The receive side of RemoteMix C+ has a hidden fine tuning
control which is adjusted by inserting a screwdriver into the
back of the unit just below the words
Null Adj
. and turning left
or right. This is a ten-turn potentiometer. This adjustment
controls the amount of transmit audio that appears on the
receive audio jacks. If you would like to adjust the mix of
transmit and receive audio on these jacks, you can fine tune
using this control. The null circuit matches the
characteristics of RemoteMix C+ with the characteristics of
the telephone line. Although much of the transmit audio can
be removed, this is not the equivalent of a mix-minus output.
The amount of transmit/receive isolation will depend on the
quality of the telephone line. This circuit is factory set to give
an equal mix on an average telephone line, and is therefore
perfect for recording both sides of a telephone interview.
To find the absolute null for any telephone line, follow this
procedure:
1.
Place a call to a quiet location.
2.
Send a signal down the line at moderate levels, this
can be a prerecorded tape of music or speech, or a
test signal.
3.
Connect the
From Phone
line output to a meter or
scope, or listen to the receive signal over the
headphones. Be sure to turn up the
Line Out
or
Headphones
volume control depending on which
method you are using to monitor the receive signal.
4.
On the back of RemoteMix C+, below the words
Null
Adj
., there is a hole for a small flat blade screwdriver. This is a
10-turn potentiometer to allow for very sensitive adjustments.
Try to remember the position of the blade, and the number of
turns you are making as you first adjust clockwise, and then
back to the original position, and then counterclockwise.
Hybrid Null Adjustment
(optional)
Hybrid Null Adjustment
(continued)
The goal is to adjust the null signal until the receive signal
contains a minimum amount of transmit audio. If, on the
other hand, you want a good mix of transmit and receive
signals, you must have someone on the far end speak or
send audio down the line as you adjust for the proper
balance.
5.
Factory Setting:
To reset to the factory setting, set the phone line
switch to
On-Hook
, and then adjust to find the null.
We have installed a terminating resistor in the
On-
Hook
position that simulates the average phone line
connection and gives the right mix of transmit and
receive audio.
The best tools for this task are 1) and oscilloscope connected
to the receive line output, and 2) a 1 kHz sine wave generator
connected to the send line input. However, the above
procedure will work with a little patience.
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