BBC iPlayer to offer HD by Christmas

7th October 2008

 

A telecoms analyst has posted a blog covering a conference called "HD Online: Can it be a Commercial Reality?". The BBC's head of the iPlayer, Anthony Rose, and Andy Quested, the Principle Technologist in the BBC's Future Media & Technology department, gave presentations, which included the following about HD being delivered on the iPlayer:

 

"Unless I misunderstood his remarks during the Q&A panel later, I believe he said we should see some HD content on the iPlayer "this side of Christmas," probably encoded at around 4Mbps."

 

"The BBC is starting [an SDTV] trial of 1.5Mbps H.264 on the iPlayer to Virgin Media's 10,000 50Mbps trial customers in Ashford, Kent."

 

"The BBC's view is that the minimum threshold bitrate for HD is >3Mbps (though it's interesting to note that their "true" broadcast HD content goes out at 16Mbps or higher)"

 

Anthony Rose also said that he was interested in working with ISPs to look at technology-based solutions to ease pressure on the networks.

 

4 Mbps H.264 on the iPlayer won't be true HD

As mentioned above, the BBC HD channel on satellite is using a bit rate of 16 Mbps at the moment, and the BBC HD channel on satellite is encoded using the H.264 video codec, which will also be used for the HD content on the iPlayer, so a valid question to ask is whether 4 Mbps H.264 could be described as being "HD" considering that the bit rate is only a quarter of that used on satellite?

When it comes to comparing the picture quality of different formats that are using the same video codec, the best parameter to compare is the number of bits per pixel, which is shown in the following table for four different formats:

 

Format Video codec Video bit rate
Mbps		 Coding mode Video resolution Frame rate Pixels per second
Mpix		 Bits per pixel
BBC HD channel on satellite H.264 16.4 CBR 1440 x 1080 25 Hz 38.88 0.42
Planned "HD" content on the iPlayer with 50 Hz frame rate H.264 4 VBR 1280 x 720 50 Hz 46.08 0.09
Planned "HD" content on the iPlayer with 25 Hz frame rate H.264 4 VBR 1280 x 720 25 Hz 23.04 0.17
Current higher quality iPlayer SDTV streams H.264 0.7 CBR 640 x 360 25 Hz 5.76 0.12

 

The BBC will most likely use the 720p format (1280 x 720 pixels) in terms of resolution, but instead of using a frame rate of 50 Hz, which is the frame rate specified for the 720p format for use on the digital TV platforms, the BBC will most likely use a lower frame rate of 25 Hz, as this halves the number of pixels per second that need to be encoded — so this doubles the number of bits per pixel as well. So because the HD content on the iPlayer most likely won't fulfill the HD-ready requirements, it won't be "true HD".

But even if the BBC uses the 720p format with a frame rate of 25 Hz, the number of bits per pixel is still less than half that used on the BBC HD channel on satellite. However, the BBC HD channel is apparently encoded in CBR (constant bit rate) mode on satellite, whereas Anthony Rose has said that VBR (variable bit rate) coding will be implemented on the iPlayer within the next few months, so you would expect that VBR would be used for the HD content as well. The reason for using VBR coding is that video is inherently extremely "bursty", which means that the required bit rate can be very low at some points, for example when there's not much moving on-screen, whilst at other times there may be a lot changing on-screen so the bit rate requirement is very high. So by using VBR coding the actual video bit rate can be high when it needs to be, but it can also be low when there isn't much changing on-screen, so bits aren't wasted. Overall, VBR provides higher picture quality for a given average bit rate level, so as the BBC HD channel on satellite is using CBR and the HD content on the iPlayer is likely to use VBR, the iPlayer HD content could have a similarly sharp picture as on the BBC HD channel on satellite.

 


 
 

Comments

720p/25 is HD

By -jm-
15th November 2008, 2:31
720 x 1280 progressively scanned is true high definition as defined by the EBU. The definition does not depend on the frame rate and a fair amount of 720p is actually shot at 24 frames per second. So you're wrong in saying that 720p/25 won't be "true HD".
 
 

By Steve
15th November 2008, 9:08
The EBU's definition of the 720p HD format is that it uses a frame rate of 50 Hz. So just because some material is shot at 24 frames/second doesn't alter the fact that everything not shot at that frame rate won't be "true HD".
 
 

By 2Bdecided
3rd December 2008, 11:49
4Mbps H.264 1280x720p25 should look really good. Vimeo currently uses 1.4Mbps, while YouTube has a 2Mbps test, both at the same resolution - so the BBC's quality will beat these services.

The main issue will be having a PC fast enough to decode the stream - many people with older computers might be in for a shock.

1280x720p50 is a none starter for a mass market PC based service, firstly because _many_ PCs can't decode this in real time, and secondly because almost all PCs have monitors running at 60Hz, 75Hz, or some other non-multiple of 50Hz. The whole point of using 50fps instead of 25fps is to get smoother motion, but this is partly destroyed by bumping it up to a non-integer multiple for display on typical PCs.

So, in summary, most of the potential advantage of 50fps over 25fps is lost on a typical PC, it would require almost double the datarate to maintain the same perceived quality, and many PCs would choke on it - the BBC would be absolutely right to stick with 25fps for HD on a PC at this time.

They _might_ want to consider a 50fps _option_ for the standard (download) service, at least for those programmes broadcast in 50i which would benefit from smooth motion.
 
 

An idea

By Tony
5th January 2009, 10:52
Maybe the BBC should be trying to work with Microsoft and Sony to provide their iPlayer HD content through the XBox360 and PS3. There are already more than 15m next gen consoles in UK homes with more than enough capability of decoding 4Mbps H.264 1280x720p 25Hz. The vast majority of these would be connected to HD Ready TVs. Xboxes as also dirt cheap at the moment.
 
 

An Idea reply

By Bob
7th February 2009, 10:52
You can already get the BBC iPlayer on the xbox.

Use either the media player/zune player as a streamer to your xbox or use the media centre extender if your version of windows supports it.

If your not using a PC with Windows installed there isn't much luck outside 3rd party apps.

I have been using the above media centre method for over a year now and it works great.
 
 

 

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