Affordable Colour Grading Monitors
One of the most searched for and hotly debated topics on colour grading forums, websites, podcasts and tech shows is that of “Affordable Colour Grading Monitors”. The search for a cheaper option, most often a high-end calibrated computer monitor or high end television, used in place of a (relatively) expensive, purpose-built, professional colour grading monitor, still remains a holy grail quest, of sorts.
In this post my aim is to dip a toe in the water, share some expert opinions (not my own), present a case for and against, and suggest some possible solutions. Along with a bucket load of caveats. The first being, I am only an editor with a blog, I am certainly not presenting myself as a professional colorist nor colour science expert, so, with that out of the way, let’s get started!
First, A Few Caveats
Here’s the thing… buying a display is a highly personal decision that has as much to do with you and your clientele’s preferences as it does with a given display technologies’ level of accuracy. Don’t buy a display because you read that I like it, because my reasons may not be your reasons.
Instead, you should evaluate the different legitimately color-critical options for yourself, and then get what suits your particular needs. This is the approach I’ve tried to take when describing the various display technologies that are currently available in chapter two of my updated Color Correction Handbook, 2nd Edition, and I think it’s the only way to be honest about this frequently debated subject. – Alexis Van Hurkman
The first thing we need to clear up are a few philosophical framing issues when discussing this topic. Firstly, as Alexis so eloquently points out, everyone is starting from a different spot and will end up at very different destinations. Budgets, creative needs, technical requirements and promises made to clients will vary wildly from person to person. So everyone should evaluate their choice in light of their options.
Secondly, the most slippery aspect of this topic is that it’s an endless rabbit hole of a search for perfection. There is always more that can be done to make the monitor, grading system, viewing environment, source footage etc. more technically accurate. Is your background wall 18% grey? Is your room lighting the right colour temperature? How black are you blacks? How recently was the display calibrated? How old is the technology? What format is the footage? What is your output device? What software are you running, on what operating system? And on and on it goes.
I would wager that *most* people who are looking to buy an ‘affordable grading monitor’, are probably not going to be trying to persuade their clients to pay them vast amounts of money per hour for their colour grading skills, founded on the promise that the display in front of them is 100% perfectly accurate. Although some may have the audacity, I think a lot of people just want to have a decent display that they can work with, reasonably confident it looks ‘pretty good’.
Notice what just happened. In one sentence we slid from ‘total perfection’ to ‘pretty good’. These days, as with so many advancing technologies, ‘pretty good’ is actually very very good. There is always better, but colour-critical, calibratable monitors which will get you most of the way there, are now very ‘affordable’, for example $1000-$3000 instead of $6000 – $30,000.
What matters most – Confidence
“The real purpose of the display is peace of mind. Because that’s what as a colourist drives you nuts. You can work on any display, but you need the confidence that the display is showing you something that is accurate to the specification, so that you can have that peace of mind.
There’s so much else that needs to fit correctly to give you that peace of mind, that could go wrong with those (cheap) displays, that unless you have a real display that does give you that peace of mind to compare it to, you’ve kind of lost… But if you don’t have that comparison device, and it’s off, you won’t know it’s off.” – Juan Salvo
This brings us full circle. What really matters most, in professional colour grading suites, is that you can point at the display and say “This is accurate”. So the question becomes, how confident are you? And how much are you willing to pay for that peace of mind?
Now, as soon as you take your project off that display and out into the real world, it doesn’t really matter anymore, as everyones iMac, TV, projector, iPad etc. will make it look completely different anyway. But at least you’re starting from a comfortable position. But again, I’d expect that the majority of people reading this are editors, photographers, cameramen, budding colorists and DITs etc. who want to have a decent display to work on and who know that for 100% accuracy they need to take their project to a reputable colorist for the final polish. Or rent a real monitor for the final pass themselves.
Which Colour Grading Monitor Should I Buy?
So what should you look for when choosing an affordable colour grading monitor? Our first port of call is this recent podcast from the Coloristos in which they discuss this particular thorny topic for a good hour or so. This is well worth a listen. One thrust of this podcast, and other things I’ve picked up online, is that OLED seems to be the future, but it’s still pretty expensive.
