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Vesta Pro (PCVC680K) picture quality tests
During my webcam CCD astrophotography practice I noticed, that picture quality, what you can get from Philips Vesta Pro webcam is depending on selected frame rate. Philips Vesta Pro enables to select from 6 frame rates: 5, 10, 15, 20, 25, 30 fps. Connection between frame rate and picture quality is evident from camera's video source setting dialog. Here is part of dialog window showing frame rate options:
The reason of above facts is limited throughput of USB. Vesta Pro is using YUV420 codec, which requires 12 bits per pixel. That means, that, for example, 640x480 pixels frame has size 460800 bytes. For 5 fps video stream it requires 2304000 bytes/s - it is more than throughput of USB. That's why there must be used some compression of video data, which are sent through USB. As my measurements confirmed, the compression is lossy.
I remind, that it is difference between shutter speed and frame rate. Shutter speed is always equal or higher than frame rate (for Vesta Pro). That means, that for lower frame rate it is possible to setup the same exposure like for higher frame rate. But in reversal it is not valid.
The next table is showing results of my tests. I
captured my red lamp (similar to Mars) light. Exposure
was set to manual settings (shutter speed 1/100s), so the
same conditions were keeped for all captured video files.
The only parameter, which was alternated, was frame rate.
To ensure the highest quality of the images on this page
with the reasonable size, the result bitmap frames were
cropped and compressed to JPG (quality 100%, smoothing 0).
Complete package with bitmaps can be downloaded here (325kB).
It would be interesting to do the similar tests with
lamps of another colors.
The most visible difference can be seen in 640x480
pixels frames - pictures with frame rate higher than 15
fps are blurred - it seems that image is only a result of
640x480 interpolation of 320x240 image.
The next table is showing the stacked 640x480 images processed by unsharp masking. The artifacts in 10 and 15 fps images are visible even though images are not magnified.
The last table compares the 320x240@5fps image with downsampled 640x480 images to the same size. It shows, that better result is achieved with downsampled image of 640x480@15fps than with 320x240 image. But downsampled 640x480@25fps image seems to be of worse quality than 320x240@5fps image. Downsampled images were a little unsharp-masked.
For better understanding YUV420 format I created 2 AVI
frames - 320x240 and 640x480 with the same scene (5
frames per second). YUV frames were extracted from AVI by
means of K3CCDTools. U, V
planes were resampled (simple double-sizing) to the size
of Y channel.
Another example of resolution power (320x240 format):
For further reading:
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Last Update: 24.2.2002