Switching the news into IRN 

13 June 2024 tbs.pm/81694


Cover of Radio Month

From Radio Month for October 1981

The ILR contribution network provides a means of routing broadcast quality audio signals from any ILR station in the UK to IRN in London, via a branching network of Post Office music lines. Automatic signal routing throughout the network is controlled remotely from London, necessitating the use of specialised equipment on the premises of each ILR company.

In the latter part of 1979, the IBA commissioned Audix to design, develop and manufacture the equipment necessary to implement this switching process. Experimental prototypes were demonstrated at the International Broadcasting Convention in Brighton in 1980, with the first production units being installed in Birmingham, Coventry and London during May this year.

From earlier experience gained in the field of tone signalling, the IBA was quite specific in its requirements for the system. The equipment was to be fully automatic in operation, under control from IRN, using the public telephone network to send control signals out to the distant contributing ILR station. The ILR station equipment would then “reply” by sending its own set of tone codes back down to IRN in London using the network of music lines switched appropriately by the self-routing tone codes.

The only action necessary by the contributing ILR station’s personnel would be to answer a telephone call from, or make a call to, IRN via a dedicated telephone handset. At no other stations in the signal route back to London were personnel to be needed and these stations could therefore be unmanned. Signalling on both the telephone and music lines was to consist of a series of three consecutive audio frequency tones, each tone being one of four possible frequencies, referred to as A, B, C and D.

To implement these basic signalling requirements, the burst of three tones has been arranged to last for 50ms, and for most purposes it is repeated 150ms after the first burst ends. This is designed to allow a high degree of immunity against false triggering by programme material or spurious pulses present on the line. Experiments done at Audix on a typical station programme output demonstrated that in 150 hours of operation, a single tone burst detector (using three frequencies in ascending order) falsely triggered 45 times, yet the double tone burst detector (within a fixed time window) was not falsely triggered once.

The choice of tone signalling system was considerably influenced by the availability of a family of triple-tone transceiver ICs specifically designed to detect and generate this type of tone code format with high accuracy and interference rejection. Much of the IBA specification was based on satisfying the operational requirements of these ICs, and ensuring that the equipment met with Post Office approval.

A map of the UK showing radio stations connected together

How the contribution network will look in 1983

The most fundamental performance requirement was that all programme audio paths must meet (and exceed) the IBA Code of Practice specifications, but since telephone lines are notorious for high losses the sensitivity of the tone code detectors had to be high enough to work even on the poorest lines, and the tone code output has to be shaped and reduced in level from the original high level square wave signals.

Although the system could have been built in a “black box” format, since its operation requires no intervention by personnel at the ILR stations, this approach would not have provided any assurance to station personnel that the equipment was functioning correctly, nor would it allow any human intervention in its operation. As a result, it was decided to provide comprehensive manual controls, self-test facilities and operational indicators to allow full manual override of the system if required.

The equipment itself can be considered in two parts: the ‘central’ IRN and the ‘remote’ ILR station units.

At IRN there are two rack frames. One frame holds two tone code generator modules (one of these is a spare) for sending codes via the telephone network to the remote ILR station, and the same ZU frame also holds a standard test and faultfinding module, plus a barrier module to protect the equipment from dangerous external voltages, and a mains power supply.

The other rack, the IRN supervisory frame, is a double height (6U) frame fitted with a maximum of 12 supervisory control modules, each one of which monitors one of the 12 incoming music lines from a separate area of the country. It also contains a mains power supply, and an alarm/multiplex module which sounds an external alarm when any new signal route is completed to IRN, and also multiplexes information about the stations currently routed into the network, to feed to the IRN microprocessor-based audio switching and control system.

The modules in both racks (as for the entire system) are eurocard size boards with DIN 64/96-way connectors and metal support rails. These plug into the frame from the front, with the majority of internal wiring being done with 64-way ribbon cable using insulation displacement connectors. In the supervisory frame, twin-screened cable is used for the audio programme paths to avoid transient interference problems, especially from digital clock and strobe lines.

At the ILR station the much larger manufacturing quantities involved meant that the frames are constructed rather differently to the one-off IRN equipment. They are ZU high frames with eurocard sized plug-in modules and a power supply, but this time using a complex motherboard for interconnection between modules (and, again, twin screened cable for the audio programme paths). Very little conventional wiring is used in these frames, making them neat, tidy and spacious inside to simplify construction and ease fault-finding and maintenance.

Equipment frame

A wide-angle view of both ILR station and IRN supervisory frames

The number of modules at any one station is determined by its position in the network. A station at the far end of a spur, like Plymouth, needs only a bare minimum, since it performs no signal routing functions for other stations. But a station such as Bristol requires many more modules, since it provides signal routing for other stations in the network as well as its own ‘local’ signalling functions.

Each ‘spur’ or area of the country has its own dedicated telephone switching code (the code sent up the telephone line by IRN to initiate a network switching sequence). Each station on the spur has its own exclusive ‘station ident’ code, which it sends down the network to IRN on completion of the routing process.

These two codes can be programmed into each frame by the addition of small wire links on a PCB fixed to the inside face of the power supply front panel. This allows codes to be easily changed without removal from the rack, and all the modules in a frame are readily interchangeable. Avoiding dedication of the modules vastly simplifies the problems of spares and replacements.

The electronics of the system fall into three areas: the audio programme path; the music line and telephone line signalling circuitry; and the control logic.

The audio path is the simplest. By using high quality changeover relays for audio routing, there is no necessity to unbalance the signal, and thus no degradation of quality due to signal processing. This, in combination with the use of twin screened cable, ensures a balanced, interference-free audio path which more than satisfies the IBA Code.

