This a recurring question that I get asked about time and time again. The main problem being that the SD pictures, shot with an HD camera look soft. So why is this and what can be done about it?
Well there are several issues to look at. First there is camera optimisation. Sadly what works for HD doesn’t always work well for SD. Secondly there is the downconversion process. If your shooting HD and simply outputting SD using the cameras built in downconverter than you really don’t have many options but if your using a software downconverter you may be able to improve the results your getting.
Starting with the camera, what can you do? Well first off let me say that a camera optimised for HD will always be a compromise when it comes to SD. As the native resolution of HD cameras increases then the problem of getting good looking SD actually gets worse. The problem is that a good high resolution camera will normally only have a very small amount of artificial sharpening via the detail or aperture circuits, because in HD it will look nice and sharp anyway. SD cameras and the SD TV system with it’s inherently low resolution and soft pictures has always relied very heavily on detail enhancement to try and make the pictures appear sharper than they really are. When you take the minimal additional sharpening of an HD camera and downconvert it to SD it all but disappears, the end result is a soft looking picture. There is no easy fix for this, you can either add additional extra thick detail correction edges to the HD pictures, which risks spoiling the HD image or you can add additional detail correction in post production. On a Sony camera the thickness of the detail correction edges is controlled using the “frequency” setting. Setting this to a negative number will thicken up the detail edges, very often you need to go all the way to -99 to get an appreciable difference. As an alternative you can add extra sharpening or detail correction in post, after the downconversion process. This is the way I would go whenever possible as I don’t want to compromise my HD pictures for the sake of the SD images.
The second issue is the quality of the downconversion. A simple rescale from HD to SD rarely works well as it can create a lot of aliasing. Aliasing is the result of taking too much detail and trying to record or represent it with too few pixels. See this article for more on aliasing. Imagine a diagonal line running through your image.
If you sample it at a high resolution, with your HD camera then the line looks reasonably good as you can see in the diagram to the left.
If you then take that HD captured edge and simply scale it down to SD, you quarter the number of samples and the end result is a jagged, stepped line. Not pretty. In addition, if the line moves through the image it will flicker and “buzz”. This is far from ideal.
A better approach is to blur the HD image before down converting using a 4 pixel (or similar) blur, or to use a downconversion programme that will include smoothing during the conversion. The final image shows the kind of improvement that can be gained by softening the image before down conversion. The blur around the edges of the line soften it and make it appear less jagged. This will result in a much more pleasing SD image. Next you then add in some detail correction to restore the apparent sharpness of the image and viola! A decent looking SD image from an HD source. In compressor to get a good downconversion you need to activate the advanced scale tools and use the “better” or “best” scaling options.
Ever since the release of the XDCAM EX cameras users have been having problems getting good looking SD pictures out of downconverted HD.?Why is this and what can be done about it? This is an issue that effects all high resolution HD cameras and is not unique to the EX’s. There are two key issues. The first is the way basic software converters handle fields in interlace material and the second is the amount of information in an HD image that must in effect be discarded to get a SD image.?At first glance you would think that starting off with lots of picture detail would be a good thing, but in this case it’s not. Let’s see if I can explain.?Imagine that you have something in you HD picture that over 4 pixels goes from light to dark, in Hd you get a gradual transition from light to dark and all looks good. Now what happens when you take those 4 pixels and convert them to SD. The 4 pixels become just 2 and instead of a stepped change from light to dark the picture now goes instantly from a light pixel to a dark pixel. If these pixels were the edge of a moving object, as it moved the pixels would be switching instantly from on to off and unless the object moved at exactly one pixel per frame you will get a flickering effect. Clearly our nice gradual transition from light to dark has been lost and if there is any motion we may now be seeing flickering edges. Niether of these look good.
Take a look at these images:
As you can see the down converted SD is very blocky and there is some strange patterning (aliasing) going on amongst the bricks of the houses in the background. This does not look good and if there was motion the brickwork would shimmer and flicker.
So what can be done?
Well the best way to improve the SD down conversion is to soften the HD image before it is down converted to prevent this single pixel light to dark switch from happening. You need to end up with an SD image where you go from full light to full dark over at least 3 pixels to prevent flicker (Twitter).
How much you will need to soften you HD by will depend on how sharp it is to start with. Simply turning down the cameras detail settings can be a big help, but even then the best results are often obtained by applying some kind of blur filter in post production. In FCP I find the flicker filter works quite well. As you can see from the frame grab below the difference in the quality of the downconvert is quite striking.
I have also found that another problem is that the detail settings on an HD camera are not optimised for SD. The detail correction edges created in HD are very thin and when these are down converted to SD they all but disappear and can cause further aliasing. The solution is to make the detail correction edges thicker (on an EX turn detail frequency down to -60 to -99) but this then looks ugly in HD. The bottom line is that a camera optimised for HD works best in HD and SD will be a compromise.
Most Recent Posts
- Weifeng WF717 Tripod Review, Cheap as chips camera support.
- NEX-FS700 Workshop Helsinki, Finland on 10th of October (Wednesday)
- Maximum Dynamic Range Picture Profile for the NEX-FS700
- NEX-EA50 Review. Great Value For The Money.
- PMW-200 Picture Profile. AC-Asia1
- Sony NEX-EA50 First Tests: Really rather good!
- Litepanels LED lighting Patent Upheld, ITC Bans Import of LED Video Lights In to USA!
- Bangkok, Thailand, FS100 and FS700 workshops. October 3rd 2012
- A to Z Index
- Alister's Bio and Info
- Alister's Blog
- Alister’s Bio and Info
- Alister’s Blog
- Camera Setup
- Contact Me
- Documentary Production Workshops
- Example Video Clips
- How To Videos
- Northern Lights Live 2012
- Once in a Lifetime Northern lights Expeditions to Norway.
- Picture Profile Guide.
- Adaptimax Lens Adapters for PMW-F3
- Alister’s Equipment Reviews A to Z.
- Alphatron EVF-035W Viewfinder Video Review.
- Atomos Samurai
- Camrade cb-single-iii camera bag.
- CamRade WS PMW F3 Rain Cover Review.
- Cineroid HDSDI EVF (EVF-4MSS) review.
- Juice Designs EX1R Baseplate.
- Manfrotto 509 Tripod Head
- NEX-FS700 In Depth Review.
- Review of the Sony PMW-200
- Sonnet QIO Review. Very, Very, Fast Offloads.
- Sonnet SDHC to SxS Camera Adapter.
- Sony FS100 and F3 Video Review
- Sony NEX-EA50 Review.
- Sony PVM-740 OLED monitor
- Tilta BS-TS03 Shoulder Mount Review
- Today3D FIZ controller. Focus, Zoom, Iris, Interaxial.
- Transvideo PMW-F3 replacement base assembly.
- Triad PL to E-Mount adapter.
- TVLogic WFM-056WP Monitor/Viewfinder
- Vinten 100 Tripod Review
- Zunow SWV-E11 11-16mm E-Mount F2.8 Zoom Lens.
- Shooting Tips
- Short Film Workshop
- Tech Notes.
- Tornado Chasing
- Training and Expeditions
- Ultimate Documentary Production Workshop
- Video Reviews
- Workshops and Training