Freetalk

Have you ever noticed that basically all webcams are connected to the host computer using the USB 2.0 bus? The ubiquitous USB 2.0 bus is cheap and convenient for such purposes. Providing 480 Mbps it’s no slouch, but it’s not exactly state-of-the-art either. This has implications when webcams are reaching for HD resolutions at decent frame rates.

Until quite recently webcams always provided an uncompressed image stream to the host computer. USB 2.0 is a serial connection standard supporting up to 480 Mbps. That’s about one third of the data rate of the production HD-SDI standard, SMPTE-292M, which is 1.485 Gbps.

Let’s do a little math corresponding to a 720p video stream as related to uncompressed HDTV.

8 bit/pixel @ 1280 x 720 @ 59.94fps = 105 MB per/sec, or 370 GB per/hr.

105 MB/s = 840 mbps

…but a lot of video conferencing gear actually uses 30 frames/second instead of 59.94 or 60 frames/second…so half that value…

720p30 = 420 mbps!

There you have it! The mathematics supports the assertion that 720p30 uncompressed “HD” video stream can be passed across the USB 2.0 serial bus. This explains how Skype, Google, ooVoo, VSee and others have been able to offer HD video using common USB 2.0 connected webcams. Understanding the limit of the USB 2.0 connection also informs us why 1080-capable webcams have not become similarly commonplace.