Getting the best out of your LCDs?
Many people who use CRT picture monitors have found the introduction of LCD displays disappointing. Getting the best out of your LCD may not be as simple as it may seem. With a wide choice of vendors and models all proclaiming broadcast quality, it can be difficult to determine which product is most suitable for you. The ‘safe choice’ of buying a big brand name product is no guarantee of picture quality or satisfaction. LCD displays are capable of giving stunning pictures and within this feature we aim to look at some of the essential criteria to be considered when assessing LCDs which will help you make a more informed choice.
The days of buying CRT picture monitors are over. They served a purpose well but it’s only the advent of the LCD which has helped reveal that the very monitors broadcasters, post production companies, etc have all relied upon were not so great. Clever use of low pass filtering ensured users saw ‘nice’ pictures but not necessarily the true signal.
The colour LCD has opened up a myriad of new applications for signal display and monitoring. The compact size, lightweight, low power consumption and affordability have led to LCD displays being used innovatively in previously unimaginable ways. The bulk of the applications intrude upon our domestic lives from satellite navigation systems to airport information systems, public messaging, medical systems, retail point of sale displays and of course, LCDs are the de facto domestic TV viewing device. LCDs are the greatest ever conveyers of visual communication information. Their use in broadcast is a consequence of their innovation which is driven by high volume consumer markets. Many LCD panels found in broadcast LCD displays are the same LCD panels found in our personal and domestic lives which have been repurposed for broadcast use.
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 erudite independent national standards bodies within the EBU region reveals 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. Some manufacturers claim to have gamma correction facilities but in truth, this often simply means that the correct gamma values are used when viewing the respective SD or HD input signals.
LCD display components are readily available ‘off the shelf’ from specialist electronics components manufacturers. Without enormous expense or time these can be engineered together to form an LCD display. Many LCD display manufacturers use the same LCD panel components originally manufactured by LG, Samsung or ABU, for example. However, one key criteria and differentiator for high quality display of SDI and 1080i HDTV signals is the scaler and de-interlacer technology. Whilst there are low cost chip sets commonly found in domestic TVs, these are not suitable for full bandwidth broadcast SDI signals some of which are interlaced signals and need to be de-interlaced to be displayed on an LCD panel. Deriving a progressive signal from interlaced signals at 50Hz or 60Hz with a high motion content requires a high level of very fast signal processing and interpolation of moving (temporal) data to generate an accurate facsimile of the original image. Poor quality de-interlacing leads to motion blur and ghosting artefacts, which are typically most often seen on lower cost broadcast LCD monitors. With small low resolution screen sizes such as 4”, 5.6”, 7” and even 8” LCDs, it is often difficult to see any artefacts but the larger, higher resolution screens are able to show artefacts more easily.
When viewing an SDI (270Mb/s) signal with 720x576 pixels on a high definition display, the original images are scaled up from 720 pixels horizontally to 1920 pixels and similarly in the vertical domain from 576 pixels to 1080 pixels. This 'magnification' or scaling, can induce undesirable artefacts too and therefore close attention should be paid to the performance of the LCD's scaler. As there are no standards to define both de-interlacer and scalers’ performance in this respect, it is essential to make a proper technical assessment. Generally, as a rule of thumb, the lower cost of the display, the less technology it incorporates and therefore the picture performance will reflect a lack of detail, clarity and possible smearing motion artefacts.
LCD displays are available in a wide choice of sizes from 1.5" to 56". The most impressive of all is Astrodesign's DM-3400 display that handles 1080, 2048 with the most stunning images from 4K signals. This display shows detail like you’ve never seen before with a realism that is almost surreal simply because there is so much detail. Your eyes can quickly become tired as your mind wants to explore all the detail on the screen and there is no apparent focal point. Even the smallest objects viewed at great distances become crystal clear. Astrodesign have proven that LCD technology is very capable of producing unimaginably good images and at a little over £30,000 the display is not substantially more expensive than a conventional 32" CRT such as a Sony BVM-A32E.
