I've been watching the feed from a webcam (Foscam 8910). When the scene is bright (lit with sunshine) the video feed is approx. 350 kBytes/sec. When I view the same scene lit very dully with just a dim bulb, the feed comes at approx. 50 kBytes/sec.

As I understand it, the video feed only transmits the changes between frames. Once the image has been transmitting for a few seconds, and there is no change whatsoever in the scene, the feed should settle down.

Why is there a seven fold difference in network traffic between light and dark scenes? Shouldn't they be identical?


Our eyes and brains are more sensitive to data in the bright areas so compression schemes generally throw out more of the data in the dark areas. If the video is mostly dark, then it compresses very well because they can throw out more data and you don't notice. But when it's bright (assuming it's not overexposed), less data can be crunched down without you noticing it.

  • Thanks for that, but I draw your attention to the 2nd paragraph. I thought that a static image would generate much lower (virtually nothing) network traffic as nothing is changing between frames..? Does video imaging not work this way? – Paul Uszak Mar 19 '15 at 14:25
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    It depends on the compression used. MJPEG compresses each frame individually for example. – neuhaus Mar 19 '15 at 14:40

Based on neuhaus' comment, I've confirmed that the Foscam uses MJPEG compression. Thus it's clear how a brighter (hence more detailed) scene would generate higher bit rates by repeatedly sending complete full frames. I thought that it would be smarter than that.

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