Choice of HDTV Compression Algorithm and Bitrate for Acquisition, Production & Distribution
The recommendations given in this document are based on the tests of production HDTV studio compression codecs and HDTV broadcast encoders performed by the EBU during the second half of 2007.
It should be noted that all the encoders tested are in a state of evolution and therefore this document is something of a snapshot of the situation in 2007/2008. Further enhancements in hardware and software can be expected.
The EBU has not performed comparative tests of different vendors’ products, but some general advice, independent of product types, can be formulated as a result of the investigations. This document relates to “mainstream” HDTV production and not “high-end” HDTV production.
For acquisition of mainstream HD material, it is recommended that:
- The acquisition format should use 4:2:2 sampling.
- No further horizontal or vertical sub-sampling should be applied.
- 8-bit bit-depth is sufficient for mainstream programme (10-bit bit-depth is preferred for high-end acquisition).
For production of mainstream HD, the EBU has found no reason to relax the requirement placed on SDTV studio codecs that ‘Quasi-transparent quality’ must be maintained after 7 cycles of encoding and recoding with horizontal and vertical pixel-shifts applied.
It is recommended that:
- If the production/archiving format is to be based on I-frames only, the bitrate should not be less than 100 Mbit/s.
- If the production/archiving format is to be based on long-GOP MPEG-2, the bitrate should not be less than 50 Mbit/s.
Users should be aware that maintaining the above criteria would ensure quasi-transparent quality up to at least 4 to 5 multi-generations.
Expert viewing tests have further revealed that:
- A 10-bit bit-depth in production is only significant for post-production with graphics, and after transmission encoding and decoding at the consumer if the content, e.g. graphics or animation, has been generated using advanced colour grading, etc.
- For normal moving pictures, an 8-bit bit-depth in production will not significantly degrade the HD picture quality at the consumer’s premises.
For distribution, the EBU has already stated that the HD picture format for HD distribution should be 720p/50 with H.264/AVC encoding. Recent EBU tests have confirmed this recommendation.
It was found that:
- H.264/AVC requires up to 50% less bitrate than MPEG-2 (content dependent).
- Interlaced image format (1080i/25) requires about 20% more bit-rate than the progressive image format (720p/50) to obtain the same subjective image quality.
- Statistical multiplexing between HD and/or SD data streams for video is recommended.
Source: EBU Recommendation R124-2008
