Cirrus Logic CS6422 - Manual (Page 15)

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3.3.1 MIC - MICROPHONE PREAMPLIFIER ENABLE
The microphone preamplifier described in Section 3.1.1, “Acoustic Interface” is enabled by default,
but may be disabled by setting Mic to ‘0’. Refer to Section 3.1.1, “Acoustic Interface” for more details
on using the Microphone Preamplifier.
3.3.2 HDD - HALF-DUPLEX DISABLE
In normal operation, the CS6422 will be in a half-duplex mode if the echo canceller is not providing
enough loop gain reduction to prevent howling. This half-duplex mode is active at power-up while the
adaptive filter begins to train. Half-duplex mode prevents howling and also masks the convergence
process.
In some cases, such as when measuring convergence speed (see Section 4.3.2, “Testing Issues”),
the half-duplex mode is undesirable. By default, the half-duplex mode is enabled.
3.3.3 GB - GRADED BETA
The room-size adjustment scheme called “graded beta,” provided for the acoustic echo canceller in
the CS6422, is controlled by GB. The network echo canceller does not support graded beta.
Graded beta is an architectural enhancement to the CS6422 which takes advantage of the fact that
acoustic echoes tend to decay exponentially with time. The CS6422 can increase the beta, or update
gain, for the coefficients of the adaptive filter which occur earlier in time and decrease it for those that
occur later in time, which increases convergence speed while maintaining stability. In order to make
this improvement, there is an implicit assumption that the decay rate of the echo is known. The graded
beta control allows the system designer to adjust this. For very acoustically live rooms, use either no
decay (00) or slight decay (11). Cars and acoustically dead rooms can benefit from the most rapid decay
(01).
3.3.4 RVOL - RECEIVE VOLUME CONTROL
Volume in the receive path is set by RVol. The volume control in the receive direction is implemented
by a peak-limiting automatic gain control (AGC) and digital attenuation at the near-end output DAC.
The AGC is discussed in detail in Section 4., “Design Considerations”. See Section 4.1.3, “AGC”for a
full explanation of how it functions.
When the reference level is set to +0 dB, the AGC is disabled. Volume control is implemented by dig-
ital attenuation in 3 dB steps from this point on down. The maximum gain is +30 dB and the minimum
is -12 dB. The lowest gain setting (1111) mutes the receive path.
The default setting for RVol is +18 dB.
3.3.5 TSD - TRANSMIT SUPPRESSION DISABLE
The Transmit Supplementary Echo Suppression function is a non-linear echo control mechanism.
Transmit Suppression introduces TSAtt (see Register 3) of attenuation into the transmit path when it
is engaged. When TSMde = ‘1’, the transmit suppressor engages when there is speech detected in
the receive path and no near-end speech is present. When TSMde = ‘0’, the default case, the transmit
suppressor engages when there is no near-end speech present. When near-end speech is present,
the suppression attenuation is removed. By default, the transmit suppression function is enabled.
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