Lectrosonics LMa - Manual (Page 4)

LMa
LECTROSONICS, INC. 4
Introduction
The design of the LMa transmitter introduces 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 ana-
log wireless systems. DSP “compatibility modes” allow
the LMa to be used with a variety of analog receivers
in addition to its native hybrid mode by emulating the
compandors found in Lectrosonics 100 Series, 200
Series and IFB transmitters, and certain receivers from
other manufacturers (contact the factory for details).
Changing the compatibility mode is accomplished with
a simple procedure using the frequency switches and
power switch.
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
front panel allow quick and accurate level settings with-
out having to view the receiver. The battery compart-
ment accepts any 9 volt battery and makes a positive
connection via self-adjusting contacts. The antenna is
a super-rugged, permanently attached 1/4 wavelength
design made of flexible galvanized steel cable.
The switching power supplies in the LMa provide
constant voltages to the transmitter circuits from the
beginning (9.3 VDC) to the end (5.5 VDC) 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
43 dB range, with a DSP controlled dual envelope input
limiter to cleanly handle signal peaks over 30 dB above
full modulation.
Digital Hybrid Technology
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
increased operating range and weak signal condi-
tions, however, the hybrid system handles this situa-
tion elegantly with rarely audible audio artifacts 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 Wire-
less ® enjoys all the benefits of conventional FM wireless
systems, such as excellent range, efficient use of RF
spectrum, and long battery life.
Frequency Agility
The transmitter section uses a synthesized, frequency
selectable main oscillator. The frequency is extremely
stable over a wide temperature range and over time.
Two rotary switches, located on the side panel provide
256 frequencies in 100 kHz steps over a 25.6 MHz
bandwidth.
Wide-Band Deviation
In the Digital Hybrid and 200 Series modes, the sys-
tem uses ±75 kHz deviation to dramatically improve
the capture ratio, and signal to noise ratio of the overall
wireless system.
Long Battery Life
The use of switching power supplies throughout the
design allows over 6 hours of operation using a single 9
volt alkaline battery and over 7 hours of operation with
a 9 volt LiPolymer rechargeable battery. The battery
contacts are spring loaded to prevent “rattle” as the unit
is handled.
Servo Bias Input and Wiring
The LMa input preamp is a radically different design than
previous Lectrosonics transmitter inputs. The improve-
ments are audible and make the transmitters easier to
use and much harder to overload. It is no longer neces-
sary on some mics to introduce pads to prevent over-
load 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 now
available to simplify and standardize the configuration.
Simplified 2-wire and 3-wire configurations for the servo
bias input only take full advantage of the preamp cir-
cuitry to maximize the signal to noise ratio, and several
other configurations are available that are compatible
with the servo bias input and earlier types.
The input will automatically switch the low frequency
roll-off to 35 Hz when the MI33ARA and MI33AST
instrument cables are connected.