Lectrosonics UM450 - Manual (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 5