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CDB6422

Enhanced Full-Duplex Speakerphone IC

Cirrus Logic 

CS6422

Suppression may cause some modulation of the

perceived background noise which may be distract-

ing to some users. As a result, it may be desirable

to limit the suppression attenuation to the minimum

necessary. The CS6422 provides TSAtt (Register

3, bits 15 and 14) to control the amount of attenua-

tion introduced by suppression in the transmit

channel. Receive suppression attenuates by 24 dB.

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.

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.

There are four controls that govern the behavior of

Transmit Suppression. These are TSThd (Register

3, bits 7 and 6), TSAtt (Register 3, bits 15 and 14),

TSBias (Register 3, bits 5 and 4), and TSMde (Reg-

ister 0, bit 4). TSThd is the primary control and

should be adjusted before changing the value of

TSBias from its default setting. TSThd sets the

ERLE expectation to be used in discriminating be-

tween near-end speech and far-end echo. This con-

trol setting will by far predominate in affecting the

manner in which Transmit Suppression behaves.

TSAtt controls the amount of attenuation added to

the transmit path when the transmit suppressor en-

gages.

TSBias is a secondary control. This is to be adjust-

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

keep it disengaged. We recommend using larger

values of TSBias relative to TSThd settings in or-

der to facilitate ease of near-end speech transmis-

sion. For example, the default setting for TSThd is

15 dB and 18 dB for TSBias.

In some scenarios, especially when the dynamic

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.

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.

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

speakerphones, and helps keep noise levels low.

One side effect of this scheme is that a constant

power signal, such as noise from a noise generator

or a tone, will eventually be attenuated when the

background noise level estimate turns off the re-

ceive suppression speech detector. See Section

4.1.1.4, “Speech Detection” from more details.

RSThd (Register 2, bits 13 and 12) sets the speech

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.

4.1.4.3 Double-talk Attenuation

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

DS295F1

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