Connectix QuickCam Frame Rates
Connectix QuickCam Frame Rates
On Wed, 7 Dec 1994
explained:
The QuickCam frame rate is determined by several factors:
Brightness setting (determines how long the CCD is allowed to gather charge for each frame)
Size of the image (when running certain applications, like the ones Connectix supplies)
Available processor bandwidth on your Mac (depends on what program(s) you’re running on your Mac)
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
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 . Thanks!
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