The Hitchhiker's Guide to Encoding: So Many Tests, and Thanks for All the Recommendations (Or the BBC and the EBU)
TuesdayMany posts have mentioned EBU documents and recommendations. The BBC is a founder and very active member of the EBU. We take part in every aspect of the EBU's technical activities. I have been a group chair and currently lead the camera work of the P-HDTV group. We not only test using EBU standards, we were part of the groups that wrote the standards and recommendations in the first place.
BBC Research and Development is one of a diminishing number of European laboratories with the expertise and facilities to carry out testing for, and make contributions to the EBU's technical committee. I am extremely proud to work with the experts in BBC R&D and for the opportunity to contribute to the work they do.
Many of you have mentioned our current frame size and said:
"1440 Horizontal pixels is not HD"
To answer this I can point to several EBU documents that cover this point:
EBU - TECH 3328 Current Status of High Definition Television Delivery Technology (May 2008)
...In addition, the Sony HD-CAM and Panasonic DVCPROHD formats record only 1440 samples per line (with camera scanning at 1920 x 1080i/25). There is arguably no point in broadcasting material derived from this format at more than 1440 samples per line (although on the HD-SDI interfaces, a 1920x1080i/25 signal is carried).
A similar situation exists with the DVCPROHD format that horizontally sub samples (down-filters) the 1280 x 720p/50 format to 960 x 720p/50 (though on the HD-SDI interface, the signal is 1280x720p/50).
This document mentions 1440 and 960 as used in production. The HDCam tape format and some cameras that record to the DVCPro100 format use pre-filtering to reduce horizontal resolution before the signal is compressed. These cameras and formats are still widely used but are gradually being replaced by cameras and recorders that can compress the full 1920 or 1280 horizontal resolution.
EBU - TECH 3333 EBU HDTV Receiver Requirements (March 2009)
The following image sampling structures shall be supported (see TS 101 154 V1.9.1, which defines further formats beyond those listed here).
1920 x 1080, interlaced, 25 frame/s (50 fields)
1920 x 1080, progressive, 25 frame/s
1440 x 1080, interlaced, 25 frame/s (50 fields)
1440 x 1080, progressive, 25 frame/s
1280 x 1080, interlaced, 25 frame/s (50 fields)
1280 x 1080, progressive, 25 frame/s
1280 x 720, progressive, 50 frame/s
1280 x 720, progressive, 25 frame/s
Tech 3333 is about receivers and shows 1440x1080 is an acceptable standard for high definition transmission.
It is worth pointing out that HD-Ready does not actually specify the horizontal resolution for an HD display. HD displays must have a minimum vertical resolution of 720 pixels and display 16:9 images correctly. There is no mention of the required number of horizontal pixels!
Several other posts have suggested the BBC is not meeting EBU standards for transmission with the current bit rate and have said:
"9.5Mbs is not in line with the EBU recommended high definition bit rates"
Again there are several EBU documents covering this point but the key thing to take account of is the date of the references. This document for example:
EBU Tech 3334 Accommodation of HDTV in the GE06 Plan (Feb 2009)
EBU tests of stand-alone MPEG-4 encoders of different vendors have suggested  the following minimum fixed bitrates in order achieve an HDTV image quality providing a significantly better quality perception compared to good quality SDTV (e.g. 6 Mbit/s MPEG-2) for a wide range, including critical content:
The key here is the  after "suggested". Reading the bibliography, point  refers to:
EBU Tech 3328 (Current Status of High Definition Television Delivery Technology)
Tech 3328 is dated May 2008, and was published long before we started tests on the new encoders for the BBC HD Channel.
Bit rates are also mentioned in an EBU presentation by Adi Kouadio (Asian Broadcasting Union symposium in March 2009).
Trends & implementations of HDTV Broadcasting
...Minimum (video) bit rate to provide HD quality (from EBU tests - BPN085-087):
EBU test documents BPN085 to BPN087 detail tests carried out on specific encoders. These tests were undertaken over the last two years but none refer to the encoder we are currently using.
Tests have demonstrated that at transmission bit rates, H264 encoders should deliver approximately a 2:1 efficiency over MPEG2 encoders. Depending on the manufacturer and their current stage of development good HD at 8-10Mbs is achievable now.
The BBC has made contributions to most of the documents mentioned above and many others besides. We are also involved in testing and trials for high definition production, contribution and transmission compression and were one of the earliest activists in the latest round of HD activity in the EBU.
The majority of the documents produced by the EBU are Technical Recommendations and are based on work done at a specific point in time by the members. After publication many groups continue to work on revisions and updates to the recommendations as the technology behind the encoders (in this case) continues to develop and improve. EBU Tech 3334 acknowledges that:
...with the expected future developments in video coding, it is assumed that HD fixed bit rate requirements will be reduced to 8-10 Mbit/s per programme. There will also be advances in the transmission system such as DVB-T2...
For some strange reason the encoder manufacturers don't all bring their latest offering to market at the same time! Two or three times a year they do show off concepts or the next generation prototypes at the main trade shows but products to the broadcasters arrive as and when they are stable and ready. This year for example, several manufactures were showing early versions of HD coding at 4-6Mbs (average not minimum).
One final point worth noting here is the life cycle of the hardware. As I mentioned earlier, the old encoders had reached the end of their life but the new encoders are at the beginning. They exploit more of the AVC toolset than the old encoders even though we are on version 1 of the firmware. We will continue to add improvements as and when they are available.
Tomorrow I want to concentrate on picture quality analysis and how we set up the encoder tests.
Andy Quested is Principal Technologist, HD, BBC Future Media and Technology.
- Read part 1 of Andy Quested's HD guide: The Hitchhiker's Guide to Encoding: Before we start
- Read part 2: The Hitchhiker's Guide to Encoding: Life, Encoders and Everything (Or a brief history of HD encoding)