User's Manual

7
Frequency-Agile UHF Belt-Pack Transmitter
Rio Rancho, NM – USA
Input Limiter
The 400 series transmitters employ a digitally-controlled analog audio limiter just before the analog-to-digital converter.
The limiter has a range of more than 30dB for excellent overload protection. A dual release envelope makes the limiter
acoustically transparent while maintaining low distortion. It can be thought of as two limiters in series, connected as a
fast attack and release limiter followed by a slow attack and release limiter. The limiter recovers quickly from brief
transients, so that its action is hidden from the listener, but recovers slowly from sustained high levels, to both keep
audio distortion low and preserve short term dynamic changes.
The audio level LEDs indicate limiter activity. The first red LED indicates that the limiter is active and that the trans-
mitter is fully modulated (audio level is between +0 and +10 dB). The second red LED indicates that the level is 10dB
or more into limiting. Occasional forays into the red are desirable for most applications, since the distortion introduced
by the limiter is so minimal, and full modulation is thus assured. We strongly recommend setting the gain of the
transmitter high enough so that the first red LED occasionally lights.
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 recorders and many sound reinforcement systems.
AUDIO LEVEL
Used to adjust the audio input level for the proper modulation.
ANTENNA
The flexible bronze cable 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 remove subsonic (or very low frequency) audio, often produced by air
conditioning systems, automobile traffic and other sources from the audio signal. Excessive low frequency content in
the audio input can cause a variety of audio problems including driving the transmitter into limiting. In sound reinforce-
ment systems, as one instance, 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 PN:28528 which is a Phillips head, 4-40 x 3/16", FL100 screw.