Lectrosonics MM400b - Manual (Page 6)

MM400B
General Technical Description
General
The 400 Series (or 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 (1.5 Volts) to the end (0.85 Volts) of battery
life. The input amplifier uses an ultra low noise op amp
for quiet operation. It is gain controlled with a wide
range dual envelope input compressor which cleanly
limits input signal peaks over 30 dB above full modula­
tion.
Note: The terms 400 Series and Digital Hybrid
Wireless™ describe the same product line are
interchangeable.
Digital Hybrid Wireless ™ 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 informa­
tion in digital form, at the cost of some combination of
power, bandwidth 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
+6V Bias Supply
proprietary algorithm is not a digital implementation of
an analog compandor but a technique which can be
accomplished only in the digital domain, even though
the inputs and outputs are analog signals.
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 resistance to interference. How­
ever, unlike conventional FM systems, the Digital Hybrid
has done away with the analog compandor and its
artifacts.
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 mic capsule being
used.
The low frequency roll-off is used to remove subsonic
(or very low frequency) audio, such as that produced by
air conditioning systems or automobile traffic, 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. For
example, in sound reinforcement systems, as one
instance, excessive low frequency content can cause
excessive power amplifier drain or even damage to
loudspeaker systems.
MM400B
Block Diagram
Phase Locked Loop
Voltage
Controlled
Oscillator
Freq
Switches
(Fits Switchcraft
plug #850.)
11001001
A-D
Converter
Digital Signal Processor
11001001
D-A
Converter
Shunt
Limiter
Bicolor
Modulation
LEDs
Microprocessor
1.5V
AA
Lithium
Switching
Power
Supply
+3.3v
+3v
+1.8v
+6v
-3v
50
Ohm
Isolator
Hi/Lo
Pass
Filter
Audio
Encoded
Audio and
Pilot Tone
4MHz
Reference
Bicolor
Power
LED
Mic
Jack
(see Input Jack Wiring for details)
Audio
Level
Input
Amp
Magnetic
Power On/Off
Switch
*US Patent Pending
LECTROSONICS, INC. 6