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4 Hybrid Technical Description
4 Hybrid Technical Description
A POTS hybrid (such as the DHY-04) has to be very flexible to maintain its
operation for the many different countries’ telephony systems, but with a
GSM hybrid some of this functionality is provided by the Mobile Network.
The network separates the send and receive traffic so there is no local echo
to eliminate. However the quality of the network separation is not perfect,
especially when the caller is using a POTS network to make the call. The
DHY-04G/HD provides the facilities to use noise gate and ducking functions
to provide significant improvements in signal quality.
Echo Cancellation
The echo part of the returned signal can be expressed in terms of delayed
sent signal, and will not normally be required when using a GSM interface
so we recommend setting DIPSwitch 7 to OFF. Occasionally the hybrid may
be called on to perform echo cancellation, e.g. when echo is experienced
from a remote call, and this is easily enabled by setting DIPSwitch 7 to the
ON position.
The algorithm keeps a record of the set of recently sent data and, in the
time domain, calculates the amount (hybrid coefficients) of signal being
returned as echo back to the unit, subtracting this from the actual return
signal to just leave the original caller signal. The echo cancellation filter has
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 250ms.
The adaptation routines constantly adjust the internal hybrid 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.
Updating Firmware
The preferable method of communicating with the DHY-04G/HD is to use
the Webserver embedded within the DHY-04G/HD Firmware.
The DHY-04G/HD operating software is stored in flash memory and can be
overwritten as new code is published. The flash memory is partitioned into
boot code, firmware, and saved parameters. Only the firmware partitions
are erased so that even in the unlikely occurrence of a power fail during the
flash update, the system can still be recovered. If no firmware file is found,
the unit will automatically start the Bootstrap code that contains a minimal
webserver allowing the unit to have new code uploaded.
If a firmware file is corrupted, but still seen by the system as valid code
then the unit will, most likely, not be able to connect to the integrated
webserver. In this case you should force a bootstrap mode by powering
up or resetting the unit whilst holding down the Line Hold button for
approximately 8 seconds. The LEDs show an alternating 2on-2off pattern
that switches to a 4on-4off pattern after 8 seconds. Now release the button
and connect to the bootstrap webserver either by discovering the unit with
the discovery app or by powering up with DIPSwitch 6 set to ON, when the
unit will be found at a fixed static IP address of 192.168.0.100.
Select the firmware file to be uploaded – a link to our website is shown to
fetch the latest version of code – and then press ‘Update’. The DHY-04G/HD
will reboot when the file transfer is complete.
