A GUIDE TO FILE BASED LEGALISATION
This guide has been introduced to help explain the complex world of technical video level compliance in the file based domain. Software legaliser plug-ins for the most commonly used within editing platforms, namely Avid Media Composer, Apple Final Cut and Adobe Premiere Pro.
WHY LEGALISE AT ALL?
When a finished programme is delivered to a broadcaster, the broadcaster will evaluate the quality of the content (Quality Assessment, or QA). It will be evaluated for many things such as its production content (violence, sex, bad language, legal issues etc) but it will also be evaluated in the technical domain where both the video and audio must meet precise technical specifications. If you do not meet these specifications the content will be rejected by the broadcaster and sent back to be re-edited until it meets those specifications.
The “BroadcastLegal” plug-in will force the content to meet the specification automatically.
WHAT ARE THE SPECIFICATIONS AND WHY ARE THEY THERE?
The video specifications relate mostly to video levels. This can be a combination of black and white levels, RGB levels and also “composite colour – PAL NTSC” levels. The level restrictions are there to ensure that the content that is seen at the editing stage will remain faithful through transmission to the consumer. It is possible to produce video content at the editing stage that will change when it is passed through the video transmission process and consequently change colour from edit to consumer. This is undesirable. Currently we only know of one published recommendation by a standards committee which is an EBU technical recommendation “R103-2000”. Many (mostly European) broadcasters’ technical compliance documents follow these recommendations. Many other worldwide broadcasters have there own specific requirements.
FILE BASED LEGALISERS IN THE EDIT
Complete Plugins target the editing platform to perform legalisation. It is recommended by most broadcasters that legalisation is in the domain of post production. Here the production staff can evaluate any colour changes that may occur as a result of the legalisation process and even decide to re-edit if the results of the legaliser are unsatisfactory. Complete Plugins legalisers have a lot of parameters to enable soft clipping and in the composite domain, constant hue clipping. The factory presets within our legaliser XML file are also optimised to keep picture quality as high as possible. Users can also choose to use our “BroadcastEvaluate” plug-in which will indicate technically non-compliant parts of the content which can then be corrected using native image level tools.
CODING AND DECODING
For those of you who are familiar with the world of SDI and the use of our SDI legalisers, you might say, things were simple and predictable compared to the file based world. All SDI video is uncompressed and there are no coders and decoders involved (Although broadcast tape machines employed some compression it was generally low compared to many file formats). This meant that once your video was legal it would stay that way unless you put it through another bit of kit with active video processing to change levels. Not so with the file based world and you need to be very careful with the quality of your coding and decoding. Codecs vary in quality enormously and the bit rate is usually the biggest driver of quality. Generally nowadays the broadcasters will specify a file delivery type of high quality. Typical High quality codecs are ”Apple Pro Res” and “AVC Intra 100”.
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Below are three snapshots. Figure 1 shows the original highly illegal material. Figure 2 shows the same material after being exported using the BroadcastLegal plug-in via a high quality codec. Figure 3 shows the same material after being exported using the BroadcastLegal plug-in via a low quality codec.
The legalisation standard used is EBUR103+PAL. You will notice that legalising with the high quality codec produces an acceptable result. The low quality codec re-introduces artefacts causing an error to be flagged on the Tektronix waveform monitor.
Any codec will introduce some compression artefacts and for this reason all of the Complete Plugins preset parameters are set to account for some error by pulling in the levels a little more than the specification allows. All the Complete Plugins preset standards do this.
The legalisation standard used is EBUR103+PAL. You will notice that legalising with the high quality codec produces an acceptable result. The low quality codec re-introduces artefacts causing an error to be flagged on the Tektronix waveform monitor.
Any codec will introduce some compression artefacts and for this reason all of the Complete Plugins preset parameters are set to account for some error by pulling in the levels a little more than the specification allows. All the Complete Plugins preset standards do this.
In Conclusion:
1. Do not export content through the BroadcastLegal plug-in using a low quality codec as it will re-introduce artefacts that may cause the content to be rejected
2. Remember that ANY codec will introduce some artefacts no matter how small. Always account for this by setting up the BroadcastLegal parameters tighter than the broadcaster recommendations.
OVERUSE OF THE LEGALISER
If you take a look at the previous section and look at Figure 1 you will notice that the levels of the original content would be considered to be a long way from any broadcaster specification. In our view and the view of most experts it is not good practise to just legalise such content as this will lose a lot of content detail and even produce artefacts from the legaliser if it is set up badly.
The editor should, particularly for quality content, keep an eye on levels using a waveform monitor set to YRGB. If the editor is keeping the content within reasonable limits the legaliser can then be used to catch any stray levels. This ensures that the content remains faithful after legalisation.