This means that if your effective connection speed is lower than the video's bitrate (see table below) then the video cannot possibly play smoothly.

(a) Effective internet connection speed - This determines your potential data delivery speed and thus your potential viewing experience. There are two elements that will dictate your viewing experience: From February 2009, all new videos are shown using H.264 and AAC by default for both Standard and High Quality viewing, allowing for higher quality videos at approximately the same file size. They also feature stereo audio encoded with AAC. 720p videos are shown in full 1280x720 and encoded with the H.264 codec. This was introduced first, more than a year before, by the website Vimeo. At the same time, the Online Video Player was changed from a 4:3 aspect ratio to widescreen 16:9. This was followed up in November 2008 by 720p HD support.

This offers a significant improvement over standard quality. They can also be viewed with the H.264 codec and stereo AAC audio by adding &fmt=18 to the end of the video URL. High Quality videos, introduced in March 2008, are shown at 480x360 with mono MP3 sound. This was, at the time, the standard for streaming online videos. The original format, now labeled Standard Quality, displays videos at a resolution of 320x240 using the Sorenson Spark codec with mono MP3 audio. The web originally offered videos in only one format, but it now has three main formats, as well as a mobile format for viewing on mobile phones.

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