Vitec Group Communications
PTX-3 UHF Digital Hybrid IFB Transmitter
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INPUT LIMITER
A DSP controlled analog audio limiter is employed before the
analog-to-digital converter. The limiter has a range of more 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: a fast attack and release limiter
followed by a slower attack and release limiter. The dual release limiter
recovers quickly from brief transients but recovers more slowly from
sustained high levels, keeping audio distortion low while preserving
short term dynamic changes. When the audio meter on the LCD
display widens slightly as it reaches zero, limiting is indicated. When
the zero changes to a letter C, severe limiting and/or clipping is
indicated.
DSP COMPATIBILITY MODES
As a DSP based design, compatibility with a variety of different analog
and digital hybrid receivers is possible. The primary noise reduction in
the IFB system is a single band compandor with pre-emphasis.
When the PTX-3 is configured for compatibility with other types of
wireless systems; the DSP emulates the appropriate audio processing
and companding for the chosen mode.
PILOT TONE SQUELCH SYSTEM
Clear-Com IFB systems use a special “pilot tone” so that valid IFB
signals can be distinguished from RF interference. During normal
operation, an IFB receiver will listen for the distinctive pilot tone,
remaining silent (squelched) until the pilot tone is detected. The pilot
tone is located well above audio frequencies and is never passed
through to the receiver’s audio output.
The benefit of the pilot tone squelch system is that the receiver will
remain muted until it receives an RF carrier strong enough for good
reception and the pilot tone from the matching transmitter. Even strong
interfering RF signals on the carrier frequency of the system will not
open the squelch on the receiver.
When the PTX-3 is operated in compatibility modes other than IFB, it
generates pilot tones as appropriate for the chosen mode.
FREQUENCY AGILITY
The PTX-3 transmitter uses a synthesized, frequency selectable main
oscillator. The frequency is extremely stable over a wide temperature
range and over time. The transmitter’s standard tuning range covers
256 frequencies in 100 kHz steps over a 25.6 MHz band. This flexibility
significantly helps avoid interference problems in mobile or traveling
applications.
