I was hired to design a product to teach Braille.  I was originally
looking at using a big EPROM driving a D/A, LPF, PA.  Based on
suggestions from this list, I used an ISD chip that handles 4 minutes of
audio at 8,000 samples per second (can be increased to 8 minutes at 4,000
samples per second).  It has an SPI interface.  Works real well!  I used
the speech development software from Quadravox to combine speech from
their libraries and some custom speech from wav files.  I then emailed
the project files to Quadravox and they sent me two programmed ISD chips
for $52.
        Use of the ISD with SPI interface reduced the pin count I needed on the
PIC (compared with driving an EPROM), so I went from a 16c74 down to a
16c61 (or 16c621 or whatever's cheapest at the moment).  The design runs
off 3 AA cells.  I used a TI bridged output power amp, a Maxim micropower
regulator (to get 3V for the ISD and PIC).  So, it's a three chip system.
 When idling, current draw is something like 20mA.  After being idle for
a minute, the PA shuts down and the processor goes to sleep.  Current is
less than 50uA.
        The ISD chip is very nice!  Sounds great!  They have some WAV files of
chip outputs available at their web site so you can evaluate the chip's
capabilities.  The little song about calling 911 is pretty cute...

Harold



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