Implementing cost effective transmitter site telemetry and changeover solutions.

1.???BACKGROUND

Modern solid-state FM transmitters offer a number of inherent redundancy features that are built-in to the equipment for maximising reliability. These functionalities include those of having multiple (paralleled) power amplifier and power supply stages and are largely dependent on the operating transmitter power output (TPO) value. However – the failure of any one of these components usually results in the broadcast service being degraded (via a loss in RF output power) along with calling for remedial action to be taken for repairing the equipment. The damaged equipment is sometimes substituted with a rotational spare unit that is placed in service for the duration of the repairs to the main transmitter.

In the days of vacuum-tube based equipment – it was customary to have a full-power ‘reserve’ transmitter coupled through an extensive array of motorised RF switches and automatic changeover equipment. Tube-based equipment has multiple single-points-of failure {or SPOF} (e.g. high-voltage power supplies, final tube power amplifier stage/s, etc.) in their operative designs – The SPOFs are typically addressed by implementing either passive-reserve (i.e. cold standby) or active-parallel approaches (dependent on RF power level) for improving the system redundancy.

Many transmission system operators provide backup (i.e. same or reduced RF output power) solutions for solid-state equipment on a mission-critical transmitter site, for the purposes of ensuring service continuity. These control systems have varying degrees of cost, complexity and/or add-on functionality (such as site telemetry) and remote accessibility (either via GSM or via a dedicated TCP/IP network). The simplest approach for increasing the system reliability is through implementing two separate antenna/feeder systems for low power main/and backup transmitters. This alleviates the use of an RF switch that can be costly. Such an approach may work well for low power systems, but for medium to high-power transmitter systems – it is no longer an elegant solution and it is appropriate to use motorised RF switches within the system.?

2.???IMPLEMENTATION CASE STUDY

LS of South Africa has developed a simplified automatic changeover system for one of its FM radio customers, based in Johannesburg. The system comprises of the following elements; namely;

·????????Existing main-and standby FM transmitters (Nautel VS Series)

·????????2G-enabled controller (Centurion Systems)

·????????Relay switching matrix, and

·????????12VDC battery backup system

A simplified flow diagram of the system is presented below;

2.1?????????Explanation of system operation

2.1.1?????Site AC failure monitoring

The system receives a stable 12VDC power supply (battery backed-up) from the dedicated PSU. The PSU continuously monitors the incoming 230VAC mains – on failure – the unit sends a ‘mains fail fault’ to the G-Ultra which in turn relays this via the GSM 2G network to the mobile devices that are authorised to receive the push notifications.

2.1.2?????Transmitter interfacing

Each of the transmitters is interfaced to the relay matrix which effectively ‘steers’ each of the transmitter/s interlock signal/s, RF ON/OFF command/s, fault summary alarm/s and remote feed switching interface information, i.e. when TX-A is selected as ‘main’ the unit can be monitored and controlled via the app.

2.1.3?????G-Ultra operation

The G-Ultra is a versatile remote control system primarily designed for domestic and professional security-type applications. It is essentially an off-the-shelf product and can be readily customised according to the requirements of the end user. The unit offers the following basic features;

·????????Four universal inputs (opto-coupler isolated) or outputs (open collector) are offered and can be configured during the setup of the unit. Configuration is carried out via a dedicated ‘app’ that can be downloaded onto an iOS or Android mobile smartphone. The ‘app’ allows each input/output function to be configured per the device (AC mains, etc.) and also permits the user to assign colour codes and icons per I/O function. Further ‘filters’ (by the way of timers) can be applied for failing and rising edge (high→low, and low→high) inputs respectively. When designated as an output – it is further possible to either select the function to ‘pulse’ or ‘latch’ – depending on the application that is presented.

·????????Two relay outputs are provided. (with normally open/normally closed contact arrangements). The relays are designed for switching low power loads only. The relays can be configured for ‘pulse’ or ‘latch’ mode.

·????????The unit offers further flexibility by which it would be controlled – i.e. it is possible to have different levels of access to the unit. The physical activations are by either the G-Remote ‘app’ buttons, missed call, DTMF or SMS directed to the unit. If a physical access number is not stored onto the profile – the G-Ultra will simply ignore requests from unauthorised users.

·????????The system incorporates a built-in log of all activations and recorded alarm conditions.

·????????G-Ultra is a 2G-enabled unit and as such uses this network for remote control and notifications. Each unit must be equipped with its own SIM card and airtime is to be loaded onto each for operation.

2.1.4?????Transmitter operation

The system continuously monitors the status of the selected transmitter. Should the fault summary output of the VS Series will go ‘low’ for any alarm condition, and be sustained for an extended period, it will require user to investigate the condition. As the alarm ‘summary indication does not detail the exact failure/s, one may expect these to be either be feed-related (loss of incoming audio), LAN network-related or actual transmitter failure. Should the transmitter have executed an internal ‘shutdown’ (through PA/PSU/VSWR fault/s) – it may be possible to do the following via the remote control;

·????????A ‘remote-reset’ command can be issued to the main (or reserve transmitter) for restoring a lockout condition (i.e. antenna VSWR condition), or for ‘forcing’ either transmitter back to its primary audio feed.

·????????On complete failure of the main TX- the app allows the ‘forcing’ of the main to backup TX, as well as being able to do ON/OFF control of either unit as has been selected.

?It is also noted that this changeover system does not operate automatically (this was a particular decision that was taken when the system was designed for the customer).The G-Ultra however, does support IO mapping, which allows a particular input condition to automatically trigger an output condition. The ‘mapping’ facility further allows for ‘filters’ to be incorporated where time delays need to be factored into the design.?

3.???ADVANTAGES AND DISADVANTAGES OF THE CHANGEOVER SYSTEM

The following table summarises the basic advantages and disadvantages of the system as has been implemented.?