MT93L16 datasheet(5/27 Pages) MITEL

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Preliminary Information

MT93L16

Howling Detector (HWLD) 4

(4. Patent Pending)

The Howling detector is part of an Anti-Howling

control, designed to prevent oscillation as a result of

positive feedback in the audio paths.

The HWLD can be disabled by setting the AH- bit to

1 in the (MC) control register.

Offset Null Filter

To ensure robust performance of the adaptive ﬁlters

at all times, any DC offset that may be present on

either the Rin signal or the Sin signal, is removed by

highpass

ﬁlters.

These

ﬁlters

have

corner

frequency placed at 40Hz.

The offset null ﬁlters can be disabled by setting the

HPF- bit to 1 in the LEC or AEC control registers.

Limiters

To prevent clipping in the echo paths, two limiters

with variable thresholds are provided at the outputs.

The Rout limiter threshold is in Rout Limiter Register

1 and 2. The Sout limiter threshold is in Sout Limiter

User Gain

The user gain function provides the ability for users

to adjust the audio gain in the receive path (speaker

path). This gain is adjustable from -24dB to +21dB in

3dB steps. It is important to use ONLY this user gain

function to adjust the speaker volume. The user gain

function in the MT93L16 is optimally placed between

the two echo cancellers such that no reconvergence

is necessary after gain changes.

The gain can be accessed through Receive Gain

Control Register.

AGC

The AGC function is provided to limit the volume in

the speaker path. The gain of the speaker path is

automatically

reduced

during

the

following

conditions:

•

When clipping of the receive signal occurs.

•

When initial convergence of the acoustic echo

canceller detects unusually large echo return.

•

When howling is detected.

The AGC can be disabled by setting the AGC- bit to

1 in MC control register.

Mute Function

A pcm mute function is provided for independent

control of the Receive and Send audio paths. Setting

the MUTE_R or MUTE_S bit in the MC register,

causes quiet code to be transmitted on the Rout or

Sout paths respectively.

Quiet code is deﬁned according to the following

table.

Bypass Control

PCM

bypass

function

provided

allow

transparent transmission of pcm data through the

MT93L16. When the bypass function is active, pcm

data passes transparently from Rin to Rout and from

Sin to Sout, with bit-wise integrity preserved.

When the Bypass function is selected, most internal

functions are powered down to provide low power

consumption.

The BYPASS control bit is located in the main control

MC register.

Adaptation Enable/Disable

Adaptation control bits are located in the AEC and

LEC control registers. When the ADAPT- bit is set to

1, the adaptive ﬁlter is frozen at the current state. In

this state, the device continues to cancel echo with

the current echo model.

When the ADAPT- bit is set to 0, the adaptive ﬁlter is

continually updated. This allows the echo canceller

to adapt and track changes in the echo path. This is

the normal operating state.

MT93L16 Throughput Delay

In all modes, voice channels always have 2 frames of

delay. In ST-BUS/GCI operation, the D and C

channels have a delay of one frame.

LINEAR

16 bits

2’s

complement

SIGN/

MAGNITUDE

µ-Law

A-Law

CCITT (G.711)

µ-Law

A-Law

+Zero

(quiet code)

0000h

80h

FFh

D5h

Table 1 - Quiet PCM Code Assignment

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