Time in Broadcasting: Time measurement in the BBC 

14 October 2022 tbs.pm/76334

 

It is not, perhaps generally realised that, in one sense, time is the basis of all broadcasting, for time is the reference against which is measured the frequency of the electromagnetic waves used for radio transmission. Broadcasting frequencies are expressed as so many kilocycles (thousands of cycles) per second, or so many megacycles (millions of cycles) per second.

Time has, of course, many other applications in broadcasting and this little booklet, produced by the British Broadcasting Corporation especially for the Centenary Exhibition of the British Horological Institute, describes some of these. It is hoped that it will be of interest to horologists and others visiting the exhibition.

 

Cover of the monograph

From ‘Time in Broadcasting’, published in October 1958 as part of the centenary exhibition of the British Horological Institute

Since its earliest days, the BBC has been closely concerned with accurate methods of time measurement. There are three main reasons for this:-

  1. One of the most important uses of broadcasting – although perhaps also one of the least spectacular – is to provide simple, accurate and regular time signals for the public, and for mariners, horologists and others whose work requires precise time measurements.
  2. The start and finish of most broadcast programmes must be precisely timed, and high-grade clocks must therefore be provided throughout the broadcasting system. In many cases, the actual changeover between programme sources on the one hand, and transmitters on the other, is carried out automatically by switches controlled from master clocks.
  3. The frequencies (or wavelengths) of BBC transmitters must be maintained at their internationally-allocated values with a degree of accuracy comparable to that of the standard sources of reference for time measurements. The equipment for measuring these frequencies must therefore be calibrated against the standard time-references.

Origin and Present-day Generation of Time Signals

The vocal imitation of time-signal “pips” is now one of the hardy perennials among music-hall “jokes”, but it is probably not generally known that the first “pips” signal was actually transmitted in this way.

When British broadcasting began in November 1922, time signals were transmitted before the news bulletins at 7.00 p.m. and 9.30 p.m. on weekdays. The studios were, of course, equipped with reasonably accurate clocks and the announcer played “Westminster Chimes” on the studio piano, taking his time from the studio clock to strike out the final chords. Later, a set of tubular bells was installed in the studio for the same purpose and these can be seen in Fig. 1 which shows the main studio at Savoy Hill. On 21st April 1923, however, Mr. Hope-Jones, a well-known authority on electric clocks, gave a five—minute talk prior to the change to British Summer Time. He concluded his talk by counting out the seconds from five seconds to ten o’clock until ten p.m. Mr. Hope-Jones afterwards suggested that a permanent service of accurate time “pips”, one per second, from five seconds before the hour until the exact hour, should be provided directly from the Royal Observatory to the broadcasting studio at Savoy Hill. This proposal was adopted and the new service of time signals was introduced on 5th February 1924.

 

Radio studio, draped in curtains

Fig. 1. Main studio at Savoy Hill

 

Initially, two Dent chronometers at the Royal Observatory (which had been purchased originally at £50 each [impossible to translate into modern values but a rough estimate would be at least £10,000 -Ed] for Transit of Venus observations in 1874) were fitted with single-pole make-and-break switch contacts made by V. Kullberg, which were operated from the escapement wheel. This switch was made to close momentarily, at one-second intervals, six times every hour and half-hour. A Post Office telephone line was permanently connected between the switch contacts on the Greenwich chronometer and the BBC control room at Savoy Hill (at Broadcasting House, London, from 1932 onwards). The time signal equipment at the control room included a 1000-c/s electrical oscillator which was kept running continuously. The closing of the chronometer contacts at each pip energised a relay which closed further contacts in the output circuit of the oscillator, and thus delivered the pips in the form of short bursts of 1000-c/s tone. The pips were not fed to the transmitters, however, unless other contacts were closed by the operation of a “G.T.S. Key” in the control room. This key was operated manually a few minutes before a time signal was scheduled.

On 29th June 1927, the BBC transmitted an extended series of dots for observers of the total eclipse of the sun which was visible in England on that date.

In 1928, as the time of the news bulletin was changed from 6.30 p.m. to 6.15 p.m., the chronometer contacts were modified to give signals every quarter of an hour.

In 1929, the two Dent chronometers were fitted with synchronizers, enabling them to be controlled by the Shortt clock which had recently come into use as the reference source for Greenwich Mean Time.

During the war, the time service was transferred from Greenwich to Abinger, Surrey, and an emergency time service was set up at the Royal Observatory, Edinburgh. At the latter station, the original Kullberg contacts on the Dent chronometer were replaced by more robust “Ritchie” contacts operated by a wheel mounted in jewelled bearings on the pendulum.

