braindrizzle

[Brian Dessent]

bcroe at juno.com wrote:

> A digital lookup table can modify a curve.  If the
> curve relating 2 different voltages ever changes
> from positive slope to negative (or the reverse), you
> not only need the level, but also the slope, to convert

This is true, but I don't really think there's much demand for a sin ->
cos transform (or any other frequency domain transform other than basic
filtering.)  What *is* useful is the ability to do nonlinear transforms,
i.e. convert an exponential response to a linear response, and vice
versa; or any other input/output relationship that cannot be expressed
with a simple gain & offset. 

> "universal" pattern where you chose the voltage
> translation you want from a table, and it tells you
> which resistors to solder into the pattern.  Inverting

The problem here is that you can only do a linear transform, i.e. only
scaling and/or adding an offset.  In the case of the WB O2 output for
example it might be much easier to interpret if the LED display was
logarithmic rather than linear.  (I have not looked at the actual
circuit or the specs of the sensor, so I don't know if this is already
being done.)  And there's the general case of just wanting to tune an
output such as throttle position sensor so that (for example) the gain
is high at low throttle/idle and yet does not saturate at WOT -- the
equivalent of replacing a linear taper pot with a audio (exponential)
taper pot.

I think the most useful "building block" would be some PIC code where
the user supplies up to say 10 points of an input-output transformation
and the PIC handles the interpolation.  This of course can be used to do
a routine linear transform (by only specifying two points) in the same
way as a gain+offset opamp circuit, but it could also be used to expand
or compress the signal to make it easier to interpret by a human or to
interface with another component.  I'm sure this is a very common
application and there ought to be u-processor code already written, it's
just a matter of settling on the details of A/D and D/A conversion and
writing up the circuit so that it can be used in the building-block
fashion.

Brian
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