Lectrosonics Inc UM400E 사용자 설명서

5

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.

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

±75 kHz deviation improves the signal to noise ratio and 

audio dynamic range of a wireless system dramatically, 

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.

Battery Options and Operating Time

Switching power supplies convert regulated battery 

voltages to operate various circuit stages with maxi-

mum efficiency. With the variety of alkaline, lithium and 

rechargeable batteries available today in the 9V 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 from the low 500 MHz band to the upper 700 

MHz band. Each block includes 256 frequencies in 100 

kHz increments. The block number is part of a simple 

formula to derive the frequency. The block number is 

multiplied by 25.6 to produce the lowest frequency in 

the block. For example, block 27 x 25.6 = 691.200.

Circulator/Isolator

The RF output circuit includes a one way circulator/isolator 

using a magnetically polarized ferrite. This device greatly 

reduces the RF intermodulation produced when multiple 

transmitters are used in close proximity to one another 

(several feet apart). The isolator also provides additional 

RF output stage protection.