Lectrosonics UM250b User Manual download pdf (Page 7)

UHF Belt-Pack Transmitter

MODULATION LEDS

Indicate the proper setting of the MIC LEVEL control.

“-20” LED -- Flickers or glows when sufficient audio is present.

“0” LED -- Lights up when the input level is high enough to cause limiting. The input limiter has a very high overload

threshold (over 30 dB). Generally speaking, some limiting is desirable in normal operation to improve the signal to

noise ratio of the system. The limiting action is not audible and does not create distortion. A highly trained ear would

hear only the compression of the peaks in the audio signal, which is desirable with most tape recorders and many

sound reinforcement systems.

AUDIO LEVEL

Used to adjust the audio input level for the proper modulation.

ANTENNA

The flexible wire antenna supplied with the transmitter is cut to 1/4 wavelength of the center of the frequency block

(the frequency range) of the transmitter. It is removable via an SMA connector. The SMA connector is a 50 Ohm RF

port which can also be connected directly to test equipment. Replacement antennas are available in pre-cut lengths

for specific frequency blocks, or as a kit with instructions to cut the antenna for any frequency block.

ADJUSTABLE LOW FREQUENCY ROLL-OFF

A 18dB per octave low frequency roll-off is provided in the audio section, with the -3dB point adjustable from 35Hz to

150Hz. The actual roll-off frequency will vary somewhat according to the low frequency response of the mic capsule

being used.

The low frequency roll-off control is used to prevent undesirable subsonic (or very low frequency) audio, often pro-

duced by air conditioning systems, automobile traffic and other sources from causing the compandor to mistrack.

Excessive low frequency content in the audio input can cause “pumping and breathing” of background noise or

modulation of the program audio in recording applications. In sound reinforcement systems, excessive low frequency

content can cause excessive power amplifier drain or even damage to loudspeaker systems. By rotating the control

clockwise, the hinge point of the roll-off is increased to reduce the level of low frequencies. In controlled situations,

such as a motion picture production set indoors where environmental noise is minimal, the control can be rotated

counter-clockwise to permit low frequency audio to be captured.

THE BELT CLIP

The belt clip may be removed for special applications by removing one screw.

USE ONLY THE SCREW THAT IS SUPPLIED

The circuitry is tightly packed into this unit. A longer screw will permanently damage the transmitter! Use only

Lectrosonics #28528 which is a Phillips head, 4-40 x 3/16", FL100 screw.

Rio Rancho, NM – USA

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Lectrosonics UM250b User Manual Page 7