Recreating the Dialup Modem
Getting Online
Who am I?
What is sound?
What does data look like?
0110100001100101011011000110110001101111001000000110101001110011011000110110111101101110011001100111010101111001
How do we get data into audio?
01011
+
An Encoding Problem!
Humans are good at encoding data.
It breaks down when you don't speak the language
Language is data encoding for speech
How then do computers speak?
Computers can not just understand language.
Computers need a language and a way to speak.
How does it work?
History Time.
Communication Needed
- Fast transmission.
- Something Easy to use.
- Something Cheap to implement.
Telegraphy
Coded Signals Line of Sight
Optical Telegraph
Both sides need to know the code
Claude Chappe
1792 - 1837~
Morse Code
Pioneered by Samuel Morse
1836
Electrical Signals over a wire.
Both sides needed to be trained in morse code.
Baudot Code
Invented by Émile Baudot
1874
5 bit code to send letters
Multiplexed transmissions
Chorded Keyboard
"Baud" Comes from Baudot
Multiplex like it is 1855
🔥240 baud
🎷🔥
How is data encoded into audio?
FSK
By Frequency Shifting a Key we can encode state.
AFSK
Audio Frequency Shift
Keying
(R)TTY
~1908
Non-Skilled operators on both ends, uses Baudot Murray encoding.
Existing Medium
- Intended to carry speech
- "hacked" to carry data
- AFSK work(s)(ed) on existing infrastructure.
Modem
~1942
Connect teleprinters over regular phone lines.
Demodulates and Modulates data to be sent over standard lines.
So JSCONF right :) ?
Recreating a Modem in the Browser with Javascript!
The Oscillator Node
Allows us to change the frequency over time.
ScriptProcessorNode
Process chunks of audio JIT with a given bin size and sampling rate
JavaScriptNode
WaveTableNode
AudioWorkerNode
processor.onaudioprocess = function(e) {
let processData = e.inputBuffer.getChannelData(0);
remainder = remainder.concat(Array.prototype.slice.call(processData, 0));
remainder = goertzel(k, binsPerBit, remainder, out);
};
Converting Data
String => Binary
Bits to Audio
Data Extraction
The hard part
Extracting Bits From the Waveform
How do we get from a wave to frequency Intensity?
Goertzel Algorithm
Allows us to extract a target frequency intensity over time
Goertzel Results in:
Goertzel filter is not quite perfect.
We want it to be as close to "binary" as we can get it.
Hamming Window
Frequency Intensity
All Together now.
What if there was noise on the line?
Noisy Signals
What is next?
- Air gapped communications.
- Multiplexed Signals.
- Dynamic JS Injection.
- ✨✨✨✨✨✨✨✨✨?
JSConf UY 2015
By Sam Saccone
