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The Simple Universal Modem is a small open-hardware circuit that converts digital signals into audio for recording, then turns playback audio back into a digital signal. It is designed for retrocomputer projects and builders using PCs or microcontrollers with audio-recording equipment. “Universal” refers to its format-agnostic demodulation approach—not guaranteed compatibility with every tape deck, cable, tape, or vintage computer.
What the modem does
A cassette recorder or other audio device can store digital information as sound. The Simple Universal Modem sits between digital hardware and that audio path: its transmit circuit makes a digital signal more suitable for recording, and its receive circuit converts recorded audio into a digital waveform for a computer or microcontroller to decode.
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Anders Nielsen’s project documentation describes the circuit and reports demonstrations; the results are the designer’s own, not independent compatibility or performance testing.
How the transmit and receive paths work
Transmit: digital signal to recordable audio
The board expects a 1200/2400 Hz frequency-shift-keyed (FSK) square wave at its digital data input. A five-stage RC filter and emitter-follower buffer shape that signal into a sine-like audio output. The purpose is to avoid feeding a harsh square wave directly into the recording device.
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Nielsen recommends connecting the output to a line-in or AUX input. A microphone input may work, but its gain should be reduced; excessive input gain can distort the signal. The project does not provide a controlled comparison across recorder models, so the correct level may require adjustment for the particular equipment.
Receive: playback audio to digital signal
For reception, the designer specifies approximately 1 V peak-to-peak at the audio input. The input is capacitively coupled, and an LM393 comparator acts as a zero-crossing detector, turning the audio into a 5 V square-wave output for downstream digital decoding.
As Nielsen puts it: “The demodulator consists of a LM393 comparator set up as a zero crossing detector, outputting a nice clean square wave for digital circuits to decode.”
What the designer demonstrated
Nielsen reports saving and playing back 1200 bps Kansas City Standard data with an ABN6502 SBC R1. The project documentation also says the receive circuit was tested with computer audio and tape using 1200/2400 Hz FSK. These are author-reported demonstrations and tests, not a guarantee that every computer, recording, encoding, or tape deck will work.
The project page describes the board as less than 50 × 15 mm. Both that size and the reported 1200 bps demonstration are project details attributed to Anders Nielsen in 2022; they are not independent measurements or benchmark results. The documentation does not establish a maximum data rate, error rate, reliability figure, supported-encoding matrix, or archival lifetime.
Check your recorder, cable, and signal levels
The project is intended for equipment that can record and play the relevant audio signal. That may include cassette decks, digital voice recorders, or reel-to-reel machines, but the documentation does not establish compatibility model by model. Before connecting anything, check the following:
- Input type and level: Prefer line-in/AUX for recording the modem’s output. If using a microphone input, reduce its gain. For reception, the designer specifies about 1 V peak-to-peak audio at the board input.
- Connector and wiring: The board’s TRRS poles are jumper-selectable because TRRS pinouts vary. The project notes distinguish common iPhone-style and Android-style wiring. Do not assume two 3.5 mm TRRS cables have the same pin assignments.
- Deck connections: Nielsen describes connecting a reel-to-reel deck with a 3.5 mm-to-RCA cable and setting the jumpers for audio input, output, and ground. Verify the cable wiring and jumper positions for your actual setup.
- Digital encoding: Your computer or microcontroller still needs to generate and decode the chosen FSK data format. The analog board does not by itself interpret arbitrary computer files or tape encodings.
- Task: Decide whether you are making new recordings or trying to recover existing ones. The project gives circuit and setup guidance, but no controlled comparison of media or playback equipment.
Building the design and handling the data
Hardware files and board status
The project page says KiCad files, Gerbers, and a PDF schematic were uploaded for people who want to make the design. It also says the PCB layout had not yet been tested at the time of that update, while Nielsen described moving the circuit onto a small breadboard and demonstrating it. Downloadable design files and a demonstrated breadboard circuit are distinct from a currently manufactured or stocked finished board; the documentation does not establish present-day board availability.
Generating and decoding FSK
Nielsen describes using a timer to generate the square wave and timer/interrupt handling to count transitions on the ABN6502. The project also suggests minimodem for converting binary data to a selected format, and notes that timer-and-interrupt approaches can suit Arduino-class microcontrollers. These are implementation suggestions, not a turnkey software package for every platform. You will need to match the signal generation and decoder to the format and hardware you intend to use.
What “universal” does—and does not—mean
The project’s demodulation approach is intended to avoid tying the receive circuit to one specific computer-data format. That does not make every recording interchangeable: signal level, FSK frequencies, software decoding, connector wiring, recording quality, and device behavior still matter. The available project details do not quantify error rates or establish a complete list of supported encodings, so treat compatibility as something to verify with your particular setup rather than assume.
Quick Recap
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