Lectrosonics UM400 - Manual (Page 7)

Frequency-Agile UHF Belt-Pack Transmitter
No Pre-Emphasis/De-Emphasis
The signal to noise ratio of the 400 system is high
enough to preclude the need for conventional pre-em­
phasis (HF boost) in the transmitter and de-emphasis
(HF roll off) in the receiver. Pre-emphasis and de-em­
phasis in an FM radio system usually provides about
a 10 dB improvement in the signal to noise ratio of the
system, but the high frequency boost in the transmitter
must be removed in a purely complementary manner or
else the frequency response of the original audio signal
will be altered.
Pre-emphasis can also cause distortion in the receiver.
As this signal is passed through the IF filters in the
receiver, distortion can be produced, most noticeably at
full modulation. De-emphasis cannot be applied until
the signal is converted into audio, so there is no way
around this problem short of eliminating pre-emphasis
altogether. Neither of these problems occur in the 400
system
Pilot Tone Squelch
The 400 system uses one of 256 different ultrasonic
tones between 25 and 32 kHz, that modulate the car­
rier to operate the receiver squelch. The pilot tone fre­
quency is chosen according to which of the 256 chan­
nels has been selected by the frequency switch setting.
The basic benefit of the pilot tone squelch system is
that the receiver will remain 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 sys­
tem. The UM400 extends this concept even further by
insuring that all transmitters in a system have different
pilot tone frequencies so that even spurious RF from
the wrong transmitters can’t open the receiver squelch.
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 30 dB
for excellent overload protection. A dual release enve­
lope 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
transmitter is fully modulated (audio level is between +0
and +10 dB). The second red LED indicates that the
level is 10 dB 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 recom­
mend 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 rein­
forcement systems.
Wide-Band Deviation
± 75 kHz deviation improves the capture ratio, signal to
noise ratio and AM rejection of a wireless system dra­
matically, compared to the more commonly used ±15
kHz deviation.
Long Battery life
Switching power supplies throughout the design al­
low over five hours of operation using a single 9 VDC
alkaline battery. (A 9 V lithium battery will provide over
10 hours of continuous operation.) The battery con­
tacts are spring loaded to prevent “rattle” as the unit is
handled.
Frequency Agility
The transmitter section uses a synthesized, frequency
selectable main oscillator. The frequency is extremely
stable over a wide temperature range and over time.
Two rotary switches, located on the side panel of the
unit, provide 256 frequencies in 100 kHz steps over a
25.5 MHz range. This alleviates carrier interference
problems in mobile or traveling applications.
Antenna
The antenna on the UM400 consists of a flexible 1/4
wavelength galvanized steel cable, detachable via an
SMA connector. The impedance of this connector is 50
Ohms.
Rio Rancho, NM