MT9300 データシートの表示（PDF） - Mitel Networks

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MT9300

Multi-Channel Voice Echo Canceller

Mitel Networks 

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MT9300

Sin

(channel N)

ST-BUS

PORT2

µ/A-Law/

Linear

Offset

Null

+

-

Non-Linear

Processor

Linear/

µ/A-Law

Disable Tone

Detector

Programmable

Bypass

Microprocessor

Interface

Double-Talk

Detector

MuteS

Narrow-Band

Detector

MuteR

Disable Tone

Detector

Rout

(channel N)

Linear/

µ/A-Law

12dB

Attenuator

Offset

Null

µ/A-Law/

Linear

Sout

(channel N)

ST-BUS

PORT1

Rin

(channel N)

Echo Canceller (N), where 0 ≤ N ≤ 31

Figure 3 - Echo Canceller Functional Block Diagram

Each echo canceller in the MT9300 has four

functional states: Mute, Bypass, Disable Adaptation

and Enable Adaptation. These are explained in the

section entitled Echo Canceller Functional States.

Adaptive Filter

For each group of echo cancellers, the Adaptive

Filter is a 1024 tap FIR adaptive filter which is

divided into two sections. Each section contains 512

taps providing 64ms of echo estimation. In Normal

configuration, the first section is dedicated to

channel A and the second section to channel B. In

Extended Delay configuration, both sections are

cascaded to provide 128ms of echo estimation in

channel A. In Back-to Back configuration, the first

section is used in the receive direction and the

second section is used in the transmit direction for

the same channel.

Double-Talk Detector

Double-Talk is defined as those periods of time when

signal energy is present in both directions

simultaneously. When this happens, it is necessary

to disable the filter adaptation to prevent divergence

of the Adaptive Filter coefficients. Note that when

double-talk is detected, the adaptation process is

halted but the echo canceller continues to cancel

echo using the previous converged echo profile.

A double-talk condition exists whenever the relative

signal levels of Rin (Lrin) and Sin (Lsin) meet the

following condition:

Lsin > Lrin + 20log10(DTDT)

where DTDT is the Double-Talk Detection Threshold.

Lsin and Lrin are signal levels expressed in dBm0.

A different method is used when it is uncertain

whether Sin consists of a low level double-talk signal

or an echo return. During these periods, the

adaptation process is slowed down but it is not

halted. The convergence speed is shown by the

CONV bit in the Status Register.

In G.168 standard, the echo return loss is expected

to be at least 6dB. This implies that the Double-Talk

Detector Threshold (DTDT) should be set to 0.5

(-6dB). However, in order to get additional

guardband, the DTDT is set internally to 0.5625

(-5dB).

In some applications the return loss can be higher or

lower than 6dB. The MT9300 allows the user to

change the detection threshold to suit each

application’s need. This threshold can be set by

writing the desired threshold value into the DTDT

register.

The DTDT register is 16 bits wide. The register value

in hexadecimal can be calculated with the following

equation:

DTDT(hex) = hex(DTDT(dec) * 32768)

where 0 < DTDT(dec) < 1

Example: For DTDT = 0.5625 (-5dB), the

hexadecimal value becomes

hex(0.5625 * 32768) = 4800h

5

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