Original Link: https://www.anandtech.com/show/11242/xiaomi-mi-mix-design-and-display
Xiaomi Mi MIX: A Closer Look at the Design and Display
by Matt Humrick on April 10, 2017 7:00 AM EST- Posted in
- Smartphones
- Mobile
- Xiaomi
- Mi
- Snapdragon 821
We recently saw the release of two new smartphones with dramatically improved screen-to-body ratios, the LG G6 and Samsung Galaxy S8. Before either of these phones were announced, however, Xiaomi’s Mi MIX gave us a glimpse of the future. A pseudo-concept phone that’s being produced in limited quantities and sold in China, it combines the latest technology with innovative design, resulting in a forward-looking phone with virtually no bezels on three sides of the screen and an amazing 91.3% screen-to-body ratio, according to Xiaomi (or roughly 84% using the chassis dimensions).
Xiaomi created the Mi MIX to show off its design prowess and demonstrate its ability to produce innovative products. Xiaomi has employed new materials (ceramic) and design elements (rounded edges on the back) in the past, but by creating something as unique as the MIX, it hopes to prove that its products are as lust worthy as those that come from Apple and Samsung.
Competition among Chinese OEMs is fierce, with most of them sacrificing smartphone profit for market share. Xiaomi in particular has a reputation for creating nice-looking phones packed with high-performing hardware that are sold at lower-than-expected prices, compensating for the near-zero profit from hardware sales with recurring revenue earned through its software ecosystem. This strategy has helped fuel its rapid growth, but as the smartphone market in Asia nears saturation, it may be time to shift its focus. Xiaomi has been aggressively diversifying its product portfolio, branching out into smart home devices, UAVs, bicycles, etc., and pushing into western markets where premium phones fetch higher profit. By creating a flashy product like the MIX, Xiaomi attracts attention and improves its brand cachet, which can help it break into new markets and establish itself as a premium player that can charge more for its phones.
Encased within the shiny ceramic is a Snapdragon 821 SoC and a large 4400 mAh battery. There’s also either 4GB of RAM and 128GB of internal storage for the standard MIX or 6GB of RAM and 256GB of storage for the Mi MIX 18K. Both configurations offer flagship-class performance.
In the short time I’ve used the MIX, the only component I’ve had issues with is the fingerprint sensor. It seems less tolerant of moisture than other sensors and returns a higher rate of false negatives. It’s certainly usable, but I often found it required two or three tries to unlock the phone.
Xiaomi Mi MIX | |
SoC | Qualcomm Snapdragon 821 (MSM8996 Pro) 2x Kryo @ 2.35GHz 2x Kryo @ 2.19GHz Adreno 530 @ 653MHz |
Display | 6.4-inch 2040x1080 IPS LCD |
Dimensions | 158.8 x 81.9 x 7.9 mm 209 grams |
RAM | 4GB / 6GB LPDDR4 |
NAND | 128GB / 256GB (UFS 2.0) |
Battery | 4400 mAh (16.94 Wh) non-replaceable Qualcomm Quick Charge 3.0 |
Front Camera | 5MP, 1/5" OmniVision OV5675 PureCel, 1.12µm pixels, f/2.2 |
Rear Camera | 16MP, 1/3.06" OmniVision OV16880 PureCel Plus-S, 1.0µm pixels, f/2.0, PDAF, Auto HDR, dual-tone LED flash |
Modem | Qualcomm X12 LTE (Integrated) 2G / 3G / 4G LTE (Category 13/12) |
SIM Size | 2x NanoSIM (dual standby) |
Network | China |
FDD-LTE | B1 / B2 / B3 / B4 / B5 / B7 / B8 |
TDD-LTE | B38 / B39 / B40 / B41 |
GSM | 850 / 900 / 1800 / 1900 |
WCDMA | B1 / B2 / B5 / B8 |
CDMA | BC0 |
TD-CDMA | B34 / B39 |
Wireless | 802.11a/b/g/n/ac 2x2 MU-MIMO, BT 4.2, NFC, GPS/GNSS/Beidou |
Connectivity | USB Type-C, 3.5mm headset |
Launch OS | Android 6.0 with MIUI 8 |
Launch Price | 4GB/128GB: ¥3499 6GB/256GB: ¥3999 |
The MIX’s 6.4-inch IPS LCD display has a 17:9 aspect ratio, which Xiaomi divides into a traditional 16:9 viewable area with a dedicated space at the bottom of the screen for the onscreen controls. Not only does this eliminate the need for capacitive buttons in the lower bezel—keeping it as slim as possible—but it avoids UI scaling issues for apps targeting 16:9 screens.
