Hi, after all the discussion recently about doing
sound on a PIC, 1bit systems etc, I came up with an
idea to refine the process of 1bit encoding to be
PIC friendly and it actually worked. It's fast enough
to do 16bit to 1bit conversion real time at up to 156
kbit/sec! It also gives good results with speech even
with quite low bitrates. Great for making cheap
talking remote controls and stuff for the visually
impaired.

Playback only requires one or two digital output pins
and RC filter and very simple bitstream code.

I played with a few systems, but the encoder that worked
best was a closed-loop math model of the RC filter.
Each new bit is decided by generating a possible
hi and lo bit result, then models the analogue voltage
that would occur on the RC filter, and picks the bit
that gives the result closest to the original wave.
Being closed loop simulation it follows the original
wave quite well and gives good sound quality for
low bitrates and very little record or playback
hardware.

I wrote some software that shows the waveform and
the encoder waveform, with some adjustments to the
encoding parameters. It models the RC filter and
even tells you values of R and C to use.

Then one button press to write the encoded bitstream
to a data file or even a PIC .asm file with the
sound data already formatted in "retlw" table
format. You get a few seconds of speech on a 8kb
PIC. Handy. :o)

It's really a "PIC sound suite" that allows you to
load your own sound waveforms in, view them, try
different encoding formats and and see how the
sound will turn out as it is plotted on screen.
Then dump the bitstream data straight into your
PIC. One thing I want to add is a RC filter attached
to the PC parallel port, so I can hear the sound
on identical hardware to the product, and know
exactly how it will sound when the PIC plays it
back later in the product.

I wanted a system where I just had to push one
button to add any sound or speech to any of my
PIC projects, and it's pretty close to that. With
the ability to record too, hmm, now I just need a
few Mb of fast static ram...
-Roman

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