AEV Stargate - Page 9

AEV STARGATE
Futurcom Srl via della Tecnica 35, 40050 Argelato Bologna ITALY
9
Figure 1
The signal is transmitted by amplitude modulating, in suppressed carrier mode, a 57 KHz (3
times the 19 KHz pilot) subcarrier with a deviation of ± 2 KHz of the RF carrier, retaining
a maximum deviation of ± 75 KHz for the multiplex, composite audio/data signal.
Data transmission speed is 1187.5 bit/s and, being biphase coded, produces a spectrum of
about ± 2 KHz around the 57 KHz frequency which is suppressed-carrier modulated.
Carrier suppression is required to enable co-existance with ARI signals, since the latter
transmits information using narrow-band (about 250 Hz) DSB amplitude modulation.
The most important waveforms produced by this modulation are shown in fig.2 below.
To avoid phase ambiguity on recovering the 57 KHz subcarrier, in the case of coherent
demodulation, the binary data signal undergoes differential encoding prior to biphase
encoding.
Fig.3 gives an example of the 57 KHz subcarrier after modulation by the data signal.