In the Coloristos’ podcast the Sony OLED PVM2541A is mentioned. This is now a discontinued product, superseded by the Third Generation OLED panels BVM A series. These are not cheap at $26,000 for a BVME250A 25″ Trimaster EL OLED. That said a PVMA250 25″ OLED monitor will only set you back $5495. For that you get a 10-bit 1920 x 1080 Trimaster EL panel with SDI and HMDI input.
So we’re getting closer to ‘affordable’ but we’re not quite there yet. We really want to get down to about $1000-$3000, and even then that’s not ‘affordable’ to some.
The Flanders Scientific CM171 drops down to a bargain price of $2495 for a 17″ 1920 x 1080 10bit LCD display, with SDI only. For comparison the 12 bit OLED CM172 costs an additional $5000 at $7495. These are the kind of solutions that Juan argues for in the Coloristos podcast. Buy cheap, buy twice the man says. And in many ways he’s right. If you’re gunning for becoming a serious colorist, this is a much better direction to head in, instead of wasting $1000 on a computer monitor, when you could save a bit longer, or rent in the meantime, for a ‘real’ colour grading monitor, that costs a bit more.
But looking at these prices, *most* people switch their thinking to a ‘high end computer display’, which is where the rest of this post will be heading.
What to look for when choosing an affordable grading monitor
There are quite a few things to take into consideration when choosing an affordable grading monitor, not least the final price tag. Here are some thing that you should weigh up when making your selection.
Display Resolution – You want to be able to monitor the kind of footage you regularly work with at it’s full resolution. If you’re often working with 4K footage then you’ll want a 4K monitor. If you’re only ever delivering HD then a 1920 x 1080 monitor will do just fine.
Inputs – Depending on the monitor you choose it will come with different inputs. The most common are SDI, HDMI and Displayport. You’ll also want to look at whether it has Thunderbolt, USB 3.0 and audio capabilities as these make the monitor even more useful day to day.
Size of display – If you’re the only one who will be looking at the display, having a 30-50″ monster will be overkill. If clients need to be viewing it from the comfort of a sofa, having a bigger display might be helpful. Personally I like something around the 27″ mark – plenty of space to work with, but not unwieldy.
Gamut & Gamma – This is the range of colours (gamut) and luma range (gamma) that a given specification requires. For example, can the monitor accurately display common specifications such as Rec 709, Rec 2020, DCI-P3, Adobe RGB etc?
Viewing Angle – What you don’t want is your image looking different when you move your head or seat around. The image should look exactly the same no matter which angle you’re viewing it from. Most displays these days declare that they have a ‘wide viewing angle’ of around 178 degrees. Although technical specifications and real world testing can be quite different things.
Contrast Ratio – This will probably make the biggest difference to your perception of the images on display. Glossy displays tend to have a higher contrast ratio than matte displays. According to chapter 2 of Alexis Van Hurkman’s Color Correction Handbook 2nd Ed. (paraphrasing here) for an LCD display 1400:1 (glossy) or 1100:1 (matte) or better, is a good ball park. For OLED 5000:1 is a good ball park.
Black Levels – Having deep blacks is what colorists are always looking for, not muddy grey ones. Deep gorgeous blacks with plenty of detail still in them. Partly this impacts on your perceived contrast and partly it’s a sign of a good display panel. OLED panels beat LCD in this and the contrast department.
Calibration – That you will be able to calibrate your monitor with either in-built tools, and/or an external probe and software should be an essential element in your choice. Otherwise you won’t be able to maintain the accuracy of your images over the lifetime of your monitor. (Which, by the way, will also be a moving target, as display’s performance changes as they age.)
UPDATE – Colorist David Torcivia has a really helpful explanation of some of these key terms in the first of his two-part post on Colour Grading Monitors. He also offers some practical suggestions on how monitors of different capabilities will alter your image, according to their specification.
In practice, you’re going to be hard pressed to find a display that offers anything besides 1.8 and 2.2 (Gamma settings) without stepping up to professional models.
Once you do find a display capable of a wide range of gamma power functions, it can be tough to know which multiple to use as BT.709 has no set standard. Typically, the deciding factor is based on the general ambiance of the room. A darker room requires a higher power (the reason why cinema projections in a completely dark room use gamma 2.6). A good rule is to use a value around 2.0 for daylight uncontrolled lighting, 2.2 for dim lighting, 2.35-2.4 for a dark controlled room, and 2.5-2.6 for pitch black.
