CS6422
CS6422
14), and the receive suppression speech detector
threshold is set by RSThd (Register 2, bits 13 and
making. The CS6422 must be <Idle> before it will
allow a state change between <Transmit> and <Re-
12). The transmit speech detectors for both half-du- ceive>.
plex and suppression default to 6 dB.
The half-duplex controller can be susceptible to
Note that constant power signals which persist for echo, so a holdover timer is provided to help pre-
long durations, such as tones or white noise from a vent false switching. Holdover will force the chan-
signal generator, will be detected as speech only as nel to remain in its current state for a fixed duration
long as the background noise level has not risen to
within the speech detection threshold of the signal
after speech has stopped. HDly (Register 2, bits 9
and 8) sets the duration of the holdover. Longer
power. When a tone has persisted for long enough, holdover will tend to make interrupting more diffi-
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.
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.
4.1.2 Half-Duplex Mode
4.1.3 AGC
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.
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.
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 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.
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
For example, Figure 11 shows how the AGC works
with a reference level of +30 dB (Word = 0000).
35
35
DS295F1
CIRRUS [ CIRRUS LOGIC ]
