Lectrosonics MM200 - Manual - Manual (Page 4)

GENERAL TECHNICAL DESCRIPTION
If the unit will be used in a wet environment (possible immersion, perspiration, rain scenes, etc.) – be certain all
connectors have intact gaskets and that a properly sealed Lectrosonics’ waterproof mic connector is installed and
used with the microphone.
GENERAL
The 200 system uses 75kHz 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 30dB above full modulation.
DUAL BAND COMPANDOR MODELING in the MM200
Traditionally, compandors have been a source of distortion in wireless microphone systems. The basic problem with
conventional systems is that the attack and decay times are always a compromise. If the time constants are fast,
high frequency transients will not be distorted, but this will cause low frequency distortion. If the time constants are
slower, low frequency audio distortion will be low, but high frequency transients will then be distorted. The 200
system introduces an entirely new approach to solving this basic problem, called “dual-band companding.”
There are actually two separate compandors in the 200 system, one for high frequencies and one for low frequen­
cies. A crossover network separates the frequency bands at 1kHz with a 6dB per octave slope, followed by separate
high and low frequency compandors. The attack and release times in the high frequency compandor are fast enough
to keep high frequency transient distortion at a low level, and the low frequency compandor uses slower time con­
stants, reducing low frequency distortion to well below that of a conventional compandor.
NO PRE-EMPHASIS/DE-EMPHASIS
The signal to noise ratio of the 200 system is high enough to preclude the need for conventional pre-emphasis (HF
boost) in the transmitter and de-emphasis (HF roll off) in the receiver. Pre-emphasis and de-emphasis in an FM
radio system usually provides about a 10dB improvement in the signal to noise ratio of the system, but the high
frequency boost in the transmitter must be removed in a purely complementary manner or else the frequency
response of the original audio signal will be altered.
+3V Bias
Transmitter
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
Isolator
Hi/Lo
Pass
Filter
Audio
Encoded
Audio +
Pilot Tone
4MHz
Reference
Bicolor
Power
LED
Mic
Jack
Supply
Audio
Level
Input
Amp
MM200 Block Diagram
4