nather@utastro.UUCP (Ed Nather) (11/04/83)
:-<Non-blank line>-:
Consider the retina, and its processing algorithm. It is certainly
true that once the raw information has been collected and in some way
band-limited, it can be processed in either fashion; but one part of
the algorithm must necessarily be implemented in parallel. To get
the photon efficiencies that are needed for dark-adapted vision
(part of the specifications for the algorithm) one must have some
continuous, distributed attention to the light field. If I match
the spatial and temporal resolution of the retina, call it several
thousand by several thousand by some milliseconds, by sequentially
scanning with a single receptor, I can only catch one in several-
squared million photons, not the order of one in ten that our own
retina achieves.
There seems to be a misconception here. It's not clear to me that "parallel
processing" includes simple signal accumulation. Astronomers use area
detectors that simply accumulate the charge deposited by photons arriving
on an array of photosensitive diodes; after the needed "exposure" the charge
image is read out (sequentially) for display, further processing, etc.
If the light level is high, readout can be repeated every few milliseconds,
or, in some devices, proceed continuously, allowing each pixel to accumulate
photons between readouts, which reset the charge to zero.
I note in passing that we tend to think sequentially (our self-awareness
center seems to be serial) but operate in parallel (our heart beats along,
and body chemistry gets its signals even when we're chewing gum). We
have, for the most part, built computers in our own (self)image: serial.
We're encountering real physical limits in serial computing (the finite
speed of light) and clearly must turn to parallel operations to go much
faster. How we learn to "think in parallel" is not clear, but people
who do the logic design of computers try to get as many operations into
one clock cycle as possible, and maybe that's the place to start.
Ed Nather
ihnp4!{ut-sally,kpno}!utastro!nather