Posted by Simon Thompson on , last updated
Staff from BBC Research and Development recently attended a workshop at the Institut für Rundfunktechnik organised jointly by the European Broadcasting Union (EBU) and the Digital Video Broadcasting (DVB) consortium. The theme of the workshop was High Dynamic Range (HDR) video. HDR isn't part of Ultra High Definition Television (UHD TV) specifications as currently defined by DVB, but it is potentially one of the key components of a future UHD TV phase.
BBC R&D were invited to present and demonstrate our proposed method for the transmission of HDR video which, uniquely amongst the proposals to the International Telecommunications Union (ITU), still presents a usable image to viewers using receiving equipment that does not have an HDR compatibility. We were able to demonstrate a system which included showing our proposed solution on professional and consumer HDR monitors with the same signal being displayed on a standard Rec.709 (PDF) professional monitor.
What is Higher Dynamic Range video?
The dynamic range of a video signal is the range between the lowest light level represented (typically the system noise floor) and the highest light level represented. Readers who have used digital cameras will be aware that often a choice has to be made about which areas of an image are represented correctly and which lose details in the dark or bright areas. HDR video aims to correctly expose the subject whilst maintaining details at both ends of the light level spectrum.
HDR is a very attractive system for broadcasters to implement as initial tests show it creates an increase in subjective quality for viewers independent of screen size and viewing distance (compare this with subjective quality results for the move to 3840x2160 for why broadcasters are quietly excited. You can find further information on this discussion in the EBU tech-i).
Why this isn't the same as High Dynamic Range Photography
The HDR techniques described at the DVB/EBU Workshop differ from the technique currently used in stills photography in two ways:
- Acquisition. Broadcast video cameras usually have much lower numbers of picture elements (pixels) than a stills camera whilst having a similar sensor size. The physical size of each pixel is therefore larger leading to a greater number of photons landing on each pixel. This results in a larger signal and better signal-to-noise ratio. Similarly, the relatively strong signals created by shadows in the scene means detail is maintained in areas where in stills cameras excessive noise would be introduced. To overcome these shortcomings, stills photographers traditionally take multiple exposures of a scene and then blend the information together into an HDR image file. This can lead to motion artefacts if used for video.
- Rendering. Currently in stills photography, due to limitations in print and screen technologies, it is usual to utlilse a "tone-mapping" algorithm to create a lower dynamic range representation of an HDR image suitable for printing or viewing on a PC screen. These tone-mapping algorithms can lead to severe distortion and poor representation of natural scenes. For HDR video, the intent is to utilise a high brightness, high contrast ratio screen to show the HDR content, reducing the need for tone-mapping. As such screens become more widely available, the author believes the use of tone-mapping in photography may also reduce significantly.
The workshop was well attended and proved to be very useful as it allowed the community working on HDR video to discuss:
- The human visual system and its response to static and dynamic changes in image dynamic range,
- The proposals currently under discussion in the ITU on how to convey this information,
- Current camera technologies,
- The effects of including HDR in acquisition, production and emission systems,
- The current state of play in the various standards bodies - ITU, The Society of Motion Picture and Television Engineers (SMPTE), DVB, EBU, Joint Collaborative Team on Video Coding (JCT-VC), and
- HDR in the consumer electronics industry.
The atmosphere of the workshop was very positive. Lots of interesting presentations and discussions occurred and it allowed everyone present to compare and contrast the proposals.
Our belief is that further work is required amongst the various standards groups to:
- identify a suitable peak brightness and suitable dynamic range to maximise the increase in subjective quality without causing physical discomfort or requiring long adaptation periods,
- identify which proposal is best suited to television including live production with mixes, fades and Digital Video Effects (DVE) image shifts, and
- write a final end-to-end specification for the delivery of HDR video.
UHD Production Training and Skills from the BBC Academy including:
This post is part of the Broadcast and Connected Systems section