Lectrosonics R400 User Manual download pdf (Page 5)

Portable Wireless Sound System

IF AMPLIFIERS AND SAW FILTERS

The first IF low noise amplifier is controlled with feed-

back regulation and drives a quartz SAW (Surface

Acoustical Wave) filter. The 244 MHz SAW filter com-

bines sharp tuning, constant group delay, wide band-

width and excellent temperature stability, far superior to

conventional LC filters. The second mixer converts the

244 MHz first IF signal down to 10.7 MHz. The second

IF is filtered through two ceramic filters for sharp selec-

tivity, then converted down to 300 kHz and fed to the

Digital Pulse Counting Detector.

DIGITAL PULSE COUNTING DETECTOR

The R400 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. The

output from the Digital Pulse Counter is an analog signal

containing the digital audio information. This signal is

fed through a low pass filter to an A-D Converter in the

Digital Signal Processing section.

DIGITAL SIGNAL PROCESSOR

The DSP reconstructs the original digitized audio from

the A-D Converter and detects the ultrasonic Pilot Tone

used to control the receiver's squelch (only in 400 Series

and 200 Series Compatibility Mode with the Pilot Tone

enabled – see DSP-Based Pilot Tone).

The DSP also incorporates an RF-controlled digital

noise filter (in addition to SmartNR

™

). This RF sensitive

variable frequency filter reduces high frequency re-

sponse under extremely weak RF conditions. The filter

does nothing until the RF signal strength drops below 3

uV, at which point it begins to roll off high frequencies.

Usable audio remains unaffected, but noise-ups or "hits"

occurring near the fringe of reception sound much less

harsh.

The reconstructed original analog audio signal is then

sent to the audio output section.

COMPATIBILITY MODES

The R400 receiver was designed to compatible with

Lectrosonics 400 Series transmitters and will yield the

best performance when doing so. However, due to the

flexibility of digital signal processing, the R400 is also

able to operate with Lectrosonics 200 Series,

Lectrosonics 100 Series, IFB and certain non-

Lectrosonics transmitters in special compatibility modes.

(Contact the Lectrosonics Sales Department for a

complete list of compatible transmitters.)

DSP-BASED PILOT TONE

The 400 Series system design uses a DSP generated

ultrasonic pilot tone from the transmitter to control the

receiver audio muting (squelch). If the Pilot Tone is

enabled, a Pilot Tone Detect signal generated by the

DSP automatically controls the receiver's squelch. Built-

in brief delays are incorporated to eliminate the thumps,

pops or other transients that can occur when the trans-

mitter 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. Using the

DSP to detect the 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.

Note

The above 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

™

The R400 employs a sophisticated squelching system 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 valu-

able 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 R400

combines several techniques to achieve an optimal

balance, removing distracting noise without the squelch-

ing action itself becoming a distraction. One of these

techniques involves waiting for a word or syllable to

complete before squelching. Another 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 R400 can deliver acceptable

audio quality from otherwise unusable signals. In the

PILOT TONE BYPASS mode, the squelch system is

disabled. Received audio remains unmuted at all times

with this setting.

SMART NOISE REDUCTION (SmartNR

™

)

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 Ohm 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 dy-

namic range of the system, the R400 is equipped with a

Smart Noise Reduction algorithm, which removes hiss

without sacrificing high frequency response.

The Smart Noise Reduction algorithm works by attenuat-

ing only those portions of the audio signal that fit a

statistical profile for randomness or “electronic hiss.”

SmartNR

™

offers significantly increased transparency

over the sophisticated variable low pass filters used in

Rio Rancho, NM – USA

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