Preliminary Information
NLPTHR
(hex)
= hex(NLPTHR
(dec)
* 32768)
MT9123
Echo Canceller Functional States
Each echo canceller has four functional states: Mute,
Bypass, Disable Adaptation and Enable Adaptation.
Mute:
The Mute state forces the echo canceller to
transmit quiet code and halts the filter adaptation
process.
In Normal configuration, the PCM output data on
Rout is replaced with the quiet code according to
the following table.
LINEAR
SIGN/
16 bits
MAGNITUDE
2’s
µ
-Law
complement
A-Law
+Zero
(quiet code)
0000h
80h
CCITT (G.711)
where 0 < NLPTHR
(dec)
< 1
The comfort noise injection can be disabled by
setting the INJDis bit to 1 in Control Register 1.
It should be noted that the NLPTHR is valid and the
comfort noise injection is active only when the NLP is
enabled.
Narrow Band Signal Detector (NBSD)
Single or dual frequency tones (e.g. DTMF tones)
present in the reference input (Rin) of the echo
canceller for a prolonged period of time may cause
the adaptive filter to diverge. The Narrow Band
Signal Detector (NBSD) is designed to prevent this
divergence by detecting single or dual tones of
arbitrary frequency, phase, and amplitude. When
narrow band signals are detected, the adaptation
process is halted but the echo canceller continues to
cancel echo.
µ
-Law
FFh
A-Law
D5h
Table 1 - Quiet PCM Code Assignment
In Back-to-Back configuration, both echo cancellers
are combined to implement a full duplex echo
canceller. Therefore muting Echo Canceller A
causes quiet code to be transmitted on Rout, while
muting Echo Canceller B causes quiet code to be
transmitted on Sout.
In Extended Delay configuration, both echo
cancellers are cascaded to make one 128ms echo
canceller. In this configuration, muting Echo
Canceller A causes quiet code to be transmitted on
Rout.
Bypass:
The Bypass state directly transfers PCM codes from
Rin to Rout and from Sin to Sout. When Bypass state
is selected, the adaptive filter coefficients are reset
to zero.
Disable Adaptation:
When the Disable Adaptation state is selected, the
adaptive filter coefficients are frozen at their current
value. In this state, the adaptation process is halted
however the MT9123 continues to cancel echo.
Enable Adaptation:
In Enable Adaptation state, the adaptive filter
coefficients are continually updated. This allows
the echo canceller to model the echo return path
characteristics in order to cancel echo. This is the
normal operating state.
Controllerless Mode
The NBSD is always active and automatically
disables the filter adaptation process when narrow
band signals are detected.
Controller Mode
The NBSD can be disabled by setting the NBDis bit
to 1 in Control Register 2.
Offset Null Filter
Adaptive filters in general do not operate properly
when a DC offset is present on either the reference
signal (Rin) or the echo composite signal (Sin). To
remove the DC component, the MT9123
incorporates Offset Null filters in both Rin and Sin
inputs.
Controllerless Mode
The Offset Null filters are always active.
Controller Mode
The offset null filters can be disabled by setting the
HPFDis bit to 1 in Control Register 2.
Controllerless Mode
The four functional states can be selected via S1,
S2, S3, and S4 pins as shown in the following table.
8-51
MITEL [ MITEL NETWORKS CORPORATION ]
