BBC Experiments With 1080p Broadcasts

A software encoder upgrade for BBC HD on the Freeview HD platform has led to intermittent audio dropouts on some Sony Bravia televisions and ssue appears to be caused by transitions between 1080p and 1080i high-def broadcasts on BBC HD, and affects Sony’s 2010 and 2011 HDTV models………..


The BBC has introduced dynamic picture switching to improve picture quality during programme broadcasts, this has unexpectedly created a problem with an audio dropout, on the 2010 and 2011 Sony Bravia TVs and taking advantage of a part of the Freeview HD spec that allows the broadcaster to dynamically select between 1080p / 25fps and 1080i / 25fps transmission and the boxes are mandated to be able to output 1080p / 50fps. The reasoning for the decision is to provide better picture quality on material that was shot that way and unfortunately some HDTVs are having trouble handling the switch and causing an audio dropout when the format shifts. The Freeview HD platform has always been specified to provide a 1080p25 capability.  This is why STBs are mandated to have a 1080p50 output to enable seamless up-conversion to a mode which is guaranteed to be present in all display devices.  A great deal of material is shot natively at 1080p25 and there are significant advantages in maintaining 1080p25 through to the viewer’s display.  Within a single programme interlaced may be used for moving credits, cross-fades and studio shots whereas  progressive may be used for location shot material. Up until now this capability has not been exploited and 1080p25 material was encoded using a 1080i25 encoding mode.  But after a software upgrade on the 22nd/23rd March the HD encoder supporting BBC HD on Freeview HD has been set-up to automatically detect progressive material and change encoding mode appropriately. The encoded bitstream can only  change at each GOP boundary to ensure decoders maintain a consistent display. This means that each coded video sequence either contains interlaced or progressive pic_struct values within the bitstream. The transitions between interlaced and progressive modes are entirely dependent upon how a programme has been made.


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