Network Synchronization

Having a well-synchronized network is crucial for the performance of the network. An unsynchronized network could lead to sampling signals at different points in time. In a less severe scenario, this can result in mixed signals that exhibit a comb filter characteristic, distorting the audio. In a worst-case scenario, the lack of synchronization could cause audio dropouts, glitches, and clicks, significantly degrading the listening experience. Therefore, maintaining precise synchronization is essential to ensure seamless and high-quality audio transmission across the network.

Milan AVB employs the Generalized Precision Timing Protocol (gPTP) to synchronize all participants within the network, including both Endstations and switches. Notably, the inclusion of Switches distinguishes Milan from other existing networked audio protocols, as it requires the Switches to be time-aware and therefore capable of understanding Network time.

Milan timing is divided into two parts: the Network Time, provided to all participants of the network via gPTP, and the timing information that controls the audio sampling clock, referred to as Media Time. To avoid confusion between the two domains, these terms clearly distinguish global synchronization from audio-specific clock control.

Network Clock Leader Election

Consider a Milan network that has just been switched on. It is likely that it consists of multiple Endstations and Switches. In a first step, a Grand Master (GM) has to be elected. This Grand Master will distribute its time to all participants allowing them to share a common understanding of time. The election process is defined in an algorithm called Best Time Transmitter Algorithm (BTCA). The algorithm is executed automatically when a change in the network is detected.

The election of the the gPTP GrandMaster is based on parameters that describe the clock quality of the device. Of course the best clock quality is elected as the GM. In case multiple devices have the same clock quality, the device with the lowest MAC address is selected. Switches are preferred over Endstations.

After the election process, the GM provides its time to all network participants.

Network Clock Device Synchronization

In gPTP, synchronization occurs at the ports of network devices, allowing for more accurate time measurements and reducing the effects of network delays and jitter.

The Peer Delay is measured by sending a PDelay Request to a neighbor and recording the time it takes to receive a PDelay Response. By also including timestamps of when the response was sent and received, the protocol can accurately calculate the link delay between two peers.

This contrasts with PTPv1, which uses the end-to-end approach that may introduce additional uncertainties in time synchronization due to varying network conditions.

Network Clock Domain

All devices synchronized to the same gPTP GrandMaster belong to a single domain. Each domain is exclusively clocked by one gPTP GM. In Milan, audio streams cannot be shared between different gPTP domains because there is no timing relationship between them.

Media Clock

To ensure accurate playback and recording, Endstations must synchronize the timing of their audio playback/recording to the shared Network Time. This synchronization is called Media Clocking.

Media Clocking can be achieved in two ways: by locking to the timing information in an Audio Stream, or by using a Clock Reference Format (CRF) Stream. Both methods allow devices to align their local audio clocks with the rest of the network. More details on the Media formats can be found in Media Transport.

Support for CRF is optional and depends on the device capabilities. Smaller devices often rely on AAF streams alone for Media Clocking.