User's Manual

Digital Hybrid Wireless Belt-Pack Transmitter
Rio Rancho, NM
5
Pilot Tone Squelch
The squelch system in a receiver is a method of muting
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 transmitter. 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.
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 excellent
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 transients, 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-em-
phasis and de-emphasis are eliminated.
LT Block Diagram
LT Transmitter
Filters
(3)
Keypad
(2) AA
Batteries
Oscillator
Oscillator
I2S
I2S
Voltage Controlled
Oscillator
Final
Amplifier
Driver
Bicolor
Modulation
LEDs
Control
PLL Ref
Encoded
Audio and
Pilot Tone
Buffer Amp
and Filter
Phase Locked Loop
DSP Clock
Switching
Power
Supply
Microprocessor
with internal clock
Tr icolor
Power
LED
AUDIO
D-A
Converter
A-D
Converter
Shunt
Limiter
Limiter Control
Gain
Adj
Input
Preamp
Input
Jack
Bias Voltage
0, 2 or 4V
Servo Bias
Supply
+5V
AUDIO
Gain
Split