Background

The BBC uses three synchronised long wave transmitters on 198 kHz (1515 m) to provide national coverage of Radio 4. These transmitter sites are located at Burghead (50 kW) north Scotland, Westerglen (50 kW) in south Scotland, with the main transmitter located at Droitwich in central England (500 kW). The stability of the long wave transmitter carrier is very accurate and can be used as a frequency reference. The transmitted carrier frequency is maintained to an accuracy of 1 part in 10¹¹.

What may not be so well known is that this long wave transmission also carries a data signal. Digital data is transmitted by directly modulating the 198 kHz carrier and provides for 16 different data channels. One data channel is used to transmit an accurate time code.

The radio audio signal and data signal are independent of each other and quality of the programme audio is not affected by the data transmission. The audio is used to amplitude modulate (AM) the carrier wave, whereas the data signal is transmitted by phase modulation of the carrier wave. This is not a recent innovation as the first trails of the radio data system took place in 1979.

Data Modulation

Data modulation of the 198 kHz carrier uses bi-phase encoding where a data bit ‘1’ is signified by 20 milliseconds of phase advance of the carrier followed by 20 milliseconds of phase retard. Conversely a data bit ‘0’ is signified by 20 milliseconds of phase retard of the carrier followed by 20 milliseconds of phase advance. The phase deviation of the 198 kHz carrier is ±22.5 degrees and this phase shift changes over several milliseconds rather than an as an abrupt phase change.

The use of bi-phase modulation avoids any net phase shift of the carrier when averaged over a period of one second or more. Thus the frequency stability of the carrier remains and its use an accurate frequency reference is not compromised.

Radio Data

The data is sent in 50 bit synchronous packets, with synchronous transmission there are no inter-packet gaps. Each 50 bit data packet contains a prefix code (1 bit) which is always transmitted as a ‘1’, channel identification (4 bits), 32 bits of data, and CRC error detection (13 bits). When there is no data to be transmitted a filler packet consisting of alternate ‘1’ and ‘0’s is transmitted.

Transmission data rate is 25 bits per second, thus each 50 bit data packet takes two seconds to transmit. Therefore there are potentially thirty self contained packets of data that can be transmitted each minute. These packets are numbered 0 to 29 for reference, with the data carried in each packet allocated to any one of 16 different data channels.

Apart from the time packet, information destined for any other channel can be transmitted in any order. Several packets of data sent sequentially and allocated to the same channel is allowed.

Time Data

Channel 0 is allocated to the time data and this information is always transmitted in packet 29. This packet is the last packet in the minute sequence so that the boundary between packet 29 and the next packet is the minute edge.

The time code transmits the time in UTC (Co-ordinated Universal Time) and the time difference between UTC and UK local time.  This time offset is rather generous as it allows a local time offset of up to ±15½ hours from UTC.

Radio Teleswitching

Apart from transmitting time data the only other known application is the Radio Teleswitching system used by the Electricity Association on behalf of the electricity supply companies.

Radio Teleswitching uses time coded data transmitted for the remote switching of night time storage and water heaters installed in the home, and to set the tariff rates on the electricity meters for off-peak electricity usage.