Lectrosonics UDR300c User Manual download pdf (Page 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 300 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 trans-

former coupled and will work without the pin 3 ground connec-

tion. 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.

1 2 3 4 5 6 7 8 9 10 11 ... 25 26

Comments to this Manuals

No comments

Lectrosonics UDR300c User Manual Page 6

Comments to this Manuals

Related products and manuals for Equipment Lectrosonics UDR300c