About Bell 103 modulation

The Bell 103 modem dates from 1962 and many conventional modems still fall back on this modulation. The originating station uses 1270 Hz bleeps for mark (or binary 1) and 1070 Hz for space (or binary 0).

Its baud rate is 300 bps, with a leading space and a trailing mark after each 8-byte and no parity bit. That is a serial communication convention called 8N1 and is the same one an old-school PC serial port uses.

This also means that each "word" is in fact 10 bits, each carrying 8 bits of data, sandwiched between start and stop bits.

Most documentation about Bell 103 insist on it being intended for carrying 7-bit ASCII, but this implementation supports Unicode UTF-8, so you can even send emoji or non-Latin alphabets like Crylic or Arabic without switching modes.

About Radioteletype (RTTY)

Radioteletype can be heard on shortwave and is used for transmitting weather information by DWD, by radio amateurs and by "diplomats". It is also used over the phone by TDD devices and is probably the most common series of bleeps and bloops you hear on the medium-, long- and shortwave bands on the radio.

This implementation of RTTY uses LSB-on-right (big-endian) ITA2 Baudot code, which is a character encoding dating all the way back to 1924. The baud rate is 50 bps, and we use the European convention of 2125 Hz for mark and 1955 Hz for space.

We don't support mode shifting to Crylic, weather symbols or other variant codepages, so only the English alphabet is supported here along with spaces and new lines (Carriage Return and Line Feed).

About this app

This is an educational tool intended for demonstrating different frequency shifting data modulation standards. Use it for teaching and learning, sending secret messages to your friends or for generating sci-fi sound effects.

There are no plans to implement proper signal processing for receiving messages (RX) and the signals generated by Bleeper have not been tested on real hardware.