DXD8/PTP User Manual (rev 2.15)
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9. General Information on PTP
9.1. TIME AND PHASE
Precise time information has always been crucial in traditional production and broadcast environments. To keep the different
video and audio sources synchronized properly, two different references were typically used, one providing time information
(time code), the other phase information (black & burst or tri-level sync for Video, and DARS or Word Clock for Audio). With
IP networks, PTP provides both Time Of Day and phase information, replacing and/or coexisting with the traditional ‘legacy’
references.
9.2. WHAT IS PTP?
PTP (Precision Time Protocol) is a method for precisely synchronizing devices over a computer network. On a LAN, PTP can
achieve clock accuracy to within a microsecond. Originally designed for network measurements, PTP has been broadly
adopted in applications ranging from real-time automations to telecom transmissions and financial transactions.
Published in 2002 by IEEE under the 1588-2002 standard (PTP v1) it was revised in 2008 as 1588-2008 (PTP v2). Version 2
brought several improvements but is not backward compatible with version 1.
9.3. A/V OVER IP APPLICATIONS
PTP has been widely adopted in A/V over IP applications. It is the protocol chosen by AES and SMPTE with PTP profiles
included in both the AES67 and ST-2059 standards. The SMPTE ST-2110 suite of standards (digital video over IP with audio,
video and ancillary data as separate streams) uses PTP for synchronization as it is based on ST-2059.
Major A/V over IP systems such as Dante, Ravenna, Q-LAN, LiveWire and Wheatnet-IP, all use PTP. Although the original
Dante used PTP v1, with the release of the Dante Domain Manager version 1.1, a Dante system has the ability to clock to a
non-Dante PTP v2 clock.
AVB (Audio Video Bridging) is a set of IEEE standards that facilitate the transport of high performance audio and video on a
LAN. One of those standards, IEEE 802.1AS, defines a specific profile of IEEE 1588-2008 with additional timing features
called gPTP (Generalized Precision Time Protocol).
9.4. PTP DESCRIPTION
Per IEEE 1588, PTP uses a hierarchical Master-Slave architecture for clock distribution. The primary timing source is called the
PTP Grandmaster. Precise timestamps are captured at the Master and Slaves and exchanged to determine the offset between
the two and the network latency so that the Slaves can synchronize their internal clocks to the Master’s.
With the PTP protocol, two activities continually occur on the network:
- All grandmaster-capable devices negotiate to select a grandmaster.
- All devices synchronize to the grandmaster.
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BMCA: The Grandmaster is elected autonomously without any user interaction. Grandmaster-capable devices broadcast
their clock “quality” and other parameters with Announce messages. They each compare the received Announce messages
and, independently of the other devices, determine the most suitable grandmaster. This determination is done through the
Best Master Clock Algorithm (BMCA), which by design ensures that all clocks reach the same conclusion. This best master
clock negotiation continues even after a grandmaster has been selected so that in case the current grandmaster fails another
clock can quickly replace it.
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SYNC: Synchronization to the grandmaster is done using a series of messages. Sync messages are broadcasted periodically
by the grandmaster and contain timestamps from its internal clock. Slaves read these messages and adjust their internal
clocks to match exactly the master’s. Every update cycle, the slaves calculate corrections for offset and network latency to
achieve high precision synchronization.
