Why typical Network Time fails
NTP (Network Time Protocol) is the standard for computer clocks. It syncs laptops to the correct time of day globally, accurate to a few milliseconds. But in broadcast audio (working at 48,000 samples per second), one sample is 20 microseconds long. Syncing audio interfaces with millisecond jitter results in severe phase distortion and dropped samples.
PTP (Precision Time Protocol) was designed for industrial automation and later adopted by Dante, AES67, and SMPTE 2110. It provides synchronization accuracy in the sub-microsecond range, effectively replacing legacy Genlock in all-IP broadcast facilities.
The BMCA (Best Master Clock Algorithm)
In PTP, you don't manually assign a master clock (though you can weight priorities). The BMCA runs automatically on the network, evaluating every PTP-capable device.
- It checks clock quality (e.g., GPS-locked hardware vs a software clock).
- It checks priority settings (Priority 1 and Priority 2 vectors).
- It checks MAC addresses in the event of a tie.
The winner becomes the Grandmaster Clock. Every other device on the network becomes a Follower, constantly listening to Announce and Sync messages to adjust their local oscillators.
L2 vs. L3 PTP Profiles
PTP can operate at different layers of the OSI model depending on the industry standard:
- Dante (L3): Uses Multicast PTP over IP (Layer 3). Requires a router or a Layer 3 switch to handle traffic across subnets, though it typically resides in a single VLAN.
- SMPTE 2110 (L2/L3): Often uses the ST 2059-2 profile, which relies on a specific set of PTP parameters (like 128 messages per second) to ensure sub-microsecond alignment for video frames.
Troubleshooting PTP Sync
If your devices aren't syncing, check these three common "PTP Killers":
- mDNS Conflict: On some networks, mDNS and PTP multicast groups can collide or be throttled, leading to "PTP Sync Loss" in Dante Controller.
- Clock Domain Mismatch: PTP devices must share the same Domain Number (default is 0 for Dante, but often 127 for other systems).
- Hybrid Mode: Mixing "Unicast" and "Multicast" PTP (common in AES67 transitions) can lead to BMCA instability where the Grandmaster flip-flops between sources.