INRIM being part of the National Calibration System as a primary metrological institute, is in charge of the realization and maintenance, for Italy, of the national standard time, the UTC(IT) scale, and takes care of the dissemination of the standard time also by means of coded signals radio broadcast by RAI, the Italian Radio and TV broadcaster. Time signals generation for RAI began in 1945 at IEN, Istituto Elettrotecnico Nazionale “Galileo Ferraris” and from then on the structure of these signals has been sometimes revised.
The signal, starting since 1951, was formed by a sequence of six acoustical pulses lasting for 100 milliseconds, each of them being made of 100 complete sinusoidal cycles of a 1000 Hz key, synchronized with second 54, 55, 56, 57, 58, and 00 of each minute. In those years the signal application was mainly acoustical: in fact it was important for paying attention to the vocal announcement of time that was broadcasted immediately after. However, automatic systems were already in use to synchronize electronic clocks on this signal; in that case, it was necessary that the clock was synchronized within the minute.
In 1979, in coincidence with second 52, a date code of “numerical – acoustical” type was introduced: it was formed by 32 successive elementary information indicating, in binary decimal code and by using 2000 and 2500 Hz musical notes, the hours, minutes, the month’s number, the day of month, the week day’s number and if the winter time (CET, the Central Europe Time = UTC + 1) or the legal summer time (CEST, The Central Europe Summer Time = UTC + 2) is in force. This innovation allowed the development of new decoding devices that can acquire the complete information of date and time and automatically update an electronic clock.
Later, also because of users urging the absence of the year information inside the signal, by that time excluded from the code to keep its duration within a second, another portion of code was added with the indication of the year, a notice on the next passage from winter time to legal time or vice versa and a warning on the possible introduction of a leap second. This last innovation, on duty from July 1994, brought the RAI coded signal, named from then on SRC (Segnale RAI Codificato), to the actual structure that presents two segments of code of analogous characteristics, but different duration (960 and 480 milliseconds, respectively), generated in correspondence of seconds 52 and 53, in addition to the six 1000 Hz key pulses in coincidence with seconds 54 to 00, excluding the second 59. The information about the second is contained implicitly in the beginning instant (52,00 s) or in the final instant (53,48 s) of the code itself.
The signal elementary packets constituting the two code segments are formed by 2000 and 2500 Hz notes being approximately two whole tons of a major third (B6 – D#7 on a piano), each lasting 30 milliseconds; they constitute the logical level “0” and “1” respectively of an elementary binary information (bit). Each one of the two code segments, generated at second 52 and second 53 respectively, is forerun by two identification bits, the first characterized by the sequence “0 1”, the second by the sequence “1 0”. Even if they are not perceived by the human ear, these sequences allow that an electronic decoding circuit recognizes them so that the following information can be expected.
The first segment formed by 32 bits, apart from the identifier, contains in binary decimal code the Italian Standard time (two bits for the tens and four for the units), the minute (three bits for the tens and four for the units), the notice of the presence of the winter time or the legal time (bit=0 or bit=1) and a parity check bit (of odd kind) referring to the first part of the message, so that the number of bits of this part, being at level “1” for these 17 elementary information, is kept odd. The month’s number (one bit for the tens and four for the units), the day of month (two bits for the tens and four for the units), the week day number (three bits) where number one corresponds to Monday, then follow, and finally an additional parity check bit, like the previous one, for the last 15 elementary information.
The second segment formed by 16 bits, apart from the identifier, contains the information about the current year (four bits for the tens and four for the units), a warning code activated six days before the change from winter to legal time or vice versa (three bits), a code warning on the introduction of the leap second activated during the month at the end of which the event is expected (two bits), finally a parity check bit, like the previous ones, relative to these 16 bits. For what concerns the notification of the next winter -to-legal time change, the three bits are kept in the “1 1 1” condition until seven days before the event; from the next day instead, the decimal equivalent of these three bits is decremented of one unit per day at each 00:00 UTC. When these three bits become zero and the winter time is in force, at 02:00 the hour will jump to 03:00, the legal time entering into force; in the opposite case being in force the legal time, the jump will be from 03:00 to 02:00. About the notification of the leap second, if the two bits are in the “0 0” condition no events are expected for that month, if they are in the “1 0” condition a one second addition is expected for end of month, while the “1 1” condition warns about the next one second subtraction. The leap second is a correction that consists of adding or subtracting a whole second to the duration of a particular day. This event, internationally coordinated by the IERS (International Earth Rotation Service), has the aim of keeping in step inside 0.9 seconds the international time scale UTC, calculated by BIPM (Bureau International des Poids et Mesures) on the basis of a weighted mean of the time of the atomic clocks that are kept in about seventy metrological laboratories all over the world, and the UT1 time indicated by the Earth’s angular position, that is the Earth’s rotational scale. Given that at present the Earth is slightly slowing down and the mean duration of a day is larger than the one indicated by atomic clocks, in the last years leap seconds were always added, this event happening usually once every one or two years; From July 1st 2012, when the last leap second was added, in total 35 seconds were accumulated.
Coded time signals, generated at INRIM by three independent devices using the signals of standard cesium clocks, are originally kept in step with the national time scale UTC(IT) inside a few millionths of seconds. The outputs of the three generators are constantly compared by an automatic system that, adopting a majority criterion in case of contrast of the information, provides once a minute for the dispatch to the Torino RAI of the signal from the device that is considered the most reliable at that moment. The signals are then disseminated from RAI to its offices for internal use and in particular sent to the Roma production centers, from where they are broadcasted from 15 to 30 times a day on radio networks (Radio 1 and Radio 3) and sometimes also on the television channels.
The devices developed to make use of this service consist of a radio receiver and a decoder, and usually also an electronic quartz clock. In case the clock is present, this is automatically put back in step at each received signal (or after a certain number of coded signals); in case there is no clock, the date information is powered in output to another electronic device. Giving that the propagation delay of the SRC from INRIM to user, depending to the position of the latter, can vary from approximately 10 to 30 milliseconds in case of terrestrial radio links, and it can reach up to 0.25 seconds values for satellite links, decoders are able to compensate this delay that usually is quite constant; the accuracy of this offsetting depends on the fact that the delay has been measured or simply estimated.
Through the years devices were developed that decode the signals, measure and elaborate statistically the time difference from the local clock and the received pulses, control dynamically the local quartz oscillator’s rate and make it available as local reference signal with a residual error of the order of a few parts per billion; they are also able to keep the local hour consistent with the INRIM hour at the millisecond level.
Retracing for the SRC history, we can summarize the following main dates:
• 1945: first dissemination of time signals with codes different than the current ones;
• 1951: Introduction of a six impulse sequence at 1000 Hz equivalent to the current ones;
• 1979: SRC first version (containing only date, time and day of the week);
• 1994: SRC current version (as the 1979 version, plus the indication of the summer/winter time and the leap seconds).
So, the year 2014 represents the 35th anniversary of the first SRC code and the 20th anniversary of the current version of the time code disseminated by RAI.