Lectrosonics UM450 User Manual download pdf (Page 5)

Frequency Synthesized UHF Belt-Pack Transmitter

Low Frequency Roll-Off

The low frequency roll-off at the 3 dB down point is vari-

able from 35 to 150 Hz Hz to control subsonic and very

low frequency audio content in the audio. The actual

roll-off frequency will vary slightly depending upon the

low frequency response of the microphone.

Excessive low frequency content can drive the transmit-

ter into limiting, or in the case of high level sound sys-

tems, can even cause damage to loudspeaker systems.

The roll-off is normally adjusted by ear while listening

as the system is operating.

Input Limiter

A DSP-controlled analog audio limiter is employed before

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 distortion. 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 en-

codes the digitized audio from the A-D converter and

adds an ultrasonic pilot tone to control the receiver’s

squelch. A pilot tone squelch system provides a reliable

method of keeping a receiver output muted (squelched)

even in the presence of signiﬁcant interference. When

the system is operating in the hybrid mode, a differ-

ent pilot tone frequency is generated for each carrier

frequency to prevent inadvertent squelch problems and

simplify multi-channel coordination.

Microprocessor, PLL and VCO Circuits

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 selected Compatibil-

ity Mode and that the correct pilot tone is added to the

encoded signal.

Compatibility Modes

Digital Hybrid transmitters were designed to operate

with Lectrosonics Digital Hybrid receivers and will yield

the best performance when doing so. However, due to

the ﬂexibility of digital signal processing, the transmit-

ters can also operate in various compatibility modes

for use with Lectrosonics 200 Series, Lectrosonics 100

Series, IFB and certain non-Lectrosonics receivers.

Contact the Lectrosonics sales department for a com-

plete list of compatible non-Lectrosonics receivers.

Batteries, Battery Life and External Power

Multiple switching power supplies, a DSP and micropro-

cessor, and the high output power all add up to signiﬁ-

cant power requirements for operation. For this reason,

high capacity rechargeable batteries with charger and

a battery eliminator are included with the transmitter to

provide useful options and ensure that the transmitter is

usable right out of the box.

The supplied rechargeable LiPolymer 9V batteries pro-

vide about 3.5 hours of operating time per charge.

Alkaline 9V batteries will run the transmitter for about

an hour and a half.

Dry cell Lithium batteries are not recommended for

use in the UM450. This type battery has an upper limit

on the maximum current it can provide. The UM450

exceeds this limit, so it will only run the transmitter for

about a half hour, or even less depending upon the age

and state of the battery.

The ISO9VOLT battery eliminator is included to allow

powering the UM450 from an external DC power source

for extended operating times.

Frequency Selection

The transmitter section uses a synthesized, frequency

selectable main oscillator. The frequency is extremely

stable over a wide temperature range and over time.

Two rotary switches, located on the side panel of the

unit, provide 256 frequencies in 100 kHz steps over a

25.5 MHz range. This alleviates carrier interference

problems in mobile or traveling applications.

Antenna

The antenna on the UM450 consists of a rugged 1/4

wavelength stranded and galvanized steel cable, de-

tachable via an SMA connector. The impedance of this

connector is 50 Ohms.

Rio Rancho, NM

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Lectrosonics UM450 User Manual Page 5

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