Lectrosonics SM - Manual (Page 5)

Super-Minature Belt Pack Transmitter
actual roll-off frequency will vary somewhat according
to the low frequency response of the mic capsule being
used.
The low frequency roll-off is used to remove subsonic
(or very low frequency) audio, often produced by air
conditioning systems, automobile traffic and other
sources from the audio signal. Excessive low frequency
content in the audio input can cause a variety of audio
problems including driving the transmitter into limiting.
Input Limiter
A DSP-controlled analog audio limiter is employed
before the analog-to-digital converter. The limiter has a
range of more than 30 dB for excellent overload protec­
tion. A dual release envelope makes the limiter acousti­
cally 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 lis­
tener, but recovers slowly from sustained high levels, to
both keep audio distortion low and preserve short term
dynamic changes.
Two bicolor LEDs indicate limiter activity. (See Operat­
ing Instructions, Adjusting Audio Levels.)
Digital Signal Processor
The DSP encodes the digitized audio from the A-D
Converter and adds an ultrasonic Pilot Tone to control
the receiver’s squelch (only in 400 Series, 200 Series,
IFB Compatibility Mode, and Mode 6 – see Pilot Tone
Squelch). It also controls the input limiter.
Microprocessor, PLL and VCO Circuits
An 8-bit microprocessor monitors user command inputs
from the Control Panel buttons and numerous other
internal signals. It works intimately with the DSP to
ensure the audio is encoded according to the selected
Compatibility Mode and the correct pilot tone is added
to the encoded signal. (See Pilot Tone Squelch.) It
also drives the LCD display and operates the PLL/VCO
circuits.
Compatibility Modes
The SM transmitter was designed to operate with Lec­
trosonics 400 Series receivers and will yield the best
performance when doing so. However, due to the flex­
ibility of digital signal processing, the SM is also able to
operate with Lectrosonics 200 Series, Lectrosonics 100
Series, IFB and certain non-Lectrosonics receivers in
special compatibility modes. (Contact the Lectrosonics
Sales Department for a complete list of non-Lectroson­
ics compatible receivers.)
Pilot Tone Squelch
A fixed frequency pilot tone squelch system (such
as Lectrosonics 200 Series) ensures that a receiver
remains muted until it receives the pilot tone from the
matching transmitter, even if a strong RF signal is pres­
ent on the carrier frequency of the system.
The 400 Series (Digital Hybrid Wireless™) takes this
a step further by using one of 256 different ultrasonic
tones between 25 kHz and 32 kHz to modulate the
carrier and operate the receiver squelch. The pilot
tone frequency is chosen according to which of the 256
channels has been selected. (See SM Screen Selec­
tions.)
This ensures 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.
Control Panel
A waterproof control panel which includes four mem­
brane switches and an LCD screen is used to set and
adjust the operational settings, and also provide a
visual feedback of overall system operation. (See Con­
trols and Functions.)
Wide-Band Deviation
A ±75 kHz deviation improves the capture ratio, signal
to noise ratio and AM rejection of a wireless system
dramatically, compared to other designs that use 30
kHz to 40 kHz deviation. This combined with a full 100
mW of power output makes a significant improvement in
signal to noise ratio and maximum operating range.
Battery Life
Switching power supplies throughout the design allow
about 4.5 hours of operation using a single AA lithium
battery. If shorter operating times are acceptable, an
alkaline or NiMH battery can be used. AA alkaline bat­
teries provide about 2 hours of operation, and a NiMH
AA battery provides about 3.5 hours of operation.
Note: A NiMH battery will give little or no warning
when it is depleted. If you wish to use NiMH
batteries in the SM, we recommend trying fully
charged batteries in the unit, noting the length of
time that the batteries will run the unit and then
using the battery timer feature available on most
400 Series receivers.
The battery contact is designed 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.
256 frequencies in 100 kHz steps over a 25.5 MHz
range are available.
Circulator/Isolator
The RF output circuit includes a one way circulator/
isolator using a magnetically polarized ferrite. This
device greatly reduces RF intermodulation produced
when multiple transmitters are used at separations of
less than five feet. It also provides additional RF output
stage protection but is rarely seen in a wireless micro­
phone transmitter due to its high cost.
Rio Rancho, NM 5