Quality control in the digital age

The days when only engineers needed to monitor signals are long gone. Today's editors and graphic designers also must understand the basics of legal color space and other technical requirements.
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London production company Post Republic employs Hamlet’s Monitor Scope 601AX for all-digital signal processing.

Digital technology has transformed our world, but we should never lose sight of the one final arbiter of quality: the viewer. The goal is to deliver content to the viewer in the home. The viewer does not care if the content was originated on DV or film, or if post has been in a high-end uncompressed hero system or in Final Cut Pro. The viewer wants a consistent signal on the television, with no shadow detail lost because the blacks have been crushed, no highlights burning out half the picture, and with loudness levels consistent from program to commercial to trailer to program. To do this, broadcasters set technical standards. If something fails to meet that standard, then it is rejected. That is as true now as it ever was.

The difference is that, in the past, the submitting producer would have a team of engineers who would discuss (or possibly argue) with the broadcaster’s team of engineers, and a solution would be reached. Now it is perfectly possible for a “broadcast- quality” program to be produced by one person — shot on DV and edited on a desktop system. A complete shooting and finishing kit costing comfortably less than E10,000 could be capable of producing excellent technical quality. But that single person is unlikely to be a trained and experienced broadcast engineer. Most likely, it will be a journalist with a story to break or a creative dramatist with a new vision to deliver. He or she may not realize that it is possible to create out of specification signals in post, but having to rework a rejected master could well break an already tight budget.

Yes, it is possible to take well-exposed footage and create out of specification footage in post. Compositing and graphics are the obvious areas, but with even the most humble desktop editing system now including color correction facilities, the risk is high. Illegal color is probably the error most likely to creep in. Television has a defined color space. Keep within that space, and every system downstream should handle the signal accurately; go outside the color gamut, and unpredictable results are likely, with the viewer seeing clipped and distorted colors.

We talk about color gamut, but actually there are a number of different definitions — the gamuts for PAL and NTSC; RGB color space; YUV color space (which includes colors lying outside the RGB gamut from which it is derived); and component digital color space, which is, of course, closely related to YUV.

A gamut error is defined as any video signal excursion outside the appropriate color space and is grounds for rejection of the material by the broadcaster. And here we have another problem: Different broadcasters have different attitudes towards gamut errors. Should we be concerned about momentary gamut errors? And how many errors does it take to reject a tape?

In the UK, for instance, the BBC’s view is that while short-term gamut errors are failures to meet specification, they may well not have any significant impact on the viewer. That is particularly true if the alternative is to use a color legalizer with relatively simple clipping, which is likely to introduce artifacts more disturbing than the original errors.

For many years, though, the ITC, which was responsible for standards setting for the independent television companies, took the contrary view that any error was unacceptable and a cause for rejection.

Following some excellent work initiated the post-production industry and manufacturers in the UK, everyone agreed on a definition that now forms part of the EBU recommendation on gamut errors. The EBU recommends that the color gamut in television program material can be accepted if both the following conditions are met:

  • When matrixed to RGB, all of the R, G or B signals should lie inside the range -5 percent and 105 percent after an IRE filter has been applied.
  • The resultant luminance signal should lie inside the range -1 percent to 103 percent.

This seems to be a sensible and pragmatic view. But this modest latitude does not take away from the basic requirement to monitor signals to ensure their compliance with good technical standards. Given that the days are long gone when an engineer would oversee all technical processes, what do we do now?

Education

In part, it has to come down to education. Editors and graphic designers must be encouraged to understand the basics of legal color space and the other fundamental technical requirements. To balance that, manufacturers must develop products that are accurate and stable, but simple and clear to use for the non-engineer. This means developing new ways of displaying the information. Staying with color gamut, many people from engineering backgrounds will be familiar with the “double diamond” display. While this shows gamut errors at the white end, it is not at all clear at the black end of the spectrum (and colors can be illegal for being too low).

More modern solutions provide displays that indicate all the gamut errors. Digital processing in the RGB domain is ideal, showing instantly and directly where the problems lie. These should preferably be linked to a gamut alarm with audible warnings of problems, as well as a logging facility against timecode. Tracking all errors over time becomes a useful reporting facility.

For those who are strictly non-technical, the ability to switch on a pattern effect that clearly highlights the parts of the picture that are in violation of gamut limits is without a doubt the most user-friendly solution. It tells the artist which areas need work or, in desperate cases, makes it clear they need to call for help.

The progression towards desktop editing tools is now unstoppable, of course, and so test and monitoring specialists have to adapt to survive. Every broadcaster worldwide maintains its quality control department, because even in the digital world, good signal housekeeping is important; bad housekeeping can lead to your precious tape speeding back to you for remedial work.

It may not seem glamorous if you are an investigative journalist working undercover or a young director at the cutting edge of drama, but getting your signal levels right is still vital. A little education — and the right tools — can save you from this most embarrassing rejection.

Steve Nunney is managing director at Hamlet.

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