CDB6420 Cirrus Logic Inc, CDB6420 Datasheet - Page 8
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CDB6420
Manufacturer Part Number
CDB6420
Description
Development Kit
Manufacturer
Cirrus Logic Inc
Datasheet
1.CDB6420.pdf
(52 pages)
Specifications of CDB6420
Silicon Manufacturer
Cirrus Logic
Application Sub Type
Speakerphone
Kit Application Type
Audio / Video / TV
Silicon Core Number
CS6420
Kit Contents
Board
Kit Features
Analog And Digital Patch Area
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
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.
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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,
CS6420
DS205PP2