Preliminary Information
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.
Disable Tone Detector
G.165 recommendation defines the disable tone as
having the following characteristics: 2100 Hz
(± 21Hz) sinewave, a power level between -6 to
-31dBm0, and a phase reversal of 180 degrees (± 25
degrees) every 450ms (± 24ms). If the disable tone is
present for a minimum of one second with at least
one phase reversal, the Tone Detector will trigger.
G.164 recommendation defines the disable tone as a
2100 Hz (±21Hz) sinewave with a power level
between -6 to -31dBm0. If the disable tone is present
for a minimum of one second, with or without phase
reversal, the Tone Detector will trigger.
The MT9122 has four Tone Detectors in order to
monitor the occurrence of a valid disable tone on
channels A and B on both Rin and Sin. Upon
detection of a disable tone, output pins TD1 or TD2
will go low as illustrated in Figure 4.
Tone Detector
TD1
Sin
Tone Detector
Echo Canceller A
Rin
Sin
Tone Detector
TD2
Tone Detector
Echo Canceller B
Figure 4 - Disable Tone Detection
Once a Tone Detector has been triggered, the
MT9122 no longer needs a valid disable tone (G.164
or G.165) to maintain Tone Detector status (e.g.
TD1, TD2 pins low). The Tone Detector status will
only release (e.g. TD1, TD2 pins high) if the signals
Rin and Sin fall below -30dBm0, in the frequency
range of 390Hz to 700Hz, and below -34dBm0, in the
frequency range of 700Hz to 3400Hz, for at least
400ms.
MT9122
Controllerless Mode
The selection between G.165 and G.164 tone
disable is controlled by the REV pin. When the REV
pin is connected to Vss, G.164 is selected. This
applies to all four Tone Detectors.
In response to a valid disable tone, the MT9122 must
be switched from the Enable Adaptation state to the
Bypass state. In an application, the Tone Detect
outputs, TD1 and TD2, may be used to switch the
echo cancellers between these two states. This is
achieved by connecting S1 and S3 pins to Vdd and
by connecting the TD1 and TD2 outputs to the S2
and S4 input pins respectively.
Controller Mode
The selection between G.165 and G.164 tone
disable is controlled by the PHDis bit in Control
Register 2. When the PHDis bit is set to 1, G.164
tone disable requirements are selected. This applies
to all four Tone Detectors.
In response to a valid disable tone, the MT9122 must
be switched from the Enable Adaptation state to the
Bypass state. This can be done in two ways,
automatically or externally. In automatic mode, the
Tone Detectors internally control the switching
between Enable Adaptation and Bypass states. The
automatic mode can be activated by setting the
AutoTD bit in Control Register 2 to high. In external
mode, an external controller is needed to poll the TD
bit in Status Register A or B. Following the detection
of a disable tone (TD bit high), the external controller
should switch the echo canceller from Enable
Adaptation to Bypass state.
The TD1 and TD2 output pins remain active, as in
Controllerless mode, and they can be used as an
interrupt to an external controller.
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.
Rin
8-23
MITEL [ MITEL NETWORKS CORPORATION ]
