Method and apparatus for delta modulator and sigma delta modulator

According to the invention, oscillating signals are generated from analog signals by providing an analog signal having a variable slope or amplitude to a circuit with a variable operating point and having a transfer function characterized by an unstable operating region bounded by a first and a second stable operating region. The unstable operating region contains a first and a second reference point. The circuit is capable of producing an oscillating signal having a variable duty cycle, the duty cycle increasing as the variable operating point is positioned closer to the first reference point, the duty cycle decreasing as the variable operating point is positioned closer to the second reference point. The variable operating point is positioned substantially within the unstable region to produce the oscillating signal. The positioning of the operating point relative to the first and the second reference points is a function of the variable slope or amplitude of the analog signal. The oscillating signal is capable of being used to directly or indirectly generate a delta modulation or sigma delta modulation signal corresponding to the analog signal.