Frequency shift keying (FSK) is a data recording or transmission method whereby binary data is recorded or transmitted as one of two frequencies. It is sometimes called double pulse modulation, but this term also includes phase modulation (PM).
In FSK, a sequence of binary ones is recorded at twice (or half) the pulse rate of a sequence of binary zeroes. The ratio of frequencies, or pulse rates, need not be 2:1 but this ratio has many advantages. It is usually a result of compromise between bandwidth requirements and frequency filtering requirements.
Among the advantages of FSK are self-clocking, i.e., the pulse timing is included in the same signal with the data. FSK has narrow bandwidths compared with other recording or transmission techniques having comparable bit densities. This feature permits the use of narrow band filtering which improves the signal-to-noise-ratio and noise immunity. The spectral density of FSK reveals that most of the power (or energy) is contained in a narrow spectrum.
Among the disadvantages of FSK is the requirement for synchronization after signal loss by using special data conditions. At high frequencies, FSK usually operates in the descending region of the gap-loss curve of the reading head which results in pulse interference and low play back voltages. To reduce noise, a-c erasure is preferrable to d-c erasure.
Another disadvantage of FSK is the complex reading circuitry. Either sharp filters or phase-lock-loop circuits are used for reliable reading and these are sensitive to speed variations of the recording medium.
The invention disclosed herein permits the reading of FSK coded data using a simple circuit for decoding. This is especially useful when using inexpensive audio recorders as data input devices in small minicomputer or microcomputer systems. Speed variations, within certain limits, can be tolerated using the circuit of the invention. Furthermore, when used with asynchronous data terminals, such as Teletypes, multiple transitions used to indicate a cell time do not require a special decoder.