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Frame Grabbing Hardware The vOICe Home Page Pipelined Design

Frame Grabbing http://www.seeingwithsound.com/framgrab.htm

1 of 4 20-04-2015 6:42 PM

Image samplingThe sampling, digitization and storage of a 64 64 pixel image takes place within a single 20 ms frame time, thereby avoiding blurred images.

A commercial 2.7 W Philips vidicon observation camera was used for input. This standard 625-line PAL camera delivers 312- and 313-lineinterlaced images every 20ms, of which only 64 lines are used for later conversion, by neglecting three out of every four lines in a centeredsubset of 256 lines. To avoid spurious digital codes, the digitization hardware applies Gray code encoding in a 4-bit flash AD-converter, and itincludes a sample-and-hold circuit for extremely rapid video signal transitions.

Frame Grabbing http://www.seeingwithsound.com/framgrab.htm

2 of 4 20-04-2015 6:42 PM

Why The vOICe hardware uses Gray codeWith Gray code encoding, only one bit changes in subsequent binary codes. Sampling during a noisy codetransition will then have at most one bit wrong, which implies a difference of only one quantization step. Withregular binary counting code, several bits may simultaneously change in subsequent binary codes (e.g., all 4bits change when going from 0111 to 1000), and having some of them wrong during a transition can cause abig error amounting to many quantization steps!

In general, the Gray code for a decimal integer i may be written as Gray(i) = i ^ (i>>1), i.e., taking theexclusive-OR of i itself and i shifted one bit to the right.

For the purpose of experimentation, as well as for technical reasons in prototyping the system, the observationmonitor was made an integral part of the experimental setup. This has the advantage of providing visualfeedback to the sighted experimenter or teacher on the field of view and on the quality of the camera image,concerning contrast and definition. For technical testing, this was essential. A disadvantage is, of course, thatthe present setup as a whole is not yet portable, although the special purpose computer for image-to-soundconversion is portable. Some further engineering will therefore be needed to arrive at a small set of battery-powered systems that can be carried around to evaluate to what extent people can learn to perceive, comprehend and make use of thesoundscapes. The technical feasibility has now been established.

Frame Grabbing http://www.seeingwithsound.com/framgrab.htm

3 of 4 20-04-2015 6:42 PM

Copyright 1996 - 2015 Peter B.L. Meijer

Frame Grabbing http://www.seeingwithsound.com/framgrab.htm

4 of 4 20-04-2015 6:42 PM