Music line signalling circuits use tone code transceiver ICs for code detecting and generating, with resistors between various pins and earth to ‘programme’ the tone frequencies used in each code.

Inputs to the ICs are taken from the balanced lines via unbalancing transformers and suitably amplified to allow for line loss. Outputs from the ICs require filtering to reduce spectral content and attenuation to reduce signal levels to within PO specification limits. Music line filtering is achieved by using straightforward second order low pass filters, and are fed to line via a 600ohm transformer-balanced line driving circuit. The telephone line switching code generator filtering is more involved. Using non-linear filtering, the square wave is level shifted, integrated into a triangle wave, sine wave shaped, and zero-crossing switched, to provide an extremely clean and transient-free signal to feed onto the telephone line via a 1OKohm transformer-balanced bridging circuit.

The logic family used throughout is C-MOS, running from a 15 volt supply rail to ensure a high noise immunity. This provides all the decoding of transceiver IC outputs and the control of their input functions. All the front panel switches and Led indicators are also controlled by C-MOS logic. In conjunction with C-MOS timer ICs and the transceiver ICs, the logic circuitry provides the exact timing pulses necessary to generate double tone code bursts in the correct time period, and similarly looks for the reception of two tone code bursts within the defined time window in the case of tone code receivers.

All the circuitry is designed to be calibration-free, with the exception of a single preset to adjust the ‘A’ tone frequency of each triple tone transceiver IC, and some presets on the telephone code generator at IRN. Even these preset controls are easily set-up and should not need re-calibration after they have left the factory. This, combined with mechanical robustness, high quality connectors and high grade components (with close tolerance values in critical cases) makes the system inherently reliable.



Manual override of the circuitry in the two main ‘controlling’ modules of an ILR station frame is made possible by turning a key-operated ‘lock switch’ on each module’s front panel – the key is available to authorised personnel only. The major functions of that module can then be manually controlled in order to set up the required switching condition, and to generate the necessary tone codes to feed down the line. The module can then either be left in the ‘manual’ state, or returned to normal automatic operation.

Operation of either lock switch in a frame also supplies power to the test module. Test modules are fitted as standard to all ILR station frames (and to one of the IRN frames) and can be used to generate or detect every code used in the entire network, using the single or double tone burst format. The ‘receive’ input can be fed from several points within the frame for internal diagnostic purposes, or from an ‘external’ connector on the rear panel, which in turn is fed from the jackfield. The tone code signal amplitude being received can also be measured to the nearest 1OdB.

The tone code test generator output can be fed into various points in the frame, or again, to a rear panel socket and hence to the jackfield Using this test module, in combination with the manual override facilities, the majority of potential faults in a frame can be easily compensated for or overriden, and the correct operation of the system can be verified without recourse to any other test equipment or tools.

The Post Office requirement that protection should be provided for Post Office lines, equipment and personnel against dangerous voltages is satisfied in the IRN equipment by external barriers, while in the case of the ILR frames, external barriers on all the audio programme path connections to the equipment are provided, and the frame power supply complies with PO standard D2000, thereby isolating mains voltages from the rest of the circuitry. Full Post Office approval has now been obtained for all equipment in the system.

The ILR contribution network offers a novel and sophisticated way of remotely controlling music line interconnections, with the signalling system employed giving extremely high immunity against false triggering by audio programme material. Aside from this specific application, the principles involved lend themselves favourably to many other areas of signalling and control, and have interesting possibilities in other fields of audio/data communication.


Current state of play in 1981

The IRN contribution network is still in the process of being established, although virtually all the necessary British Telecom circuits are now in.

By late September, according to LBC chief engineer Roger Francis, all the Midlands stations, those in Wales and the West Country, those in Scotland, those in the South-East and Piccadilly in Manchester had all been brought into the network, with Downtown in Belfast and those on spurs from Manchester (Hallam, Pennine, Aire and City) due to join in the following month or so. Hereward Radio in Peterborough is expected to enter the network when Chiltern Radio in Luton goes on air later this year.

The remote station switching gear from Audix, however, had not yet been installed – it is due to arrive at most stations this month or next – except at BRMB, Beacon and Mercia, where it was reported to have performed well. The result is that at most stations the actual switching is still being done manually by newsroom staff. “It’s a fairly simple operation,” says Francis, “but it’s a slight handicap, and an added chore.” Nevertheless, reaction from local news editors to the improved quality of much of the material they are now getting from IRN – and, as an added bonus, the feeling of being “part of a network” which the presence of a direct STD line to the IRN newsroom provides – has been good.

Francis reports only one minor teething problem – a tendency for the encoded tones sent up the line from IRN to get lost under the high level of background noise generated in the IRN newsroom, and being picked up by the telephone handset there. The problem has been solved by incorporating a mic cut button on the handset of the IRN phones.

So far the network has proved its worth by being in regular use. The Midlands and Scotland in particular are providing plenty of material – three or four items a week in the case of the Scottish stations – and another two or three items a week are coming from the rest of the country. IRN is paying remote stations for contributions in the normal way, although the rest of the cost of the network – both capital and running costs – is being met by the IBA out of secondary rental.

Francis expects that use of the system will increase slightly with the arrival of the automatic switching fear, which will make it easier to operate.


You Say

1 response to this article

Tim 25 June 2024 at 2:58 am

Interesting article, thanks.
BTW it mentions ZU high frames. That should be 2U, i.e. 2 standard rack units high. Yes, I’m nitpicky:)

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