Low Frequency Roll-Off; Input Limiter; Digital Signal Processor; Microprocessor, Pll And Vco Circuits - Lectrosonics SM Series Instruction Manual

SM Series

Low Frequency Roll-Off

A 12 dB per octave low frequency roll-off is provided in the
audio section, with the -3 dB point at 70 Hz. The actual
roll-off frequency will vary somewhat according to the low
frequency response of the microphone being used.
This 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 fre-
quency 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 (A-D) converter. The limiter has a range
of more than 30 dB for excellent overload protection. A dual
release envelope makes the limiter acoustically transpar-
ent 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.
Two bicolor LEDs indicate limiter activity. (See Operating
Instructions, Adjusting Audio Levels.)

Digital Signal Processor

The DSP controls the input limiter, 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).

Microprocessor, PLL and VCO Circuits

A 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 Compatibil-
ity Mode and that 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 transmitters were designed to operate with
Lectrosonics 400 Series receivers and will yield the best
performance when doing so. However, due to the flex-
ibility of digital signal processing, SM units are also able
to operate with Lectrosonics 200 Series, Lectrosonics
100 Series, IFB and certain non-Lectrosonics receivers
in special compatibility modes. (Contact the Lectroson-
ics Sales Department for a complete list of compatible
non-Lectrosonics 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 present 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 Selections.)
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 membrane
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 Controls and Functions.)

Wide-Band Deviation

A ±75 kHz deviation improves the capture ratio, signal to
noise ratio and AM rejection of a wireless system dramati-
cally, 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 bat-
tery in the SM. If shorter operating times are acceptable,
an alkaline or NiMH battery can be used. In the SM, AA
alkaline batteries provide about 2 hours of operation, and
a NiMH AA battery provides about 3.5 hours of operation.
The 250 mW SMQ has slightly more operating time with
two batteries than the SM, and the SMD provides more
than double the operating time than the SM.
Note: A NiMH battery will give little or no warning
when it is depleted. If you wish to use NiMH batteries,
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 in each frequency block.

Circulator/Isolator

The RF output circuit includes a one way circulator/isolator
using a magnetically polarized ferrite. This device greatly
reduces the RF intermodulation produced when multiple
transmitters are used at separations of less than five feet.
It also provides additional RF output stage protection.
LECTROSONICS, INC.