It was announced on 10 February 2009, that the signal would be encoded with MPEG-4 AVC High Profile Level 4, which supports up to 1080i30/1080p30, so 1080p50 cannot be used. The system has been designed from the start to allow regional variations in the broadcast schedule. Services are statistically multiplexed bandwidth is dynamically allocated between channels, depending on the complexity of the images with the aim of maintaining a consistent quality, rather than a specific bit rate. Video for each channel can range between 3 Mbit/s and 17 Mbit/s. AAC or Dolby Digital Plus audio is transmitted at 384 kb/s for 5.1 surround sound, with stereo audio at 128–192 kbit/s; audio description takes up 64 kbit/s, subtitles 200 kbit/s and the data stream, for interactive applications 50 kbit/s. Recording sizes for Freeview HD television transmissions average around 3 GB per hour. Between 22 and 23 March 2011, an encoder software change allowed the Freeview version of BBC HD to automatically detect progressive material and change encoding mode appropriately, meaning the channel can switch to 1080p25. This was extended to all of the other Freeview HD channels in October 2011.