True Story No19


Sorry I missed the bit about who you wanted shooting down

Specifications are very useful because they give you all the information you need to design something. If the end result is wrong but it meets the specification then the spec was probably wrong or something important was forgotten about when the spec was written.

A couple of very clever engineers were given the job of designing a radio interfacing system.

The system interconnected lots of different radios and radar operators via a computer, which did the organising, and a collection of switches which did the routing for microphones and headsets.

Say for example a radar operator wanted to launch a missile at an aircraft in his sector, he would call the SAM chap and give him the instructions. To do this his microphone would need connecting to a particular transceiver tuned to the frequency of the SAM operators radio.

Or if the radar chap wanted to guide an interceptor towards an enemy aircraft he needed to talk to the pilot and therefore be connected to an Air-Ground-Air type of radio transceiver.

The system being designed allowed all this to happen.

Instructions for doing the various tasks were carried out by merely touching a screen scanned by infra-red detectors which would recognise the screen position (where there was a legend such as "Connect to AGA") and allocate and set up a free radio of the type required.

The young chaps, whom this story is about, had recently joined us from another site and were very keen to show us how professional they were.

We hadn't bothered much with specifications until then and if we did they were usually written after we'd done the designing.

This always ensured the spec matched the end result.

Anyway the two spent ages writing extremely clever specifications full of dB's and complicated technical expressions.

And of course no-one in the Industry had the right bits required to meet the specifications and therefore lots of tenders had to be issued for the design of audio filters, special matching transformers and voice operated gain adjusting amplifiers and the like.

At last suppliers for all the relevant bits were identified and the bits ordered and bread-boarding commenced.

Costs were very high but the end result was a set of equipments that would perfectly match their specifications.

The proof was there because all sorts of exotic and expensive test gear had been acquired, and where it wasn't available, specifications for new stuff was produced, and other engineers hired to design and build it. All the components met their specs. Some had been rejected and sent back for rework but eventually everything was just right.

radar consolesa "voice"switch

Artificial ears, special voice recordings and suchlike were gathered together and finally a complete trial system was constructed.

Everything worked and everything met the specifications to the nth degree.

Orders were placed for thousands of components and detailed manufacturing drawings proceeded apace.

Later, Jim, an engineer who was just passing the door of the special laboratory, wherein the works had taken place, and the first "public" demonstration was to be made, was summoned at random.

"Put on the headphones Jim and listen to this".

It was the first live test with a real microphone and a real headset.

First a direct connection from the microphone to the headset…..then a connection via the new system.

"What do you think?" asked the pair of proud engineers.

"The first test was really good but the second was a bit muffled", said Jim.

A second opinion was sought; someone with good hearing was needed.

"What do you think", they asked.

"It's really good said second opinion, really clear".

"That's without our system", was the annoyed response, "Listen to THIS!".

"It's very muffled", said second opinion.

What they'd forgotten was that over the years, air traffic control headsets and microphones developed in pace with each other.

Neither was very good individually but together, results were quite acceptable.

To give good fidelity into a typical ATC headset the audio into it was nothing like flat but our new system processed the audio into a perfectly flat chunk having a variation of less than 0.5dB across a band of 300Hz to 3000Hz with a cut off at the sides like Beachy Head.

Unfortunately the headset needed lots of top, which the ATC microphone had, but which our system had removed!

By then we'd ordered thousands of special filters and transformers, the Drawing Office had drawn up all the circuit boards and manufacturing was already under way.

Too late!

Maybe the Radar Operators developed a special squeaky voice to make themselves understood, I don't know.

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