Studio Technologies 46A - Manual (Page 7)

Model 46A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 7
microprocessor control to achieve sig-
nificant trans-hybrid loss. This return-loss
“null” is achieved by making a series of
adjustments to account for the resistive,
inductive, and capacitive conditions that
are present on the connected 2-wire party-
line circuit. The party-line’s conditions are
the sum of the impact made by the type
and quantity of cable, the connected user-
devices, and the intercom power source.
Whenever a user presses one of the
Model 46A’s auto null buttons digital
circuitry adjusts the analog hybrids to
rapidly achieve their maximum return-loss.
The nulling process takes less than 10
seconds for each channel of an interface.
However, it’s important to note that while
the nulling process is automatic, it only
takes place upon user request. This can
lead to more stable and consistent audio
performance when a hybrid is exposed to
the varying 2-wire conditions often found
in broadcast applications. The param-
eters obtained during the nulling process
are stored in non-volatile memory; mains
power interruptions won’t require the auto
nulling function to again be performed.
A sine-wave audio tone is generated for
use during the auto nulling process. The
frequency is software-controlled to maxi-
mize the ability of the hybrid circuits to
reach a “deep” null. In addition, at the
beginning of each auto null sequence a
short period of 24 kHz tone is sent to the
associated 2-wire party-line interface. This
serves as a microphone disable (“mic kill”)
signal for user devices such as the RTS
BP325. By automatically disabling “open”
microphones the auto nulling process can
achieve better performance.
4-Wire Interfaces
Associated with the 4-wire portion of the
Model 46A’s interfaces are analog line-level
inputs and outputs. These are intended to
interconnect with a variety of 4-wire devices,
including matrix intercom systems, audio-
over-fiber transmission systems, and other
specialized audio equipment. The input
and output circuitry is transformer-coupled
to minimize the chance of hum, noise, or
ground “loop” issues. A key characteristic
of the Model 46A’s design is the ability to
select the 4-wire input and output nominal
levels. This helps to ensure compatibility
with virtually all audio equipment. Front-
panel-accessible DIP switches allow the
nominal levels to be configured from among
four choices: 0, +4, +6, or +8 dBu. The
0 dBu setting was specifically provided for
compatibility with Clear-Com matrix inter-
com systems. The +4 dBu setting allows
“standard” audio signals to be directly
connected. Digital matrix intercom systems
from Riedel® can be effectively connected
using the +6 dBu setting. And the +8 dBu
setting allows proper level matching with the
popular ADAM™-Series of matrix intercom
systems from RTS. This setting also applies
to the related RVON-I/O VoIP product.
The Model 46A contains eight 5-segment
LED level meters. Four of the meters are
provided for each interface, with two dis-
playing the level of the signal being received
from the 4-wire source and two displaying
the level being sent to the 4-wire output.
During installation and setup the meters
are invaluable in helping to confirm that the
nominal level DIP switch settings have been
properly made. During normal operation
the meters offer rapid confirmation of audio
signal flow in and out of the unit. Additional
LED status indicators are also provided,