The Hitchhiker's Guide to Encoding: The Salmon of Style (Or how programmes styles can change your view)
The last area to talk about is programme style and the techniques used by producers. The HD Channel is expanding the range of programmes made and transmitted in high definition all the time. As the number of programmes grows we are bound to show some that don't appeal to everyone in the audience, just as any multi-genre channel would. Similarly some of the techniques and styles used will not match some viewers' expectations of what is or is not HD.
Posts have suggested we are changing the bit rate depending on the programme because individuals have seen something they really like the look of while others have suggested programmes are up-converted when they don't like the look. Neither is true. The bit rate has been constant since the change in August and the amount of up-conversion (or non-HD) allowed is still 25%. There is though a correlation between comments posted and some of the production techniques used to make HD programmes.
Techniques that cause the most debate are:
- Motion portrayal (using 25p or so called "Film Motion")
- Depth of field or focus
- Noise & Grain
- Softness & Smear
Motion Portrayal - Many programmes are shot using a camera's 25p option i.e. film motion. Drama and Natural History moved to HD from film and wanted to keep the same look. Technically there is more detail in a progressive image than the equivalent interlace image. I have mentioned this before in several posts.
Temporal resolution describes the motion of a sequence i.e. the duration of each frame in a moving sequence and therefore how far an object moves between frames.
Spatial resolution describes the detail of the image and is about the content of a frame.
The BBC R&D white paper WHP169: High Frame-Rate Television gives a very interesting insight on the relationship between the two.
At the moment though we leave the decision about motion type to the Producer and Director of Photography of the programme but there are times when we do comment on the inappropriate choice of 25i or 25p.
Caption: These two images are from the white paper. They are frame grabs of a moving train shot at 300fps (bottom image) and translated to 50fps (top image).
It is worth reading the paper if you want to know how the detail you can see in an image is very much dependant on the frame rate - unless there is no movement. However we are not talking 25 to 50fps to make the difference in clarity you see here it's more like 25 to 300fps!
Depth of Field - Drama productions often use focus to move the point of interest in a frame. Programmes such as Cranford and Wallander also use feature film style cameras with a very small or shallow depth of field which allows the point of focus to play a major role in the story.
There have been many comments about HD being pin sharp and some people believe an HD image should be in focus from the nose of the person in close up to the trees on the horizon. I think many people would find that incredibly distracting and (as some of the posts lead me to believe) people would just be looking at the image quality not the programme. Focus is a very useful programme-making tool, and when used well it adds to the look and feel of a programme.
- Depth of Field entry on Wikipedia
- Alan Roberts' book Circles of Confusion is a far more in-depth look at HD cameras and covers depth of field in Chapter 11.
Noise & Grain - as the range of programmes on the channel increases, we get programmes that are deliberately made to look "dirty" either by style of shoot or during post production. This technique is often used in the cinema too. However if overdone for television it will stretch the encoder and cause unpredictable quality changes. Programmes that do this will usually fail a technical review.
Softness & Smear - some programme makers do not want extremely sharp images and choose to soften the picture either with lens filters or in post production. Both series of Criminal Justice for example used softening to create a very distinctive look. However if this is overdone it will significantly reduce the image resolution and can increase the amount of noise in the image. We do try to limit the amount of lens filtering programmes use but if extreme image softening is required, we encourage people to it in post production so that if the end result is unsatisfactory at least we can ask for it to be removed.
Motion blur (or smearing) is another matter. It usually occurs when the camera shutter is not set appropriately. A programme shot at 25p should use a shutter speed around 1/50th sec (or 180˚). If the shutter is not turned on or is set too long, the images will smear. This doesn't look very nice and we do try and stop people doing it. If the producer wants to add motion blur, again it is always better to do it in post production. Doing it in camera is usually fatal!
It has been a marathon blog and has taken me a long time to write! But there was a lot to go through and a lot to check before I published. Over the last six days I have covered most of the technical issues raised in the four key picture quality blogs. I hope it has answered the questions you've asked and laid to rest some of the rumour.
Running through the sections day by day
I decided audio is always forgotten or left 'till last so I put it right at the front. I covered the recent audio issues we've had and some of the measures put in place to prevent them happening again if at all possible. More programmes are being delivered in surround sound now and I hope some of the recommendations due from the EBU next year will give people more confidence to try it out.
There is one thing that does crop up from time to time that I need you to watch (or should that be, listen) out for. Very occasionally the 5.1/2.0 switching gets stuck. It works very well for months then for no apparent reason sticks in 5.1. This has no effect on the audio but it's annoying for anyone using the switch information go to the Pro Logic option on an AV amplifier. Please keep letting me know if you spot it though and which programme is affected.
On Monday and Tuesday I looked at the history of HD encoding on the channel and the EBU recommendations covering programme making and transmission decision we made when the channel started. We review these decisions regularly and changes are made as and when the technology allows us to so.
Wednesday's and Thursday's posts covered how the tests are set up and the much requested PSNR and expert viewing tests results. It is fairly clear now that the new encoder is a lot more efficient than the old and more importantly, capable of many further upgrades as the technology continues to develop.
There is no doubt this series will cause much comment and raise more questions about image quality and encoding generally. I will try and answer as many as I can but in the end we may have to admit we will never be able to satisfy all of you when it comes to what is or isn't high definition.
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)
- Part 3: The Hitchhiker's Guide to Encoding: So Many Tests, and Thanks for All the Recommendations (Or the BBC and the EBU)
- Part 4: The Hitchhiker's Guide to Encoding: Mostly Testing (Or how to set up an encoder test)
- Part 5: The Hitchhiker's Guide to Encoding: And Another Test...(Or PSNR and all that...)