Lectrosonics UM400 - Manual (Page 6)

UM400
General Technical Description
Introduction
The 400 system uses 75 kHz wide deviation for an ex­
tremely high signal to noise ratio. The switching power
supplies provide constant voltages to the transmitter
circuits from the beginning (9.3 Volts) to the end (5.5
Volts) of battery life. The input amplifier uses an ultra
low noise op amp for quiet operation. It is gain con­
trolled with a wide range dual envelope input compres­
sor which cleanly limits input signal peaks over 30 dB
above full modulation.
Digital Hybrid 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
to improve the signal to noise ratio, at the cost of subtle
artifacts (known as “pumping” and “breathing”). Wholly
digital systems defeat the noise by sending the audio
information in digital form, at the cost of some combina­
tion of power, bandwidth and resistance to interference.
The Lectrosonics Digital Hybrid 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 informa­
tion via an analog FM wireless link. This proprietary
algorithm is not a digital implementation of an analog
UM400 Block Diagram
+5V Bias
compandor but a technique which can be accomplished
only in the digital domain, even though the inputs and
outputs are analog signals. (As of this writing, the pat­
ent is still pending, so we cannot reveal detailed infor­
mation about the algorithm at this time.)
Channel noise still has an impact on received signal
quality and will eventually overwhelm the receiver. The
Digital Hybrid simply encodes the signal to use a noisy
channel as efficiently and robustly as possible, yield­
ing audio performance that rivals that of wholly digital
systems, without the power and bandwidth problems
inherent in digital transmission. As always, these
advantages come at a cost. The Digital Hybrid system
requires fairly intensive digital processing in both the
transmitter and the receiver. These processors cost
money, take up space and consume power. The Digital
Hybrid system also requires that the underlying RF link
be of excellent quality, with better frequency response
and distortion characteristics than that required by con­
ventional systems.
Because it uses an analog FM link, the Digital Hybrid
enjoys all the benefits of conventional FM wireless
systems, such as excellent range, efficient use of RF
spectrum, and long battery life. However, unlike con­
ventional FM systems, the Digital Hybrid has done away
with the analog compandor and its artifacts.
Phase Locked Loop
Voltage
Controlled
Oscillator
Freq
Switches
11001001
A-D
Converter
Digital Signal Processor
11001001
D-A
Converter
Shunt
Limiter
Bicolor
Modulation
LEDs
Microprocessor
9V
Battery
Switching
Power
Supply
+3.3v
+1.8v
+9v
-3v
Hi/Lo
Pass
Filter
Audio
Encoded
Audio +
Pilot Tone
4MHz
Reference
Bicolor
Power
LED
Mic
Jack
Supply
Audio
Level
Input
Amp
5
4
3
2
1
Final
Amplifier
50
Isolator
LF
Roll Off
<--See 5-Pin Input Jack Wiring for details.
LECTROSONICS, INC.