NAME
	fodtmf - Transfer files over audio.

ETYMOLOGY
	File over DTMF (dual-tone multi-frequency signaling)
	~    ~    ~~~~  ~    ~    ~     ~

SYNOPSIS
	fodtmf-send [OPTIONS] [< FILE]

	fodtmf-recv [OPTIONS] [> FILE]

DESCRIPTION
	fodtmf-send shall send the file FILE over audio
	encoded with dual-tone multi-frequency signaling.

	The message will be enhanced with hamming code
	for error correction, and optionally with
	redundant frequencies to help combat noice in
	the environment.

	At the end of the transfer, an 'end of transmission'
	byte will be send. If the message is cancelled, a
	'cancel' byte will be send. The end of the message
	will be padded with this byte until a full error
	correcting code has been constructed.

	For any 'data link escape' byte, any 'end of
	transmission' byte, and any 'cancel' byte in the
	transmitted file, a 'data link escape' byte will
	be inserted directly before it.

	fodtmf-recv shall receive a file sent using
	fodtmf-send.

OPTIONS
	-f MULTIPLE
		In addition to any tone with the frequency
		F, also send, simultaneously a tone with the
		frquencies F times MULTIPLE. This is adds
		redundancy to improve the reliability of
		the transfer. Redundant tones are not added
		by default because they make the tones less
		pleasant.

	-r N
		In each error correcting code, include N
		parity bytes. By default, this is three. The
		higher this value is, the less redundancy the
		code will have. The number of bytes per code is
		calculated by 2^N - 1, and the number of data
		bytes per code is calculated by 2^N - N - 1.

		There are two exceptional values, that cases
		the message to be transferred without any
		hamming code for error correction:

		0	Do not include any redundancy at all,
			apart from any specified using the
			flag -p.

		1	Only one redundancy byte per code,
			apart from any specified using the
			flag -p.

		This value must be non-negative.

		If this value, plus 1 if -p is used, equals
		1, the receiver may be able to spot errors,
		but has no chance of repairing them, unless
		additional error correction mechanisms are
		added. If it equals 0, the receiver will
		be unable to spot errors unless additional
		error correction mechanisms are added.

	-p
		Include one extra parity byte per error
		correcting code. This byte will be the
		parity of all data bytes.

	-o FD
		For fodtmf-send:
		Write the audio to file descriptor FD rather
		than playing it back. This is intended for
		development purposes only. The written file
		will not contain any metadata to hint how
		it is encoded and how it should be played
		back.

		For fodtmf-recv:
		Write a list, of all codes (by index) that
		where not properly received and could not
		be corrected, to file descriptor FD.

	-s FD
		Read input from file descriptor FD rather
		than from the microphone. This is intended
		for development purposes only.
		Only available for fodtmf-recv.

	Each option can only be used once.

NOTES
	It may be a good idea to compress the file before
	transmitting it. For example

		xz -e9 < your-file | fodtmf-send    ## sender
		fodtmf-recv | xz -d > your-file     ## receiver

	But note well, which compresion algorithm is most
	efficient depends on the file.

FUTURE DIRECTIONS
	afodtmf (Advanced File over DTMF) will be written. It
	will make the computers communicate with each other
	bidirectionally (limited to one-to-one communication)
	to repair errors that could not be correct using
	the error-correcting code.

RATIONALE
	I really don't know precisely why. But it sounded fun.

SEE ALSO
	scp(1), nc(1)