications is

achieved in the CS6420 using a digital signal pro-

cessing technique called “Echo Cancellation.” The

end result is a more natural conversation than half-

duplex, with no awkward breaks and pauses, as if

both parties were speaking to each other directly.

Echo Cancellation reduces overall loop gain and

the acoustic coupling between speaker and micro-

phone. This coupling reduction prevents the annoy-

ing effect of hearing one’s own delayed speech, the

effect being worse when there is delay in the sys-

tem, such as vocoder delay in digital cellular

phones.

The CS6420 is a complete system implementation

of a Digital Signal Processor with RAM and pro-

gram ROM, running Echo Cancellation algorithms

developed at Crystal Semiconductor using custom-

er input, integrated with two delta-sigma codecs.

The CS6420 is intended to provide a full-duplex

speakerphone solution with a minimum of design

effort while displacing existing half-duplex speak-

erphone chips.

FUNCTIONAL DESCRIPTION

The CS6420 is roughly divided into four external

interface blocks. The analog interfaces connect the

chip to the transmit and receive paths. Certain con-

trol functions are accessible through the microcon-

troller interface. Two pins accommodate either a

crystal or an externally applied digital clock signal.

Analog and digital power and ground are provided

through four pins.

Analog Interface

In a speakerphone application, one input of the

CS6420 connects to the signal from the micro-

phone, sometimes called the near-end input or

transmit input, and one output connects to the

speaker. The output that leads to the speaker is

sometimes called the near-end output or receive

output. Together, the input and output that connect

to the microphone and speaker are referred to as the

Acoustic Interface.

The signal received at the near-end input is then

passed to the far-end output or transmit output after

acoustic echo cancellation. This signal is sent to the

telephone line. The signal from the telephone line

is received at the far-end input, also called the re-

ceive input, and this signal is passed to the receive

output after network echo cancellation. Together,

the far-end input and output form the Network In-

terface.

The analog interfaces are physically implemented

using delta sigma converters running at an output

word rate of 8 kHz, resulting in a passband from

DC to 4 kHz. Because the inputs are analog to dig-

ital converters (ADCs), certain design consider-

ations must be kept in mind: specifically, anti-

aliasing and full-scale input voltage. The ADCs ex-

pect a single-pole RC filter with a corner at 8 kHz,

8

DS205PP2

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