Lectrosonics UCR411a User Manual download pdf (Page 7)

UHF Digital Hybrid Wireless™ Receiver

Rio Rancho, NM

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

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