Broadcasting – All in Good Time

I was involved in the technical side of broadcasting in South Africa from 1965 till 1996. It was a fascinating 31 years and I was privileged to see amazing developments in broadcasting equipment. But SABC was not just a user of broadcasting equipment. It was a very successful manufacturer.

The early SABC modeled itself on the BBC which was widely acknowledged as a world leader in excellence, both in production and in the research, development and manufacturing of broadcast equipment. In the 1960s we were importing a lot of equipment from America, Switzerland and the U.K. but we had some brilliant engineering brains in South Africa and they started the SABC manufacturing workshops in Judith’s Paarl in Johannesburg. There we would manufacture from absolute scratch, everything from turntables to tape recorders to mixing desks and even microphones. We even improved on some of the technical equipment we had bought in and some of the SABC innovations were taken up and used by the international community. I will tell more about the things we manufactured in a later article but today I would like to talk about the the master clock systems used over the years by the SABC.

Time keeping has always been important in broadcasting. Listeners want to switch on the radio station of their choice at a given time and hear the program or news bulletin they were promised for that time slot. People also want to set their watches to a universal time signal so that everyone’s watches correspond. It’s important for meetings etc. In the early days of broadcasting, anything within 1 second of the correct astronomical time was good enough.

Image result for image of gents of leicester pendulum master clock
Pre-1973 master clock at SABC

the effective length that decides its period is the distance from the knife edge to the centre of gravity. ie, roughly to the centre point of the bob. The old Gents master clock had a compound metal pendulum shaft and a bob of a different metal. The bob could slide freely up and down the shaft and the bottom of the bob rested on and was carried by the bottom plate of the shaft. The theory was that on a hot day, the shaft would expand and get longer, thereby slowing the clock down but the bob would expand from the bottom upwards and compensate for the change in the shaft length. It worked pretty well but it was not perfect. As a result, the clock would drift slightly out of time as the weather changed. We at the SABC used to listen in every day on a short wave radio to the time -signals from the astronomical observatory. If our clock was more than 1 second slow, we would place a small weight of about 1 or 2 grams on the top of the bob. This was above the centre of gravity of the pendulum and therefore raised the centre of gravity minisculy. This was just enough to bring it back into time for the day. If Our clock was running a bit fast by the observatory, we would add the small weight to the bottom of the bob, lower the pendulum centre of gravity and slow the clock down a tad. The system worked but it was very tedious.So the engineers at the Judith’s paarl workshops designed and built a system that would electronically listen to the time pips from the observatory and then use a relay mechanism to mechanically place a fairly heavy metal marble onto, or remove it from a platform affixed half way up the pendulum shaft. This then adjusted the effective length of the pendulum and altered its speed. The system worked brilliantly and never needed manual intervention any more.

The SABC ran on these pendulum clocks until TV arrived on the scene in 1973. But TV stations require far more accurate time-keeping than one can ever get from a mechanical device. So the old clocks were replaced by a form of atomic clock called a Rubidium standard. This is a whole nother story and maybe I will get to that in another article. The Rubidium was incredibly accurate but even that became redundant years later when things like cell phone systems required an even greater degree of exactness.

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s