In part 2 of his post David considers several professional monitors that he thinks you should be saving up for including brands such as Flanders Scientific, Dolby, Panasonic, Sony, HD2Line and a few others. All in all, very much well worth listening to what a professional colorist has to say on the matter.
Affordable Colour Grading Monitors – a few options
At long last we have come to the point where I round up a few potential high-end computer monitors and suggest that you use these as a pretty good way of viewing your images. Hopefully given all the caveats, conditions and explanations that have preceded this point, you’ll understand me when I say these displays will all do a fine job of displaying your work pretty accurately at an affordable price.
It goes without saying that there are plenty of other excellent monitors out there from brands like LG, Asus, JVC, Panasonic etc. I’ve chosen to focus on three of the main brands that I would most likely choose from, when selecting a new monitor, to use for editing and grading on my Mac Pro.
HP DreamColor Z27x
Price – $1254 | Size – 27″ | Resolution – 2560 x 1440 | Contrast Ratio – 1000:1 | Viewing Angle – 178° | Gamuts – sRGB D65 (100%), RGB D50 (100%), Adobe RGB (100%), BT.709 (100%), BT.2020 (68%), DCI-P3 (99%) | Inputs – HDMI, Displayport |
The HP DreamColor Z27x is a great looking 27″ matte display that ships with a range of handy factory calibrated presets for various colour management needs. It has a lot of great features to benefit professionals working at all stages of the digital production pipeline, including the ability to handle 4K.
In Tom Parish’s excellent interview with HP DreamColor architect Greg Staten and Spectracal’s CTO Derek Smith you can learn a ton about the development of the HP DreamColor Z27x based on a close collaboration with numerous industry professionals and Spectracal, makers of calibration software CalMan.
Aside from holding your images beautifully, the monitor will down convert 4K (4096 frame for frame at 24hz) to fit into the display, and you can also pop in to see pixel for pixel 4K with a dynamic scroll, and crop the image to various aspect ratios to use the monitor on set for checking focus and colour. If you’re interested in buying a HP DreamColor then this interview will fill you in with a lot of insider details and the rational for a lot of the new features, such as 1D LUTs, internal calibration, display performance etc.
Another great resource for anyone interested in the HP DreamColor Z27x is this review from DP Shane Hurlbut in which he compares it to the old DreamColor 2480zx. Being able to assign your own sub menu items to the 4 smart keys on the display bezel is a handy timesaver.
With all these new OLEDs and new HD wireless systems, frame delay has become a huge factor. You have a very shallow depth of field with HD, and assistants are using monitors to focus. The response time on this monitor is 7ms, which is amazing. Your monitor is your eyes into all of these new exciting sensors. Picking the right one can be a daunting task. This one is tried and true. – Shane Hurlbut
Finally, for a very thorough analysis of the Z27x visit Tom’s Hardware to see what they made of the monitor. The whole 8-part review is very complimentary and is based on their own testing. Bestowing the monitor with an Elite award they conclude that:
At this price point, there is no other monitor that is as accurate or capable as the HP Z27x. We expect to see it populating film editing bays everywhere since nothing else even comes close for less than five figures.
As a quick aside Tom’s Hardware has only given the Elite award to one other monitor in their ‘Professional’ category, the NEC PA272W a 27″ LCD panel. Although it’s not true 10-bit (but is 8-bit w/FRC) Tom’s hardware were pretty excited about it so I thought I’d mention it, but I would still prefer the HP over the NEC.
Another review of the Z27x that’s worth a read comes from Studio Daily who cover all the bases as well as drawing out an interesting point about the level of calibration the monitor can achieve:
According to HP, the internal calibration engine—using either color checker patterns or saturation sweeps—has a Delta E color accuracy rating of around 0.5. The Delta E standard takes into consideration both the hue and saturation of the colors. A Delta E rating of 2.6 is considered to be the point at which a typical human eye cannot see a difference between two colors. A trained colorist might see differences down to a Delta E rating of 1.0 to 1.5. A Delta E rating of 1.0 is considered to be the limit of what any human eye can physically discern.
I was lucky enough to be lent a demo Z27x from HP and have written up a much fuller review of the monitor here. (Coming Soon) But based on all of these reviews and its spec, the HP DreamColor Z27x is a fantastic, versatile monitor at an affordable price.
