Lectrosonics LM User Manual
INSTRUCTION MANUAL
LM/IM
Frequency-Agile UHF Belt-Pack Transmitters
Featuring
Digital Hybrid Wireless™ Technology
(US Patent Pending)
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Purchase Date:
Rio Rancho, NM, USA
www.lectrosonics.com
LM/IM
LECTROSONICS, INC. 2
Frequency-Agile UHF Belt-Pack Transmitter
Table of Contents
Introduction ............................................................................................................................................................................................4
Digital Hybrid Technology .....................................................................................................................................................................4
Input Limiter ..........................................................................................................................................................................................5
No Pre-Emphasis/De-Emphasis ........................................................................................................................................................... 5
Pilot Tone Squelch ................................................................................................................................................................................5
Wide-Band Deviation ............................................................................................................................................................................5
Long Battery Life .................................................................................................................................................................................. 5
Frequency Agility ..................................................................................................................................................................................5
Antenna ................................................................................................................................................................................................5
Controls and Functions ......................................................................................................................................................................... 6
Mic Input Jack.......................................................................................................................................................................................6
Power ON/OFF Switch ......................................................................................................................................................................... 6
Power LED ............................................................................................................................................................................................ 6
Frequency Select Switches ..................................................................................................................................................................6
Modulation LEDs ..................................................................................................................................................................................6
Audio Level ...........................................................................................................................................................................................6
Antenna ................................................................................................................................................................................................6
Belt Clip ................................................................................................................................................................................................6
Battery Installation .................................................................................................................................................................................7
Operating Instructions ..........................................................................................................................................................................8
Selecting Compatibility Mode ...............................................................................................................................................................8
Attaching a Microphone or Musical Instrument and Adjusting Audio Levels ........................................................................................8
Adjusting the Transmitter Frequency ....................................................................................................................................................9
5-Pin Input Jack Wiring ........................................................................................................................................................................ 10
RF Bypassing .....................................................................................................................................................................................10
Line Level Signals .............................................................................................................................................................................. 11
Wiring Hookups for Different Sources ................................................................................................................................................11
Wiring Diagram for MI33A Instrument Cable ......................................................................................................................................11
Troubleshooting ...................................................................................................................................................................................12
Specifications and Features ............................................................................................................................................................... 13
Service and Repair .............................................................................................................................................................................14
Returning Units for Repair ..................................................................................................................................................................14
The LM and IM transmitters are FCC type accepted under Part 74: 536-608 MHz and 614-806 MHz
Rio Rancho, NM 3
LM/IM
Introduction
Thank you for selecting the Lectrosonics LM or IM
frequency agile, belt-pack transmitter.
The design of these transmitters introduces at a moderate cost the advanced technology and features of a
Lectrosonics belt-pack transmitter. The design is based
on the Digital Hybrid Wireless™ system (US Patent
Pending) which combines a digital audio chain with an
analog FM radio link. This digital process eliminates the
compandor and its problems, yet preserves the extended operating range of the finest analog wireless
systems. The IM is offered as part of the IS400 package and differs slightly from the LM in that its low-end
frequency response has been optimized for use with
musical instruments.
Both transmitters also preserve your investment. They
can emulate the compandor found in Lectrosonics 100
Series, 200 Series and IFB transmitters, and certain
transmitters from other manufacturers. (Call Lectrosonics for more details.) Changing the compatibility mode
is accomplished with a simple procedure that does not
require opening the unit.
Mechanically, the assemblies are rugged, machined
aluminum packages with removable, stainless steel wire
belt clips. Their input jack is a standard Lectrosonics 5pin type for use with electret lavaliere mics, dynamic
mics and line level signals. LEDs are provided on the
front panel to make quick and accurate level settings
without having to view the receiver. The battery compartment accepts any 9 volt battery (alkaline or lithium
recommended) 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.
Only the LM and IM transmitters are covered in this
manual. Companion receivers are covered in separate
manuals.
The Digital Hybrid Wireless™ system uses 75 kHz wide
deviation for an extremely high signal to noise ratio. The
switching power supplies provide constant voltages to
the transmitter circuits from the beginning (9.3 VDC) to
the end (5.5 VDC) of battery life. The input amplifier
uses an ultra low noise operational amplifier (op amp)
for quiet operation. It is gain controlled with a wide
range dual envelope input limiter that tames input 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 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
decoding it in the receiver, yet still sending the encoded
information 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. (The patent is
still pending, so we cannot publish detailed information.)
Channel noise still has an impact on received signal
quality and will eventually overwhelm the receiver. 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 wholly digital systems, without the power, noise and
bandwidth problems inherent in digital transmission. As
always, these advantages come at a cost. The Digital
Hybrid Wireless™ system requires fairly intensive
digital processing in both the transmitter and the
receiver. These processors cost money, take up space
and consume power. The Digital Hybrid Wireless
system also requires that the underlying RF link be of
excellent quality, with better frequency response and
distortion characteristics than that required by conventional analog wireless systems.
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. However, unlike
conventional FM systems, Digital Hybrid Wireless™ has
done away with the analog compandor and its artifacts.
TM
LECTROSONICS, INC. 4
Frequency-Agile UHF Belt-Pack Transmitter
Input Limiter
The transmitters employ 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 both keep audio distortion low
and preserve short term dynamic changes.
No Pre-Emphasis/De-Emphasis
Because the signal to noise ratio of the hybrid system is
so high, there is no need for conventional pre-emphasis
(HF boost) in the transmitter and de-emphasis (HF roll
off) in the receiver. Thus, the possible distortion problems associated with pre-emphasis and de-emphasis
are eliminated.
Pilot Tone Squelch
The hybrid system uses one of 256 different ultrasonic
tones between 25 and 32 kHz, that modulate the carrier
to operate the receiver squelch. The pilot tone frequency is chosen according to which of the 256 channels has been selected by the frequency switch setting.
The benefit of a 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 RF
signal is present on the carrier frequency of the system.
The 400 Series extends this concept even further by
insuring that all transmitters in a system have different
pilot tone frequencies so that even spurious RF from
the wrong transmitters can’t open the receiver squelch.
Wide-Band Deviation
By employing ±75 kHz diviation, Lectrosonics dramatically improves the capture ratio, signal to noise ratio
and AM rejection of our 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 13 hours of operation with
a 9 volt lithium battery. The battery contacts are spring
loaded to prevent “rattle” as the unit is handled.
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 of the
unit, provide 256 frequencies in 100 kHz steps over a
25.5 MHz range. This alleviates carrier interference
problems in mobile or traveling applications.
Antenna
The antenna consists of a permanently attached
flexible, unbreakable 1/4 wavelength galvanized steel
cable.
1
2
3
4
5
Mic
Jack
+5V Bias
Supply
Hi/Lo
Pass
Filter
Bicolor
Modulation
LEDs
<--See
Shunt
Limiter
Phase Locked Loop
11.3 MHz
Reference
5-Pin Input Jack Wiring
Input
Amp
Audio
Level
Freq
Switches
Audio
11001001
Microprocessor
for details.
A-D
Converter
Digital Signal Processor
D-A
Converter
11001001
Vol tage
Controlled
Oscillator
Encoded
Audio +
Pilot Tone
9V
Battery
Final
Amplifier
Block Diagram
Switching
Powe r
Supply
Bicolor
Powe r
LED
+3.3v
+1.8v
+9v
+5v
-3v
Rio Rancho, NM 5
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