General Technical Description; Introduction; Audio Input Interface; Dsp-Controlled Input Limiter - Lectrosonics IFBT4-VHF/X Instruction Manual

IFBT4-VHF, IFBT4/E01-VHF, IFBT4-VHF/X

General Technical Description

Introduction

This version of the IFB "base station" transmitter oper-
ates in the television broadcast band from 174 to 216
MHz (US TV channels 7 through 13). It will tune across
the entire band, so clear frequencies can be found
almost anywhere.
The VHF spectrum has not been impacted by the
spectrum auctions and re-packing as much as the
UHF spectrum, so the idea behind this product design
is to operate an IFB system in the VHF band and free
up space for wireless microphones in the UHF band.
The IFBT4 features a graphics type backlit LCD
display with a menu system similar to those provided
on other Lectrosonics receivers. The interface can be
Locked to prevent a user from changing any settings
but still allow browsing of the current settings.
The unit can be powered from any external DC source
of 6 to 18 Volts at 200 mA or from the provided 12 Volt
power supply with a locking power connector. The pow-
er inlet has an internal self-resetting fuse and reverse
polarity protection.
The housing is contructed of machined aluminum
extrusion with a durable electrostatic powder coating.
The front and rear panels are machined aluminum with
an anodized finish and laser etched engraving. The
included antenna is a right angle, ¼ wavelength whip
with a BNC connector.

Audio Input Interface

XLR
Input
Connector
2
3
1
Power
Input
Voltage
6-18VDC
Regulator
200mA
Polyfuse
Power
OFF
TUNE
XMIT
4
IFBT4-VHF Transmitter
Mode
Set
Input
Switches
Amplifier
(Rear
Panel)
LF
Roll-Off
Shunt
Limiter
Graphic LCD Display
UP
Front
Panel
DOWN
Controls
MENU
Phase Locked Loop
+5 V
11.3 MHz
Reference
+3.3 V
+1.8 V
+15 V
-15 V
Audio Input Interface
The standard 3 pin XLR connector on the rear panel
handles all audio inputs. The four DIP switches allow
setting the input sensitivity for low levels, such as mi-
crophone inputs, or for high level line inputs, balanced
or unbalanced. The switches also offer special settings
to provide the proper input configurations to match to
Clear Com, RTS1, and RTS2 intercom systems. Pin 1
of the XLR input connector is normally connected to
ground but an internal jumper can be moved if a float-
ing input is desired. Phantom supplied microphones
may be connected without the need for DC isolation at
the input of the transmitter.
A user-selectable low frequency roll-off can be set for
35 Hz or 50 Hz as needed to suppress low frequency
acoustic noise or to extend the frequency response.

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 excel-
lent overload protection. A dual release envelope
makes the limiter acoustically transparent while main-
taining low distortion. It can be thought of as two limit-
ers 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 distor-
tion low and preserve short term dynamic changes in
the audio.
Block Diagram
A-D
Converter
Audio
Input
Amp
11001001
Audio
Level
Digital Signal Processor
Microprocessor
D-A
Converter
Filter
Amp
11001001
11.3 MHz
Reference
Voltage Final
Controlled Amplifier
Oscillator
LECTROSONICS, INC.