Non-linear Opto-Electrical Transfer Functions for High Dynamic Range Television

Modern video displays provide improved performance compared to, now obsolete, cathode ray tube (CRT) displays. In particular they are now starting to provide brighter images and deeper blacks. In other words they are starting to provide significantly higher dynamic range than CRT displays. A television system that can take advantage of this would be able to offer improved pictures that are more realistic and have greater impact. However, this cannot easily be achieved whilst retaining the conventional non-linear transfer functions, known as gamma curves, which were designed for CRT displays. Instead enhanced “gamma” curves are needed in both cameras and displays to support improved picture quality. Ideally an enhanced gamma curve should be simple, broadly compatible with the existing conventional gamma curves, and based on relative, not absolute, brightness. Such enhanced gamma curves would minimise the change to both existing equipment and television production process, thereby simplifying and minimising the cost of a transition to high dynamic range video. This paper discusses the background to the conventional non-linear gamma curves used for video. Based on this discussion it suggests a new, simple, and compatible, gamma curve to support the production, exchange and end user distribution of high dynamic range video. For end user distribution this proposal would increase the dynamic range by about a factor of 50. This increase would likely be sufficient for domestic use for the foreseeable future. This suggestion was proposed to ITU (International Telecommunication Union) in March 2014. The proposal to the ITU highlighted the inconsistencies between existing standards for gamma curves in cameras and displays. So this paper goes on to reformulate the ITU proposal in a simpler, but essentially equivalent, way that removes these inconsistencies. Additional key words: OETF, EOTF, HDR, UHDTV ______________________________________________________________________________ a The proposal was submitted to the International Telecommunication Union, the United Nations specialized agency for information and communication technologies, Study Group 6 (Broadcasting Service), Working Party 6C (Programme production and quality assessment). © BBC 2014. All rights reserved. Except as provided below, no part of this document may be reproduced in any material form (including photocopying or storing it in any medium by electronic means) without the prior written permission of BBC except in accordance with the provisions of the (UK) Copyright, Designs and Patents Act 1988. The BBC grants permission to individuals and organisations to make copies of the entire document (including this copyright notice) for their own internal use. No copies of this document may be published, distributed or made available to third parties whether by paper, electronic or other means without the BBC's prior written permission. Where necessary, third parties should be directed to the relevant page on BBC's website at http://www.bbc.co.uk/rd/pubs/whp for a copy of this document. White Papers are distributed freely on request. Authorisation of the Chief Scientist or General Manager is required for publication.