Lmb Block Diagram; No Pre-Emphasis/De-Emphasis; Pilot Tone Squelch - Lectrosonics LMb Instruction Manual

LMb, LMb/E01, LMb/E06, Lmb/X
DSP-controlled Input Limiter
The transmitter employs a digitally-controlled analog
audio limiter prior to the analog-to-digital converter.
The limiter has a range greater than 30 dB for excel-
lent 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
keep audio distortion low and preserve short term
dynamic changes in the audio.

No Pre-Emphasis/De-Emphasis

Because the signal to noise ratio of the Digital Hybrid
Wireless system is so high, there is no need for con-
®
ventional pre-emphasis (HF boost) in the transmitter
and de-emphasis (HF roll off) in the receiver. Thus,
the potential distortion problems associated with pre-
emphasis and de-emphasis are eliminated.

LMb Block Diagram

+5V
Servo Bias
Supply
Input
Jack
Input
Preamp
Gain
Adj
Shunt
Limiter Control
Limiter
Oscillator
Oscillator
4
Bias Voltage
0, 2 or 4V
A-D
Converter
HI/Lo
AUDIO
Pass
Filter
I2S
Microprocessor
Bicolor
Modulation
LEDs
Micro Clock
PLL Ref
Phase Locked Loop

Pilot Tone Squelch

The squelch system in a receiver is a method of mut-
ing the audio when the RF signal conditions are too
poor to produce usable audio. While not all squelch
systems work the same way, the goal is always the
same: to discard noise and unusable audio.
The Digital Hybrid Wireless system employs a DSP-
generated supersonic tone (pilot tone), using it as
a kind of signature, so the receiver can mute even
strong signals that aren't from the appropriate trans-
mitter. The pilot tone also helps to ensure that the
receiver is quiet when the transmitter is turned on
and off.
256 different pilot tones provide reliable operation in
multi-channel wireless systems. A different tone is
generated every 100 kHz across the tuning range of
the system, so that a tone is not repeated until it is
25.6 MHz above or below the previous one.
D-A
Converter
Encoded
Audio and
Pilot Tone
I2S
(2) AA
Batteries
Keypad
Voltage Controlled
Oscillator
Control
Buffer Amp
and Filter
Switching
Power
Supply
Tricolor
Power
LED
AUDIO
LMb Transmitter
Driver Final
Amplifier
LECTROSONICS, INC.
Filters
(3)