A Primer on an Alternative Path for DTV
Readers may recall that your blogger started his consulting business in Paris after retiring from FCC. Thus events and policies in Europe are of more interest to him than to many on this side of the pond.
The FCC 12/2/09 public notice on "Data Sought on Uses of Spectrum" raised the following issue,
"There may be opportunities for broadcasters to share 6 MHz channels in a market without significantly disrupting the free over-the-air television service that consumers enjoy today."
While this might seem like a strange concept, the chart at the top of this page shows it is reality in Europe. The purpose of this post is to summarize and give links to additional data on the European situation in order to stimulate discussion in the US spectrum community.
DTV developed first in the US while the original concept of analog HDTV developed in Japan. As in many cases, the early adopter had fewer options than the later adopters and the US ATSC group selected 19 Mb/s 8-VSB as the over-the air transmission standard. The Europeans later chose the DVB-T standard. DVB-T is an OFDM standard somewhat related to both WiMAX and LTE.
(8-VSB uses a high speed 6 MHz signal with 11 million broadband signals/s so each group of bits is sent sequentially. OFDM divides the TV channel into many parallel smaller channels and then send the binary data in parallel over the parallel channels. In a perfect world they would be similar in performance, but the real world has propagation problems that have to be corrected at the receiver and they have different approaches to correcting such problems. An added complication is that European TV channels are 8 MHz wide.)
A senior staffer at the FCC's UK counterpart, Ofcom, has provided the following factual summary of the UK use of multiplexes to carry several video and audio streams on one DTV signal. "DTT" is used in UK-speak for "digital terrestrial TV" ). Ofcom has no position on this US domestic issue:
The DTT platform in the UK comprises six multiplexes. The BBC and Arqiva Ltd. each operate two multiplexes, and the remaining multiplexes are operated by Digital 3&4 (carrying services from ITV, Channel 4 and Teletext Ltd.) and SDN Ltd. (a wholly-owned subsidiary of ITV plc.) respectively.
Information on the programme services carried on each multiplex is available at www.dmol.co.uk: DMOL manages common technical aspects of the DTT platform, and is made up of representatives of the four multiplex operators. The composition of the multiplexes is slightly different in those areas of the UK which have been through digital switchover (DSO), and listings for the pre-DSO and post-DSO configurations are provided on DMOL’s site. Each DVB-T multiplex carries between four and nine individual standard definition video streams.
In September this year, the BBC’s second multiplex, multiplex B, was cleared of its standard definition services in those areas which had been through DSO, and will be converted to operate as a DVB-T2 multiplex as DSO progresses across the UK. The DVB-T2 multiplex will also be broadcast on temporary frequencies as a ‘seventh’ multiplex from five of the UK’s major TV transmitters (these transmitters do not switch over until later in the regional DSO sequence, but serve significant populations). We expect that multiplex B will have capacity for up to four HD services (three services at launch).
The BBC has announced a schedule for launching the DVB-T2 multiplex, including ‘retrofit’ launches at transmitters which have already been through switchover, and the ‘early launch’ sites using temporary frequencies. The schedule is available at www.bbc.co.uk/pressoffice/pressreleases/stories/2009/11_november/16/freeview.shtml.
More information on the multiplex reorganisation and capacity allocation process for HD services on DTT is available on our website at www.ofcom.org.uk/radiocomms/digital/hd_on_dtt.’
A senior source at Arqiva recommends Chapter 4 and 8 of Ofcoms's "The Future of Digital Terrestrial Television - Enabling new services for viewers" 2007 consultation (NOI) as a good summary of present and future plans for DTV in the UK.
Note that the UK has started an evolution from DVB-T to DVB-T2 as the modulation of DTV/DTT multiplexes. DVB-T2 can deliver 40.2 Mb/s in an 8 MHz channel to fixed receivers. (Delivery to mobile receivers is less efficient since extra coding/error control is needed.) DVB-T operates 18 or 24 Mb/s at present in the UK, although they all will become 24 Mb/s when the DTV switchover is finished. DVB-T2, though now operate in the UK at 40.2 Mb/s. Original DVB-T receivers can not receive information from DVB-T2 transmissions. (Most of the cost of higher end DTV receivers is not in the receiver electronics, but in the display. So an external receiver with an interface such as HDMI could add upgraded performance to existing TV sets.)
A parallel issue to channel modulation is digital coding. ATSC started with MPEG-2 picture coding, but now supports H.264/MPEG-4 AVC. Europe is actively pursuing MPEG-4 to increase the capacity of existing multiplexes.
The Ofcom Future of DTT report, cited above, says (p. 5-6)
An improved video and audio coding compression standard called MPEG-4. This is expected (over time) to operate at up to double the efficiency of the coding standard that is used at the moment on DTT, MPEG-2. This means that a DTT multiplex could carry up to twice as many services using MPEG-4 as can currently be achieved using MPEG-2, whilst maintaining similar picture quality.
A new transmission standard, known as DVB-T2. This is expected to deliver an increase of at least 30% in the capacity of a DTT multiplex over the current standard, whilst maintaining the same coverage. This standard is a development of the existing DVB-T standard used in the UK since 1998. DVB-T2 is still undergoing development by DVB5 in Geneva, but is expected to be finalised in spring 2008.
It is important to note that MPEG-4 and DVB-T2 differ in one important respect. MPEG-4 can be introduced within a multiplex (so it can offer a mix of services coded in MPEG-2 and MPEG-4). But the introduction of DVB-T2 requires a whole multiplex to be converted from DVB-T. This is, of course, a larger step-change.
The introduction of these two technologies could, if combined, increase the capacity of a multiplex by up to 160%. This is a very large increase. It is the equivalent of raising the number of Standard Definition (SD) services that can be carried on a DTT multiplex from around eight currently to around 13-15 at DSO, and over 20 in the longer term. HD is generally regarded as unfeasible on DTT in the UK without use of MPEG-4: but with the use of these two technologies combined, a single DTT multiplex could in time offer at least four HD services. (Emphasis added)
While Europe is using MPEG-4 with DVB-T and DVB-T2 transmissions, it is also usable with the ATSC 8-VSB technology to get more video waveforms/TV channel.
It isn't that far away!