defined by the central segment. In addition to the framing information, Brazilian specification includes the spectral composition for the hierarchical transmission, the means of sending multiple signals, and multiple tables in different layers. The specification group has proposed three different emission masks to cope with the Brazilian planning scenario. All high power stations shall operate with critical mask except in the absence of installed or planned adjacent channel. All the other stations shall operate with the sub-critical mask above, except for the following conditions: In the absence of either digital or analogue adjacent channels, planned or installed in the same locality, the emission mask might be non-critical; In the presence of analogue adjacent channel, the emission mask shall be critical; In the presence of digital adjacent channel with a power difference higher than 3 dB, the emission mask shall be critical; In the presence of digital adjacent channel with a distance between the radiating systems longer than 1.300 feet (or 400 meters), the emission mask shall be critical. These documents are also officially available at ABNT website. Summary Transmission requirements established for the Brazilian digital television standards comply with the general characteristics existing in its Japanese counterpart, while addressing the challenges of broadcasting within a more polluted and unregulated spectrum environment. The additional requirements set by the document ABNT NBR 15601:2007 - Digital terrestrial television – Transmission can be seen as evolutions suggested for better overall performance levels in the transition period between analog and digital television. References External links ABNT website http://www.abnt.org.br/tvdigital () SBTVD Forum SBTVD Forum website http://www.forumsbtvd.org.br/ () ARIB Association of Radio Industries and Businesses website http://www.arib.or.jp/english/ () DiBEG Digital Broadcasting Experts Group website http://www.dibeg.org/ () ABNT standards