Lectrosonics UDR200c - Manual (Page 6)

6
RF ONLY NOISE FILTER
A small switch on the rear of the receiver will allow the noise
filter to be set for RF ONLY. In this mode the filter is held out
of the audio frequency range unless the RF level drops to very
low levels. At weak RF levels, the filter operates in the TRI
MODE state until the RF level rises back to acceptable levels.
This has the desirable effect of softening dropouts. We recom-
mend using the RF ONLY setting when it is desirable to pick
up high frequency background noise, such as for a location
shot for a movie. This might be machinery noise, compressed
air, etc. If the desirable background noise is at a low level, the
Tri Mode Dynamic Filter will identify this as undesirable hiss
and effectively remove it.
2:1 EXPANDER (Dual–Band Compandor)
“Dual-band Companding” is a complimentary system, that is,
whatever is done in the transmitter must be exactly mirrored in
the receiver. The transmitter compresses the audio signal in
two separate audio bands using four separate time constants
to avoid the inevitable trade-offs in attack and decay times that
occur in a single-band compandor. The companion circuit in
the receiver then re-expands this compressed signal restoring
the original dynamic range and frequency characteristics of the
signal.
The mixed audio signal leaves the Opti-Blend circuit and is fed
through a 23 kHz low pass filter where all the high frequency
noise (including the 32 kHz pilot tone) is filtered out. After the
23 kHz low pass filter, the signal is split into two parts via a 1
kHz low pass filter and a 1 kHz high pass filter. The separated
signals are then processed in separate channels of the 2:1
expander. Each channel of the 2:1 expander is optimized for
its respective frequency band. By optimizing the compandors
for high and low frequencies we can handle sounds such as
clicks, sibilants and fast transients produced by plucked or
struck instruments in the high frequency section without com-
promising the performance in the main voice range. The two
outputs of the 2:1 expander are then summed in an op-amp
and sent to an audio amplifier as one signal.
PILOT TONE MUTE (SQUELCH)
The 200 system utilizes a separate ultrasonic tone modulation
of the basic carrier to operate the receiver squelch. In the
transmitter, a 32.765 kHz tone is injected into the audio signal
after the microphone preamp, just after the compandor. The
supersonic pilot tone is filtered out of the audio signal immedi-
ately after the detector in the receiver so that it does not
influence the compandor or various gain stages.
The basic benefit of the pilot tone squelch system is that the
receiver will remain squelched (muted) until it receives the pilot
tone from the matching transmitter, even if a strong RF signal
is present on the carrier frequency of the system. Once a pilot
tone is detected, the receiver will remain open during all signal
conditions. If the transmitter signal degrades to the point
where hiss and noise may become objectionable, the Opti-
Blend circuitry and Tri Mode Dynamic Filter will work to reduce
or eliminate the unwanted noise. Since the pilot tone keeps
the receiver audio output open, as soon as the transmitter
signal returns to normal, the audio signal is instantly available
with no delays. A conventional squelch system, on the other
hand, can briefly interrupt the audio during a near dropout
condition.
The pilot tone mute circuit drives a relay which physically
disconnects the output amplifier from the output audio trans-
former. The relay then connects the transformer primary to
ground to prevent hum pickup in the transformer due to an
open primary winding. This provides complete muting of the
audio and the noise. The pilot tone function may be bypassed
with a rear panel push button. Once pushed, the pilot tone
mute is “latched” in a disabled condition until the receiver is
powered off then back on. When the pilot tone is disabled,
there is still a “squelching” function provided by the Opti-Blend
circuitry and Tri Mode Dynamic Filter. These circuits can
provide approximately 50 dB of muting during weak or no
signal conditions when the pilot tone is disabled. If you hear
moderate hiss when the transmitter is off, the pilot tone has
probably been bypassed.
AUDIO OUTPUT
The audio is a fully floating, transformer coupled, balanced
signal. Pin 2 is “high” or in phase with the transmitter input. A
precision attenuator allows you to adjust the output level from
+15 to -40 dBu. The output always runs “wide open” and is
only attenuated by this output level control. Neither the noise
level in the receiver nor headroom is affected. You can, how-
ever, over drive or under drive attached audio equipment.
A ground lift switch is available to disconnect pin 1 of the 3 pin
XLR if a hum loop is encountered. Never, never cut off the
ground plug on the AC cable. The ground lift switch will ac-
complish the same audio results without the safety hazard.
If a single ended output is necessary, pin 3 (or pin 2) must be
tied to ground, preferably at the end of the cable with the
single ended connector. Other Lectrosonics equipment is not
transformer coupled and will work without the pin 3 ground
connection. This fully floating output will not.
The headphone output is a high quality signal derived from 4
high current op amps operating in parallel to provide a low
noise, low distortion signal. This output can be used as an
unbalanced output if needed. Unlike the rear panel balanced
output, this output can be overdriven if the headphone volume
control is set too high. As with any headphone output, keep
the sound levels in the headphones at a moderate level.
When the output is not being used, turn it all the way down.
Be careful to not turn up the headphone gain if the receiver is
muted or squelched. As with any output that is muted, there is
no way of knowing what the level will be when the receiver
audio is opened.