Mike Richardson of Harris sheds light on the importance of gamut monitoring in a multi-format environment.
Many articles have been written addressing colour space gamut differences between analogue composite, RGB and digital video.
Without repeating much of what has already been written, gamut "errors" are a product of the conversion of video signals originally produced for one video format into another. For instance, colour TV cameras produce RGB signals that are (still!) often encoded into composite video.
These two different ways of representing video - RGB and composite - also contain different colour representations.
Each one has an allowable range (or "gamut") that defines its colour space. In the broadcast world we are most often interested in discussing colour space conversions between the native colour space of SD and HD video (Y Cb Cr) and between RGB and composite.
The mathematics behind colour space conversion is relatively straightforward, but out of the scope of this article. It is enough to understand that converting between colour spaces is a simple mathematical process, involving nothing more than algebraic manipulation.
ITU and SMPTE standards contain all the appropriate detail, including the actual coefficients for conversion.
Beyond the mathematical curiosity, there are practical reasons for monitoring gamut excursions. Any video captured by a real-world device such as a camera will be captured as RGB since the camera sensors are RGB in devices.
Even if the camera has SD or HD outputs in Y Cb Cr colour space, these sensors are still in RGB space. At the opposite end of the chain, the video will be viewed in RGB colour space. All monitors and televisions, regardless of technology, are RGB devices.
Digital broadcast technology was designed to include more chrominance range (larger gamut), as well as more picture detail when compared to analogue.
An engineer working solely with hardware that conforms to the ITU BT.601 (SD-SDI) serial digital video specification will never encounter a need to compensate digital video for colour space limitations.
Equipment that is designed properly will not have any "gamut-related" problems.
However, as soon as the engineer needs to prepare SD-SDI video for conversion to high definition serial digital or for broadcast by an analogue transmission channel, gamut measurements and possible adjustments may be necessary.
While a programme produced in analogue video (composite or RGB) usually will not have gamut problems when converted to SD-SDI video, the process of converting an SD-SDI signal to analogue can easily produce gamut errors.
Perfectly legal combinations of the three components in Y Cb Cr colour space can quickly become highly illegal in composite colour space.
A triple ramp test signal is a simple illustration.
Composite gamut violations can be generated from legal Y Cb Cr combinations. One main reason is that in composite colour space, the allowable chroma levels (saturation) of a particular picture element are tied to the luma value of that same element.
For example, high saturations are allowed with mid-range luma values, while low saturations are needed at both high and low luma levels. This restriction does not apply to component video formats such as Y Cb Cr.
A similar situation occurs when comparing Y Cb Cr colour space to that of RGB. Highly saturated picture elements riding on a high (or low) luma value can once again cause illegal RGB gamut excursions.
Monitoring gamut errors in a serial digital video signal is more challenging than measuring most other parameters.
Many serial digital test and measurement instruments are not capable of displaying a composite representation of the serial digital signal; therefore, they can only display the native YCBCR or RGB waveforms.
The traditional instruments used to measure and monitor video are vectorscopes and waveform monitors. Using only a waveform monitor and/or vectorscope to monitor or measure gamut is nearly impossible.
The vectorscope plots a polar diagram of the phase and amplitude of the chrominance content of the video signal, while the waveform monitor is designed to show the amplitude of the video signal as a voltage per unit of time.
However, gamut problems are caused by combinations of the luma (Y) and chroma (Cb and Cr) components, and neither a waveform monitor nor a vectorscope effectively shows these particular combinations of components.
An engineer could attempt to analyse a vectorscope and waveform monitor (composite display) and then guess when the gamut parameters are proper; however, this "measurement" would be inefficient at best and at worst - impossible.
Some waveform displays do permit simple and limited gamut monitoring. For instance, a waveform monitor that displays an HD signal in composite or RGB representations allows a very coarse gamut evaluation of these colour spaces.
However, a great deal of information is not conveyed: What portion of the picture is in violation? What is the colour composition at the point of violation? These questions often cannot be answered intuitively.
For this reason, specialised gamut displays have been created. In most cases they convey a great deal of extra information about gamut violations, well beyond that which could be done with a simple waveform monitor.
Some of these specialised displays can show not only which component is in violation, but also the overall image colour at that point. This is crucial because gamut errors can be counter intuitive.
For instance, it is common for the blue component to have a negative gamut error in highly saturated red areas, where a red error would be expected.
Other specialised displays feature exact error highlighting on a picture representation of the video, allowing specific pixel errors to be located.
These and other specialised features allow more in-depth gamut monitoring than is possible with traditional waveform and vector displays.
Mike Richardson is director of Product Development for Videotek Test and Measurement at Harris Corporation.
Harris' Videotek name is synonymous with test and measurement instruments that are designed for mission-critical broadcast environments.
Gamut monitoring is available on a number of Harris test and measurement devices including the TVM series of multi-function, multi-format signal analysers; the VTM Series of user-configurable, field-upgradable, multi-format test and measurement consoles and the Videotek QuiC media analysis server.
The QuiC system automates the QC process for video and audio content as it's ingested into, stored on, or played out from server networks. It enables broadcasters to analyse files faster than real-time, and correct certain file problems on the fly.
As part of the migration towards the emerging 1080p HD format, Harris also introduced Videotek OPT-3GB, a 3 Gb/s video input module for the TVM and VTM product range.
With the inclusion of a 3 Gb/s signal source, the module offers an affordable, field-upgradeable solution, and includes a colour-bar generator for verifying 1080p signal paths.
Its new 7RU AVM-717 self-contained test and measurement solution provides waveform/vector/picture/data/gamut monitoring and audio monitoring, and supports HD, SD, composite analogue and 1080p.