Corprit D157 portable USB3 monitor review: input lag and upscaling tested using the piLagTesterPRO
This 15.6" 1080p monitor features both standard HDMI video input and also video over USB3, and is designed be easily packed up with your laptop as a dual monitor for the on-the-go crowd. It was made in 2021 and is commonly sold for $130-170 on Amazon. I did a brief video review, or you can read more more detailed review below.
Overview
USB 3 isn't fast enough for raw 1080p video, so your laptop has to compress the video signal before sending it. This works quite well, and is fast enough to send full screen video (see my video review for an example). It doesn't overstress the CPU (even on my older laptop), and the USB transport only adds about 25ms lag, which is certainly tolerable for desktop use and some gaming. You do have to install a driver for this, of course. If that's a problem or if you want to minimize lag, the solution is simple: plug in the HDMI cable and then the lag drops to about 6ms (see below for full details)
If you don't have USB3 it is possible to send the video over USB2 but unless you are just doing office work or email this offers a poor experience. The lag is still low, but in order to achieve this they switch to 8 bit color, and websites suffer, especially if they have photos or video. Most laptops sold in the last 5 years have USB3.
One of the compromises of having a portable screen is that it doesn't have a traditional stand, instead using a folding folio design like a tablet would have. It works as well as any tablet folio case, giving near 80 degree to a full 45 degree viewing angles, and I haven't had a desire to go out side that range. Speaking of tablets, this does not support touch, which is a shame, though in all honesty I'm not sure if I'd use it much.
Power consumption is about 10 watts. For comparison, my older, moderately "gaming" laptop uses 18 watts just surfing the web with no monitor attached. So this will definitely cut into your battery life.
It has built in sound, and the sound quality is actually a bit better than I expected, exceeding my laptop's speakers (though that's a very low bar).
One thing to consider is that the resolution of this display is pretty high relative to what older laptops like mine have to offer. Since the screen is only 15.6", that makes text pretty small if you use the default font size. If your laptop screen is similarly high-resolution you can set windows to use a larger font, but that's not a good solution if your laptop display is not "high dpi" (the two screens have to share a single font size). I find myself wishing for a lower resolution, perhaps 1366x768, like my laptop. You can set the monitor to any resolution you want, of course, but non-native is going to look blurry. Not that blurry, though, it's totally tolerable.
Image quality
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| TN laptop on the left, IPS monitor on the right |
The screen looks great - it's an IPS panel and has great viewing angles. I took some pictures comparing viewing angles to my laptop's TN display and as you can see it's fantastically better.
The monitor seems to accept all 60hz resolutions, from 240p and 480i up to 1080p. It does not go above 60hz.
For all resolutions there are no cropping or stretching options - you always get a 16:9 aspect ratio, meaning that 4:3 gets stretcher a lot. But on the upside, they have zero cropping and zero aliasing in 480, 720, and 1080, which is great. Why can't more displays get this right?
Input Lag
This display has no game mode, scene, or color preset.
I used a piLagTesterPRO to measure input lag. This device sends a frame of video over HDMI and measures how long it takes to display it. This monitor was able to properly sync to the refresh rate of the Pi, producing consistent lag values each time it was turned or or inputs were switched.
Full Test Results
I report two kinds of values. 1st response measures how long it takes for the TV to start responding (I use a 5% change in display brightness). This overly optimistic value doesn't tell how long it takes to see anything useful, but matches what other reviewers call input lag. full response is a more realistic measure of lag, and requires the display to reach 80% of full brightness. This combines both input lag and response time, and is closer to what you would actually experience in a game.At all resolutions this display does pretty well, taking 5ms or to start responding, and about 17ms more to finish drawing. It takes a full 15ms to paint the whole screen, so the realistic lag is around 34ms. The display deinterlaces by using the bob method, which means zero added lag in 480i, at the cost of minor up/down flicker particularly visible on horizontal lines and text. This is the same method the OSSC uses, and is great for action games but not so good for RPGs and other text-heavy games. Consider it an alternative to buying a OSSC for the gamer on the budget.
I measured the responses time for black to white, but also various levels of gray to gray. Response time was fairly constant, which is a good thing.
Results compared to other displays
To allow quick comparison between many displays I've summarized the results across all the displays I've personally tested with the piLagTester Pro. Min lag is the first response at the top of the screen, real lag is the full response at the bottom. The tested monitor is in bold.
This monitor ranks pretty well. I've sorted this list by 720p real lag, and it's near the top of everything I've tested. In part that's because it's a computer monitor, and most of the tests I've done are on TVs which almost always have more lag. Among computer monitors, it only beats the average by 2ms. It has very low lag, but relatively high response time, giving a realistic lag value that's good but not exceptional.
Somewhat uniquely for a computer monitor, it does support 480i, making it a great choice for retro gaming on the go (it's such a small screen that I doubt you'd want it for home use), with zero extra lag. Only the Panasonic Plasma TH-58PE75U does better without the addition of extra scaling hardware.
Conclusion
This is a pretty unique device and it most makes sense if you want a portable display. There's nothing wrong with using it for other purposes, but there are certainly cheaper and bigger displays out there if I didn't need the portability.
Other models
Amazon is full of portable USB3 monitors. I suspect many are made by the same factory, just with different brands silkscreened on. The only particular reason to choose this one is that I tested it. You can find the product I tested here:
(not an affiliate link! I make nothing from this).
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