Several potential reasons can cause a video you upload to your YouTube account to play back in grainy quality. Identifying the reason behind your grainy video will allow you to solve the problem -- which in some cases requires you to upload a new version of the video -- and view the video in its highest possible quality.
If the original video is of low resolution or poor quality, uploading it to YouTube amplifies any defects it has. YouTube allows users to upload videos up to 2 gigabytes in size, so unless your video file exceeds this upper limit, don't resize it yourself using a video editor, as this lowers the resolution. Although larger files take longer to upload, the end result is one of higher playback quality and minimized graininess.
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Playback Quality Settings
Each YouTube user has the option to select a default playback setting to be applied to all videos. If your video plays in low quality, make sure your playback setting doesn't automatically play videos in low quality. To check the setting, click your username in the top, right corner and choose "Account" from the drop-down menu, then select "Video Playback Quality." Choose "I have a fast connection. Always play higher quality videos when that option is available."
Individual Video Settings
Even if your quality is set to be high, YouTube may default to a lower-quality version of a video if, for instance, your connection is temporarily slow. To make an individual video higher quality, click the button in the lower right corner of the video. This will be labeled with a number -- 360, for example -- followed by the letter "p," which indicates the video's resolution. Choose the highest resolution number possible; 480 and 720 are the highest YouTube will play, as of October 2011.
Device and Connection
No matter how high the quality of an uploaded video file, or what your playback settings are, a video may appear grainy on certain devices or via certain connections. If you access YouTube video an old mobile phone, for example, its screen resolution may prevent your video from displaying in its highest quality. Alternatively, if you attempt to view the video on a connection that's slower than your usual one, YouTube may be unable to play the video in its highest quality.