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.
8
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,
DS205PP2
CIRRUS [ CIRRUS LOGIC ]
