By Barry Flynn, Contributing Editor
Last month in Paris, Thomson Video Networks proved that packing five HDTV channels into a French DVB-T multiplex without compromising quality was technically possible using current video compression technology.
This relatively low-key demonstration has potentially far-reaching consequences, however.
The immediate aim was to prove to the TV regulator, the CSA, that the countryâ€™s entire DTT platform could move to HD within a few years despite the continued mandation of the DVB-T standard and the prospect of Franceâ€™s existing eight DTT multiplexes being reduced to six by spectrum sell-offs.
But Thomson also reckons that by 2018, improvements on todayâ€™s state-of-the-art compression technology could allow this number to be reduced to five, allowing one DTT multiplex to carry UHDTV.
This harks back to a two-year-old CSA initiative, whereby the regulator called for the law to be changed to permit the adoption of the more efficient DVB-T2 standard on the French DTT platform, in order to pave the way for the launch of a UHDTV multiplex in 2018. That request was denied by the then French administration.
Thomsonâ€™s demonstration, which was overseen by Franceâ€™s HD Forum, broadcast HD channels from Arte, Canal+, France TÃ©lÃ©visions, NRJ, and TF1 within a single DVB-T multiplex running at 24.8 Mbps. Transmitting for six days from NRJâ€™s Paris broadcasting centre, the five channels were compressed by a Thomson ViBE EM4000 premium HD/SD encoder into variable bit-rate MPEG-4 streams, statistically multiplexed and passed through a Thomson NetProcessor to modulate them into a single DVB-T multiplex.
Rohde & Schwarz and Tektronix lent support with modulation and monitoring, while the signals were â€œsubjectively evaluatedâ€ on 45- and 56-inch displays provided by Samsung, LG, Sony and Toshiba.
Claude Perron, Thomson Video Networksâ€™ CTO, explains that there are around 30 DTT channels spread across Franceâ€™s eight multiplexes today, which could be neatly shoe-horned into six HD multiplexes using the compression ratios adopted in the demonstration.
â€œWhat we have shown is that with attractive digital compression technology, our most recent models of digital encoder with the latest software versions are capable of transmitting six multiplexes today [each] carrying five HD channels, with full resolution, and without degradation of the quality when compared to the quality that exists today on HD channels,â€ he said.
While Thomson encoding equipment is ubiquitous across the French DTT network, Perron says that first-generation hardware is still mostly in use, â€œwhich is why today there are only three HD channels [at most] per multiplex in France.â€
HD was launched on the French DTT platform in October 2008 with five channels. Today there are 11, six of which are carried on two dedicated HD DTT multiplexes.
In France, MPEG-2 compression is currently specified for standard-definition free-to-air DTT channels, with MPEG-4 reserved for pay-DTT and HD. The switch to an all-HD environment would therefore entail â€œmoving everything to HD using MPEG-4,â€ notes Perron, â€œwhich means switching off MPEG-2 â€“ thatâ€™s the first message.â€
The second message is that first-generation encoders need to be swapped out for second-generation ones of the type used in the demonstration, since software upgrades alone will not generate the required efficiency increase.
According to Perron, the CSA believes this second switch-over process (analogue switchoff took place at the end of November 2011) could take place between 2016 and 2018. â€œThe installed base of TV sets and adapters is now well-enough equipped [with MPEG-4], but there are still lots of first-generation MPEG-2 adapters around.â€
But if an all-HD platform is the aim, why not switch off DVB-T at the same time and use DVB-T2 instead?
â€œBecause there is a phenomenon which is much too significant with the installed base,â€ says Perron. â€œIt is completely unthinkable to move to DVB-T2. Consumers wouldnâ€™t understand. Theyâ€™ve just equipped themselves with flat screens with integrated tuners â€“ theyâ€™re not keen to be told we need to migrate to DVB-T2 now and add an adapter to receive the same thing [i.e, existing HD channels].â€
However, â€œwhat is being played out, what we are hoping, and what is understood at the level of the CSA, is an evolution of the terrestrial platform to go towards UHDTV,â€ believes Perron.
â€œTo do that, we would need to get rid of yet another one of the six multiplexes, and to reserve that one for the introduction of 4K,â€ explains Perron. â€œWe would do that with HEVC and DVB-T2. And here people would understand that UHDTV requires new equipment, new TV screens and displays, etc., so there would be no problem about migrating [one multiplex] to DVB-T2.â€
That would imply compressing the 30 HD DTT channels onto five multiplexes instead of six, but Perron says Thomson is convinced that â€œin around two or three years, MPEG-4 compression technology will further evolve [â€¦] to improve the performance of these machines,â€ making it possible to squeeze a sixth HD channel onto each of the five putative HD multiplexes.
Perron concedes that this move would also require more than just an encoder software upgrade. â€œWe think that to migrate to this next step [â€¦] will require even further encoder replacements.â€
From the feedback that Perron has received so far, he believes the demonstration has convinced the CSA that at least the first step towards an all-HD DTT future is possible, if not the second one to make room for a dedicated UHDTV multiplex.
As for the broadcasters, Christophe Cornillet, head of the expertsâ€™ panel at HD Forum member NRJ, which transmitted the demonstration multiplex, describes the demonstration as a â€œreal successâ€, not just technically, but in terms of peopleâ€™s subjective reaction to the HD picture quality. â€œA number of VIPs came to see it,â€ he says, including â€œall the CTOs of the main TV channels in addition to public bodies such as the culture ministry, the CSA and the ANFR [Franceâ€™s spectrum management body].â€
Cornillet acknowledges that maintaining full HD resolution (at 1920 x 1080i) and broadcast quality for viewers on large flat screens â€œconstituted a real challenge, but we made it thanks to the qualities of the MPEG-4 encoders from Thomson Video Networks.â€
He believes that â€œif the CSA validates our vision and our solution, it is obvious that the French channels will ask their multiplex operators to upgrade the DTT headends with this kind of encoder.â€
Coincidentally or otherwise, the CSA this week released its response to a request from the current French prime minister, Manuel Valls, for its view on how to address the loss of DTT frequencies in the 700MHz band (see previous story).
Besides confirming that this would entail a reduction from eight to six multiplexes, the CSA said that in order to maintain the attraction of the DTT platform, all the DTT channels must be allowed to follow the move towards HD becoming the default transmission standard; adding that the administration needed to send a clear message to manufacturers to encourage the adoption by consumers as soon as possible of â€œreceivers compatible with DTTâ€™s future standards (DVB-T2 and HEVC)â€. If an all-HD DTT environment is possible using a combination of DVB-T and MPEG-4, that would appear to put UHDTV on DTT firmly in the regulatorâ€™s sights.