The Development of New 'Video Factory' Profiles for BBC iPlayer

The Video Factory Project is a BBC in-house programme designed to encode, package and deliver all iPlayer video content. The move to cloud based encoding on Video Factory gave us the opportunity to re-design and consolidate the number of video encoding profiles being used by BBC iPlayer. This blog details some of the work undertaken by BBC R&D in designing this new profile set and how these new profiles map through to end user devices.

The Video Factory Project is a BBC in-house programme designed to encode, package and deliver all iPlayer video content. The move to cloud based encoding on Video Factory gave us the opportunity to re-design and consolidate the number of video encoding profiles being used by BBC iPlayer. This blog details some of the work undertaken by BBC R&D in designing this new profile set and how these new profiles map through to end user devices.

The new profiles were designed to improve the picture quality available on devices without significantly increasing the associated bit-rate and therefore distribution cost. They were also designed to exploit the benefits of Adaptive Bit-Rate Streaming[1], with just noticeable changes in video resolution within an Adaptive Bit-Rate adaptation set.

A series of new profiles were defined to cover representations for all devices.They range from the low bit-rate (sub 64kbps), 192x108 at 6.25 frames per second (fps) through to a HD 1280x720 50fps encode. In addition a high bit-rate (8000kbps) 1920x1080, interlaced at 25fps, full HD encode is generated, but has not yet been made available. It can be enabled for TV devices in the future.

The profiles were designed to be encoded with a 3.84 second chunk size. This enables video and audio access units to be aligned for HTTP Live Streaming (HLS), where audio and video frames are multiplexed together within a single MPEG-2 transport stream, thereby helping some decoders have a clean switch between profiles. During subjective evaluation done within R&D, we identified that large screen devices such as TVs benefited more from higher frame rates than small screen devices such as tablets, where spatial resolution is of greater importance. In addition higher framerate 50Hz television streams still exhibit motion artefacts on most mobile, tablet and desktop devices due to their 60Hz screen refresh rate. So separate profile sets were developed for the two classes of devices.

To enable resolution, bit-rate and encoding optimisations to be made to the new encoding profiles, a test tape of representative content at varying encoding difficulties was produced by BBC R&D. This test tape was designed to test the encoder performance, using a range of content and included clips, from popular shows such as Strictly Come Dancing, East Enders and Top Gear.

The profiles were first evaluated using x264. x264 is an open source application and software library that can be used to encode video streams in the H.264/MPEG-4 AVC compression format. x264 was used as it gave us greater control over encoding parameters, compared to those available to us in commercially available encoders. The encoder will deinterlace the video source where required. The videos were chunked for parallel, 2-pass Variable Bit-rate (VBR) encoding. 

Encodes were then created using a commercial encoder from Elemental as used for the Video Factory project. The encoding optimisations and parameters for each profile were then validated against these new encodes by the commercial encoder from Elemental.

The following tables detail the different Video and Audio Profiles.

The audio that goes with a given video format depends on the device capabilities.

Once the details of the new Video Factory profiles had been confirmed, it was important to work out which of these new profiles to present to all the different device types. To do this we first needed to audit where the existing BBC iPlayer assets were being used and map the new equivalent profiles across to devices. We then made some key decisions around which particular devices would benefit from other additional new profiles, for example making the 1280x720 profile at 50fps available to tablets.

Main Profile gives around a 30% bit-rate saving over Baseline Profile but is not supported on some older mobile phones. So it was decided that both H.264 encoded Main and Baseline profiles should be made available at key bit-rates.

At around 3 Mbit/s a 960x540 profile at 50fps will be made available to Connected TVs and set top boxes. The Elemental encoders are used to convert the 1920x1080 interlaced content to 960x540 for progressive encoding at 50fps. Although the 960x540 profile has a reduced spatial resolution compared to the outgoing 1280x720 at 25fps, subjective assessments shows it delivers significantly better pictures on TV screens across a wide range of popular content (such as EastEnders and Top Gear) due to its higher frame rate. The 50fps, 1280x720 profile, however, will be available to those with 5Mbit/s broadband connections.

Additional lower bit-rate profiles will be made available to computers on wifi. This enables video playback to continue, even when an individual's available bit-rate is reduced by users sharing a connection. These profiles will also cater for similar bit-rate restrictions on public wifi connections.

The new profiles are currently being tested and rolled out across BBC iPlayer services. If you are watching on mobile, tablet or desktop, you are probably already getting the new profiles.

The video factory team have recently published a blog post detailing the updates to the creation and distribution for on demand video.

[1] Ted D. Monchamp, Adaptive Bit-Rate Streaming, Minimising end-user buffer times in real-time video delivery, Rivier Academic Journal, Vol 9, No. 2, 2013. http://www.rivier.edu/journal/ROAJ-Fall-2013/J789-Monchampbit-rate-streaming.pdf