BBC iPlayer TV streams now available at higher quality

3rd September 2008

 

The BBC has added higher quality versions of the BBC iPlayer TV streams, giving viewers the option of watching in 'normal' or 'higher' quality. The new higher quality iPlayer TV streams are using higher bit rate levels and newer video and audio codecs than the normal quality versions — the normal quality streams use a bit rate of 500 kbps, with the video using the On2 VP6 codec (which is the video codec that YouTube uses), and the audio uses 128 kbps MP3. The new higher quality streams, however, are using a bit rate of 800 kbps, and they're using the new MPEG-4 H.264 video codec and the audio is using 96 kbps AAC+ — AAC+ is the audio codec that's been adopted for the new DAB+ system.

H.264 is the video codec that's being used for HDTV channels on digital satellite, and the HDTV channels that will launch on Freeview in regions that have completed digital switchover will use H.264 as well — the BBC HD channel is assured a place on Freeview, and ITV and Channel 4 have recently applied to Ofcom for capacity to allow their HD channels to be launched on Freeview as well. H.264 is also the video codec that will eventually replace MPEG-2 for standard-definition TV (SDTV) channels on satellite, cable and Freeview, and Sky has applied to Ofcom for permission to launch a mini pay-TV package on the Freeview platform, and that will use H.264 if it does indeed see the light of day.

However, as well as it being suited to the higher bit rate levels used for HD and SDTV, H.264 was designed to perform well right down to the very low bit rate levels that are used for video that's viewed on very small screens, such as mobile TV channels, and the latest versions of a few mobile phones and MP3 players support H.264 video too — most notably, Apple's iPhone, iTouch and the latest versions of the iPod, and I have to say that I was very impressed with the quality of H.264-encoded video when I tried it on the latest version of the iPod nano recently.

H.264 is typically quoted as being about twice as efficient as MPEG-2, which means that H.264 can produce the same level of picture quality at half the bit rate that MPEG-2 requires. However, MPEG-2 becomes less efficient at lower bit rate levels, so the 700 kbps or so bit rate for the video on the higher quality iPlayer TV streams (i.e. 800 kbps minus the 96 kbps for the AAC+ audio) would probably work out to be the equivalent of say 1.6 - 1.7 Mbps if MPEG-2 were used. This would put the quality of the new iPlayer TV streams to be a bit lower than on SDTV channels on Freeview, satellite and cable, where the lowest bit rates tend to be around 2 Mbps per channel — BBC1 uses bit rates of about 4.5 Mbps, though, so the bit rates do vary a lot.

However, iPlayer TV streams tend be viewed on computer moniters, which typically have a screen size of around 19", whereas normal TV is viewed on TVs that typically have a screen size of 26" or larger, and the bit rate required to provide a constant level of quality is dependent on the screen size you're watching it on, so the new higher quality iPlayer TV streams should have a picture quality that's roughly similar to that of the lowest bit rate TV channels on Freeview, satellite and cable, and from the couple of higher quality iPlayer TV streams I've watched so far, that's roughtly how I'd describe them — it'll also depend on the quality of the computer monitor and your video card on your computer, though, as well as on the quality of your TV set.

The BBC has said that it's planning on moving to automatically detecting how fast a user's Internet connection is, then the most appropriate stream will be delivered based on this information. I can't say that I like the idea of a computer deciding for me what quality stream I want to watch, so I'd prefer it if they provided Auto, Normal and High options so that people could at least manually choose which stream to watch.

The iPlayer TV streams are also using 96 kbps AAC+ for the audio, as opposed to 128 kbps MP3 being used for the standard quality iPlayer TV streams. Anthony Rose, who's in charge of the BBC iPlayer, said that the new AAC+ audio streams are at higher quality than the MP3 streams, which they are. However, the MP3 streams on the 'normal quality' iPlayer TV streams are encoded in "dual channel" mode, and they would be at significantly better quality if they were encoded using joint stereo instead.

 


 

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