DVB-T2 Extends DVB-T Terrestrial DTV Performance
September 19, 2008
The new and enhanced terrestrial DTV standard from DVB, DVB-T2, attracted a lot of interest at IBC this week. The DVB-T2 and DVB-C2 standards were introduced in a Monday conference session.
DVB-T2 offers greater bandwidth efficiency than DVB-T, allowing transmission of more HDTV channels within the same spectrum. Several manufacturers showed products supporting DVB-T2. NXP Semiconductors, ENENSYS and set-top box manufacturer Pace presented an end-to-end demonstration of DVB-T2.
The BBC said it will use DVB-T2 to launch regular terrestrial HDTV broadcasts in November 2009. Broadband TV news reported that Catherine Smadja, BBC's head of strategy, termed DVB-T2 an "absolute necessity" at a DVB Group press briefing.
"DVB-T2 is for us an absolute necessity because DTT is the number one platform in the U.K.," she said. "We have to offer HD as a public service broadcaster on DTT, not only on cable and DTH. But how to get there? There is no business model; HD will only add to the costs. There might develop a market for HD pay-TV, but not for free-to-air. Luckily, the DVB Group solved the spectrum scarcity problem."
I'll cover DVB-T2 in more detail in a future RF Technology column in TV Technology, but here are some of the major differences setting it apart from DVB-T.
DVB-T, in the current mode deployed in the United Kingdom, uses 64-QAM modulation with an FFT size (number of carriers) of 2 K to provide a capacity of 24.1 Mbps. DVB-T2 can use 256-QAM modulation with up to 32 K carriers to provide up to 35.9 Mbps bandwidth. Use of higher order modulation requires more powerful error correction. The UK DVB-T system uses 2/3 rate convolutional coding and 8 percent Reed-Solomon coding. DVB-T2 can use a 3/5-rate low-density parity check (LDPC) error correcting code along with 0.3 percent BCH coding. The pilot carrier structure is modified in DVB-T2 to drop the overhead for scattered pilots from 8 percent to 1 percent. The 16 K and 32 K OFDM modes available in DVB-T2 allow a maximum guard-interval of over 500 microseconds, making it ideal for large, nationwide single frequency networks without increasing the guard-interval overhead. The guard-interval is flexible, allowing more payload bandwidth when long guard intervals aren't needed.
Connected TV posted an article by Barry Flynn entitled IBC 2008 Report: DVB-T2 could deliver up to 70 percent efficiency savings in SFN environment.
Connected TV asked Peter MacAvock, executive director of the DVB Project, if the ATSC/DVB "wars" were now over.
MacAvock said the "wars" between the two standards would continue, noting there had been significant political pressure from the United States to prevent Colombia from choosing the DVB-T standard. He added that the relationship between the two sides was friendlier now.
None of the articles or papers I was able to find compared the signal-to-noise (SNR) performance of DVB-T with DVB-T2. The use of more complex modulation (256-QAM versus 64-QAM) alone would significantly degrade the SNR threshold. ATSC holds the advantage (about 5 dB) over DVB-T in SNR performance at 19.39 Mbps, but the more powerful error correction coding in DVB-T2 and some other tricks I haven't described should offset, to some extent, the use of 256-QAM. The large guard interval makes it easier to use single frequency networks to improve SNR in urban areas for indoor/portable reception. Compare the power ratings for transmitters supporting DVB-T and ATSC and you'll see that the same transmitter can put out more power with ATSC than DVB-T. This is due to the high peak-to-average power ratio of DVB-T. DVB-T2 uses techniques to reduce the peak-to-average ratio, allowing more transmitter power and providing another way to offset the deterioration in SNR threshold at higher data rates.
It will be interesting to find out if there is a mode that would trade off some of that 30 Mbps plus bandwidth for robustness and allow a DVB-T2 system to carry a 19 Mbps payload with the same SNR performance as ATSC.