All devices on an AVB network have a clock which is automatically synchronized using the Precision Time Protocol (PTP). However, this does not mean that every AVB talker on the network automatically uses a synchronized sampling rate for the audio stream it sends ("Media Clock") . For example, one device might generate an 48 kHz media clock with an external oscillator; another device might derive an 48 kHz media clock from its PTP clock; yet another device might synchronize to an ADAT lightpipe input; etc. While these sampling rates are nominally the same, the clocks are not synchronized. To generate AVB streams with synchronized media clocks in multiple devices, a media clock has to be generated in one device and distributed to the others. At the moment, the only way to distribute a media clock is to act as a talker and provide a stream (which does not neet to contain any actual audio channels) to other devices, which may then recover the media clock from this stream and generate their own talker stream with the same media clock. In the XMOS AVB reference implementation, if an input stream is connected to the listener, the media clock is automatically recovered from this stream and used for the talker . For IEEE1722a a stream format for the distribution of a global media clock has been proposed .  Understanding IEEE's deterministic AV bridging standards, http://www.embedded.com/print/4008284  XMOS AVB Media Clocks, http://xcore.github.io/sw_avb/clocking.html  1722A Global System Clock Streams (aka Media Clock Streams), http://grouper.ieee.org/groups/1722/contributions/2013/1722a_rsilfvast_Global-System-Clock-Stream-Principles_15-Apr-2013.pdf
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http://www.avnu.org/files/static_page_files/C5E0B5F8-1D09-3519-ADB32F1F88E6C057/AVnu_SWAPIs_v1.0.pdf contains more information about best practices for media clock handling. "Many real world applications (a mixer for example) require a Listener to receive audio from multiple Talkers (many microphones for example). In these situations, a common media clock for all Talkers eliminates the need for sample rate conversion (SRC) operations inside the Listener. The implication is that some sort of “House Media Clock” equivalent for AVB is required. Although not formally part of the AVB specification suite, the simplest method of distributing a “House Media Clock” is via a designated AVB stream that contains the canonical media clocking information - any stream can be designated as the house clock reference stream. Any Talker device that can talk to a mixing device should also be capable of being a Listener so that it can Listen to “House Media Clock” AVTP packets and adjust its internal media clock to match the “House Media Clock”."