CDB6422 Просмотр технического описания (PDF) - Cirrus Logic

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CDB6422

Enhanced Full-Duplex Speakerphone IC

Cirrus Logic 

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).

DS295F1

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