How to choose a broadcast display


How to choose a broadcast display…

10 years ago, buying a CRT monitor was simply a case of buying the latest version of your facilities favourite brand in either Grade I or II. The advent of the LCD, HD and the demise of the CRT means we have now to try and decipher all the marketing jargon to work out which display best fits our needs.

To help de-mystify the process, the following primer will give you an insight into some of the ways you can test displays and ask the salesmen those awkward questions!

Standards

The EBU document EBU TECH 3320 entitled 'User requirements for video monitors in television production' sets out the guidelines for broadcast grade 1, grade 2 and grade 3 broadcast LCD displays in a way analogous to grade 1 and grade 2 terminology used for CRT monitors. Testing by national standards bodies within the EBU region shows that NO LCD display has met the necessary standard for grade 1 performance. Only one manufacturers complete product range meets grade 2 whilst all the rest were equal to or below grade 3 level. It is therefore not surprising, perhaps mainly for commercial reasons, that many manufacturers do not refer to the EBU TECH 3320 document in their marketing literature.

ITU-R Recommendation BT.709, more commonly know by the abbreviations Rec. 709 or BT.709, sets out standards for the format and chromaticity of high definition television signals following on from the D65 colourimetry set out in earlier standards adopted in CRT monitors. As the gamma for SDI and HD-SDI are different, it is vital that a broadcast LCD with pretensions to meet broadcast standards displays the input signals correctly using the appropriate gamma setting. Gamma is therefore a vitally important parameter to be considered for high quality signal monitoring. In a situation similar to the ITU TECH 3320 recommendation, most manufacturers produce broadcast LCD displays which perform 'near' to BT.709 standards with exception of teutonic German manufacturer Penta who’s monitors are designed and calibrated to REC709 colouimetry.

What is the display being used for ?

Rather than looking at specific ranges, it is important to understand what we are going to use the display for as money can be easily wasted by using an inappropriate display, it is not always the most expensive display that gives the best results.

For quality control situations such as telecine, camera control or anywhere we need to be confident that whatever artefacts we are seeing is an actually what is being generated and not a function of the monitor, we need to look at a range that satisfies the Grade II EBU TECH EBU 3320 and Rec.709 standards.

For general viewing and framing, such as a monitor wall, where we are looking at the content, a grade III display will work well, but you need to be confident that when you get a new display for the expansion that will happen in the future, that the colourimetry will match your existing displays, so does it conform to Rec.709? Is a multi-split display a possibility?

For situations such as a machine room where the presence of the signal and the levels of the signal are more important, savings can be made using displays that incorporate their own waveform and audio monitoring.

LCD displays are much more stable in their colourimetry than CRTs were and they should give stable operation of 1-2 years, but for especially critical colour sensitive areas, check to see how easy is it to calibrate them and how deep can you calibrate the gamma curve for absolute precision.

A bit about LCDs…

LCD panels (as in the glass panels) are manufactured by only a few electrical component manufacturers and then integrated into broadcast monitors and it is about how well they engineer this, more than the quality of the glass that affects how well the monitor performs. The key aspects to performance are:-

  • Scaling – this is the process where an input signal is converted to match the number of pixels on the LCD panel, known as resolution. For an SD-SDI 625i/25 interlaced signal this needs to be mapped onto the number of pixels on the panel; the SDI-SDI signal is 768 x 576 pixels which need to be displayed as 1920 x 1080 pixels on a full HD display. Scaling can also reduce the pixel count to display for example an HD signal (1920x1080 pixels) on a small monitor, of say 800 x 400 pixels.

  • De-Interlacing – an LCD display is driven progressively, which means the 625i/25 signal needs to be de-interlaced before it can be displayed.

Neither of the standards mentioned above check for this so it is down to you when you are testing the displays so you need to be thorough.

What to look for and how to check it…

Scaling and de-interlacing will have most affect on the artefacts that are seen on the picture and can manifest themselves as softness and on fast moving material, ghosting and blurring respectively. When you test monitors don’t just use static images. Motion graphics can be a great test for displays as they are can generate repeatable motion in different directions, so use roller captions and crawls in as many directions as possible. Some displays will even display 50Hz signals at 60Hz, so beware of artefacts induces by converting 50Hz to 60 Hz and try to test on different frame rates if you can. To get over some of these issues some monitors have an additional processing delay is introduced which can be significant if the display is used in a gallery and cut decisions are made using it.

Multi-split displays

If you only have many sources that require monitoring or space is an issue, consider using a multi-split, either as an external unit feeding a large display or if you only have 4 sources an integrated unit that accepts 4 inputs and displays them on a single screen.

The only things to be aware of are how much delay does the multi-split introduce and is this important and how easy is it to use, control and reconfigure.

Specification Wars
As there are just a small handful of manufacturers of LCD panels this results in the specifications for broadcast monitors being very similar and identical in many cases.
As already highlighted the panel’s specifications are only one factor in a monitors performance. Almost all LCD panels are 8 bit and beware of specification wars and marketing hype regarding 10 bit panels as these are significantly more expensive and are designed with a much wide gamut than 8 bit panels and as a consequence it can require 2 bits of signal processing to remap the colourimetry for broadcast applications leaving just 7 0r 8 bits bits for displaying pictures on a 10 bit panel. The message is that specifications are only a guide and be sure to ascertain true details of the display resolution

Integration

Once you have chosen your monitors on picture quality you will need to check on how easy they are to ingrate. Question to ask are…

  • Are the correct inputs included or extra?
  • What are the mountings?
  • What are the facilities for remote control, Ethernet, GPI etc
  • Are Under Monitors Displays available within the units as cost savings could be made
  • Does it support dynamic IMD/UMDs
  • When a software upgrade is released, how easy is it to apply it?
  • Does the supplier want to sell you the only monitors that his company makes or will they work with you to engineer the best solution?
  • How easily can you calibrate the colourimetry after +2 years when the CCFL tubes begin to age and begin to give a colourcast to the picture

This article has just briefly highlighted some of the criteria to be considered when choosing a LCD picture monitor and we hope that it has given you an insight that will enable you to make the best of them. We are all trying to make the most of budgets and a little time spent testing displays in a ‘shoot out’ can pay dividends in the quality of your installation and on the bottom line. Whilst there are a plethora of cameras, edit systems and production equipment, the resulting pictures have to have the correct colours and therefore picture monitors are the most vital element across the whole broadcast signal chain.

Tags: iss032 | monitor | display | lcd | crt | ebu tech 3320 | rec 709 | multiviewer | N/A
Contributing Author N/A

Article Copyright tv-bay limited. All trademarks recognised.
Reproduction of the content strictly prohibited without written consent.

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