Lectrosonics MM400a - Manual - Manual (Page 5)

Frequency Agile UHF Miniature 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-
emphasis (HF boost) in the transmitter and de-emphasis
(HF roll off) in the receiver. Pre-emphasis and de-
emphasis 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 the pre-emphasized signal is passed through the IF
filters in the receiver, distortion can be produced, most
noticeable 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.
LOW FREQUENCY ROLL-OFF
A 12 dB per octave low frequency roll-off is provided in
the audio section, with the -3 dB point at 70 Hz. 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 is used to remove subsonic (or
very low frequency) audio, often produced by air condi­
tioning 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
reinforcement systems, as one instance, excessive low
frequency content can cause excessive power amplifier
drain or even damage to loudspeaker systems.
INPUT LIMITER
The 500 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 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.
PILOT TONE SQUELCH
The 400 Series wireless system utilizes an ultrasonic
tone between 25 and 32 kHz to operate the receiver
squelch. The pilot tone squelch system keeps the
receiver 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. The
“pilot tone” frequency is different for each of the 256
available carrier frequencies to prevent the pilot tone
from being transferred to the wrong receiver via an
intermodulation product.
WIDE-BAND DEVIATION
A ±75 kHz deviation improves the capture ratio, signal to
noise ratio and AM rejection of a wireless system
dramatically, compared to other designs that use 30 kHz
to 40 kHz deviation. This combined with a full 100 mW
of power output makes a significant improvement in
signal to noise ratio and maximum operating range.
BATTERY LIFE
Switching power supplies throughout the design allow
about 4.5 hours of operation using a single AA lithium
battery. (an alkaline AA battery will provide about 2
hours - a NiMH AA battery will provide about 3.5 hours)
The battery contacts 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.
CIRCULATOR/ISOLATOR
The RF output circuit includes a one way circulator/
isolator using a magnetically polarized ferrite. This
device greatly reduces RF intermodulation produced
when multiple transmitters are used at separations of
less than five feet. It also provides additional RF output
stage protection but is rarely seen in a wireless micro­
phone transmitter due to its high cost.
Rio Rancho, NM – USA 5