Acoustic FSK data link — browser to browser, over the air, no cables.
UltraModem is a pure JavaScript and Web Audio API implementation of a data-over-audio communication system. It allows two devices running a web browser to transmit and receive short text messages through the air using high-frequency sound, effectively turning their speakers and microphones into a modem.
This project runs entirely in the browser, with no server-side components, frameworks, or build steps required.
- Open the application on two separate devices (e.g., a laptop and a phone). You can start at the main
index.htmlpage. - Navigate to Sender Mode: On one device, choose ▲ SENDER.
- Navigate to Receiver Mode: On the other device, choose ◉ RECEIVER and click the ◉ LISTEN button to activate the microphone.
- Position the devices: Place the devices a few feet apart in a quiet room, with the speaker of the sender facing the microphone of the receiver.
- Transmit Data: On the sender device, type a message (up to 64 characters) and click ▲ SEND.
- Receive Data: Watch the receiver's UI as it locks onto the signal, decodes the bits, verifies the checksum, and displays the final message.
UltraModem uses Frequency-Shift Keying (FSK) to encode binary data. A 0 is represented by a tone at 18500 Hz and a 1 is represented by a tone at 19500 Hz. These near-ultrasonic frequencies are at the edge of human hearing, making the transmission relatively unobtrusive.
-
Sample-Accurate Timing: Instead of relying on JavaScript's
setInterval(), which is prone to event loop jitter, the receiver uses aScriptProcessorNode(orAudioWorklet). This allows it to process audio in exact, sample-accurate chunks, ensuring the demodulator never loses sync with the incoming bitstream. -
Efficient Goertzel Algorithm: To detect which frequency is present in an audio chunk, the demodulator uses the Goertzel algorithm. Unlike a full Fast Fourier Transform (FFT) which calculates the magnitude of a wide spectrum of frequencies, Goertzel is highly optimized for detecting the strength of a few specific, pre-determined frequencies. This makes it significantly more efficient and provides better signal-to-noise performance for this use case.
-
Robust Preamble Synchronization: A transmission begins with a
101010...preamble. The receiver continuously scans the incoming audio for this pattern. To achieve a lock, it correlates the recent history of detected bits against the expected preamble pattern at multiple sub-bit phases. Once a strong correlation is found, the receiver is "locked" and knows the exact sample index where each subsequent data bit begins.
Each data packet is carefully structured for robust transmission:
| Preamble | Start Marker | Length (8 bits) | Data (N*8 bits) | Checksum (8 bits) | End Marker |
|---|---|---|---|---|---|
1010101010101010 |
11111111 |
0...N |
... |
... |
00000000 |
- Preamble (16 bits): An alternating sequence of 1s and 0s that allows the receiver to detect a signal and synchronize its clock.
- Start Marker (8 bits): A unique pattern that signals the end of the preamble and the start of the payload.
- Length (8 bits): An unsigned integer specifying the number of bytes in the data payload.
- Data (Variable): The raw text message, with each character converted to its 8-bit ASCII representation.
- Checksum (8 bits): A simple XOR checksum calculated from the data payload, used for error detection.
- End Marker (8 bits): Signals the end of the packet.
The core transmission parameters can be adjusted in main.js:
FREQ_0/FREQ_1: The frequencies for bit0and bit1.BIT_MS: The duration of each bit in milliseconds. A shorter duration increases the data rate but may reduce reliability. The default is 50ms (20 bits/sec).PREAMBLE: The bit pattern used for synchronization.
No build process is required. Simply serve the project directory with a local static file server.
While any server will work, we recommend using the Node.js-based http-server as it is a standard tool in modern web development and provides more robust features than Python's built-in server.
Recommended (Node.js):
npx http-serverAlternative (Python):
python -m http.serverOr with Node.js:
npx http-serverThen open http://localhost:3000 (or the appropriate port) in your browser.
This project is open source and available under the MIT License.
Copyright © 2026 Ronen Singha. All rights reserved.