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We’ve been dying to know what broadcast format the BBC is planning to use for its HD broadcasts, which start in December. We know the service will be broadcast in MPEG-4 using a DVB-T2 transmission system, but until now, the Big British Castle hasn’t been talking about the video size — would we be getting 1080i or 720p? The first good news is there are currently no plans for any 720p material. But the BBC shocked us by confirming 1080p is a distinct possibility.
If we were to get 1080p on Freeview HD, it would be 1080p25. That means there would be 25, 1,920×1080-pixel frames per second. The BBC explains, “1080p25 uses less capacity than 1080i25 and less capacity than 1080p50.” This is because compressing interlaced material is much harder than compressing progressively scanned video. For video-derived material 1080p50 would be more appropriate, but would use much more bandwidth, which Freeview doesn’t have.
We asked the BBC if it would show films in 1080p24, or 24p as it’s often called. Graham Plumb, Acting Controller BBC Distribution, told us, “There is no 24p in the Freeview HD system. It’s not practical to switch between 25p and 24p on programme boundaries as this would cause displays to get upset.” A frame-rate sync can cause problems for TVs, especially those that don’t support the standard. But we would expect the same would be true of switching between 1080i and 1080p. Movies will therefore be shown, as they always have been in the UK, with a 4 per cent speed-up, making 24fps into 25fps.
The news that 1080p could be available via Freeview HD is really rather exciting. For movies and high-quality drama it’s fantastic, because that material is well suited to 1080p25. Studio and video-based material will use 1080i25, which is better for fast-moving action, although it does require that the TV receiving it does a good job with de-interlacing the signal.
The BBC has ruled out 720p completely, which we’re pleased about too. Auntie also tells us the Freeview HD boxes will upscale to 1080p50, which will suit people with TVs that don’t do de-interlacing well, assuming the hardware in the Freeview box is up to the job.
Update: The BBC has been in touch to point out that its Freeview HD service will have an image size of 1,440×1,080 pixels, not 1,920×1,080. This is how it broadcasts its 1080i signal on freesat, so it’s natural that it’s continuing to use this format.
