Causes
- Panel and scaler timing at the maximum refresh rate — the S27AG55 is a 165Hz VA panel whose scaler is being pushed hardest at its top timing. On this family, faint evenly spaced horizontal lines appear at 165Hz on flat mid-grey, blue, and sky-coloured areas, and they vary in strength from unit to unit. They usually disappear at 144Hz, which is what makes them look like a fault when the panel is simply out of margin.
- Response Time overdrive creating line shimmer — with Response Time set to Extreme, the panel's overshoot alternates from row to row on fast horizontal motion and on scrolling text, producing a crawling line pattern that is easy to mistake for a dead panel. Extreme also adds bright trailing edges that emphasise the effect.
- A DisplayPort 1.2 link that is only just holding 165Hz — 1440p at 165Hz is near the top of HBR2 bandwidth. A long or thin cable, an extension, or an adapter causes bit errors that the scaler hides as brief line artifacts, sparkles, or a thin band that flickers in one part of the screen.
- Sharpening or scaling in the game or driver — in-game sharpen filters, NVIDIA Image Scaling, AMD RSR, and non-native resolutions make the scaler double-process the image, which exaggerates row-level artefacts and can produce visible line structure on textured surfaces. This is the most likely cause when the lines appear only in one game and never on the desktop.
- VA gamma shift and inversion near panel edges — VA panels show a slight horizontal banding pattern from pixel inversion, and it becomes visible at high brightness or when viewing off-axis. It is not a defect, but it looks like lines on a white document or a grey test screen.
How to Fix
- Check whether the lines are in the monitor's own menu — press the joystick to open the OSD and look at the menu background. If the OSD itself shows the same horizontal lines, the artefact is inside the panel or scaler and no software setting will remove it, so go to the warranty step below.
- Drop the refresh rate to 144Hz — in Windows Settings → System → Display → Advanced display set the G5 to 144Hz, and set System → Refresh Rate in the OSD to match. Most units that show lines at 165Hz are clean at 144Hz, and 144Hz versus 165Hz is a difference of about 1.3ms per frame.
- Set Response Time to Standard — open Picture → Response Time in the OSD and choose Standard. Test again in a fast game and while scrolling text; if the crawling lines stop, the overdrive setting was the cause and Faster or Extreme should stay off at high refresh rates.
- Use a short, certified DisplayPort 1.2 cable — power down, replace the cable with the one in the box or another certified DP 1.2 cable under 2m with no extension or adapter, and plug it directly into the GPU. Re-seat both ends until the latch clicks. Line artefacts that appear and vanish with movement of the cable are always a link problem.
- Turn off extra sharpening and scaling — in the game disable sharpen filters and resolution scaling, set the render resolution to native 2560x1440, and in the GPU control panel disable image scaling and any custom sharpen. Confirm the OSD reports 2560 x 1440 as the incoming signal, not a scaled mode.
- Lower brightness and use a neutral picture mode — set Picture → Picture Mode to Custom, bring brightness down to roughly 35–45, and set Color Tone to Standard. Reducing backlight drive makes inversion banding on white pages much less visible.
- Update the GPU driver and retest at 120Hz — install the latest driver, then try 120Hz as well. If the pattern changes shape or spacing with the refresh rate, it is timing-related and the practical fix is 144Hz; if it stays fixed in the same rows at every rate, it is panel-level.
- Claim the warranty if the lines persist — with the OSD showing lines, at 144Hz, on a known-good cable and a second source device, the panel or scaler is defective. Photograph the pattern on a flat grey background, note the serial number from Support → Information, and open a service request with Samsung.
