EA915X DIGITAL TELEPHONE HYBRID
INSTRUCTIONS FOR USE
EELA AUDIO Het Riet 8A
5431NM CUIJK The Netherlands
Page 9
order not to damage the slot of the trimpots.
Send level.
The hybrid is set out in such a way that when putting the SND control in the centre position the
nominal input level of the SEND input is set to +6dBm. The adjustment range of the SND control
is + en -12 dB with reference to this nominal level. The send level has to be aligned as follows:
Apply to the hybrid input by means of the controls of the mixing console a nominal signal. Now
activate the hybrid by pressing the HYBRID_ON switch. Adjust by means of the SND control the
level in such a way that only the peaks occurring in the signal make the LIM-led go on. Convince
yourself of the fact that a continuos overloading limiter gives a poor result for the signal to the
caller, whereas a limiter handling an occasional peak sounds good. Now the send level has been set correct
and the hybrid can be switched off.
Receive level.
Make sure there is NO send level present at alignment of the receive level. The hybrid is set out in
such a way that when the RCV control is placed in the centre position the output level is set to
+6dBm with a signal present on the telephone line of -12 dBm. The control range of the RCV
control is + and - 12 dB. It is important to align the receive level very accurate to make sure
there is no overload on the input of the A/D converter. This results in a considerable and
aggressive distortion of the receive signal. Because the level of the various telephone connections
can vary, it is wise to test the maximum level by making several calls.
With the highest level found, the RCV control has to be set in such a way that this gives an
output level of the hybrid to the mixing console (-input) of about +6 dBm. Again, do not give too
much gain in this path to prevent overload of the electronics. It is easier for a too low signal to be
corrected with the gain or the gain of the input channel of the mixing desk.
Adjustment of the hybrid damping (echo-cancelling).
The alignment of the hybrid circuitry is done by means of the R-bal and C-bal controls and
ensures an minimal crosstalk in these analogue circuits before the digital echo canceller takes care
of any residual unwanted signal. The best way to perform this alignment is by using a "pink
noise" signal because it contains all frequencies with equal energy thus enabling a wideband
adjustment. If no such signal is available, a wideband music passage can be chosen.
Now switch the hybrid in bypass mode by pressing the BYP-switch which is situated behind the
frontpanel and can only be operated by inserting e.g. a small screwdriver through the hole in the
frontpanel marked BYPASS. Judge the contribution of the (send) signal to the caller appearing
because of the crosstalk in the receive signal. Subsequently use the R-bal and C-bal trimpots to
adjust this contribution to a minimum.
Now switch off the bypass mode. By pressing the UPDATE switch the digital echo canceller will
minimize the residual crosstalk. For getting the best result, it is important that, during the update procedure
of the digital echo canceller, there is no receive signal present. This can be achieved by covering the
receiver at the caller's end and asking the caller to be silent for some seconds. If the result is not satisfactory,
the procedure for adjusting the R-bal and C-bal can be repeated, using another setting.
The hybrid is now completely aligned and ready for use.
