zl50211 Zarlink Semiconductor, zl50211 Datasheet - Page 13

• Manufacturer Part Number: zl50211
• Description: 256 Channel Voice Echo Canceller
• Manufacturer: Zarlink Semiconductor

Each EVP has two Tone Detectors per channels (for a total of 64) in order to monitor the occurrence of a valid
disable tone on both Rin and Sin. Upon detection of a disable tone, TD bit of the Status Register will indicate logic
high and an interrupt is generated (i.e., IRQ pin low). Refer to Figure 5 and to the Interrupts section.
Once a Tone Detector has been triggered, there is no longer a need for a valid disable tone (G.164 or G.165) to
maintain Tone Detector status (i.e., TD bit high). The Tone Detector status will only release (i.e. TD bit low) if the
signals Rin and Sin fall below -30 dBm0, in the frequency range of 390 Hz to 700 Hz, and below -34 dBm0, in the
frequency range of 700 Hz to 3400 Hz, for at least 400 ms. Whenever a Tone Detector releases, an interrupt is
generated (i.e., IRQ pin low).
The selection between G.165 and G.164 tone disable is controlled by the PHDis bit in Control Register 2 on a per
channel basis. When the PHDis bit is set to “1”, G.164 tone disable requirements are selected.
In response to a valid disable tone, the echo canceller 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 is activated by
setting the AutoTD bit in Control Register 2 to high. In external mode, an external controller is needed to service the
interrupts and poll the TD bits in the Status Registers. Following the detection of a disable tone (TD bit high) on a
given channel, the external controller must switch the echo canceller from Enable Adaptation to Bypass state.
1.6
In systems with very low echo channel return loss (ERL), there may be enough feedback in the loop to cause
stability problems in the Adaptive Filter. This instability can result in variable pitched ringing or oscillation. Should
this ringing occur, the Instability Detector will activate and suppress the oscillations.
The Instability Detector is activated by setting the RingClr bit in Control Register 3 to “1”.
1.7
Single or dual frequency tones (i.e. DTMF tones) present in the receive 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 by detecting single or dual tones of arbitrary frequency, phase, and amplitude. When
narrow band signals are detected, adaptation is halted but the echo canceller continues to cancel echo.
The NBSD will be active regardless of the EVP functional state. However the NBSD can be disabled by setting the
NBDis bit to “1” in Control Register 2.
Instability Detector
Narrow Band Signal Detector (NBSD)
Rin
Sin
Rin
Sin
Figure 5 - Disable Tone Detection
Tone
Tone
Tone
Tone
Zarlink Semiconductor Inc.
Echo Canceller A
Echo Canceller B
Detector
Detector
Detector
Detector
ZL50211
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Status reg
Status reg
TD
TD bit
ECA
ECB
bit
Data Sheet