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 up to seven 100 MBit/s switches 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.

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 (1 second 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.

Table 1. Recommended presentation time offset (estimated maximum transfer time) depending on network size, in samples, for Gigabit networks

Rate (Hz)

1 switch
(~0.3 ms)

≤3 switches
(~0.6 ms)

1 ms

2 ms
(default)

44100

14

27

44

88

88200 (x2)

28

54

88

176

176400 (x4)

56

108

176

352

48000

15

30

48

96

96000 (x2)

30

60

96

192

192000 (x4)

60

120

192

384

In the event that the chosen offset is too low, the audio stream may experience drop-outs or distortion.

The RME Digiface AVB shows the remaining offset ("input delay") for the first incoming stream, which is useful to verify that a shorter setting for the existing network can be used without risking dropouts.
In AVB networks, the latency is always specified by the talker and guaranteed by the listener. This behavior is plug and play and does not require any user interaction or monitoring for the entire duration of the stream.