Clear-Com Cellcom Instruction Manual download pdf (Page 9)

Vitec Group Communications

PTX-3 UHF Digital Hybrid IFB Transmitter

1-3

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.

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Clear-Com Cellcom Instruction Manual Page 9

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