Studio Technologies 220 2004 - Manual (Page 12)

Model 220 User Guide Issue 2, August 2004
Studio Technologies, Inc. Page 13
talkback output connector is isolated from
the Model 220’s chassis and circuitry.
By making pin 1 “float,” an often-feared
“ground loop” problem shouldn’t arise.
The talkback outputs are intended to drive
lengthy cable runs that are frequently part
of a remote broadcast application. While
the output circuitry is not intended to be
“on-air” quality, overall audio performance
should be very good. Devices connected
to the talkback outputs can range from
amplified loudspeakers to analog inputs
on intercom systems, and input channels
associated with audio consoles. Connect-
ing the talkback outputs to devices that
allow easy control of the signal level can
be helpful. For example, connecting to
a spare input module on an audio con-
sole provides the flexibility to add gain or
attenuate as required. Then a talkback-
associated output connection on the
audio console can connect to the final
destination(s).
As previously mentioned, the talkback
outputs on multiple Model 220 units can
be directly connected together. Using
a simple “Y” or “W” cable, this passive
summing (adding together) of talkback
signals allows one audio cable to serve
as a master talkback path. One side ef-
fect from using this passive summing
technique is that signal attenuation will
occur. The audio quality won’t suffer, but
an audio “pad” is created. If two talkback
outputs are connected together, a signal
attenuation of 6 dB can be expected. Con-
necting three talkback outputs together
will result in 9.5 dB of attenuation. And
four talkback outputs “multed” together
will lead to 12 dB of attenuation. In most
cases this attenuation won’t pose a prob-
lem. Typically a device that receives the
talkback signal, such as an amplified
loudspeaker, will have an adjustable input
sensitivity.
IFB Input
The Model 220’s IFB input is designed
to directly connect with “wet” (DC-biased)
IFB circuits. These circuits provide DC
power and one or two channels of audio
over a standard 3-conductor microphone-
style cable. Typically, the IFB circuit’s in-
terface connector is a 3-pin male XLR-type
wired so that common is on pin 1, DC with
channel 1 audio is on pin 2, and channel
2 audio is on pin 3. Some IFB circuits may
only have one audio channel. In this case,
audio will generally be on pin 3 with pin 2
providing only DC power.
The power supplied by an IFB circuit,
normally in the range of 28 to 32 volts DC,
is usually sufficient to operate the Model
220’s circuitry. The acceptable input range
is 24 to 32 volts, with a required current
of 125 milliamperes. Note that the speci-
fied input voltage is given when measured
directly at the Model 220’s IFB input con-
nector, not at the source of the IFB circuit.
The one or two audio signals provided
by the IFB circuit can serve as the audio
sources for the headphone outputs.
In North American field and in-studio
broadcast applications it is common to
find RTS 4000-series IFB equipment be-
ing used to provide the IFB circuits. The
Model 220 can be directly connected to,
and function correctly with, one of these
circuits. For reliable operation, especially
when using lengthy cable runs, it’s strong-
ly recommended that no other device be
connected to a 4000-series IFB circuit that
is specified for connection to a Model 220.
This requirement is due to the current-
limited DC source that is supplied by the
4010 IFB Controller.