Our review unit uses a Sharp fte716 IPS LCD panel that appears to use IGZO (indium gallium zinc oxide) thin-film transistors (TFTs) in the backplane. IGZO transistors have several advantages over the amorphous silicon (aSi) or low-temperature polysilicon (LTPS) varieties.
Every liquid crystal (LC) requires circuitry, including a transistor and a capacitor, to control its operation. These components can block some of the light transmission from the backlight, however, reducing the display’s power efficiency. The more light that passes through the backplane, the less power the backlight needs to consume to reach a given brightness.
Most IPS LCD panels use aSi TFTs. These are not naturally transparent, although, they can be made thin enough to allow at least some light to pass through them. IGZO TFTs, however, are fully transparent, which improves power efficiency by allowing more light to pass through.
IGZO’s electron mobility is also significantly higher than aSi and almost as good as LTPS, which allows smaller transistors to carry the same current. The MIX’s panel uses the smaller TFTs to increase the aperture ratio (the ratio of a pixel’s transparent and opaque areas), which further improves efficiency by allowing more light to pass through.
Most LCD displays need to continually refresh the screen, even when displaying a static image, because high leakage current in aSi and LTPS TFTs causes the capacitors in the pixels to discharge. Continuously driving the pixels in this manner increases power consumption, not just within the panel itself, but also by forcing the SoC to continuously push the contents of the frame buffer over the memory bus. To help mitigate this issue, some panels support panel self-refresh (PSR), which embeds a small amount of RAM in the panel itself to hold a local copy of the frame buffer. When displaying a static image (which is most of the time when not playing a game or watching a video), the panel can use its local copy to refresh the screen, reducing the power the SoC and busses would normally consume.
The Mi MIX’s panel does not support panel self-refresh, which would normally be a serious omission for a flagship phone; however, IGZO TFTs have significantly less leakage current than aSi and LTPS, allowing their pixels to discharge more slowly and retain their image state longer. A technology called idling-stop driving (IDS) takes advantage of this by significantly reducing the refresh rate of the screen when displaying static content, essentially achieving a similar power advantage as a display with PSR.
IGZO’s benefits not only reduce a display’s power consumption, but they are directly responsible for the Mi MIX’s most eye-catching feature: its edge-to-edge display. IGZO allowed Sharp to integrate the supporting circuitry that normally surrounds the active area of the screen into the panel itself, allowing Xiaomi to nearly eliminate the bezel on three sides.
The Mi MIX is a stunning looking phone whose nearly edge-to-edge display and shiny ceramic body make it stand out as something special. Xiaomi’s use of ultrasonic and piezoelectric technology streamlines the front by hiding two components—proximity sensor and earpiece speaker—behind the screen, fueling the illusion that you’re holding nothing but a 6.4-inch display. As the phone’s centerpiece, display performance is critical for the Mi MIX, and our analysis on the next page shows that it looks as good as the rest of the phone.
Display Performance
The Mi MIX’s most prominent feature is its 6.4-inch IPS LCD display. In addition to its larger than average size, this display deviates from the usual 1920x1080 16:9 aspect ratio. Similar to what LG and Samsung have done with their G6 and S8 displays, Xiaomi has stretched the MIX’s display vertically, giving it a 17:9 aspect ratio and a 2040x1080 resolution with a sharp 360ppi pixel density. The extra 120 pixels added to the height are used for the onscreen controls, leaving a standard 16:9 window for content.
Xiaomi’s MIUI offers several different display options that affect white point, gamma, color gamut, and accuracy. For starters, there are three different color settings that are self explanatory: Standard (the default), Warm, and Cool. There are also three different options to control the display’s contrast. The default “Automatic contrast” mode claims to automatically “optimize contrast for available light.” The two other modes, “Increased contrast” and “Standard” (which leaves contrast constant), are not available when using the Warm and Cool color settings.
Chinese OEMs, including Xiaomi, have traditionally struggled with display calibration. Unbalanced RGB settings that result in poor grayscale performance and blue-tinted screens and wide-gamut panels or color compression that produce oversaturated colors are common issues. To see if the Mi MIX suffers from similar problems, we’ll use an X-Rite i1Pro 2 spectrophotometer for color measurements and an i1Display Pro colorimeter for luminance measurements, profiling it with SpectraCal's CalMAN software.
The Mi MIX gets plenty bright at 586 nits, falling just below the Mate 9 and the iPhone 7 Plus, which reaches up to 618 nits when using its auto-brightness boost (the value in the chart above is for manual mode). The MIX does not have a brightness overdrive feature when using auto-brightness, but its outdoor visibility is still very good. At its lowest setting, the MIX’s brightness drops to a mere 1 nit, which makes it usable even in a pitch-black room.
