John Valentine
Review: LG-31MU97 4K Cinema display
Date10 January 2015
CategoriesReview

This is a blog article, which expresses informal views.

What is the LG 31MU97?

It's a 30.5" 'True 4k' (4096 × 2160 pixels) G-BR LED backlit IPS-type display, factory calibrated to Adobe-RGB and other professional profiles.

Not reviewed yet?

I've had opportunity to work with the new LG 31MU97, announced Q4 2014, and available now. There seem to be very few reviews from mainstream technical sites, so perhaps it's perceived that this monitor will not have popular appeal. We'll see why this might be the case...

Mini-Review

We'll consider writing more in-depth when time allows. For now, here's a summary:

Best bits

  • The G-BR LED lighting gives excellent source spectrum, resulting in strongly-saturated colours from the IPS filters. This puts it miles ahead of a TN panel, and is better than IPS panels with WLED lighting.
  • It has a massive desktop space (in common with 4K-type displays), all at a usable pixel density.
  • Semi-matte coating allows well-defined images, without much reflective glare or diffused washing out.

Good

  • On-axis colours are visibly consistent; calibration seems visibly good with most content.
  • Pixels are still visible, about the same density as 15.5" HD (1080p) display, or a 28" 2160p display, so for some users, UIs are still usable without OS scaling.
  • Pixel response can be improved in settings; see below.

Might affect your decision

  • If you're using this with a Mac, do some research first; at the time of writing, it is reported[1] that the OS cannot use the full capabilities of the display.
  • Dark content, or colour-critical content needs to be viewed at a distance over 0.8m. (photos soon).
    • Poor dark uniformity at the sides due to off-axis IPS light bleed, even from a normal viewing position (where the edge of the screen is over 15° from perpendicular). More noticable than with IPS monitors from 2007. This remains undetected by on-axis measurements/calibration, so technical reviews won't necessarily quantify the colour shifts.
    • Notable colour-shifting to the left and right, in all secondary and tertiary colours, when viewed off-axis. Again, undetected by on-axis measurements/calibration, due to the directional response of the pixel sub-structure.
  • Signal loss at full resolution, DisplayPort, even with the supplied DisplayPort 1.2 cables. This usually stablizes after 2 minutes of use. Note that some systems can run at 59Hz or 50Hz, which might alleviate bandwidth and cable quality issues.
  • If you need 10-bit colour, research your graphics card first. 10-bit or 'True 4K' capabilities might be inaccessible to most users. For 10-bit display, you need, for example, a recent nVidia Quadro card, whose output bypasses on-CPU graphics. 4096×2160 8-bit (24 bpp) currently saturates DisplayPort 1.2, so check whether 10-bit (30 bpp) native resolution is even possible using this monitor's ports.
  • Is this best value? Most users won't notice the extra 256 pixels (7%) of width, above the ordinary 3840 pixels. Unless you're editing 3840×2160-pixel content 1:1 with an app that has a very narrow sidebar, it might not be worth it.
  • Signal processing latency is subjectively noticable, and a maximum fixed-interval refresh rate of 60Hz might make this monitor unsuitable for fast-reaction gaming. Note that improving the pixel response does nothing about signal latency.

Negatives

  • Long 'blank screen' period when changing display resolutions (typically 5-10 seconds).
  • Visible background rippling on flat mid-greys, probably due to uneven backlight diffusion.
  • Flickery energy-saving and dynamic contrast feature; I ended up disabling them.
  • Slight blue shadow on the outermost 6mm of all edges of the panel (minor).
  • The display's menus are controlled by a single joystick, and are slow. Although there are some shortcuts, I personally found it tedious to reach simple controls.
  • The pixel response overdrive has a noticable overshoot when fully-lit pixels change to ~90%-lit pixels, e.g. in Elite Dangerous, when turning with a star in view. Otherwise, it works well, and makes mouse cursors sharper when moving.
  • In my experience, waking from sleep can result in a 1024×768 display, and Windows might not be able to detect available resolutions (one machine could: Windows 8 with integrated CPU display; another could not: Windows 8 with nVidia GTX-970). Switching the monitor off, then on again, fixes this. Your mileage may vary.
  • Usually provided in 'sales mode', so you'll typically need to change many settings before you're happy: turn the brightness down to 60%, select Adobe-RGB, remove the sharpening, eliminate the brightness/flicker artefacts caused by dynamic contrast and energy-saving.

Summary

This display is nicely balanced for professional users who need good colour reproduction, and work at over 0.8m from the display, in neutrally-lit conditions. Coders, photographers, and graphic design geeks will love it, but overall, this monitor hits a difficult niche, and feels over-specified and compromised outside that niche. Gamers will feel the compromises, and should look to the emerging 1440p-IPS-sync or 4K panels if purchase is imminent. Developers who test anything that changes the display resolution frequently, or switch tasks with full-screen apps, will be annoyed by the slow 'signal lock'.

We're concerned that the target market would benefit from the 10-bit 'True 4K' panel technology, but we're not sure that this display can actually deliver it in this package, due the to bandwidth bottlenecks of the inputs (we're happy to be corrected on this point).

Overall, it's a very nice try, is nearly mature, and ticks some difficult boxes. Unfortunately, it's a bit rough in some areas.

It's recommended you do some detailed research, to assess whether the benefits are worth the compromises, for your situation.

Using 4K with the Intel HD4600

I've succeeded running an LG 31MU97 monitor from a GTX-970 without problems, but it using with Intel's HD 4600 takes some tweaking, and there are problems with bandwidth and 10-bit support.

The oft-quoted "Intel HD4600 or higher: 4096x2160@60Hz" is not possible; the HD4600 does not have enough bandwidth. It can support 3840x2160@60Hz, or 4096x2160@56Hz with CVT-RB timing which you can create as a custom resolution using Intel's custommodeapp.exe.

If you have a notebook with discrete card (like a Quadro K-series) that relays its output through the Intel HD4600, then you'll be limited to a 24 bpp signal, and will be unable to use 30 bpp, and will be throttled by the bandwidth limitations of the HD4600. /p>

So, to take advantage of the 30-bit display of the 31MU97, you'll need a direct output from the discrete card; a digital pass-through will be the weak link. I also suspect that 4096x2160x30bpp@60Hz exceeds the bandwidth of DisplayPort 1.2, so take care to check ad claims against the capabilities of all components.