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Technical: Freeview HD multiplex pushed to the limit

TECHNICAL  The encoders on the BBC-run multiplex that carries the Freeview HD service has tonight been put to the test, with 3D programming from BBC HD going head to head with live Paralympic coverage on Channel 4 HD and C4 PG Extra.

Tonight, 3D coverage of the "Last Night of the Proms" has aired on the Freeview HD multiplex without another channel on the multiplex being temporarily taken off air, as has happened during previous 3D events when only four HD channels have been active.

All five services are being transmitted in 1920x1080 resolution.

Sample snapshot of bitrates during the Last Night of the Proms coverage. Larger figures are video bitrates.

With 40 Mbps of capacity, services on the HD multiplex are "statmuxed". This involves variable bitrates for the services. If one service is showing fast moving images which requires more bandwidth, but another is showing a programme with plenty of static images which needs less bandwidth, bitrates are adjusted accordingly. When more than one channel is showing the same content, or the content is deemed to be special enough to be broadcast at high quality, services have been adjusted on Freeview to improve the bitrate and picture quality. Traditionally, fast moving programming featuring lots of colours and lights (e.g. X Factor final or the Olympic Opening Ceremony) need special treatment so as not to suffer from macro blocking (lots of lego-like blocks in the picture. While X Factor finals are not deemed worthy enough to enjoy bandwidth increases, major national events such as the Olympics Opening Ceremony have benefitted from other services going into standby to free up more bandwidth.

Despite new encoders being used on the Freeview HD multiplex, so far this year, the BBC has been cautious not to overload the HD multiplex, with services having been closed down (i.e. reduced to static captions) for the duration of some key events, such as the Queen's Diamond Jubilee celebration and the Olympic Opening Ceremony. Even during the Paralympics Opening Ceremony, a static caption was shown on the fifth HD stream, preserving more capacity for other channels to use.

The BBC has generally provided its viewers with services at much higher bitrates than commercial providers. Under pressure not to be seen as being "wasteful" with its available broadcast capacity (both on terrestrial and satellite), and without being able to justify resources to purchase additional capacity, the BBC has of late been forced to start reducing bandwidth. Notably in Scotland, BBC Radio is taken off air every evening to make way for BBC Alba. As all SD services moved to one Freeview multiplex, some services have been cut or transmit in 3/4 resolution on Freeview (e.g. BBC Parliament). Now on HD, the chance to boost bandwidth during special events is coming to an end. Channel 5's decision not to accept the fifth slot on the Freeview HD multiplex gave the BBC a reprieve for a few months, allowing it to add a fifth stream to use for itself during the Olympics, and to switch on/off during major events. Channel 4's current occupancy of the slot gives the BBC's technicians a chance to see what a fully loaded Freeview HD multiplex looks like during nights like tonight and will no doubt be looking out for responses from viewers to see if they have perceived a worsening of picture quality during the evening. For the first time ever, it can't shut down a service to provide a 3D service better quality. This will become the norm, when the fifth slot on the HD multiplex is used on a permanent basis by another channel. Analysis of the usage of the multiplex tonight by the five channels may well provide the answer as to whether channels on the multiplex should stay at 1920x1080, or be reduced back down to 1440x1080.


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