Abstract: A modem which can automatically switch between protocols at different signalling rates. The protocal with non-standard signalling rates has a training sequence which initially has distinctive frequency-domain characteristics. Thus, as soon as one modem begins to send a training sequence, the other modem can distinguish which training sequence has been started. Thus, speed changes between protocols at different signalling rates can be remotely initiated.

Abstract: Characteristics of a pulse sequence, such as frequency, are controlled in a simple circuit without requirement for specific storage components. The cumulative effect of a sequence of control signals is maintained by providing a phase shift between a pair of waveforms. The phase shift between the waveforms is used to provide a number of control pulses to the circuit for varying the desired characteristic. In a particular embodiment, control signals are externally provided for varying the phase shift between two waveforms. In response to the phase variation, a number of control signals are provided for adding or deleting pulses to a pulse stream. The use of feedback control loops results in repeated addition and/or deletion of pulses to or from the pulse stream, thereby varying the output frequency in accordance with a one time input of the external control signals.