Whilst the main use of an LCD display is viewing pictures, this technology opens doors to new applications and ways of working. This is exemplified by Astrodesign who predominantly are a test and measurement manufacturer also applying their expertise in the broadcast sector. They have combined their T&M and broadcast video processing expertise together into very unique LCD displays. Their WM Series are LCD picture monitors with integrated waveform monitoring and vector displays, such is the versatility of LCD technology, negating the need to have two separate pieces of equipment. Furthermore, the sharpness of the pictures and clarity of detail is in excess of the performance of equivalent CRT displays.
How to get the most out of your LCDs
Firstly, the LCD is a solution to a problem. The wide range of LCDs in different styles and rack mounting formats means that there are many more opportunities to use LCDs than there were CRTs. Focus on the application and make a short list of products that may be suitable; don’t just include the big brands, as displays from more specialist LCD manufacturers may often outperform others. If you need a range of sizes and styles it is often best to choose one common brand as it will be beneficial to have a common control system.
The market is generally split into two sectors; general purpose viewing monitors and reference quality displays. Your aim will always be to get the best picture quality display whatever the application. Whilst audio de-embedders, safe area markers and waveform display etc, may be attractive facilities, the key criteria is picture quality. It can be overlooked but it is particularly important that the colourimetry is accurate and repeatable, so if you buy one model for a monitor wall or gallery and then purchase the same model six months later, make sure that it will give you the same colourimetry. Take time to objectively test and compare LCD displays and if in doubt ask your supplier for advice on how they do this. Traditional test signals are only a guide to performance and static signals do not allow you to check the quality of the scaler nor for any motion artefacts associated with de-interlacing. You will need ‘real video’ at different line rates with high motion content to make a proper assessment. Check also the LCD graphics display driver is working at 50Hz and not 60Hz as is common on some low cost LCDs as 50Hz to 60Hz conversion induces motion artefacts and frequently softens the picture, reducing detail.
Calibration and alignment
LCDs do not suffer from ‘drift’ in alignment as beset even the best CRT monitors and should give accurate performance for 1-2+ years depending upon actual use. For more critical applications such as telecine, DI and editing, users may wish to recalibrate their LCDs on an annual basis. You can use a probe to measure and monitor manual adjustments to a given colour standard. The rule of thumb is that the more expensive the probe, the more accurate it will be in measuring the LCD. Low cost probes will give you readings but these may not necessarily be accurate enough for broadcast work. Some popular probes measure light outside of the visual spectrum which is irrelevant and can give misleading results. The king of LCD probes is the Konica Minolta CS-200 which measures RGB and not just grey scale, with accuracy unachievable on lower cost counter parts.
Penta Media who manufacture an extensive range of very high quality LCD displays, all of which are guaranteed to be ITU REC709 compliant, have a very useful and unique utility which works with the Konica Minolta CS-200 to automatically calibrate their displays to a given choice of standards including ITU REC709. Furthermore, you can choose a standard alignment or deep alignment which adjusts 36 points within the gamma curve to ensure absolute precision. The whole process takes 5 - 10 minutes and does not require any great engineering expertise as it is completely automatic, eliminating human error. This facility is particularly important as it allows post houses to guarantee to there clientele that their LCD give exactly the correct colour rendition. It might also be argued that all critical camera work and OBs would benefit by using LCDs aligned to ITU REC709, after all you often can’t go back and shoot it again.
LCDs are software based devices analogous to Avid or Final Cut Pro editors, so check with the manufacturer how often they release new software with additional features and bug fixes and at what cost, so that you can get the most out of your LCD. Check also how to install software updates because some manufacturers require you to buy expensive upgrade installation tools. It is better to have network capable LCDs as this will dramatically improve the upgrade installation time to something more manageable.
In this feature we have shared some of our extensive experience and expertise with you and hope that it has been useful and thought provoking. This article has covered numerous ‘basics’ and we welcome all comments and additional questions.
Oxygen DCT is both a manufacturer and distributor of LCDs with over 100 different models and counts nearly all UK national broadcasters amongst its client base. The company’s precision LCDs have established a following at the highest levels of London’s Soho post, vfx, telecine and DI houses. Oxygen DCT is a leading light in the LCD display sector and supports it’s clients with an unprecedented choice of products, experience and expertise to match.