Gamut errors: Who cares?


TV-Bay Magazine
Read ezine online
Gamut errors are the most common video signal problems. This is because colour television or cinematography depends on being able to represent every pixel on the screen in terms of red, green and blue. We want to deliver perfect RGB signals representing all the possible colours in our pictures. Virtually all display technologies use RGB primary colours to match the RGB colour sensitivities of the human eye. A gamut error occurs if we process video from a non-RGB colour space which overloads or creates negative value in one of the RGB channels.
If the signals sent along the chain from camera to screen were always in RGB there would theoretically be little room for any gamut errors. Studio RGB levels leave space top and bottom to allow for transient overshoot. Black is level 16. 235 (in 8 bit digital) is the maximum in red, green or blue.
If bright red goes through the system at 255, that is allowed but typically the screen would clip to normal saturated red at 235. Worse, it is possible to have negative values if R, G or B is below 16 (which does not represent any real colour except black. In RGB connected paths, there is only a small range of 14 or 15 levels either end of each colour signal that are illegal, causing clipping of the image viewable range.
Historically, it was realised it would be more economical to transmit colour images as colour difference components: YUV (or 'YPrPb'). The most important part of a component video signal is the luminance (Y), made by adding each of the RGB signals in the right proportions. This then has the full image detail in black and white – but no colour.
The colour difference signals U and V are made by subtracting the blue and red respectively from Y. The colour information is thus distilled into an apparently simple format almost like semaphore when you look at them on a vectorscope: the angle between the U and V signals now points to the pixel colours on a circular scale.
In the component signal world, we pass the YUV presentation instead of RGB because it is more efficient. This extends to MPEG2 and H264 (MP4) and many other formats. RGB only gets reconstituted at the display screen. The big problem with component formats is the range of the colour difference signals. U and V are allowed to go positive or negative but the allowable range depend on how much luminance is for any given pixel. Any processing step like upconversion, contrast enhancement or a colour balance operation introduces a possibility of the balance in the YUV proportions being wrong. They can be really very wrong if, for instance, the luminance is low to black and yet colour signals are at saturated levels. This could produce very negative RGB values that mean nothing to human eyes. These types of gamut error are not necessarily picked up by viewing a standard vectorscope. Other measurement presentations or a gamut error detector that shows the offending areas for any RGB or YUV excursions as in the Real-Check SoloQC are necessary in a QC environment if you don’t want your work rejected.
Robin Palmer is Managing Director of Cel-Soft and is currently involved with software solutions for 3D & TV quality control and measurement technology.

Tags: iss065 | soloqc | celsoft | gamut error | real-check | N/A
Contributing Author N/A

Read this article in the tv-bay digital magazine
Article Copyright tv-bay limited. All trademarks recognised.
Reproduction of the content strictly prohibited without written consent.

Articles
Managing Technological Change
Alan Wheable Continual technological change in the broadcast and media industries can make it difficult to plan for the mid to long term. Typically, broadcasters and media organisation are still implementing the last set of changes to working practices when the next changes come along.
Tags: iss133 | omnitek | ip | waveform | vectorscope | ultra tq | Alan Wheable
Contributing Author Alan Wheable Click to read or download PDF
What is next in OTT
Mary Kay Evans In the past year alone, we’ve seen a substantial increase in the amount of OTT content that’s being streamed. In the first quarter of 2018, there’s been a 114 percent year-over-year growth in streaming video hours, and those numbers are only expected to rise. With OTT revenue predicted to reach $16.6B in 2018, a 40% gain over last year, there’s no question that OTT is booming, and that there’s never been a more critical time to pay attention to the space.
Tags: iss133 | ott | verizon | cisco | Mary Kay Evans
Contributing Author Mary Kay Evans Click to read or download PDF
Live audio contribution over wireless networks
Pablo Rodrigues Altmann Wireless “live” contribution from anywhere at any time, this is the ultimate goal for most news reporters. The old favorite of using ISDN is becoming harder and much more expensive to access easily. Today most of us use wireless daily, in particular Wifi and HSPA/LTE 4G networks. These are now mature technologies and widely used for many applications in the home and whilst out and about. However there are still some professionals who prefer to persevere with the traditional, costly and limited ISDN communication technology before jumping into the wireless era.
Tags: iss133 | adrl | wireless audio | isdn | cellular audio | Pablo Rodrigues Altmann
Contributing Author Pablo Rodrigues Altmann Click to read or download PDF
Taking on a self employed placement year
Joshua Round The idea of being self-employed or freelancing has always been somewhat terrifying for me. There is a level of uncertainty and responsibility that comes with the freedom of being self-employed, the likes of which makes me wonder why I chose to give it a go for my placement year as part of my university course - (BSc) Television and Broadcasting.
Tags: iss133 | placement year | university | student | education | portsmouth | Joshua Round
Contributing Author Joshua Round Click to read or download PDF
IBC in a post Brexit world
Peter Savage 2 Cast your mind forward and we are not in 2018 but next year and, yes, it’s you and me walking to the departure lounge to catch the plane to IBC just as I, and perhaps also you, have done for the last 25 years. (By the way, where is my long service award – and perhaps a new pair of shoes as I must, surely, have walked the equivalent of five Caminos covering the 12 halls in the Rai). We are at the gate and my imagination kicks in as I hypothesize on what the trip might look like next year. I leave it to you to decide which is closest to what might be to come.
Tags: iss133 | azule | brexit | ibc | finance | Peter Savage 2
Contributing Author Peter Savage 2 Click to read or download PDF