The BBC proposed to ensure compliance with copy-protection standards on the upgraded Freeview HD multiplex by compressing the service information (SI) data, which receivers need to understand the TV services in the data stream. In August 2009 the BBC wrote to Ofcom after third-party content owners asked the BBC to undertake measures to ensure that all Freeview HD boxes would include copy protection systems as required by the Digital TV Group's D-Book, which sets technical standards for digital terrestrial television in the UK. In early February 2011 it was announced that 1,000,000 Freeview HD set-top boxes had been sold. Thus equipment sold as Freeview HD before April 2011 may not deliver surround sound to audio equipment (some equipment may, but this is not mandatory) later equipment must be capable of surround sound compatible with most suitable audio equipment. As of June 2010 the DTG D-Book includes the requirement for mandatory transcoding when sending audio via S/PDIF, and for either transcoding or multi-channel PCM audio when sending it via HDMI in order for manufacturers to gain Freeview HD certification from April 2011. Transcoding from AAC to Dolby Digital or DTS and multi-channel output via HDMI was not originally necessary for Freeview HD certification. Neither AAC nor Dolby Digital Plus codecs are supported by most home AV equipment, which typically accept Dolby Digital or DTS, leaving owners with stereo, rather than surround sound, output. In the case of BBC HD, the main audio is coded as AAC-LC and only the audio description is encoded as HE-AAC. Hardware restrictions allow only a single type of audio decoder to operate at any one time, so the main audio and the audio description must use the same encoding family for them to be successfully combined at the receiver. To ensure provision of audio description, broadcasters typically use the AAC codec.
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