Connectix QuickCam Frame Rates

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Connectix QuickCam Frame Rates

On Wed, 7 Dec 1994 Robert Munafo explained:

The QuickCam frame rate is determined by several factors:

QC Brightness setting (determines how long the CCD is allowed to gather charge for each frame)
QC Size of the image (when running certain applications, like the ones Connectix supplies)
QC Available processor bandwidth on your Mac (depends on what program(s) you're running on your Mac)
QC Maximum Mac serial port speed (depends on Mac model)

The actual frame rate you get will be the minimum of the frame rates allowed by these constraints. (The chain is only as strong as its weakest link). The formula is something like this:

framerate = MIN(1/exposuretime, 1/imagesize, bandwidth, portspeed)

exposuretime is an exponential function of the brightness slider setting: if the slider is a scale from 0 to 100, each 10 units is about a doubling of the exposure time.

imagesize is in pixels -- I think. Sometimes when the exposuretime is really high, my QuickCam frame rate seems to depend on the *linear* size of the image: twice as high, half the frame rate (rather than 1/4 as you'd expect)

bandwidth is proportional to the processor's Speedometer rating for "general" tasks. As I said, it depends on the application. CU-SeeMe, for example, chews up a lot of time doing compression, deompression and audio.

portspeed depends on Mac model and sort of (but not always) follows the processor speed.

On 4 Jan 1995 Tim Dorcey said:

I believe this has been mentioned on the list before, but it deserves re-emphasis. Unlike most cameras, the Connectix QuickCam does not have an "auto-iris," (i.e., the ability to automatically adjust to available light). Although there are certain advantages to this, a problem results with flourescent light fixtures, which apparently cycle in the amount of light they give off, producing oscillations in the brightness of the displayed video image. Aside from the annoyance of the oscillations, this behavior defeats the compression algorithm by causing the entire image to change with each frame (in the worst case), so frame rates drop dramatically. I just saw this in action, dropping what should have been 10 fps or so down to 1 fps. The only solution I can think of if you are in this situation is to smooth out the flourescent cycle with some other light source. Even if you add in the price of incadescent bulb and lamp, the Connectix is still a good deal. Note also that auto-irises are not without problems, as slight changes in the subject matter can change the amount of light reaching the lens, causing it to open or close, and introducing spurious changes in parts of the image that have not changed at all.

Have you found errors nontrivial or marginal, factual, analytical and illogical, arithmetical, temporal, or even typographical? Please let me know; drop me email. Thanks!

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