Does anyone have any experience with USB 2.0 capture devices?

July 31, 2004 2:34 PM

I'm thinking of buying a USB 2.0 video capture device. I used to use a PCI capture card but now my machine has no empty slots. Does anyone have any experience with USB 2.0 capture devices? Is there enough bandwith to keep the video/audio quality good? (I have a PC)
posted by mildred-pitt to computers & internet (7 comments total)

As long as it's truly full speed USB 2, you'll be okay, I think it has a bandwidth of 480Mbps (60MB/sec).
posted by wackybrit at 3:12 PM on July 31, 2004

to clarify wackybrit's point, there is a mode of USB 2.0 that allows for operation at USB 1.x's speed of 12 MB/sec. This enables manufacturers to stamp "USB 2.0" on all products, regardless of whether they do 480 Mb/sec or 12 Mb/sec. That having been said, the 12Mb/sec speed of USB 1.1 devices is sufficient if the device in question does MPEG4 or similar compression before it sends data down the wire. You can get fairly decent video at 1Mb/sec with a good codec and 320x240 resolution.
posted by Kwantsar at 3:50 PM on July 31, 2004

What about DVD NTSC bandwith? Will USB1.1 support it?
posted by Keyser Soze at 5:37 PM on July 31, 2004

Keyser -

Uncompressed images require x_resolution * y_resoultion * color_depth_in_bits bits per frame. Multiply that number by the number of frames you want per second (30 for NTSC TV, 25 for PAL, 24 for cinema, I think). The result of that computation is the number of bits-per-second of bandwidth you need for raw, uncompressed video.

Now, as you know, decent lossy compression will compress the frames by a ratio of 10:1. High quality video compression will take that frame-compression rate, and compress the video stream (the sum of all the compressed frames) at a ratio of 10:1. Therefore the total compression ratio for the video stream should be 100:1 or better.

So, a 720p stream at 720x1280 and 24 bits per pixel will require 21 Mbits per frame. At 30 FPS, you will need 632 Mbits per second. With 100:1 video compression, this should drop down to 6.32 Mbits per second. It won't fit down a T1, but it should be possible to fit down a USB 1.1 pipe as long as nothing else is contending for the bandwidth.

Note that 100:1 compressed video is very *good* but not perfect. Using mpeg compression will creat artifacts, much like using 3200 speed black and white film adds substantial grain to photographs and film. If your goal is creativity, this should not be problematic. It may be a bonus.

I see that I just did the math for HDTV, not DVD. But now you know how to do the math, so do it yourself for whatever video source you want to play with. Also, look for devices that use the Phillips TriMedia chip. It seems to be the cheapest solution for decentish MPEG4 work. The better webcams use it (ie, the ones that actually stream MPEG4 and not just motion-JPEG) as do some security and videoconferencing devices.
posted by Kwantsar at 11:34 PM on July 31, 2004

oh, and by "better webcam" I actually meant "better ethernet webcams." The DLink DCS-2000 is an example.
posted by Kwantsar at 11:36 PM on July 31, 2004

I'm looking for good quality 640x480 30fps (normal TV size). I need to make a DVD of all our current VHS tapes for our shows- and don't want to lose much quality.

Has anyone worked with a specific brand of USB 2.0 video capture? Pinnacle?

Nice info, kwantsar.
posted by mildred-pitt at 2:20 AM on August 1, 2004

USB 2.0 is comparable to firewire, in other words you have a crapful of bandwidth to work with. Enough for TV/DVD quality encoding. Your final results will probably have more to do with the quality of the hardware capture device and the final MPEG-2 encoder.

I think going with a decent PCI capture card is the best way to go. Especially if it can do stuff like MPEG-II encoding on the fly. If you're out of slots you could invest in say a soundcard with an ethernet connector to save a slot or some other weird combo card.
posted by skallas at 8:42 PM on August 1, 2004