Since the beginning of time, we have accustomed our brains to seeing events before hearing the sounds that accompany them. This is because sound travels at 330 meters per second, and light at almost 300 million meters per second. When the situation is such that the sound is heard before being able to appreciate the accompanying visual representation (such as when listening to the audio before watching the video), the observer is upset, as the situation does not accord with the natural law of physics.
The delay between audio and video (A/V) occurs within transmission networks when both signals are processed separately. The processing of high-bandwidth digital video signals can span several fields to result in an output signal, while audio uses much less bandwidth than video, and requires a shorter processing period to produce an output signal. It is necessary, therefore, to take into account the processing time of each of the signals when designing the new A/V infrastructure, and also insert fixed delays in the audio signal to prevent it from being heard before the visual signal appears.
The increasingly complex function of routing, distributing and processing digital signals in multiple channels of audio and video has caused increasing problems in trying to maintain synchronization between both signals within the system. Small, undetectable delay errors between audio and video accumulate through the system, resulting in a very noticeable error at the end of the distribution channel. Therefore, it is important to measure and establish what the level of synchronization is at various points throughout the system.
Usually, the operator who "manually" listens and observes the program material is the one who must determine the presence of errors. To achieve this, he must find visual clues within the image to determine if the sound heard corresponds to it and offers synchrony. The most common and obvious method of doing this is to listen to a person speak, observe the movement of the lips and check the synchronization between it and the sound. When there are no visual clues in the image to establish synchronization, as in the previous example, it is difficult for the operator to determine if there is a delay in the production of the signals.
