4
UM700
Block Diagram
5
4
3
2
1
Freq
Switches
Shunt
Limiter
From
DSP
LF
Roll-Off
Preamp and
Supersonic
Filter
Audio
Gain
+5V Bias
Supply
Mic
Jack
Buffer and
Lo Freq
Filter
Analog to Digital
Converter
Digital Signal
Processor
To uP
Modulator
Drive
Amp
Power
Amp
1
2
3
1/4 Wave
Antenna
50
Audio
Level
Oscillator
Filter
Synthesizer
To
uP
uP
EEProm
Multicolor
Audio Level
LEDs
Multicolor
Battery Status
LED
Switching
Pwr Supply
Delay Control
from uP
9V
Polarity
Protection
Diode
Power
Switch
4V Main
Pwr Supply
Isolator
GENERAL TECHNICAL DESCRIPTION
GENERAL
The 700 Series wireless system uses digital modulation for an extremely high signal to noise ratio and freedom from
distortion and compandor artifacts. The input preamplifier uses an ultra low noise op amp for an excellent signal to
noise ratio. A dual-envelope limiter is implemented by the DSP firmware to cleanly limit input signal peaks up to
30dB above full modulation. The transmitter circuits are all regulated to allow full output power from the beginning (9
Volts) to the end (5.5 Volts) of battery life.
The UM700 transmitter is comprised of a number of functional sub-systems as shown in the block diagram below.
DIGITAL MODULATION
Digital modulation is used to eliminate compandor artifacts and provide an extremely high signal to noise ratio. A
proprietary compression algorithm is used to contain the RF energy into a narrower bandwidth than other digital
systems to allow the 700 Series system to utilize frequencies in FCC Part 74 for broadcast and motion picture
production. For applications that require secure operation that prevent eavesdropping, encryption can easily be
implemented.
AUDIO PROCESSING
The input circuitry includes an adjustable low frequency roll-off filter ahead of the limiter and gain stage to customize
the low frequency response for varying conditions. In controlled environments with minimal low frequency ambient
noise, such as a film production set, the frequency response can be extended to 30 Hz. For applications with
excessive low frequency energy, such as in a moving vehicle or rooms with HVAC noise, the low end can be rolled
off as high as 150 Hz to reduce the effects of the noise.
A “dual-envelope” limiter with 30 dB of range is next in the signal chain. This special limiter is software controlled,
being driven by the DSP to minimize “overshoot” and slow attack times that can occur in other types of limiter circuits.
The compressor works with two different time constants to cleanly handle a wide variety of dynamics. A fast peak
limiter prevents overload distortion from sharp, infrequent transients in the audio signal, and an averaging limiter with
longer time constants prevents “pumping” (gain modulation) that can occur with repeated peaks that are closely
spaced. Supersonic filtering is provided to prevent excessive high frequency energy from entering the A/D converter.
