Storing Serial Data On An Audio Cassette


The purpose of this project was to store serial data sent from a computer to an audio cassette.
The maximum achieved Baud rate using this setup at which no errors occured during the period over which it was tested was ~300 Baud.
At this Baud rate, the maximum amount of data which can be stored on a cassette is ~270KB
The modulator circuit ANDs the serial data with a high frequency carrier, making an amplitude modulated signal, which can be recorded to the tape.
The demodulator circuit rectifies the output from the cassette player, before partly smoothing it with a capacitor.
Since the carrier wave is of a higher frequency than the data, the variation in the amplitude of the carrier after smoothing is less than that of the data.
Now a comparator is used to detect this difference in voltage, and convert the data represented thereby to a square wave of suitable form and amplitude to be recieved by the computer as serial data.

The Waveforms image shows an example of what the waveforms at various points should approximately look like.

R1 changes the input voltage level to the cassette recorder.
R2 changes the comparator threshhold. Change this until correct waveform appears at RXD.
Comparator supply voltage should be +-12V

It is advisable to ensure that the voltage at the output of the comparator does not go below 0V or above 12V
so as to reduce the risk of damage to the computer.

The 10KHZ square wave input can be adjusted to give a higher frequency, allowing a higher Baud rate to be achieved.
However, the maximum frequency response of a cassette tape (or perhaps of the cassette recorder itself) is about 30KHZ.

You can download PuTTY, a serial terminal application, here, or you can find the latest version here.

Flow control must be turned off.

Modulator schematic


Demodulator schematic



  • U1-AND gate
  • U2-Comparator
  • R1-100K linear potentiometer
  • R2-100K linear potentiometer
  • D1-Germanium/Schottky diode
  • C1-10-100nF capacitor


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