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BBC multiplex goes down to five active TV services to protect Opening Ceremony picture quality

UPDATE  The BBC's standard definition Freeview multiplex was reduced to transmitting only five active video streams across most of England and all of Wales during the Opening Ceremony in order to preserve good picture quality on BBC One. In Scotland, there were just 6 active video streams using the multiplex's bandwidth.

Fifteen minutes into the Opening Ceremony, video bitrates of between 5 and 6Mbps was observed on BBC One's standard definition service on Freeview. Meanwhile, the BBC News Channel was enjoying video bitrates of around 3.5 Mbps.

On Freeview HD, BBC One HD's videostream was  getting up to 8Mbps of capacity. Later in the evening, both BBC HD (showing 3D coverage) and BBC One HD saw Freeview HD bitrates rise further, with the video element of the bitrate reaching 10Mpbs and beyond for short bursts, benefitting at times from either ITV and C4 not showing native HD content, e.g. ITV News. Around midnight, all BBC and both ITV and C4 services were transmitting HD content and there was a squeeze on demand. 301 HD, which was showing a caption, saw its small bitrate allocation being reduced further.

As reported here first, last Friday, the BBC has had to cut the picture from BBC Red Button 301 and remove the "no commentary" option on Freeview, due to what BBC Director of London 2012 referred to as a "quirk" of digital TV. Simulcasting the same event on BBC One, BBC News and BBC Red Button (and for a while during the BBC Two news at 10:00pm) will require more bandwidth than usual, as services usually get their bitrates adjusted based on the material they show.

When several services sharing the same bandwidth start showing fast moving and detailed material, they will all require more bandwidth at the same time, squeezing the bitrates of other services. With little extra bandwidth available, the channels would have started to go blocky under bitrate starvation.

Under these circumstances, it would not have been possible to maintain a reasonable picture quality for the showpiece event, which is was watched by millions across the UK.

So BBC Red Button 301 in standard definition was transmitting the "audio description" Opening Ceremony commentary as an audio only service, in a simulcast with BBC Radio 5 Live Olympics Extra. Whilst technically still a videostream, its static caption means it was not an active stream, and was not taking up much bandwidth.

Meanwhile, newly relaunched BBC Red Button 302 had been put into hibernation for the night, but is now up and running again.

The following video (TV) streams were active on the BBC-A Freeview multiplex during the Opening Ceremony:
BBC Three
BBC Four (reduced resolution)
BBC News (reduced resolution)

In Scotland, BBC Alba was on air until midnight.

This is not the first time this year that services have been "shut" to protect BBC One picture quality. During the Jubilee celebrations, BBC HD's usual Preview was cancelled, as the channel was sent into hibernation, reducing active services on the Freeview HD multiplex to just three.


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