Re: QuickCam frame-rate

Robert Munafo (munafo@vgi.com)
Wed, 7 Dec 1994 11:58:10 -0500


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.

At 21:37 12/6/94, Gary Vankirk wrote:
>
>I'm just wondering if everybody else is getting the same thing I'm getting
>with the QuickCam; really S-L-O-W frame rate (on a PB165c) and a lot of
>"smearing" of the image? (This is with the included QuickMovie.)

Robert P. Munafo munafo@vgi.com
mrob@world.std.com
rpm%mrob@ursa-major.spdcc.com