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AMTOR ( Amateur Teleprinting Over Radio ) is an artificial word and describes a digital operating mode in amateur radio . It was specially developed for shortwave telex traffic in order to compensate for the disturbances (fluctuations in field strength, interference) occurring on these frequencies.

In AMTOR is an evolution of the amateur radio teletype (RTTY). In contrast to RTTY , this is a process with high transmission security, since with an AMTOR connection the receiving station is requested to acknowledge error-free reception after three characters have been sent ( ARQ , automatic repeat request), or the characters are redundant in the case of blind transmission sends ( FEC , forward error correction). With ARQ, both transmitters operate alternately and must be able to switch quickly (approx. 20 ms) from sending to receiving.

The principle of the transmission of telex characters using the ARQ or FEC method exists in commercial marine radio under the name SITOR , SPECTOR or MICROTOR and has been modified accordingly for amateur radio.

To carry out AMTOR operation, controllers are required that are connected between the terminal (usually a normal PC ) and the transceiver . There is also free software that generates the signals with a sound card and no longer requires a special controller.

modulation

AMTOR uses AFSK with two tones and a synchronous transmission with error correction like PACTOR but with 7 bits per character.

As with RTTY, the signals are transmitted by frequency shift keying with the same tones and the same shift. The higher frequency is called the mark and the lower is called space . They are apart by the shift (frequency difference). The shift is 170 Hertz on short wave and 850 Hz on ultra short wave (VHF / UHF). The blocks of three are sent at 100 baud. At 100 baud, 1/100 second, i.e. 10 milliseconds, is required for one bit. A 7-bit character takes 70 ms and a block of three takes 210 milliseconds. The blocks of three are sent at an interval of 450 ms, so that the remote station has a gap of 240 ms to send its control character. The time for the receive-send switchover must therefore be very short in this operating mode. The bandwidth is 500 Hz. AMTOR is being replaced more and more by the newer and better PACTOR process.

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