Digital Hybrid Wireless
®
UHF Belt Pack Transmitter
Rio Rancho, NM
3
Introduction
The design of the LMb transmitter delivers the
advanced technology and features of Digital Hybrid
Wireless
®
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 DSP “compat-
ibility modes” to allow the unit to be used with a vari-
ety of analog receivers by emulating the compandors
found in earlier Lectrosonics analog wireless and IFB
receivers, and certain receivers from other manufac-
turers (contact the factory for details).
The housing is a rugged, machined aluminum pack-
age 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 instrument 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, permanently attached 1/4 wavelength
design made of flexible galvanized steel cable.
The switching power supplies in the LMb provide
constant voltages to the transmitter circuits from
the beginning to the end of battery life, with output
power remaining constant over the life of the battery.
The input amplifier 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 compand-
ors 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 information in digital form, at the cost of
some combination of power, bandwidth, operating
range and resistance to interference.
The Lectrosonics Digital Hybrid Wireless
®
system
overcomes channel noise in a dramatically new way,
digitally encoding the audio in the transmitter and de-
coding it in the receiver, yet still sending the encoded
information via an analog FM wireless link. This pro-
prietary 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 thresh-
old. 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
equipment.
Wide-Band Deviation
Deviation has a direct effect on the dynamic range
and signal to noise ratio of an FM system. The
greater the deviation, the wider the dynamic range
and the better the signal to noise ratio. In the Digital
Hybrid Wireless
®
mode, the system operates with a
maximum of +/-75 kHz of deviation. The resulting 180
kHz of occupied bandwidth just fits inside the gov-
ernment specified spectral mask of 200 kHz. In order
to make this possible, an advanced, DSP-controlled
limiter is used to prevent signal peaks from exceed-
ing the maximum allowed bandwidth, but not cause
distortion.
The result of all this is that the audio quality is out-
standing, rivaling a hard-wired microphone. The
system delivers a very wide dynamic range, and can
handle loud signal peaks without distortion.
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 avail-
able to simplify and standardize the configuration.
Simplified 2-wire and 3-wire configurations provide
several arrangements designed for use only with
servo bias inputs to take full advantage of the pre-
amp circuitry. Other wiring schemes are compatible
with Servo Bias and conventional inputs.
A line level input wiring provides an extended fre-
quency response with an LF roll-off at 35 Hz for use
with instruments and line level signal sources.
