Lectrosonics UCR411a - Manual (Page 7)

UHF Digital Hybrid Wireless™ Receiver
Rio Rancho, NM 7
IF Amplifiers and SAW Filters
The first IF low noise amplifier is controlled with feed-
back regulation and drives the first of two quartz SAW
(Surface Acoustical Wave) filters. The 244 MHz SAW
filters combine sharp tuning, constant group delay,
wide bandwidth and excellent temperature stability, far
superior to conventional LC filters. The 244 MHz first IF
signal is converted down to 10.7 MHz, filtered through
two ceramic filters for sharp selectivity, then converted
to 300 kHz.
Digital Pulse Counting Detector
The UCR411A 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
Note: This description applies only in 400 Series
mode. In 100 and 200 Series mode, and Mode 6,
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.
The 400 Series system design uses a DSP-generated
ultrasonic pilot tone to control the receiver audio muting
(squelch). Brief delays when the associated 400 Series
transmitter is turned on or off, eliminate thumps, pops
or other transients that can occur when the power is
switched 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 intermodu-
lation products. The DSP-generated pilot tone also
eliminates the need for fragile crystals, allowing the
receiver to survive shocks and mishandling much better
than older analog-based pilot tone systems.
Supersonic Noise-Based
Dynamic Filter and SmartSquelch ™
In addition to SmartNR, all 400 Series receivers are
equipped with a supersonic noise-based dynamic filter
and squelch system. The incoming audio is monitored for
energy above 22 kHz, besides the pilot tone. Excessive
high frequency energy indicates that the received signal
is too weak to achieve an acceptable signal-to-noise ratio.
Under marginal conditions, a variable lowpass filter is rolled
in dynamically, masking the noise while preserving as much
of the transmitted signal as possible. When the channel is
too noisy even for the filter, the audio is squelched.
This noise-based filter and squelch system replaces a
more or less equivalent system used for many years, which
based its operation on RF signal strength. Performance of
the two systems is virtually identical, but the noise-based
system requires no calibration and there is no better way to
track the signal-to-noise ratio than to measure it directly.
The UCR411A employs a sophisticated squelching sys-
tem in an attempt to deliver the cleanest possible audio
during marginal conditions of reception. Any squelching
system faces inevitable trade-offs: squelch too much
and valuable audio information may be lost, squelch
too little and excessive noise may be heard; respond
too rapidly and the audio sounds “choppy,” respond too
sluggishly and syllables or entire words can be cut off.
The UCR411A 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
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 UCR411A can
deliver acceptable audio quality from otherwise unus-
able signals.
In the Pilot Tone Bypass mode, the squelch system is dis-
abled. Received audio remains unmuted at all times with
this setting. (See Front Panel Controls and Functions.)
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 UCR411A has been meticulously designed using
the best available low noise components and tech-
niques. 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 transmitter’s mic preamp, or the (usually) greater
noise from the microphone itself. (To put this in per-
spective, 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 and thus
increase the effective dynamic range of the system, the
UCR411A 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’ 195 and 200 Series designs,
much greater transparency is thus obtained. Desired
high frequency signals having some coherence such as
speech sibilance and tones are not affected.
The Smart Noise Reduction algorithm has three modes
- OFF/NORMAL/FULL - selectable from a user setup
screen. 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 re-
produced at the receiver. When switched to NORMAL,
the factory default setting, enough noise reduction is