ttyplay -- play audio files out of the serial port.

INSTALLATION
------------
To compile, install libsndfile and libsamplerate.  Your Linux
distribution may have packaged them for you.  Make sure to also
install libsndfile-devel and libsamplerate-devel.

Then type 'make'.

USAGE
-----
The ttyplay executable takes the name of a .wav file and plays it over 
/dev/ttyS0.  You can also supply the name of a second wav file and it
will write the simulated output to that file.

Audio will appear between TX (DB9 pin 3) and GND (DB9 pin 1) on the
first serial port.  An RC filter will help remove noise, but is not
necessary for legibile sound.

You can change the baud rate in the source code.  Default is 115200.
Generally, higher baud rates sound better.  My laptop supports up to
460800 baud.

THEORY
------

The input audio is first resampled to the baud rate of the serial port.

Then, a sigma-delta modulator modifies the audio samples into
a binary bitstream.  This modulator has been modified to take into
account the start and stop bits of the serial bitstream.  These are
within the feedback loop, so their error is minimized / noise shaped.

Finally, the computed bytes are written to the serial port.

SPECIAL CONSIDERATIONS
----------------------
This hack depends on the serial port transmitting bytes "back to
back".  On a slow system with high baudrates, this may not be the
case.  

Do not connect headphones to the output if you value your hearing.

A loudspeaker presents a pretty heavy load to the serial driver.  I
haven't blown my serial port up, but I'm not responsible if you kill
yours.  It would be better to connect to the input of an amplifier,
but keep the volume down.

If a headache develops, discontiue use.

Good luck.

Mark
markrages@gmail
10/22/2005