CS6422
CS6422
Suppression may cause some modulation of the
keep it disengaged. We recommend using larger
perceived background noise which may be distract- values of TSBias relative to TSThd settings in or-
ing to some users. As a result, it may be desirable
to limit the suppression attenuation to the minimum
der to facilitate ease of near-end speech transmis-
sion. For example, the default setting for TSThd is
necessary. The CS6422 provides TSAtt (Register 15 dB and 18 dB for TSBias.
3, bits 15 and 14) to control the amount of attenua-
In some scenarios, especially when the dynamic
tion introduced by suppression in the transmit
channel. Receive suppression attenuates by 24 dB.
range of volume control is significantly large, we
also recommend the use of different combinations
of TSThd and TSBias setting relative to output vol-
ume of the acoustic interface. Specifically, higher
volume levels may call for larger values of TSThd.
4.1.4.1 Transmit Suppression
When TSMde = ‘1’ (Noise Guard ‘off’), the trans-
mit suppressor attenuates the transmit path when
only far-end speech is present. When TSMde = ‘0’
(Noise Guard ‘on’), the suppressor attenuates when
the transmit channel is idle, that is, when no near-
end speech is present.
TSMde controls the Noise Guard feature. When
TSMde = ‘0’ (Noise Guard enabled), the transmit
suppressor is engaged when no near-end speech is
present. When TSMde = ‘1’ (Noise Guard dis-
abled), the transmit suppressor is engaged only
when far-end speech is present in the absence of
near-end speech.
The purpose of Transmit Suppression is to mask re-
sidual echo by inserting additional loss/attenuation
in the transmit path in the scenario when far-end
speech is present; the residual echo, if any, in dou-
ble-talk is masked by near-end speech, assuming
reasonable levels of ERLE.
4.1.4.2 Receive Suppression
The receive suppressor is nominally attenuating
unless far-end speech is present. This behavior is
more consistent with behavior observed in modern
There are four controls that govern the behavior of
Transmit Suppression. These are TSThd (Register speakerphones, and helps keep noise levels low.
3, bits 7 and 6), TSAtt (Register 3, bits 15 and 14),
One side effect of this scheme is that a constant
TSBias (Register 3, bits 5 and 4), and TSMde (Reg-
power signal, such as noise from a noise generator
ister 0, bit 4). TSThd is the primary control and
or a tone, will eventually be attenuated when the
should be adjusted before changing the value of
background noise level estimate turns off the re-
TSBias from its default setting. TSThd sets the
ceive suppression speech detector. See Section
ERLE expectation to be used in discriminating be-
4.1.1.4, “Speech Detection” from more details.
tween near-end speech and far-end echo. This con-
RSThd (Register 2, bits 13 and 12) sets the speech
trol setting will by far predominate in affecting the
detection threshold of the suppressor’s speech de-
tector. This control is normally set to the same val-
ue as RHDet. See Section 4.1.1.4, “Speech
Detection” for more details.
manner in which Transmit Suppression behaves.
TSAtt controls the amount of attenuation added to
the transmit path when the transmit suppressor en-
gages.
4.1.4.3 Double-talk Attenuation
TSBias is a secondary control. This is to be adjust-
In full-duplex hands-free to full-duplex hands-free
scenarios (where a call exists between two full-du-
plex speakerphones), stability problems can arise at
higher volume levels due to the acoustic coupling
ed after the system designer is more or less satisfied
with the behavior of Transmit Suppression with the
TSThd set. It affects the ease with which a near-end
talker may disengage Transmit Suppression and
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DS295F1
CIRRUS [ CIRRUS LOGIC ]
