19 October 2017

Radio Teleswitch, an example of AM Signalling System (AMSS)
(by ANgazu)

BBC Radio-4 (LF 198 Khz) is a radio station that broadcasts a great variety of programs. At first glance, it looks like any other AM commercial emitter, but there is a feature that makes it different: its carrier is PSK modulated, transmiting  data to switch electric meters and consumer appliances to take advantage of best electric tariffs [1].
This sample was recorded using TWENTE sdr in USB mode so to preserve the carrier, its spectrogram is as any standard AM broadcast (Fig. 1) and it's occupation is about 12 Khz as most AM comercial stations.

Fig. 1
When zooming on carrier (Fig. 2), there is a  signal using about 50 Hz BW on each side of the carrier and once filtered, the modulation speed is 50 symbols/sec (Fig.3).

Fig. 2
Fig. 3
In this case, carrier is the AM carrier so we  can go to  the signal constelation that shows that the carrier phase is shifted  ±  22.5º (Fig. 4). This small shift is suitable  for retrieving data and is small enough to avoid interferences with the intended AM signal.

Fig. 4
To  demodulate this signal, you have to filter out the carrier and proceed as in any BPSK signal. ACF is about 2 seconds, this means that there are 30 frames within each second and everyone carries 100 bits. Signal is "manchester" coded so we will have 50 bits of information per frame (Fig. 5). The signal is idling most of the time (01010101…) 

Fig. 5

[1]