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In general, a video extender is not necessary if the cable you purchase is long enough and fits your needs. In some cases you suddenly require to extend the distance between the source and the display, video extenders will be a cost-efficient solution to you.

A longer cable can deliver the video input signal but the quality is not as good as you want, or the performance might be unstable. For example, a classic HDMI copper cable over 5M (16 feet) probably delivers a lower quality signal compared to the shorter cable with the same specification. Purchasing a higher specification HDMI cable may be a solution but more expensive and the maximum cable length may not fit your needs.

Video extenders convert an input signal to a higher voltage for transmission over longer distances and then convert the input signal back to the original video standard. The input and the output signal will match on a video extender, as the same video standard is maintained end-to-end. Video extenders help solve the issue you encountered and generally it is more cost-efficient.

There are some quick checks if you are trying to troubleshoot issues with a video cable.
You may test the connected devices to make sure that the following components are working correctly and are not the source of the issue:
·      Video cable
·      Video source
·      Video destination
Try the following methods to test your video setup components:
·      Use the video cable, video source, and another video destination (display) to setup to see if the problem is with the components.
·      Use a different video cable, video source, and the original video destination in your setup to see if the problem persists. 
Ideally, you should test a component that you know works in another setup. Otherwise, the issue might happen because of the port function of your video source, or a driver issue.
When you test the video setup components, it is recommended that you do the following:
·      Investigate that the source resolution matches the capabilities of the video destination. For example, if the video destination is only capable of 720p, then the video source cannot output 1080p, as that exceeds the capabilities of the video destination.
·      Test at a lower resolution, for example, 1024 x 768.
·      If you are using 4K resolutions, test with a shorter cable and test each of them individually.
If you are experiencing issues with audio over HDMI/DisplayPort, investigate the following:
·      The output audio over HDMI/DisplayPort setting on the source.
·      The compatible audio format on the destination and the source file format on the source.
·      The destination's volume was raised.
·      Take out the 3.5mm device if it is connecting to the hub.
·      Note: HDMI/DisplayPort to DVI/VGA cables, adapters and converters do not support audio.

We suggest you to rearrange the monitors in Windows to match the physical location of them on your desk. 
Go through the following:
Windows 10 / 8
1. Press the Windows Key plus X and select Control Panel.
2. Click Appearance and Personalization, then click Display.
3. Click the Resolution or Adjust Resolution option from the left column, which will display your monitors as numbered icons.
4. Click Identify to display the numbers on each connected monitor, showing how the operating system has assigned their positions.
5. Click and drag the monitor icons to best match the physical location of monitors.
6. Click OK to apply the changes and close the window.
Windows 7 / Vista
1. Click the Start button and select Control Panel.
2. Click Appearance and Personalization, then click Display.
3. Click the Resolution or Adjust Resolution option from the left column, which will display your monitors as numbered icons.
4. Click Identify to display the numbers on each connected monitor, showing how the operating system has assigned their positions.
5. Click and drag the monitor icons to best match your physical setup.
6. Click OK to apply the changes and close the window.
Windows XP
1. Click the Start button and select Control Panel.
2. Double-click Display.
3. Click the Settings tab, which will display your monitors as numbered icons.
4. Click Identify to display the numbers on each connected monitor, showing how the operating system has assigned their positions.
5. Click and drag the monitor icons to best match your physical setup.
6. Click OK to apply the changes and close the window.

Go through the following:
Windows 10 / 8
1. Press the Windows Key plus X and select Control Panel.
2. Click Appearance and Personalization, then click Display.
3. Click the Resolution or Adjust Resolution option from the left column.
4. Expand the drop-down menu next to “Multiple displays” and select Duplicate these Displays.
5. Click OK to apply the changes and close the window.
Windows 7 / Vista
1. Click the Start button and select Control Panel.
2. Click Appearance and Personalization, then click Display.
3. Click the Resolution or Adjust Resolution option from the left column.
4. Expand the drop-down menu next to “Multiple displays” and select Duplicate these Displays.
5. Click OK to apply the changes and close the window.
Windows XP
1. Click the Start button and select Control Panel.
2. Double-click Display.
3. Click the Settings tab.
4. Un-check the box beside Extend my Windows desktop onto this monitor.
5. Click OK to apply the changes and close the window.