I read in sci.electronics.design that Tom Potter
wrote (in ) about 'Cheap, portable,
optical scanner', on Sat, 19 Mar 2005:
I would appreciate any input and ideas about
producing an audio signal that models a surface, in a small portable,
inexpensive package.

A pickup cartridge for vinyl records is made for this job. You would
need an amplifier to connect it to the MP3 player.

There are two sorts of cartridge - piezo electric and magnetic. The
piezo type has an output proportional to stylus displacement, but the
sensitivity steps down from the low-frequency value at 500 Hz and below
to half that value at 2 kHz and above (roughly). The piezo type is a
capacitive source and requires an amplifier with a high resistive input
impedance.

The magnetic type has an output proportional to stylus speed, with the
above stepped response superimposed on that. It has an inductive and
resistive source impedance and requires a specific resistive load,
normally 47 kohms.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk