Table of Contents
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Input Limiter
A DSP-controlled analog audio limiter is employed be-
fore the analog-to-digital (A-D) converter. The limiter
has a range of more than 30 dB for excellent overload
protection. A dual release envelope makes the limiter
acoustically transparent while maintaining low distor-
tion. It can be thought of as two limiters in series,
a fast attack and release limiter followed by a slow
attack and release limiter. The limiter recovers quickly
from brief transients, with no audible side effects, and
also recovers slowly from sustained high levels, to
keep audio distortion low and while preserving short
term dynamics.
Signal Encoding and Pilot Tone
In addition to controlling the limiter, the DSP also
encodes the digitized audio from the A-D converter
and adds an ultrasonic pilot tone. The pilot tone
squelch system provides a reliable method of keep-
ing a receiver output muted (squelched) even in the
presence of significant interference. When the system
is operating in the hybrid mode, a different pilot tone
frequency is generated for each carrier frequency to
prevent inadvertent squelch problems and simplify
multi-channel coordination.
Microprocessor and DSP
A microprocessor monitors user command inputs
from the control panel buttons and numerous other
internal signals. It works intimately with the DSP to
ensure the audio is encoded according to the select-
ed Compatibility Mode and that the correct pilot tone
is added to the encoded signal.
Compatibility Modes
The transmitter was designed to operate with Lectro-
sonics Digital Hybrid receivers and will yield the best
performance when doing so. Due to the flexibility of
digital signal processing, the transmitter can also be
used with IFB receivers.
Control Panel
The control panel includes four membrane switches and
an LCD screen to adjust the operational settings. Multi-
color LEDs are used to indicate audio signal levels for
accurate gain adjustment and for battery status.
Wide-Band Deviation
±50 kHz deviation improves the signal to noise ratio
and audio dynamic range of a wireless system dra-
matically, compared to other designs that use ±30
kHz to 40 kHz deviation. Wide deviation combined
with a high powered transmitters makes a significant
improvement in signal to noise ratio and operating
range.
Rio Rancho, NM
Digital Hybrid
Battery Options and Operating Time
Switching power supplies convert battery voltages to
operate various circuit stages with maximum efficien-
cy. With the variety of alkaline, lithium and recharge-
able NiMH batteries available today in the AA format,
there are many choices to maximize operating time or
minimize cost as needed for any application.
Frequency Blocks
Lectrosonics established a "block" numbering system
years ago to organize the range of frequencies avail-
able in the UHF spectrum. Each block includes 256
frequencies in 100 kHz increments. The block number
is part of a simple formula to determine the frequency.
The block number is multiplied by 25.6 to produce the
lowest frequency in the block. For example, block 24 x
25.6 = 614.400 (MHz).
RF Isolator
The RF output circuit includes an expensive, special
device called an isolator to significantly reduce IM (in-
termodulation) that occurs when two transmitters are
operated in close proximity to one another. The device
is located between the final amplifier and the antenna.
(the antenna in the HM plug-on transmitter is formed
between the housing and the attached microphone).
External RF signals that enter the antenna are routed
to a load and do not travel backward into the amplifier.
The isolator also provides additional RF output stage
protection against static shock.
®
Plug-On Transmitter
5
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