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CDB6420

Full-Duplex Speakerphone Chip

Cirrus Logic 

CS6420

performing adequately. The CS6420 implements a

half-duplex mode to guarantee communication

even when the echo canceller is disabled.

When the CS6420 is first powered on, or emerges

from a reset, the echo canceller coefficients are

cleared, and the echo cancellers provide no benefit

at this point. The half-duplex mode is on to prevent

howling and echo from interfering with communi-

cation. Once the CS6420’s adaptive filters have

adapted sufficiently, the half-duplex mode is auto-

matically disabled, and full-duplex communication

can occur.

The half-duplex mode allows three states: transmit,

receive, and idle. In the transmit state, the transmit

channel is open and the receive channel is muted.

The receive state mutes the transmit channel. The

idle state is an internal state which is used to en-

hance switching decision making. The CS6420

must be idle before it will allow a state change be-

tween transmit and receive.

The half-duplex controller can be susceptible to

echo, so a holdover timer is provided to help pre-

vent false switching. Holdover will force the chan-

nel to remain in its current state for a fixed duration

after speech has stopped. HDly (Register 2, bits 9

and 8) sets the duration of the holdover. Longer

holdover will tend to make interrupting much hard-

er, but will be much more robust to spurious

switching caused by echo.

AGC

The CS6420 implements a peak-limiting AGC in

both the transmit and receive directions in order to

boost low-level signals without compromising per-

formance when high amplitude signals are present.

The technique effectively results in dynamic range

compression.

The AGC works by setting a reference level based

on the value represented by TVol (Register 1, bits

9-5) for the transmit direction and RVol (Register

0, bits 9-5) for the receive direction. If the signal

from the input is above this reference, it is attenu-

ated to the reference level with an attack time of

125 µs. This attenuation level decays with a time

constant of 30 ms unless another signal greater than

the reference level is detected. After the attenua-

tion, a post-scaler scales the reference level to full-

scale (the maximum digital code), which amplifies

all signals by the difference between the reference

level and full-scale.

For example, Figure 9 shows how the AGC works

with a reference level of +30 dB (Word = 00000).

Any signal greater than 30 dB below full-scale (a),

is scaled down to 30 dB (b). This signal is then

scaled up +30 dB (the reference level) to provide

the final output (c). Note that the combination of at-

tenuation and gain results in less than +30 dB total

gain being applied. If the input signal is below 30

dB below full-scale (d), no attenuation is done and

the full +30 dB of gain is applied to the signal (e).

When the reference level is set to +0 dB, the AGC

is effectively disabled. Volume control is imple-

mented by digital attenuation in 3 dB steps from

this point on down. The maximum gain is +30 dB

and the minimum is -60 dB in 3 dB steps. The low-

est gain setting (11111) mutes the path. The signal

scaling takes place in between the two cancellers,

and so does not disturb the echo canceller as chang-

ing gain in the echo path would (see the Adaptive

Filter section for more details).

Suppression

Echo cancellation is somewhat of a misnomer in

that echo is merely attenuated, not entirely can-

celled. Some residual echo still exists after the

summing node. This residual echo, though very

low, may be audible when the near-end talker is not

speaking. Suppression further attenuates the ech-

oed signal.

The CS6420 employs supplementary echo suppres-

sion which adds attenuation on top of the cancella-

tion to remove the residual echo. For example, the

DS205PP2

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