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DHY-03 User Handbook
Impedance Matching
The line to the central exchange will present certain impedance to the hybrid, which will
mean that the echo response to any signal will be modified according to the frequency and
level of that signal. By impedance matching the signal we are able to predict, and therefore
subtract, a simple estimation of the echoed signal. The AC impedance and DC impedance
parameters can also be both set via the serial port parameters.
Echo Cancellation
The echo part of the returned signal can be expressed in terms of delayed sent signal.
The hybrid coefficients are a measure of the amount of signal that is expected to be echoed
from each delay slot (TAP). There are 2 echo cancellation routines running on this unit.
The first is a simple 8 TAP unit running in the telephone interface ICs. It allows the data
presented to the DSP to be easily filtered into a signal with caller input and a signal with just
sent signal echo. The second filter is a variable length TAP with dynamic adaptation running
on the on-board 24bit DSP. The length of this TAP is dependent of the expected delay in any
echoes from the central exchange. The default length is 24ms, but the unit is capable
of calculating delays up to 127ms.
The adaptation routines constantly adjust the internal DSP coefficients to achieve the best
echo cancellation. However the dynamic performance of this algorithm (i.e. the speed
that the echo cancellation is optimised) is inversely dependant on the maximum delay
(number of TAPs) allowed, so increasing the delay will result in a slightly poorer dynamic
performance.
Calibration of DHY-03 Hybrid
Where the dynamic performance or the rejection ratio is poor then the performance of the
DHY-03 can be enhanced by tuning the simple hybrid for best response and by making sure
that the starting point for the DSP coefficients is optimised, both on the actual line to be
used. This calibration is be done by setting bits in the Calibration Register (parameter 4) via
the serial port interface.
Calibration Level
Parameter 4
Function
1
1
Calibrates DSP coefficients for faster settling time
2
4
Calibrates Telephone Interface Hybrid coefficients
for current AC termination value. DSP coefficient
calibration follows
3
8
Calibrates Telephone Interface Hybrid coefficients
for all AC termination values. DSP coefficient
calibration follows
Table 2-2 : Calibration Parameter Settings.
When the DHY-03 has been set to perform a calibration a call must be established on the
actual line to be used by the system. Once a call is established then the DHY-03 will allow
10 seconds for the call conditions to stabilise and the caller to mute their handset, so that
any signal present on the line is due to echo from the DHY-03 sent signal. The DHY-03 will
CONFIGURATION & CONTROLS
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