Lectrosonics T4 - Manual (Page 5)

Multi-Frequency IFB Transmitter
Rio Rancho, NM 5
A user-selectable low frequency roll-off can be set for
35 Hz or 50 Hz. The recommended 50 Hz default set-
ting helps to remove wind and traffic noise, air con-
ditioner rumble, and other sources of undesired low
frequency audio. The 35 Hz setting offers a fuller range
of sound in the absence of adverse conditions.
Input Limiter
A DSP-controlled analog audio limiter is employed
before the analog-to-digital converter. The limiter has a
range of more than 30 dB for excellent overload protec-
tion. A dual release envelope makes the limiter acousti-
cally transparent while maintaining low distortion. It can
be thought of as two limiters in series: a fast attack and
release limiter followed by a slower attack and release
limiter. The dual release limiter recovers quickly from
brief transients but recovers more slowly from sustained
high levels, keeping audio distortion low while preserv-
ing short term dynamic changes. When the audio
meter on the LCD display widens slightly as it reaches
zero, limiting is indicated. When the zero changes to a
letter C, severe limiting and/or clipping is indicated.
Audio DSP and Noise Reduction
Lectrosonics IFB systems use a single band compan-
dor and pre-emphasis for exceptional IFB audio quality.
The IFBT4 performs these traditionally analog functions
entirely in the digital domain, maintaining historical
compatibility while requiring fewer adjustments. When
the IFBT4 is configured for compatibility with other
types of wireless systems, such as the Lectrosonics
100, 200 and 400 Series and some other brands, the
DSP stops IFB companding and instead performs the
appropriate audio processing for the chosen mode.
While all of these modes have won acclaim for perform-
ing well in their respective spheres, the 400 Series
mode (Digital Hybrid Technology) offers objectively
superior audio performance and is recommended when
the receiver is capable of supporting it.
Pilot Tone Squelch System
Lectrosonics IFB systems use a special “pilot tone” so
that valid IFB signals can be distinguished from RF
interference. During normal operation, an IFB receiver
will listen for the distinctive pilot tone, remaining silent
(squelched) until the pilot tone is detected. The pilot
tone is located well above audio frequencies and is
never passed through to the receiver’s audio output.
The benefit of the pilot tone squelch system is that the
receiver will remain muted until it receives the pilot tone
from the matching transmitter, even if a strong interfer-
ing RF signal is present on the carrier frequency of the
system.
When the IFBT4 is operated in compatibility modes
other than IFB, it generates pilot tones as appropriate
for the chosen mode.
Frequency Agility
The IFBT4 transmitter uses a synthesized, frequency
selectable main oscillator. The frequency is extremely
stable over a wide temperature range and over time.
The transmitter’s standard tuning range covers 256 fre-
quencies in 100 kHz steps over a 25.6 MHz band. This
flexibility significantly helps avoid interference problems
in mobile or traveling applications.
Power Delay
When powering the transmitter on and off, and when
switching between the XMIT and TUNE modes, intel-
ligent circuitry adds brief delays in order to allow time
for circuits to stabilize, both locally and in the matching
receiver. These delays prevent clicks, thumps or feed-
back from entering the sound system.
Microcontroller
The microcontroller oversees most system operations,
including RF frequency and output, DSP audio func-
tions, buttons and display, and more. User settings
are stored in non-volatile memory, so they are retained
even when the power is turned off.
Transmitter
The IFBT4 transmitter operates at a high RF power
level to ensure a clean signal free of dropouts and
noise. All transmitter circuits are buffered and filtered
for excellent spectral purity. The IFBT4’s clean signal
reduces the chances for interference in multiple trans-
mitter installations.
Antenna System
The 50 Ohm BNC output connector will work with stan-
dard coaxial cabling and remote antennas.