Eizo ColorEdge CG277
Price – $2244 | Size – 27″ | Resolution – 2560 x 1440 | Contrast Ratio – 1000:1 typical | Viewing Angle – 178° | Gamuts – Adobe RGB 99%, DCI-P3 99% | Inputs – HDMI, Displayport, DVI |
I’ve always been a fan of Eizo monitors whenever I’ve come across them in various post houses that I’ve worked at in the past, and always felt they were a cut above, but now that I look at a straight cost comparison, especially in light of the Flanders Scientific CM171 at $2495, they seem pretty expensive for what you get. But price isn’t everything.
First of all, Eizo do have new, as of April 2015, 4K monitors to add to their top of the line CG range; the CG318-4K and the CG248-4K. But for the basis of comparison I’ve opted to leave out 4K monitors for now. To get a good sense of the full line up of Eizo monitors check out the three minute video above from Jigsaw24.com. Essentially the basic range are the CS models, then the CX and then the top of the line CG range.
Colorist Jason Myres (one of the Coloristo crew) has put together a really valuable post on his efforts to get a workable grading set up out of a Eizo CX271 which is the thousand dollar-ish cheaper sibling to the CG277, although Jason has the following to say about their comparative capabilities:
The CX271 is a 27-inch 2560×1440 display that’s nearly identical to the Eizo CG277 ColorEdge display. They both use the same 27-inch LED-Backlit IPS panel, case design, input options (DVI, DisplayPort, HDMI), Continuity Equalization, 16-bit internal look up table, etc. However, for around $1500 the CX271 is almost $1000 cheaper than the CG277 mostly because of differences in the back-end electronics that are largely calibration and scaling related. The good thing is you have a choice; if you need those features you can choose the CG277, but if you don’t, you can save money while getting the same hi-performance IPS panel and 90% of the options that are included in the CG277.
That said, the CG277 adds 4K input and scaling to 2K, external 3D LUTs, a monitor hood for sun shading and full calibration capabilities using the built-in sensor. Both monitors come with the ColorNavigator calibration software. The CG277 is also an 8bit+FRC (Frame Rate Control) panel where as the HP Z27x is a true 10-bit panel. Whether you’ll see any difference is a matter of hot debate, and the kind of thing you want to see for yourself. The new 4k monitors are apparently true 10-bit. – And obviously this whole thing only matters if you want to monitor in true 10-bit!
Another post professional who has put the CG277 to good use is FCPX ninja Sam Mestman, who reviewed the monitor in this post over on fcpworks.com. One of the things that Sam points out is that your Rec709 calibration might need a little re-working straight out of the box. Another consideration is the lack of an SDI input (favoured by older broadcast professionals) but he argues that
There’s no SDI in… for me, I don’t really care about this… but it’s important to a lot of people for some reason. If you understand how to get your HDMI signal in properly (and there’s not much to know), there aren’t really any advantages to SDI… in fact, I think HDMI is far more manageable and flexible for the average person.
It’s meticulously built, has a gorgeous, color-accurate image, and a deeper feature set than all five of my prior monitors put together, while still being really easy to use. Products this well rounded are hard to come by, but easy to spot in that they are so nice to use, they basically disappear into the background as you work. And what makes the CX271 in particular so great, is that it does all of this at a price point that’s within reach for many users, while still performing at a world class level in every category.
Personally I’ve always enjoyed working in front of an Eizo monitor, and think they’re a great brand, but I would probably plump for the HP (by a slim margin) over the Eizo. If I was going to spend $2.5k I would shoot for the Flanders anyway. That said both the CG277 and the CX271 are extremely capable monitors.
Dell UltraSharp 27 Premier Color Monitor U2713H
Price – $697 | Size – 27″ | Resolution – 2560 x 1440 | Contrast Ratio – 1000:1 typical | Viewing Angle – 178° | Gamuts – Adobe RGB 99%, DCI-P3 99% | Inputs – HDMI, Displayport, Mini Displayport, DVI |
By far and away the cheapest of the monitors in this mini selection, the Dell UltraSharp U2713H comes with a very similar spec to the other two contenders. It’s also the oldest model monitor, I believe early 2013 while the others are 2014. You can check out the official spec page for the monitor here.
