CS6422 datasheet(35/48 Pages) CIRRUS

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CS6422

14), and the receive suppression speech detector

threshold is set by RSThd (Register 2, bits 13 and

12). The transmit speech detectors for both half-du-

plex and suppression default to 6 dB.

Note that constant power signals which persist for

long durations, such as tones or white noise from a

signal generator, will be detected as speech only as

long as the background noise level has not risen to

within the speech detection threshold of the signal

power. When a tone has persisted for long enough,

the background noise level will be equal to the

power estimator level, and so the tone will no long-

er be considered speech. This duration is dependent

upon the power difference between the signal and

the ambient noise power, as well as NseRmp. It

should be noted that the CS6422 has a tone detector

to prevent updates when tones are present and allow

tones to persist regardless of the speech detectors.

4.1.2

Half-Duplex Mode

In cases where the system relies on the echo cancel-

ler for stability, a fail-safe mechanism must be in

place for instances when the echo canceller is not

performing adequately. The CS6422 implements a

half-duplex mode to guarantee communication

even when the echo canceller is disabled.

When the CS6422 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 CS6422’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: <Trans-

mit>, <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 enhance switching decision

making. The CS6422 must be <Idle> before it will

allow a state change between <Transmit> and <Re-

ceive>.

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 more diffi-

cult, but will be more robust to spurious switching

caused by echo.

4.1.2.1

Idle Return to Transmit

When enabled, this feature causes the CS6422 to

return to <Transmit> mode from an <Idle> state

when operating in half-duplex.

This simulates

full-duplex-like behavior during the periods of

half-duplex operation at the beginning of a call.

4.1.3

AGC

The CS6422 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

11-8) for the transmit direction and RVol (Register

0, bits 11-8) 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 11 shows how the AGC works

with a reference level of +30 dB (Word = 0000).

CS6422

DS295F1

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