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RTU-292 Operations Manual
4.3.6.1 Adaptive Hybrid
The adaptive hybrid is the central function of the RTU-292. Its purpose is to separate and
isolate the telephone receive signal by subtracting the telephone send signal from the mixed
telephone send and receive signals. If this subtraction is done perfectly, the telephone receive
signal is completely isolated. The digital adaptive hybrid in the RTU-292, while not achieving
perfection, comes close, and gives as much as 40 dB isolation when measured with a
broadband noise source. Comparable isolation for a conventional hybrid would likely be only
10 dB, measured in the same way.
The adaptive hybrid samples the telephone send signal at A/D 2, samples the mixed telephone
send and receive signals on A/D 1, and outputs the isolated telephone receive signal to the
transmitter output amplifier on D/A 1.
The hybrid function also contains a threshold detector that allows VOX operation when balance
has been achieved.
4.3.6.2 Noise Generator
A pseudo-random noise generator is implemented to speed hybrid adaptation. During the
adaptation cycle, the input receiver audio is switched off by gate U8C, and the noise generator
switched on at gate U8A. The wide bandwidth of the noise assures that the hybrid is adapted
over the full audio bandwidth.
4.3.6.3 Peak Detector
A peak detector monitors the receive audio bus and signals at A/D 2 to flash the PEAK LED on
audio peaks which exceed 0 dBm. This function aids in setting up the RTU-292 with the
proper audio levels.
4.3.6.4 VOX
A VOX (Voice Operated Xmit) function looks at the output of the adaptive hybrid and triggers
if the audio exceeds a preset level. Once triggered, it stays active for a short time to prevent
VOX drop out on pauses between syllables. This is called Hang Time, and is adjustable via
Option Switch SW2-6. The VOX sensitivity is adjustable via Option Switches SW2-4 and
SW2-5.
4.3.6.5 Audio Delay
The output of the adaptive hybrid is delayed before being output to the transmitter output
amplifier. The purpose of the delay is to insure that the transmitter is keyed a sufficient time
before the audio arrives so that the entire first speech syllable is transmitted.
4.3.6.6 DTMF Detector
A DTMF detector is implemented in DSP software that listens to the telephone line signal after
the hybrid.
4.3.6.7 DTMF Generator
A DTMF generator is implemented in DSP software that outputs DTMF digits to the telephone
line for dialing. The output is from J13-32 DTMF, which is routed to gate U9D so it may be
mixed with the signal to be sent to the telephone line in U1A.
JPS Communications, Inc.
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