5.4. 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 the receiver side (
listener
). This is
achieved by adding an
offset
to the current time and sending the resulting timestamp with each sample
transmitted. 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 sample 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, most AVB products will use this offset. In smaller networks
with less hops or 1 GBit/s link speed, the offset can be adjusted to lower values, such as 0.3 ms, 0.6 ms
or 1 ms. In the event that the chosen offset is too low, the audio stream may experience drop-outs or
distortion.
If the digital outputs of networked devices should to be phase aligned, is necessary to choose a value
that is a multiple of a sample length (1 second divided by sampling rate). For example, given a
requirement of 1ms latency with a sampling rate of 44100 Hz: calculate the number of samples for the
given latency (in this example, 44.1 samples), and then multiply the rounded value (44) with the sample
length (1s/44100). Rounded to the next 100ns, the result is 997700ns.
Table 1. Recommended presentation time offsets ≤ 2ms, in nanoseconds
Rate
2ms
1ms
0.6ms
0.3ms
44100
1995500
997700
634900
317500
48000
2000000
1000000
625000
312500
88200
1995500
997700
623600
317500
96000
2000000
1000000
625000
312500
176400
2001100
997700
623600
311800
192000
2000000
1000000
625000
312500
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.
The M-1610 Pro offers a freely adjustable presentation time offset for each output stream.
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.
5.4.1. Adjusting the Network Latency
The default presentation time offset of 2 ms for outgoing streams can be changed when lower latencies
are required.
This setting can only be accessed on the web remote (see Section 8.5.1, “Finding the Device on a
Network”), where it can be found in the outgoing stream settings.
RME M-1610 Pro User’s Guide
5.4. AVB Network Latency |
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