Lectrosonics R400a - Manual (Page 5)

UHF Wireless Diversity Receiver
Rio Rancho, NM 5
Digital Pulse Counting Detector
The R400A receiver uses an elegantly simple, yet high-
ly 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 sig-
nal is fed through a low pass filter to an A-D Converter
in the Digital Signal Processing section.
Frequency Tuning Groups
The R400A provides four “factory set” compatible
frequency groups (A through D) and two user program-
mable frequency groups (U and V).
The factory groups have been selected to avoid in-
termodulation problems. Each group includes eight
channels.
The user programmable frequency groups can have up
to 16 frequencies per group.
Microprocessor,
PLL and VCO Circuits
An 8-bit microprocessor monitors user command inputs
from the front panel control buttons and numerous other
internal signals such as RF level, audio levels, pilot tone
levels and external power voltage. The microprocessor
also drives the LCD display, controls the squelch and
audio output attenuator, and operates the PLL/VCO
circuits and the antenna phase switch.
Digital Signal Processor
The DSP reconstructs the original digitized audio from
the A-D Converter and detects the ultrasonic 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
3uV, 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.
Smart Tuning (SmartTune ™ )
A major problem facing wireless users is finding clear
operating frequencies, especially in RF saturated envi-
ronments. SmartTune™ overcomes this problem by au-
tomatically scaning all the frequencies available in the
receiver’s frequency block and tuning the receiver to the
frequency with the lowest RF interference, significantly
reducing setup time.
Compatibility Modes
The R400A receiver was designed to operate with
Lectrosonics 400 Series transmitters and will yield the
best performance when doing so. However, due to the
flexibility of digital signal processing, the R400A is also
able to operate with Lectrosonics 200 Series, 100 Se-
ries and IFB transmitters, 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 en-
abled, a Pilot Tone Detect signal generated by the DSP
automatically controls the receiver’s squelch. Built-in
brief delays are also incorporated to eliminate the
thumps, pops or other transients that can occur when
the transmitter 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 multichan-
nel 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 other modes requiring pilot tones,
only one pilot tone frequency is used
on all channels.
Smart Squelch ™
The R400A employs a sophisticated squelching system
in an attempt to deliver the cleanest possible audio dur-
ing 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 R400A 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 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 R400A can deliver ac-
ceptable 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.