Lectrosonics UCR401 User Manual download pdf (Page 8)

UCR401

LECTROSONICS, INC.

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

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