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Encrypted Digital Wireless System

Introduction

The 700 Series encrypted digital wireless microphone
system uses a digital audio chain and an encrypted
digital RF communications link for excellent sound
quality and data security. The applications for this
system include high-end motion picture, studio and
stage, and boardrooms, courtrooms and conference
rooms where security is a concern. While these
applications are very different from one another, this
single system is able to provide significant benefits in all
of these areas of operation.

Overall System Design

The 700 Series system uses state-of-the-art techniques
to offer superb audio quality and formidable security.
Audio is sampled at 44.1 kHz using a 24-bit A/D con
verter in the transmitter. (For perspective, a CD uses
the same sample rate, but only 16 bits per sample.)
High-entropy encoding, cryptographically secure
encryption, efficient modulation/demodulation,
decryption and decoding all take place in the digital
domain, using highly optimized proprietary techniques.
If digital audio output is desired, there is no need to
convert back to analog at all; otherwise, the first and
only conversion back to analog occurs at the receiver’s
output.

In the transmitter, the audio first passes through a DSP-
controlled, dual-envelope analog limiter to prevent
distortion from occurring on high level peaks.  The audio
is then digitized and fed to a DSP.  The DSP uses a
proprietary audio encoding scheme to lower the bit rate
and provide the high entropy required for secure
encryption.  The bit stream is then encrypted, appor
tioned into packets, and sent over the air using a
proprietary digital modulation technique.

In the receiver, the digital baseband signal is demodu
lated to recover the original bit clock and data stream.

UDR700 Block Diagram

The DSP separates out the packet headers and de
crypts the audio data. The audio data is then decoded
to recover the original audio signal present in the
transmitter.

UDR700 Encrypted Digital Receiver

Rota-Versity

™

Reception

The UDR700 receiver uses a proprietary technique to
implement a unique and very effective type of diversity
reception called Rota-Versity

Rota-versity is a microprocessor controlled antenna
phase matching technique.  The UDR700 uses a four
way phase shifter to combine the power of the two
antennas in any of four phase angles, 90 degrees apart.
Hundreds of times per second, all of the four phase
angles are tried.  The angle offering the best reception
is always used for the audio data.  The result is that the
diversity system “tracks” the phase shifts between the
two antennas, keeping phase cancellation loss under 1
dB at all times.  Multi-path dropouts are minimized and
operating range is maximized by using both antennas,
in phase, at all times.

RF Section

The RF section of the UDR700 is a traditional dual-
conversion superheterodyne receiver.  Once the
antenna power is combined, the RF signal is passed
through dual tracking filters. These 7 MHz bandpass
filters are centered on the operating frequency as it is
tuned through the 25.6 MHz frequency block for excel
lent selectivity and interference rejection.  The first
tracking filter is followed by a GaAs FET attenuator that
applies from 2 to 32 dB of attenuation to provide very
high overload protection.  The amount of attenuation is
controlled by the RSSI (Received Signal Strength
Indicator) feedback from the second IF.

Input

Bandpass

Filter

RF
Amp

4-Way
Phase

Combiner

0/90/180/270

Antenna

Kill Switch

A/B/Rot.

Input

Bandpass