ITC Reaches interim decision on DTT transmission mode

20th September 2002

The Independent Television Commission (ITC) is today amending its technical standards to authorise those digital broadcasters who wish to broadcast in '16-QAM' transmission mode to press ahead. This follows the readvertising of licences for three digital terrestrial multiplexes and the success of tenders based on the '16-QAM' transmission mode.

In the light of technical assessments, including laboratory trials, and following the licence awards the ITC has said it is minded to mandate '16-QAM' as the transmission mode for all DTT services. A consultation on this proposition closed on 12 September.

Proponents of a change to '16-QAM' argued strongly that the benefits in terms of coverage and signal robustness justifies making '16-QAM' the mandatory transmission standard. Other respondents argued strongly that the research evidence for this was not strong enough to outweigh the disbenefit of reducing the number of services available. The ITC was impressed by the argument from some respondents of the need to make the decision based on an assessment of the two modes in the domestic environment.

An independent study will therefore be commissioned rapidly to undertake tests in dual mode, '64-QAM' and '16-QAM' transmission in October when both modes will be in use.

The ITC will shortly be announcing field trials and research to provide practical in-home evidence of the performance of the two transmission modes. The most critical issue to assess will be the potential for differential performance between the public service channels carried at '64-QAM', and those at '16-QAM', in poor reception areas and in the presence of impulse interference (e.g. from thermostats, car ignitions, refrigerators etc).

This research will allow the Freeview service to launch with its preferred mode and the industry to assess the impact of transmission mode change based on real evidence. The ITC is minded to choose the '16-QAM' transmission mode but intends to make a final decision on this issue for the New Year based on the outcome of this research.

Notes to Editors

  1. In their recent applications for Multiplex Service Licences B, C and D, five of the six applicants, including all the existing multiplex operators, proposed a change to a '16-QAM' transmission mode from the current '64-QAM' transmission mode. Such a change would have a beneficial effect on reception equivalent to more than doubling the powers of the transmitters, thereby over-riding many reception problems. However, a consequence of the change of transmission mode would be that fewer television services could be transmitted on each multiplex. It is this trade-off between quality of reception and range of services that the ITC has to balance in making a decision.
  2. Although all the existing multiplex operators proposed a mode change, this was in the event of them winning one or more of the vacant multiplex licences. The ITC recognised that the unsuccessful applicants would want to reassess their position in the new DTT operating environment, and this is why the ITC held the consultation into the proposed transmission mode change.
  3. BBC and Crown Castle, who between them operate four of the six digital terrestrial television multiplexes, plan to move to a '16-QAM' transmission mode from the launch of their new service, known as 'Freeview'. The remaining two multiplex operators, Digital 3 and 4 and SDN, will be permitted to continue transmission at '64-QAM' during this interim period.