Lectrosonics LMa User Manual download pdf (Page 4)

LMa

LECTROSONICS, INC.

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.

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