User's Manual

Digital Hybrid Wireless Belt-Pack Transmitter
Rio Rancho, NM
3
Introduction
The design of the LMb transmitter delivers the ad-
vanced technology and features of Digital Hybrid Wire-
less
®
in a Lectrosonics belt-pack transmitter at a modest
cost. Digital Hybrid Wireless
®
combines a 24-bit digital
audio chain with an analog FM radio link to eliminate a
compandor and its artifacts, yet preserve the extended
operating range and noise rejection of the finest analog
wireless systems. Along with providing peerless audio
quality with wide frequency response and dynamic
range in Nu Hybrid mode, the technology used in the
LMb includes compatibility modes for Lectrosonics
Mode 3 and IFB receivers.
The housing is a rugged, machined aluminum package
with removable, stainless steel wire belt clip. The input
jack is a standard Lectrosonics 5-pin type for use with
electret lavaliere mics, dynamic mics, musical instru-
ment pickups and line level signals. The LEDs on the
top panel allow quick and accurate level settings without
having to view the receiver. The unit is powered by two
AA batteries. The antenna is a super-rugged, perma-
nently attached 1/4 wavelength design made of flexible
galvanized steel cable.
The switching power supplies in the LMb provide con-
stant voltages to the transmitter circuits from the begin-
ning to the end of battery life, with output power remain-
ing constant over the life of the battery. The input ampli-
fier uses an ultra low noise op amp for quiet operation.
Input gain is adjustable over a 44 dB range, with a DSP
-controlled dual envelope input limiter to cleanly handle
signal peaks over 30 dB above full modulation.
About Digital Hybrid Wireless
All wireless links suffer from channel noise to some
degree, and all wireless microphone systems seek to
minimize the impact of that noise on the desired signal.
Conventional analog systems use compandors for
enhanced dynamic range, at the cost of subtle artifacts
(known as “pumping” and “breathing”). Wholly digital
systems defeat the noise by sending the audio informa-
tion in digital form, at the cost of some combination of
power, bandwidth, operating range and resistance to
interference.
The Lectrosonics Digital Hybrid Wireless
®
system over-
comes channel noise in a dramatically new way, digitally
encoding the audio in the transmitter and decoding it
in the receiver, yet still sending the encoded informa-
tion via an analog FM wireless link. This proprietary
algorithm is not a digital implementation of an analog
compandor but a technique which can be accomplished
only in the digital domain.
Since the RF link between transmitter and receiver is
FM, channel noise will increase gradually with in-
creased operating range and weak signal conditions,
however, the Digital Hybrid Wireles system handles
this situation elegantly with rarely audible audio arti-
facts as the receiver approaches its squelch threshold.
In contrast, a purely digital system tends to drop the
audio suddenly during brief dropouts and weak signal
conditions. The Digital Hybrid Wireless
®
system simply
encodes the signal to use a noisy channel as efficiently
and robustly as possible, yielding audio performance
that rivals that of purely digital systems, without the
power, noise and bandwidth problems inherent in digital
transmission. Because it uses an analog FM link, Digital
Hybrid Wireless
®
enjoys all the benefits of conventional
FM wireless systems, such as excellent range, efficient
use of RF spectrum, and long battery life.
Frequency Agility
Frequency selection is provided in 100 kHz or 25 kHz
steps. Frequencies are displayed on the LCD in MHz
and in a hex code used with earlier Lectrosonics equip-
ment.
Servo Bias Input and Wiring
The LMb input preamp is a unique design that delivers
audible improvements over conventional transmitter
inputs. The transmitters are easier to use and much
harder to overload. It is not necessary to introduce pads
on some mics to prevent overload of the input stage,
divide the bias voltage down for some low voltage mics,
or reduce the limiter range at minimum gain settings.
Two different microphone wiring schemes are available
to simplify and standardize the configuration. Simpli-
fied 2-wire and 3-wire configurations provide several
arrangements designed for use only with servo bias
inputs to take full advantage of the preamp circuitry.
Other wiring schemes are compatible with Servo Bias
and conventional inputs.
A line level input wiring provides an extended frequency
response with an LF roll-off at 35 Hz for use with instru-
ments and line level signal sources.
DSP-controlled Input Limiter
The transmitter employs a digitally-controlled analog
audio limiter prior to the analog-to-digital converter.
The limiter has a range greater than 30 dB for excellent
overload protection. A dual release envelope makes the
limiter acoustically transparent while maintaining low
distortion. It can be thought of as two limiters in series,
connected as a fast attack and release limiter followed
by a slow attack and release limiter. The limiter recovers
quickly from brief transients, so that its action is hidden
from the listener, but recovers slowly from sustained
high levels to keep audio distortion low and preserve
short term dynamic changes in the audio.