Method and apparatus for synchronization between real-time sampled audio

Abstract

A method and apparatus for synchronization between an originating node and an answering node. Given the fact that it is impossible to expect that independent time bases at the originate and answer node will oscillate at exactly the same frequency, the time base at the answer node is constrained to maintain an average frequency which is identical to that established by the originate node. The answer node monitors the status of a buffer in respect of almost-full and almost-empty conditions. A binary timing control signal is generated which switches to one state in response to an almost-fill condition and switches to an opposite state in response to an almost-empty condition. When the buffer is in an almost-full state, the operating clock at the answer node is controlled so as to be slightly higher than the frequency established by the originate node. In this way, a buffer which is in an almost-full state will tend to be emptied more rapidly (by the answer node's clock) than it is filled from the originate node. On the other hand, when the buffer is in an almost-empty state, the operating clock (of the answer node) is controlled so as to be slightly lower than the clock established at the originate node. In this way, the answer node buffer will be emptied more slowly than it is being filled from the originate node. Since both nodes must actually be capable of both originate functions and answer functions, the clock at each is capable of operating at three different frequencies, the center frequency corresponding to originate node operation and the higher and lower frequencies are used in answer node operations. In a preferred embodiment, the pulse train output from an oscillator is subjected to deletion of a smaller or larger number of pulses-per-unit time. The high frequency operation (at an answer node) uses the unmodified pulse train from the oscillator. Originate node functions use the pulse train output from the oscillator which is processed by deleting a first number of pulses-per-unit time. The lower frequency operation (at an answer node) is implemented by processing the pulse train from an oscillator by deleting a larger number of pulses-per-unit time.