Usually this happens. In System Settings, display you will find an extra unknown display.
This is often because you use a poor quality video cable, for example, a few dollar VGA lines.
The solution has a 2, good quality one to change the strip (with magnetic field interference) VGA line or replace the Hdmi,dvi line, and the second is to write the following bash script to add 1080p display mode:
/home/bruce/bin/set-display.sh
#!/bin/bashxrandr--newmode "1920x1080_60.00" 173.00 1920x1080 2048 2248 2576 1083 1088 1120-hsync +vsyncxrandr--addmode VGA1 "1920x1080_60.00"
Then edit/etc/profile, add:
# set VGA resolution/home/bruce/bin/set-display.sh
Log off or restart. The problem is solved.
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Resolution Ubuntu14.04 External Monitor does not specify the best resolution for the problem