AVB Network Latency

All devices in an AVB network share the same time. This allows the sending device (talker) to specify the precise point of time when its audio samples should be played out at any receiver (listener). This is achieved by adding an offset to the current time and transmitting the resulting timestamps along with the audio payload. The timestamp is called "presentation time" and has nanosecond precision. For comparison, a single sample at 48 kHz has a duration of over 20800 ns.

The receiver compares the incoming presentation time of each frame to the current time and buffers the sample until the presentation time is has come.

The offset (maximum transit time) is specified by the AVB standard as 2 ms for class A traffic, which is enough time for the signal to pass through a very large network under full load with over ten Gigabit switches ('hops') along the way. By default, certified AVB products will use this offset, even though it results in a significantly higher latency than necessary in most cases. In smaller Gigabit networks, the offset can be adjusted to lower values, such as 0.3 ms, 0.6 ms or 1 ms. It can be adjusted for each stream individually.

If the digital outputs of listener devices should to be phase aligned to the talker, is necessary to choose an offset that is a multiple of a sample length (which is 1 divided by sampling rate). Using the web interface, the offset can be conveniently set in samples for each stream to ensure phase alignment between talker and listeners across the network.

In the event that the chosen offset is too low, the audio stream may experience drop-outs or distortion. However, the maximum transit time to a specific listener is known, and an ATDECC controller can show a warning if the intended presentation time is earlier than the calculated maximum transit time for that stream.