Digital Single-Tone Generator-Detectors

Devices for pure tone generation and detection have widespread applications. The most notable examples are Touch-Tone® signaling, frequency shift keying (FSK), a n d multifrequency (MF) signaling. Associated with such devices are problems of stability and predictability, which in practice are dealt with on an individual basis, using techniques peculiar to the particular application. When these devices are realized digitally, the above problems are manifest from errors due to operational roundoff. Generally, tones for signaling are analog signals of the form A sin cot {A is the amplitude, 2t/co is the period, and u/2t is the frequency). Devices that generate these tones are usually oscillators of various kinds. Because of the requirement of structural stability, in practice these devices are limit cycle oscillators. These are simulations and realizations in hardware of nonlinear differential equations that have limit cycles. Because of the complexity of these equations, the amplitude and frequency are not easily predicted from given values of resistors and capacitors in the network. For detection of these tones, linear analog filters are frequently used. These are also used as generators, when the duration of the signal is not too long compared to the period. However, passive linear analog oscillators require inductors which are bulky, and the frequency and amplitude of these oscillators can vary with changes in value of the inductors and capacitors due to environmental conditions.