Introduction to DVB-T Technology, Standards and RF Testing

By: By Wai Hoong Sun, Rohde & Schwarz Systems & Communications Asia, Singapore
( 1 Sep 2007 )

The adoption of Digital Terrestrial Television (DTT) is well underway in Europe. Already, DTT penetration has reached 25 percent in the UK and Finland with substantial penetration in Sweden and Italy.¹ Governments in European countries have also set time lines for the switch-off of analog transmission, known as Analog Switch-Off (ASO) dates. Consumers would have to switch over to DTT by the ASO dates.

In Asia, China has announced its plans to have DTT broadcast for the Beijing 2008 Olympics. China has an estimated 370 million television sets and an average of 40 million sets are being sold each year.² India, with an estimated 70 million households has already initiated DTT test transmission in 2000.³ With huge populations, burgeoning middle classes and fast growing economies, China's and India's appetite for DTT are insatiable.

There are numerous advantages that DTT has over analog television. DTT utilizes the frequency spectrum more efficiently enabling broadcasters to pack in more programs into the same bandwidth, resulting in reduced cost per program. For the consumers, it translates to more choice of programming and offers the possibility of interactive television. There are two main standards in DTT. They are Digital Video Broadcasting - Terrestrial (DVB-T), initiated by the European authorities, and the US-developed Advanced Television Systems Committee (ATSC). This article focuses on DVB-T. It will look into its technology, the various standards and RF testing.

MPEG-2 CODING
Without any data reduction or compression, a Standard Definition Television (SDTV) signal sampled at 10 bit resolution has a data rate of 270Mbps. For a High Definition Television (HDTV) signal, the data rate is even higher. In DVB-T, the high data rate is compressed using MPEG-2 coding to about 2Mbps to 15Mbps. Data integrity is improved by employing error protection. The compressed stream is then packetized into transport streams that are 188 bytes long, of which 4 bytes belongs to the header and the payload is 184 bytes long. Important information pertaining to the attached payload is located in the header.

COFDM MODULATION
DVB-T is based on the Coded Orthogonal Frequency Division Multiplex Modulation (COFDM). In DVB-T, this multi-carrier modulation can consist of around 2000 or 8000 carriers. The carriers are themselves modulated with appropriate modulation schemes. For DVB-T, the modulation schemes used are QPSK, 16-QAM or 64-QAM.

COFDM provides good immunity for the modulated signal against moving (Doppler) and static reflections (echoes). A DVB-T system with around 2000 carriers would allow for a wider intercarrier spacing and thus a bigger Doppler shift before the transmitted data is adversely affected. For protection against echoes, guard intervals are inserted between the transport stream packets. A modulation with around 8000 carriers would result in longer guard intervals and thus offer better protection against echoes.

DVB-T RECEIVER
The block diagram of a DVB-T receiver is shown in Fig. 1. The RF signal, received by the tuner is first demodulated to intermediate frequency (IF) 1. Normally, IF1 would be at a frequency of 36MHz. It is then passed through a surface acoustic wave (SAW) bandpass filter, where adjacent channels are suppressed to low enough levels to avoid interference. A mixer down converts the signal at IF1 to IF2. IF2 is normally around 5MHz. To avoid aliasing at the analog to digital converter (ADC), a low pass (LP) filter is used to filter off frequency components that are above half the sampling frequency.

At the DVB-T demodulator block, the incoming signal is sampled, demodulated and then formed into MPEG-2 transport stream packets. This is followed by the MPEG-2 decoder block where the transport stream is converted back to standard video and audio signals.

DVB-T TEST STANDARDS
DVB-T test standards ensure that the receiver is compatible and operable in the environment that they are intended for. DTG D-Book (UK), EICTA E-Book (Continental Europe), DGTVi D-Book (Italy) BSMI (Taiwan), NorDig (Scandinavia) and AS4933.1 (Australia) are among the test standards that are used to test DVB-T receivers intended for their respective countries or regions (in brackets).

RF TESTING
In general, the crucial RF tests to be performed are sensitivity testing, minimum carrier to noise ratio (C/N) testing, immunity to analog and digital signals testing, impulse interference testing and echo testing.

Sensitivity and minimum C/N testing ensure that the front-end noise level of the receiver and noise due to non-linear effects (intermodulation products) of the transmitter are within acceptable levels. Front-end noise of receivers can consist of phase noise from the local oscillator, quantization noise of the ADC, etc. In the operating environment, it is very likely that there are already transmissions of analog television signals. Immunity test against adjacent, image and co-channel analog channels are conducted to ensure that the receiver is robust against interference from analog television signals. Immunity test against adjacent digital channels are also performed to make sure that the receiver is not vulnerable to such interference.

Short burst of pulses (impulses) from domestic appliances and car ignition systems may enter the receiver through poorly screened cables or antenna. To ensure that the receiver is robust against such interference, impulse interference testing is performed by inserting pulses with different widths, burst lengths and pulse intervals together with the wanted signal.

Due to reflections off buildings, hills and static objects, there may be more than one path from the transmitter to the receiver. This may lead to echoes. Tests are performed to check that the receiver is not susceptible to this interference.

CONCLUSION
Efficient spectrum utilization in DTT systems will be the main impetus for its adoption by most countries as frequency spectrums are limited and expensive. European governments have already set ASO dates while the United States will be fully DTT in 2009.&sup9; In China, the Beijing Olympics in 2008 will be broadcasted in DTT. DTT allows broadcasters to pack in more programs into the same bandwidth, resulting in reduced cost per program.

Consumers, on the other hand, can look forward to more choices in programming and the possibility of interactive television. For manufacturers, this transition is a great opportunity to increase their revenue. However, to take advantage of this, they may need to have good product development, fast time to market product cycles and sufficient production capacities.


REFERENCES
[1] Digital Terrestrial Television Action Group. "Analogue Switch-off. Strategies to End Analogue Terrestrial Television in Europe", DigiTAG 2006.

[2] Xinhua News. "China Makes 1st Self-Designed Digital TV Chip", 27 December 2004, from http://news.xinhuanet.com/english/2004-12/27/content_2385870.htm.

[3] Tew, Barry. "India, Country-by-Country DVB Information", 20th February 2006, from http://www.dvb.org/about_dvb/dvb_worldwide/india.

[4] Fisher, Walter. "Digital Television. A Practical Guide for Engineers", Springer 2004.

[5] International Electrotechnical Commission. "EBook", Draft Version 2.0.2.

[6] Digital TV Group. "Digital Terrestrial Television, Requirements for Interoperability", Version 4.0.

[7] Mason, Arthur OBE. "Digital Video Broadcasting Standards for Satellite, Terrestrial and Cable Television Transmission (Invited)", IEEE MTT-S Digest 1998, pp. 61-66.

[8] Rohde & Schwarz. "R&S TA-DTV DVBT Technical Information" Version 03.00 May 2006.

[9] Broadcast Engineering, Beyond the Headlines. "Analog TV shutoff date set", Feb 3 2006, from http://broadcastengineering.com/newsletters/bth/20060206/.


About the Author
You can reach Wai Hoong Sun at solutions.asia@rohde-schwarz.com.
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