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What’s an i-Frame? What’s an i-Frame interval?



What’s an i-Frame? What’s an i-Frame interval?

Curious about some of the things your Slingbox does “under the hood” so you can watch your TV anywhere? In this article, we look at i-Frames and i-Frame intervals.

i-Frames and i-Frame intervals are a part of video compression; they’re used to compress video so that it can be sent over the Internet. Video requires a lot of network capacity to transmit, so video data is compressed to make streaming over the Internet possible. The compression technology that your Slingbox uses makes the best use of available network capacity at any moment. It does this by analyzing the video stream frame-by-frame, and then finding the best ways to transmit the best possible picture using the least possible data.

What exactly is an I-Frame?

An I-Frame is a piece of transmitted video that shows a complete representation of the picture that you’re watching at a particular moment. For the frames that follow the I-Frame, the Slingbox internal software analyzes this video to see if it can “describe” it only in terms of how it has changed from the I-Frame, rather than re-describing the entire picture all over again. This way, the amount of data needed to transmit the video is reduced.

Here’s a graphic that represents the “thinking” that happens “under the hood”:

iFrames_illustration.ashx

In this graphic, the first and last frames are the I-Frames, as they contain the entire image of the car and the background. However, since the only thing going on in this piece of video is the car moving across the static background, the Slingbox only sends data about how the car should move. It cuts down on data transmission by not “reinventing the wheel” and re-sending new data about the unchanging background … instead, it specifies that the background should stay the same for those frames.

However, this can’t go on forever. Eventually, the background will change, or a different car will drive by, or whatever. That’s when the Slingbox transmits another I-Frame to serves as a new “reference” that it can use to describe the following frames using as little data as possible.

In a perfect world, there would be unlimited network capacity, and the Slingbox could transmit only I-Frames for maximum video quality. But it’s not a perfect world! Picture quality needs to be balanced against available network capacity. That’s why the video quality is higher when intervals between I-Frames are shorter, but the video needs more network capacity. When intervals between I-Frames are longer, the video transmission uses a lot less bandwidth, but the video quality is lower.

On top of that, some rapidly-changing video needs lots of I-Frames for decent quality. Other video can get by with only a few. That’s why it’s best to let the Slingbox adjust the I-Frame rate … and many other quality settings! … by keeping the Video Quality setting on Auto wherever possible.

http://www.slingbox.com/en-US/Support/KB/KB-2000081.aspx