Lectrosonics UCR401 - Manual (Page 8)

UCR401
LECTROSONICS, INC. 8
incorporates recent squelching history and recent signal
strength, adjusting squelching behavior dynamically for
the most serviceable result under variable conditions.
Using these and other techniques, the UCR401 can
deliver acceptable audio quality from otherwise unus-
able signals.
Smart Noise Reduction (SmartNR ™ )
Note: The SmartNR setting is user selectable
only in 400 Series mode. In other modes, noise
reduction is applied in such a way as to emulate
the original analog system as accurately as
possible and is not user adjustable.
The UCR401 has been meticulously designed using the
best available low noise components and techniques.
Nonetheless, the wide dynamic range of digital hybrid
technology, combined with flat response to 20 kHz,
makes it possible to hear the -120 dBV noise floor in
the mic preamp, or the (usually) greater noise from the
microphone itself. To put this in perspective, the noise
generated by the recommended 4 k bias resistor of
many electret lavaliere mics is –119 dBV and the noise
level of the microphone’s electronics is much higher. In
order to reduce this noise the UCR401 is equipped with
a Smart Noise Reduction algorithm, which removes
hiss without sacrificing high frequency response.
The Smart Noise Reduction algorithm works by at-
tenuating only those portions of the audio signal that fit
a statistical profile for randomness or “electronic hiss.”
Because it isn’t simply a sophisticated variable low pass
filter as in Lectrosonics’s 195 and 200 series analog de-
signs, much greater transparency is obtained. Desired
high frequency signals having some coherence are not
affected, such as speech sibilance and tones.
The Smart Noise Reduction algorithm has three modes,
selectable from a user setup screen: OFF, NORMAL,
and FULL. When switched OFF, no noise reduction is
performed and complete transparency is preserved. All
signals presented to the transmitter’s analog front end,
including any faint microphone hiss, will be faithfully
reproduced at the receiver. When switched to NORMAL,
enough noise reduction is applied to remove most of
the hiss from the mic preamp and some of the hiss from
lavaliere microphones. The noise reduction benefit is
significant in this position, yet the degree of transparen-
cy maintained is exceptional. When switched to FULL,
enough noise reduction is applied to remove most of
the hiss from nearly any signal source of reasonable
quality, assuming levels are set properly at the transmit-
ter, and some high frequency environmental noise. The
optimal setting for each application is subjective and
selected while simply listening.
IF Amplifiers and SAW Filters
The first IF stage at 244 MHz employs two state-of-the-
art SAW (surface acoustic wave) filters. The use of two
filters significantly increases the depth of filtering while
preserving sharp skirts, constant group delay, and wide
bandwidth. Though expensive, this special type of filter
allows primary filtering as early as possible, at as high a
frequency as possible before high gain is applied to the
signal for maximum image rejection.
Since these filters are made of quartz, they are very
temperature stable. After the SAW filter, the 244 MHz IF
signal is converted to 10.7 MHz IF and then to the low
frequency of 300 kHz. Only then is the majority of the
gain applied, just before the signal is converted to audio
with a pulse counting detector. Although 300 kHz is very
unconventional for an IF in a wide deviation (±75 kHz)
system, it offers outstanding AM rejection figure over
a very wide range of signal strengths and produces an
excellent noise improvement at low signal strengths.
Digital Pulse Counting Detector
The UCR401 receiver uses an elegantly simple, yet
highly effective digital pulse detector to demodulate
the FM signal, rather than a conventional quadrature
detector. This unusual design eliminates thermal drift,
improves AM rejection, and provides very low audio
distortion.
DSP-Based Pilot Tone
The Digital Hybrid system design uses a DSP gener-
ated ultrasonic pilot tone to control the receiver audio
muting (squelch). Brief delays are applied to eliminate
thumps, pops or other transients that can occur when
the power is turned on or off. The pilot tone frequency
is different for each of the 256 frequencies in the tuning
range of a system (frequency block). This eliminates
squelch problems in multichannel systems where a
pilot tone signal can appear in the wrong receiver via
intermodulation products. The DSP generated pilot tone
also eliminates fragile crystals, allowing the receiver to
survive shocks and mishandling much better than older
analog-based pilot tone systems.
Note: This description applies only in 400 Series
mode. In 200 Series mode, only one pilot tone
frequency is used on all channels, emulating
the original crystal-based system. In other
compatibility modes, no pilot tone is used.
Smart Squelch ™
Any squelching system faces inevitable trade-offs:
squelch too aggressively and valuable audio information
may be lost. Squelch too little and excessive noise may
be heard. Respond too rapidly and the audio will sound
“choppy.” Respond too sluggishly and syllables or entire
words can be cut off.
The UCR401 combines several techniques to achieve
an optimal balance, removing distracting noise, without
the squelching action itself becoming a distraction. One
of these techniques involves waiting for a word or syl-
lable to complete before squelching. Another technique