Lectrosonics MM400b - Manual (Page 7)

Watertight Miniature UHF Belt-Pack Transmitter
Input Limiter
The MM400B transmitters employ a digitally-controlled
analog audio limiter just before the analog-to-digital
converter. The limiter has a range of more 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 tran­
sients, 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 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
distortion.
Digital Signal Processor
The DSP constructs the original digitized audio from the
A-D Converter , adds an ultrasonic Pilot Tone to control
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
defined 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 MM400B transmitter was designed to compatible
with Lectrosonics 400 Series receivers and will yield the
best performance when doing so. However, due to the
flexibility of digital signal processing, the MM400B is
also able to operate with Lectrosonics 200 Series,
Lectrosonics 100 Series, IFB and certain non-
Lectrosonics analog receivers in special compatibility
modes. (Contact 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
carrier frequency of the system. The “pilot tone”
frequency is different for each of the 256 available
carrier frequencies to prevent the pilot tone from being
transferred to the wrong receiver via an intermodulation
product.
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.
Magnetic Power ON/OFF Switch
A magnetic power switch is used to control the applica­
tion of power to the MM400B 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.)
When the Power ON/OFF switch is configured as a
power switch, the ON/OFF positions are used in the
conventional manner, i.e., ON turns the transmitter on,
and OFF turns it off.
When the Power ON/OFF switch is configured as an
audio mute switch, placing the switch in the OFF
position mutes the audio. Placing the switch in the ON
position turns on the audio.
Battery Life
Switching power supplies throughout the design allow
over 6 hours of operation using a single AA lithium
battery. (An alkaline AA battery will provide about 1.75
hours and a 2200 mAh NiMH AA battery will provide
about 4 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 on the under a
waterproof cover on the bottom of the unit, provide 256
frequencies in 100 kHz steps over a 25.5 MHz range.
This alleviates carrier interference problems, a definite
problem in mobile or traveling applications.
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 within a few feet of
each other. It also provides additional RF output stage
protection but is rarely seen in a wireless microphone
transmitter due to its high cost.
Rio Rancho, NM 7