User's Manual

Frequency-Agile UHF Belt-Pack Transmitter
Rio Rancho, NM
5
are eliminated.
Pilot Tone Squelch
The DSP in the transmitter generates one of 256 differ-
ent ultrasonic tones between 25 and 32 kHz to operate
the receiver squelch (audio muting). The benefit of a
pilot tone squelch system is that the receiver will remain
quiet 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 also
eliminates noise (pops, thumps, etc.) from occurring
when the transmitter is powered on and off.
The pilot tone frequency is chosen according to which
of the 256 channels has been selected by the frequen-
cy switch setting. This ensures that all transmitters in
a system have different pilot tone frequencies so even
spurious RF from the wrong transmitters can’t open the
receiver squelch.
instrument cables are connected.
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 keep audio distortion low and preserve
short term dynamic changes in the audio.
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
Freq
Switches
Bicolor
Modulation
LEDs
9V
Battery
Bicolor
Power
LED
+5v
LMa Block Diagram