Black level and contrast ratio are measured at maximum brightness, which makes direct black level comparisons more difficult because a display’s black level increases with brightness; however, after normalizing for this effect, the Mi MIX still achieves a very good black level for an IPS display, falling just behind the Mate 9 and LG G5. Its black levels are on par with the iPhone 7 Plus and Xiaomi Mi5 and a little better than the HTC 10 at the same brightness level.
The phones with OLED panels are not shown in these charts because they are able to achieve a black level of zero and a mathematically infinite contrast ratio by being able to completely turn off individual pixels.
When using the Standard color setting, the MIX’s display takes on a slight blue tint. Its roughly 7,400K average white point is certainly cool, but not nearly as extreme as Huawei’s phones, including the Mate 9 and Honor 8, or other Chinese OEMs or even LG’s flagships. The Cool color setting is a bit more extreme, increasing the average white point to 8,249K by boosting the green and blue primaries at the expense of red. This gives the screen a very noticeable aqua-colored tint. The only other color mode, Warm, comes very close to hitting the D65 target white point by turning down the blue output, but there’s still an imbalance between the green and red primaries that gives the screen a bit of a green cast.
The Standard and Increased contrast modes have a well-calibrated gamma curve. Things get more interesting when using the Automatic contrast setting with the Standard, Cool, and Warm color modes (Automatic contrast is the only available option when using the Cool or Warm color modes). In an effort to improve screen viewability in different lighting conditions, the MIX dynamically adjusts the gamma curve, improving black levels or emphasizing highlights as necessary, which is not the same thing as Content Adaptive Backlight Control (CABC) that actually adjusts brightness by varying the backlight intensity.
You can see in the charts above how the MIX modified the gamma curve to improve the screen’s black level based on the ambient light during the test (a mixture of ambient daylight and incandescent bulbs). In direct sunlight, the gamma curve looks very different, with gamma dramatically reduced below 50% luminance to brighten darker areas of the screen, making more detail visible that would otherwise be overpowered by the ambient light. In such extreme conditions, being able to see what’s on the screen is far more important than accuracy, and this feature does help in these scenarios.
The MIX comes with a wide color gamut panel that covers the DCI-P3 color space, which extends coverage for the red and green primaries beyond sRGB (shown by the inner triangle in the graphs above). This leads to oversaturated colors—particularly blues, magentas, and reds—because of Android’s lack of color management. People who prefer more vivid colors will not be disappointed here. If you prefer a more accurate portrayal of color, you will not be disappointed either. The MIX’s Standard contrast mode is actually an sRGB mode in disguise. It comes up a little short for pure blue, and the cool white point shifts color hues slightly (particularly the magenta and cyan secondaries), but overall this mode keeps ΔE2000 saturation error below the acceptable threshold of 3 with the exception of cyan.
Both the Cool and Warm color modes target DCI-P3 too, but exhibit greater saturation error as a result of hue shifts caused by the RGB imbalances shown in the grayscale graphs above.
When using the Standard contrast (sRGB) mode, the Mi MIX displays colors very accurately, rivaling even Samsung’s current offerings. All of the tested colors have a ΔE2000 error below 3 (the yellow line above which error becomes noticeable to the untrained eye) and shades of green, yellow, and orange have an error below 1 (the green line below which error is basically imperceptible).
Using any of the other modes, which target a wider color space than sRGB, predictably results in greater error due to oversaturation. Note that the color error will vary slightly for the Cool and Warm color modes, along with the Standard mode with Automatic contrast, based on ambient lighting because of dynamic changes to the gamma curve. The results above should be pretty close, however, because most of the color error arises from oversaturation and hue shift caused by RGB imbalance.
The Mi MIX’s visual and functional appeal both hinge on the performance of its 6.4-inch edge-to-edge display. It’s a risky move, but fortunately it delivers. The high-quality panel has good viewing angles with no backlight bleed. There’s some IPS glow when viewing off center, but it’s not as bad as what we’ve seen from other displays we’ve tested recently. Xiaomi’s combination of color settings and display modes should satisfy most everyone; the wide-gamut modes deliver rich-looking colors while still keeping intermediate color blends looking fairly natural. It’s also great to see a reasonably well-calibrated sRGB mode for people who prefer greater accuracy.
This is a really great display overall. The only area where the MIX falls a little short is white balance, which is a little cool by default. It’s not nearly as bad as what we’ve seen on other Chinese phones, though, and it probably will not bother most people. Xiaomi provides the Cool and Warm color modes to shift the white point, but it would be nice to see finer grained controls.