In 1945 the BBC six-pips signals were also sent to the service commands for synchronization of artillery barrages and other military purposes, and a 24-hour watch on the system had to be maintained to ensure that no failure of any part of it could pass unobserved.

 

Electrical diagram

Fig. 2. Schematic diagram of modified circuit

 

Towards the end of the war, the circuits linking the Observatory and the BBC’s London control room were modified in such a way as to dispense with a mechanical relay, and also to operate an automatic alarm system if the line should fail. This system was installed at Abinger and Fig. 2 shows the basic arrangement. One wire of the Post Office line is normally connected at the Observatory end to a terminal which is held at 70 volts negative to earth. When the pips are signalled, the Observatory end of the wire is transferred by the changeover switch to another terminal held at 70 volts positive to earth. At the BBC end of the line, the output of the 1000-c/s oscillator is connected to the network of metal rectifiers D 1 — D 4. During the silent periods the rectifier network is so biased that it interposes a high resistance in series with the oscillator output. On the other hand, it practically short-circuits the output of the oscillator. The combination of these two effects prevents the oscillator tone from passing through to the programme feed. As each pip is signalled, however, the line is switched to a positive potential and the biasing of the rectifiers is reversed. When this happens, the oscillator output is passed to the programme feed. The two wires of the Post Office line are connected together at the BBC end, so that a failure of either wire will cause the voltmeter V at the Observatory end to read zero, and the absence of voltage operates an alarm device.

By 1949, quartz oscillator clocks had become accepted as the standard source of reference, and the Dent chronometers, together with the Shortt clocks from which they were synchronized, were replaced by Muirhead Type D-306-A phonic motors driven from the quartz oscillator.

The oscillator works at 100 kc/s, and this frequency is divided down to 1000 c/s by regenerative electronic divider circuits. The phonic motor is driven by the 1000-c/s frequency, and in addition to a gear train with an overall ratio of 1000 to 1, giving “pips” once every second, this motor includes gearing to control all the time signals normally emanating from the Observatory, i.e. Rugby rhythmic signals, BBC six pips time signals, and hourly signals for the control of the Post Office Speaking Clock. The BBC signal is controlled by one set of contacts which closes from 54½ to ½ seconds each minute, and another set which closes for about ½ minute each quarter hour. Thus the pips signal is turned on each quarter hour for a six-second period beginning half a second before the first pip, and ending half a second after the last pip, and is transmitted through the Post Office line to the BBC.

After the Royal Greenwich Observatory had moved to Herstmonceux, the system was modified to provide fully-electronic generation of the pips, using another electronic divider of the dekatron type – in place of the phonic motor – to divide from 1000 c/s down to a pulse occurring once per second. Up to this time (July 1957) it had been difficult to operate electronic dividers below 1000 c/s with sufficiently high phase stability. The adoption of electronic division down to 1 c/s makes it possible to apply corrections of less than one millisecond by a phase-shift transformer. The phonic motors are, however, retained for the sake of the gearing and contacts which switch on the different time signals at the appropriate times.

 

 

The next step was to eliminate the remaining human element which was still required to operate the G.T.S. Key in the control room before pips were due to be transmitted. This and other facilities are now provided by the “Automatic G.T.S.” equipment which is operated by half-minute pulses from the master (pendulum) clock in Broadcasting House. The pulses drive a system of relays and selectors which automatically connect the time signal source to the Home or Light programme “feeds” fourteen minutes before time-signals are scheduled to be transmitted on either of these programmes. The half-minute pulses are divided down to one-minute pulses by integrating relays which pass each alternate pulse. A magnetically-operated eight-pole twelve-way switch of the uniselector type produces a pulse every fifteen minutes from the one-minute pulses, and the fifteen-minute pulse, in turn, moves a larger motor uniselector switch one step every quarter hour.

This motor uniselector has sixteen wipers, each of which makes 96 contacts every 24 hours. One of these wipers connects the time-signal amplifier to the Home programme feed at any quarter hour which has been selected by making a connection to one of the contacts swept by that particular wiper. In the same way, another set of contacts (swept by a different wiper) determines the times at which the time signal is transmitted on the Light Programme. Since the schedule of time signals differs on weekdays and Sundays another small uniselector is provided which moves one step every day, and transfers the Home and Light programme feeds to different contacts on the large motor uniselector every seventh day. These “seventh-day” contacts are wired up for the different times of the G.T.S. on Sundays.

Other contacts on the motor uniselector are made to illuminate a lamp display in the Technical Operations Manager’s office, showing the day, hour and minute, and thus enabling him to check at any time whether the G.T.S. equipment has slipped out of step with the control room clock.

 

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