LM/IM
LECTROSONICS, INC.
Servo Input
The LMA input is a radically different input system com-
pared to previous Lectrosonics transmitter microphone
inputs. It is so superior that this input system will eventu-
ally be utilized by all Lectrosonics UHF transmitters. The
improvements are audible and make the transmitters
easier to use and much harder to overload. It is no
longer necessary on some mics to introduce pads to
prevent overload of the input stage, divide the bias
voltage down for some low voltage mics, or reduce the
limiter range at minimum gain settings.
Input Limiter
The transmitter employs a digitally-controlled analog
audio limiter prior to the analog-to-digital converter.
The limiter has a range greater than 30 dB 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.
No Pre-Emphasis/De-Emphasis
Because the signal to noise ratio of the hybrid system is
so high, there is no need for conventional pre-emphasis
(HF boost) in the transmitter and de-emphasis (HF roll
off) in the receiver. Thus, the possible distortion prob-
lems associated with pre-emphasis and de-emphasis
are eliminated.
Pilot Tone Squelch
The hybrid system uses one of 256 different ultrasonic
tones between 25 and 32 kHz, that modulate the carrier
to operate the receiver squelch. The pilot tone frequen-
cy is chosen according to which of the 256 channels
has been selected by the frequency switch setting. The
benefit of a pilot tone squelch system is that the receiv-
er 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 system.
The 400 Series 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.
Antenna
The antenna consists of a permanently attached flex-
ible, unbreakable 1/4 wavelength galvanized steel cable.
Freq
Switches
Bicolor
Modulation
LEDs
9V
Battery
Bicolor
Power
LED
LMa Block Diagram
