Lectrosonics MM400c - Manual (Page 7)

Watertight Miniature UHF Belt-Pack Transmitter
Rio Rancho, NM 7
Input Limiter
The MM400C transmitters employ a DSP-controlled an-
alog audio limiter just before the analog-to-digital con-
verter. The limiter has a range of more than 30 dB for
excellent overload protection. A dual release envelope
makes the limiter acoustically transparent while main-
taining low distortion. It can be thought of as two limit-
ers 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 keep audio distortion low
and preserve short term dynamic changes.
Two bicolor LEDs indicate limiter activity. (See Operat-
ing Instructions, Adjusting Audio Levels.) Generally
speaking, some limiting is desirable in normal operation
to improve the signal to noise ratio of the system. The
limiting action is not audible and does not create distor-
tion.
Digital Signal Processor
The DSP constructs the original digitized audio from the
A-D Converter, adds an ultrasonic Pilot Tone to con-
trol the receiver’s squelch (only in 400 Series and 200
Series Compatibility Mode with the Pilot Tone enabled
– see Pilot Tone Squelch), and implements the user
selected Compatibility Mode.
Microprocessor, PLL and VCO Circuits
An 8-bit microprocessor monitors user command inputs
and numerous other internal signals. It also drives the
Modulation LEDs, controls the Pilot Tone and operates
the PLL/VCO circuits.
Compatibility Modes
The MM400C transmitter was designed to be compati-
ble with Lectrosonics 400 Series receivers and will yield
the best performance when doing so. However, due to
the flexibility of digital signal processing, the unit is also
able to operate with Lectrosonics 200 Series, Lec-
trosonics 100 Series, IFB and certain non-Lectrosonics
analog receivers in special compatibility modes. (Con-
tact the Lectrosonics Sales Department for a complete
list of compatible transmitters.)
Pilot Tone Squelch
The 400 Series wireless system uses an ultrasonic tone
between 25 and 32 kHz to operate the receiver squelch.
The pilot tone squelch system keeps the receiver muted
until it receives the pilot tone from the matching trans-
mitter, even if a strong RF signal is present on the car-
rier frequency of the system. The “pilot tone” frequency
is different for each of the 256 available carrier frequen-
cies to prevent the pilot tone from being transferred to
the wrong receiver via an intermodulation product.
Wide 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.
Magnetic Power/Mute Switch
A magnetic switch is used to control the application of
power to the MM400C circuits and as an audio mute
switch. The function of this switch can be set by the
user. (See Controls and Functions, Magnetic Power
ON/OFF Switch.)
Battery Life
Switching power supplies throughout the design allow
5.5 hours of operation using a single AA lithium battery.
(An alkaline AA battery will provide about 1.5 hours
and a 2500 mAh NiMH AA battery will provide about
4.5 hours of operating time.) The battery contacts are
spring loaded in order 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 16-position rotary switches, located under a wa-
terproof cover on the bottom of the unit, provide 256
frequencies in 100 kHz steps over a 25.5 MHz range.
Circulator/Isolator
The RF output circuit includes a one way circulator/iso-
lator using a magnetically polarized ferrite. This device
greatly reduces RF intermodulation produced when
multiple transmitters are used within a few feet of each
other by blocking RF from returning back into the output
stage. It also provides additional RF output stage
protection which is rarely seen in a wireless microphone
transmitter due to its high cost.