There is an incredibly detailed and extensive review of the monitor on pcmonitors.info which runs through all the bases including colour gamut, contrast and brightness, luminance uniformity, viewing angles and more. If you’re considering this monitor it’s well worth the time. In the above image the red triangle is the measured gamut, green is sRGB and purple Adobe RGB.
When it comes to colour reproduction the monitor put in an excellent performance. Dell included some nicely setup factory-calibrated presets which emulated both the Adobe RGB and sRGB colour spaces successfully. Gamma tracking and white point under these presets was pleasing and colours looked very much as they should with exceptional consistency across the screen. In our testing we had an opportunity to test both of these presets (with brightness lowered somewhat) and compare the look they gave to a range of game and movie titles. The U2713H exhibited excellent shade variety and portrayed things in the intended way under its sRGB preset. Because this content is specifically designed for the sRGB, using Adobe RGB increases the saturated and vivid look. Some people will like this look and be pleased to see the monitor outputting some of the more vivid colours that our eyes see in the real world but can’t be captured in sRGB.
The Dell doesn’t have quite the same features as the other monitors, and with it’s older technology it’s not at the cutting edge of things. But that said it is a highly affordable option with excellent colour reproduction. As suggested in the tweet below, it might make for an excellent second screen for your GUI.
@patinhofer I'm very much enjoying my Dell 27" U2713H for the Resolve UI. Looks nice after sRGB calibration, too.
— Alexis Van Hurkman (@hurkman) December 1, 2014
Further Options for Affordable Grading Monitors
Dave Dugdale shares this monitor shoot out comparing the LG 34UM95-P, the Asus PA279Q and the HP Z27x (who comes up with these monitor model numbers anyway!). You can check out the full write up over on LearningDSLRVideo.com for more details. Here is how Dave sums up how he made his selection…
I want to repeat that all three look very similar, but noticeably more accurate than my old monitor. Really splitting hairs! If money is an issue for you, buy the Asus and use the extra money upgrade one of your lenses, you will notice more of a difference. While the HP might be the best overall, it was such a minor difference I decided to go with the LG because of all the screen real estate.
Monitoring 10-bit Video on Mac OSX
UPDATE – 2016
As of Mac OS X El Capitan (10.11) and higher you can now natively monitor 10-bit on a Mac. For a great primer on how to do this and a really helpful 10-bit test file to download to test your particular system, check out this very clear post over on Imagescience.com.au.
There are several components to the complete video path:
Your actual digital image file -> OS -> Software (e.g. Photoshop) -> Video Card Digital Signal -> Monitor LUTs -> Panel Depth
Each and every one of these can have a different bit depth.
This makes things much easier but you still need to ensure you are 10-bit capable at every stage of the path your video takes. Here is a direct download link to the 10-bit test ramp image. If you don’t see any banding you’re successfully viewing the file in a 10-bit environment.
Here is an important factoid that you need to know if you want to be monitoring 10-bit video on your Mac Pro or other Apple hardware. As of OSX Yosemite 10.10, the Mac operating system is still 8-bit natively. (Windows 7 and up, is 10-bit native, I believe.) This means that if you want to view 10-bit video on a Mac you need to have 10-bit all the way through your signal path from your footage to your display.
On a Mac both DaVinci Resolve and FCPX will, for example, output 10-bit footage through a 10-bit capable I/O device via a 10-bit cable to a 10-bit monitor. For example, outputting your 10-bit footage from DaVinci Resolve via a Thunderbolt connected Blackmagic Design Ultrastudio Express hooked up to a 10-bit capable monitor via HDMI or SDI will give you 10-bit on a Mac.
For a really clear and detailed explanation of all this check out this thread on the BMD User Forum. The Thunderbolt connection on the Mac Pro is just an extension of the bus (like slotting a card in your old tower) so until OSX becomes 10-bit native, you can’t just hook up a Thunderbolt/Displayport cable from the Mac Pro to the monitor and have it work.
If you only want to monitor 10-bit video, then the Blackmagic Design Mini Monitor is a cheaper solution than the dual Input/Output of the Ultrastudio Express. The AJA T-Tap is a similar device but it might be worth checking the compatibility first. As part of my testing of the HP Z27x Dreamcolor display, Blackmagic Design were kind enough to lend me a Ultrastudio Express too, and you can read my write up of it in that post too. (Coming soon)
Another resource you might want to check out is Apple’s official support page for 4K monitors and UHD TVs.
