Introduction - Lectrosonics UDR700 Operating Instructions Manual

UDR700
The 700 Series encrypted digital wireless microphone system
uses a digital audio chain and encrypted digital RF communi-
cations link for excellent sound quality and data security. The
applications for this system include high-end motion picture,
studio and stage, as well as 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 high quality 24-bit A/D converter
in the transmitter. (For perspective, a CD uses the same
sample rate, but only 16 bits per sample.) High-entropy en-
coding, cryptographically secure encryption, efficient modula-
tion, demodulation, decryption and decoding all take place in
the digital domain, using highly optimized proprietary tech-
niques. 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-con-
trolled, dual-envelope analog limiter to prevent distortion from
occuring on high level peaks. The audio signal is then digitized
at a sampling rate of 44.1 kHz 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, apportioned into packets,
and sent over the air using a proprietary digital modulation
technique.
In the receiver, the digital baseband signal is demodulated to
recover the original bit clock and data stream. The DSP sepa-
rates out the packet headers and decrypts the audio data. The
audio data is then decoded to recover the original audio signal
present in the transmitter.
DIVERSITY RECEPTION
In analog systems, several techniques are possible to provide
diversity reception in an effort to minimize dropouts in multi-
path environments. The different design approaches include
passively mixing multiple antennas with a single receiver,
selecting the output of two separate receivers (each with its
own antenna), and actively switching between two antennas
feeding a single receiver. Each approach has its merits and
weaknesses.
Active analog antenna combining techniques utilize both
antennas at the same time, with 180 degree phase switching
to help keep the received signals in phase. Under some
conditions, a faint click may be heard in the audio noise when
the phase switch takes place, since it occurs in the RF signal
path ahead of the amplifiers in the receiver.
The popular misnomer "true diversity" refers to a dual receiver
design where each receiver has its own antenna, and the
audio output is selected from the receiver with the strongest
RF signal. In essence, a "true diversity" receiver relieves multi-
path dropout problems, but only uses one receiver or antenna
at any given moment.
4

INTRODUCTION

A ratio diversity receiver is a more advanced version of a "true
diversity" design in that it uses both receivers at the same time,
blending the audio outputs of the two receivers. The ratio
blending process takes place at higher RF levels than a switch-
ing type to anticipate dropouts before they occur. Since both
antennas and receivers are used simultaneously, operating
range is also increased.
A more effective diversity technique, however, is one that
utilizes both antennas simultaneously, with a phase controller
to ensure the antennas always add to one another, regardless
of the relative phase of the signals arriving at the antennas
themselves. This is precisely what the digital Rota-versity
technique in the UDR700 receiver does.
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 sec-
ond, 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 power from the antennas
is combined, the RF signal is passed through dual tracking
filters for excellent selectivity and interference rejection. It is
then mixed down to a first IF of 244 MHz, chosen for superior
image rejection, with dual SAW filters for excellent selectivity.
The second IF is at 10.7 MHz, with a quadrature detector to
recover the digital baseband signal.
DIGITAL DEMODULATOR
The digital demodulator consists of a PLL clock recovery
circuit and a bit slicer. The recovered bit clock and data stream
are fed to the DSP. In addition, the recovered bit clock is used
to derive the timings for the receiver's digital audio circuits, so
the receiver audio is synchronous with the transmitter audio,
sample for sample.
DIGITAL SIGNAL PROCESSING
The DSP uses the packet headers as a timing reference to
recover the digital audio data. The data is then decrypted and
decoded to recover the original audio samples. The packet
headers also contain transmitter battery status information,
which is extracted by the DSP.
AUDIO OUTPUT
The digital audio samples are sent to the AES-3id transmitter
for digital audio output, and are also sent to the D/A converter,
for conversion to analog. The converter output is filtered and
amplified, then fed to a transformer, attenuator and XLR jack.
The analog output is also sent to a separate headphone amp
for monitoring.
LECTROSONICS, INC.
TM