User's Manual

4
MM400 Block Diagram
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 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 information in digital form, at the cost of some combination 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 information via an analog FM
wireless link. This 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. (As of this
writing, the patent is still pending, so we cannot reveal detailed information 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, yielding audio perfor-
mance that rivals that of wholly digital systems, without the power and bandwidth problems inherent in digital transmis-
sion. 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 conventional 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 resistance to interference. However, unlike conventional
FM systems, the Digital Hybrid has done away with the analog compandor and its artifacts.
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 intro-
duces an entirely new approach to solving this basic problem, called “dual-band companding.”
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
+3V Bias
Supply
Audio
Level
Input
Amp