Calibrating a Grading Monitor
Getting into the detail of how to calibrate a display is beyond the realms of this already lengthy post. But clearly an integral part of purchasing a monitor that can be calibrated, is that you do and fairly regularly. Some high end computer monitors feature in-built calibration devices, although one would (safely?) assume that a dedicated probe will deliver far better results. Here too probes and calibration software can vary hugely in price, for example Light Illusion’s Light Space CMS is about $4000, and Spectracal’s CalMan Studio is about $1595.
For a clear and practical overview of calibrating a monitor head over to this post from colorist Alexis Van Hurkman, on his process for grading his plasma display with LightSpace CMS and a probe. For more resources on monitor calibration check out this previous post – Colour Grading The Technical Stuff.
Monitor calibration is an obscure corner of the already obscure profession of color correction. However, once you know how things work, automated calibration should be a simple and straightforward procedure. Essentially, you use color management software to control both a color probe and a pattern generator (which can be either hardware or software) that work together to measure your display. The pattern generator outputs a series of color patches to the display you’re calibrating, the color probe measures each patch, and the software saves the resulting measurements. – Alexis Van Hurkman
Pat – When we talked about this four years ago not only was it a dark art but it was expensive. The meters were expensive, the software was expensive, and the thing you needed to generate the test patterns were expensive. Now with DaVinci Resolve Lite there is a built in test pattern generator to perfectly test your signal path and use it with Lightspace or CalMan.
Bram- Especially for Resolve users there is literally no better way to test your actual signal path and make sure that not only is your monitor calibrated but the signal is getting to the monitor in the way you intend.
Colorist Patrick Inhofer from Tao of Color.com, recently interviewed Bram Desmet, CEO of Flanders Scientific, on the detailed technical ins and outs of calibrating a monitor. If you want a lot more details than I’ve included in this post, and to learn a tremendous amount about the current state of grading monitors and calibrating, you should definitely take the time to listen to both interviews.
In Part 2 of the interview, the topics they cover include:
- Is self-calibration of your reference monitor attainable ‘for the rest of us’?
- The new fast profiling options in CalMan and LightSpace
- DaVinci Resolve’s test patch generator
- Low cost hardware test patch generators
- What is the point of reference monitors when ‘grandma’s TV’ is blue?
- Can a pro colorist rely on a sub-$1000 probe for accurate calibrations?
Part 1 of Patrick’s interview is also well worth a listen and covers similar topics to what I’ve covered in this post, but in much, much, greater technical detail!
- What is the lifespan of an LCD and OLED?
- What is 10-bit FRC and is it still being used?
- Are there 8-bit OLEDs?
- Are consumer panels 8-bit or 10-bit?
- Are there different types of OLED technologies FSI can choose from?
- What’s in the near future for OLED technology?
Previous Posts For More Resources
Although these posts are now quite a bit out of date, but you can look them up if you wish.
UPDATE – Five More Even More Affordable Grading Monitors
Colorist David Torcivia, who has produced some great DaVinici Resolve tutorials, has put together a list of his choice of 5 affordable entry level monitors he thinks could be a viable option for new colorists.
These monitors (24″ and greater) were selected on the reliability of their brands (ie no Korean no name panels as good as they may be provided you’re lucky) and color accuracy first and foremost while coming in under $1000 (prices are time of writing – April 2016). Resolution, refresh, looks, and gimmicks like GSync/Freesync trail far behind in the criteria.
Be sure to head over to David’s post to get his expert considerations on each monitor and a few potential other options, but if you just want some quick links to snap one up, here are the headlines.
ASUS Pro Art PA249Q – 24″, 1920 x 1200, 10bit, 99% Adobe RGB
Dell UP2516D – 25″, 2560 x 1440, 8bit FRC, 100% Adobe RGB, DCI-P3, 100% REC 709
BenQ PG2401PT – 24″ 1920 x 1200, 10bit, 100% Adobe RGB
LG 31MU97Z-B – 31″ 4096 x 2160, 10bit, 99.5% Adobe RGB, DCI-P3 97%. – This is the monitor I chose for my home edit suite, check out the full review here.
Philips 276E6ADSS – 27″ 1920 x 1080, 8bit, 99% Adobe RGB. This is a new ‘Quantum dot’ monitor.