Spot: filling gaps in DVB-T networks with digital repeaters

By Alain Untersee

Review contents:

Digit is on a horizon The standardization of the digital TV broadcasting transmitter features Digital and digital-analog (hybrid) TV-transmitters Transmitters for digital terrestrial TV broadcasting Low-power DVB-repeaters from ELTI Company Spot: filling gaps in DVB-T networks with digital repeaters NEC transmitters for digital TV Transmitters of Rohde & Schwarz for broadcasting Radio broadcasting transmitters of Technosystem and Screen Service Italia Transmitters of Thales Broadcast & Multimedia for digital television broadcasting Measurement of path parameter of digital TV radio transmitters

Abstract

Repeaters for digital TV broadcasting can use either analogue or digital techniques. The latter one uses another way of repetition called digital repetition. Based on professional DVB-T receiver, they fully regenerate the signal in order to provide a higher quality and new capabilities.

Digital repeaters, due to their properties, provide an efficient solution to increase the coverage of a DVB-T network

Introduction

DVB-T broadcasters strategically launch their networks with high power transmitters in order to quickly ensure an attractive coverage to TV operators and then, in a second step, increase their coverage by placing low power repeaters in the shadow area.

• To repeat a DVB-T signal digitally is called a digital repetition. This new type of repeater uses a professional DVB-T receiver to recover the programme stream (and correct all errors) carried in the RF channel, performs a new modulation followed by an up conversion and amplification. It means that the entire signal is regenerated.

Quality of the broadcasted signal

Digital repeaters

Digital repeater

The D-AISY network with digital repeaters

In case of a digital repetition, the entire signal is regenerated; it means that repeaters, as transmitters, ensure the quality of the broadcasted signal as long as it is able to demodulate it.

The output signal quality is independent of the input signal quality:

• Phase noise is linked to the local oscillator only,
• A weak link, in a digital repetition chain, is erased by the following repeater,
• Several digital repeaters can be cascaded without any cumulative degradation.

Limits of digital repeaters

Control of channels for digital repeater

The delay inside a digital repeater is taller than the guard interval. So, the signal cannot be repeated on the frequency of the main transmitter: main transmitters and repeaters cannot operate in a single- frequency network (SFN) even with 8K carriers and a guard interval of 1/4.

Benefits of digital repeaters

Broadcasting in area with complex terrain relief

Local multiplexing on a base of digital repeater

• As long as the repeater is able to demodulate the RF channel, we have seen in the above chapter that output signal quality is independent of input signal quality.
• Output MER > 33 dB
• In theory, thanks to the forward error correction (FEC) and the output signal quality, digital repeaters can be cascaded ad infinitum. It is an efficient solution to broadcast in valleys. TV viewers and distant repeaters share the broadcasted signal.
• The demodulation process, down to the programme stream, allows broadcasters to insert a local multiplexor in order to customize the content for a local broadcasting. More and more, local communities claim their local programmes. Digital repeaters offer a flexible solution to the network.
• Shadow areas can be covered by several repeaters. Repeaters operate together in SFN mode without any external references (10 MHz and 1 PPS). In their internal memory, digital repeaters can buffer the signal so as to optimise overlaps.

Conclusion

Single frequency network with digital repeaters

As phase noise, MER, etc are critical parameters in DVB-T modulation, implementing repeaters for extending coverage requires care.

Digital repetition is an innovative concept, which helps to increase the DVB-T coverage whilst maintaining the highest quality and providing a greater flexibility.

Harris Broadcast Europe has developed SPOT: a digital repeater that implements all these concepts.

Digital Spot repeater

Thanks to our high-integrated technology, these benefits are available in a very compact device, at a competitive price.

Because it demodulates the incoming digital television stream and remodulates it, SPOT offers a number of unique benefits over a simple repeater. Whereas an analogue repeater simply boosts an existing signal, SPOT achieves full error recovery, and thus the viewer benefits from uncompromised quality.

A second benefit is that SPOT also can be connected to a local remultiplexer, enabling the insertion of local programming. Harris’s single-frequency network technology is also incorporated, to achieve maximum spectrum and coverage efficiency.

SPOT also can be used as a low-power standalone transmitter for pilot networks and evaluation. It is ideal for temporary broadcasts such as special cultural and sporting events. All SPOT models are housed in a compact 4U cabinet, and include an uninterruptible power supply for fast restart

Broadcasters trust in this concept and have already ordered our SPOT technology to increase their coverage.

Higher power repeaters are available with the Harris DTV660 product range.

Material is provided by Harris Broadcast Europe