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AVB Networking
2.7 Clocking
StudioLive™ Series III
AVB Networking Guide
2.7
Clocking
All the audio traffic on an AVB network is synchronized using a global clock so
that audio can be played and recorded while remaining in time from multiple
sources. Obviously, the more audio traffic on a network, the more critical
this becomes. For users familiar with traditional digital audio devices (ADAT,
S/PDIF, etc.) the idea of a global clocking device will not seem unfamiliar.
PreSonus AVB devices have two clocks: one wordclock and one PTP clock.
All AVB devices on the network are synchronized to a common reference time using
the IEEE 802.1AS Precision Time Protocol (PTP). Each stream includes a presentation
time that all the devices in the network use to align their playback by comparing the
presentation time in each stream packet. An advantage to this design is that AVB
networking supports multiple simultaneous sample rates and sample clock sources
which is important for applications where audio and video need to be synchronized,
even though they travel along different paths with different sample rates.
2.7.1
Wordclock
Analog audio is transferred through a cable as a continuous electrical
waveform at almost the speed of light. Because of this, audio signal traveling
from one analog audio device to another arrives nearly instantaneously,
for all practical purposes. Therefore, you don’t have to synchronize analog
audio passing from one analog device to other analog devices.
Transferring digital audio is a very different matter. Computers and other
digital devices operate one step at a time, which happens very quickly but it’s
not instantaneous, and digital signals are not inherently in perfect time. While
uncompressed digital audio plays at a fixed rate (i.e., the sampling frequency), digital
clocks are not perfect; their frequency can drift, and they almost always have at
least some irregular errors, known as jitter. Therefore, two devices, each following its
own clock, are highly unlikely to stay in agreement about precisely when a sample
starts and ends. The result is usually an artifact, like a pop or a glitch in the audio.
To avoid this problem, all digital devices in communication with one another
need to follow a single master clock. That means the master clock must send a
signal that essentially says, “everyone start at this moment and follow me!”
Even if the master clock’s timing is imperfect, all the slave devices will
follow the timing errors exactly and will stay in sync with each other,
eliminating timing-related artifacts. In general, the better the master clock,
the better the resulting audio will sound, so whenever possible, use the
best clock you have, or experiment with your rig to find the best result.
Whenever digital audio devices are synchronized, it is necessary to designate
one device as the “master” wordclock device to which all other digital devices
are synced, or “slaved.” Once you’ve determined which device is to be your
master clock, you will need to sync the remaining digital devices.
The problem of designating a master wordclock is not handled by AVB and
must be set with an AVB controller. Depending on the device, this can be done
manually, by the user, or managed automatically. For example, when setting up
a StudioLive rack mixer as a stage box from a StudioLive console mixer, the rack
mixer is automatically setup to slave from the console mixer’s media clock.
Multiple unrelated wordclocks can co-exist on an AVB network. While there are a
few ways to match wordclocks between a talker and listener, PreSonus AVB devices
currently only support recovering the media clock by listening to the first AVB stream.
