DTMF-based data transfer

Usage

• get your byte string
• append a CRC8 checksum as a last byte
• convert to hex string
• convert each hex digit to DTMF tone code, replacing f -> # and e -> *
• generate DTMF sound sequence: 100ms per tone, no silence between tones (see test.wav for example)
• build and run the binary via arecord -f FLOAT_LE -r 48000 | ./go-r2d2, it will start waiting for input
• play generated sound, if all is OK binary will terminate and output message to STDOUT
• retry playing without restarting binary in case of failure
• run binary with --debug 2 for maximum debug

Encoding

Sample python encoder:

import crc8

h2d = lambda x: "*" if x == "e" else ("#" if x == "f" else x)


def dtmf_encode(message):
    """
    encode given message bytestring as dtmf_codes with laste symbol of crc32 checksum
    """
    bytes_msg = map(ord, message + crc8.crc8(message).digest())
    print 'bytes are', bytes_msg
    hex_msg = "".join("{:02x}".format(a) for a in bytes_msg)
    print 'hex is', hex_msg
    return "".join(map(h2d, hex_msg)).upper()

print dtmf_encode('hello, world')
# outputs 68656C6C6#2C20776#726C6456

Build

Just run go get && go build for local build.

Run env GOOS=linux GOARCH=arm64 go build to build for arm, for example.

Test

A sample record and a script is provided. Run go build && ./test.sh and observe exit code.