Precise time synchronization is critical for all applications in broadcast Blackburst/ Tri-Level Sync, AES, Timecode, Test Patterns and Precision Time Protocol (PTP) are widely used method to distribute accurate time across broadcast facilities, ensuring frame-accurate video switching and synchronization.
The simplest way to ensure that your Blackburst /Tri-Level Sync, AES, Timecode, Test Patterns and PTP remain synchronised is by having all reference sources generated from the same oscillator in your Sync Pulse Generator and that unit is locked to a GPS reference, like the Leader LT4670.
That way, if the Sync Pulse Generator loses its GPS reference, both BB/TLS and PTP references will use the Sync Pulse Generators ‘signal oven controlled internal’ oscillator and go into ‘Stay-in-Sync’ mode.
And, when the GPS reference returns, a ‘slow-lock’ function is available to eliminate the shock that occurs when genlock is performed again based on ‘Stay-in-Sync’.
However, when the primary Leader LT4670 Sync Pulse Generator loses its GPS reference, how it recovers and maintains time synchronization can significantly impact broadcast system stability. One of the key techniques to mitigate timing disruptions is slow syncing—a method of gradually adjusting the clock rather than making abrupt corrections.
Why Does GPS Loss Matter for Broadcast Blackburst/ Tri-Level Sync, AES, Timecode, Test Patterns and Precision Time Protocol (PTP)?
Broadcast facilities rely on a high-precision time source to synchronize clocks across a broadcast facility. When the GPS reference is lost due to signal blockage, hardware failure, or environmental factors, the Primary Leader LT4670 Sync Pulse Generator must rely on its internal oscillator. Without GPS correction, even the most stable oscillators can drift, introducing timing errors that propagate through the video and audio signal chain.
The Risks of Fast Clock Adjustments
A common approach to correcting time discrepancies is to apply sudden adjustments to the system clock. However, this can lead to serious issues:
- Audio/Video Sync Issues: Abrupt time corrections can lead to noticeable lip-sync errors and discontinuities in video playout.
- Broadcast Network Instability: Downstream devices expecting smooth, continuous time updates may struggle to adapt to abrupt changes, resulting in synchronization failures.
- Playout and Frame Drops: Fast corrections can introduce frame drops or jitter, disrupting seamless video and audio playback.
The Role of Slow Syncing
To mitigate these risks, slow syncing allows the system clock to adjust gradually, avoiding sudden jumps and maintaining network stability. This method involves:
- Gradual Clock Steering: Instead of instant corrections, the clock frequency is slightly adjusted over time to align with the correct time.
- Filtering and Smoothing: Using algorithms to average time error measurements and apply smooth corrections.
- Holdover Strategies: Leveraging high-stability oscillators and error compensation techniques to maintain accuracy during GPS loss.
Benefits of Slow Syncing
- Minimized Audio/Video Disruptions: Broadcast playout systems experience a seamless transition without abrupt time shifts.
- Increased Resilience: The system remains stable even under prolonged periods of GPS loss, preventing frame drops and jitter.
- Improved Synchronization Accuracy: By avoiding phase jumps, downstream clocks maintain better consistency with the master clock, ensuring seamless content delivery.
Implementing Slow Syncing in Broadcast PTP Systems
To effectively implement slow syncing in a broadcast facility, the Leader LT4670 Sync Pulse Generator utilises the following best practices:
- Use High-Quality Oscillators: A stable oscillator with low drift ensures that the system can maintain accuracy when GPS is unavailable.
- Enable Holdover Mode: Configure the PTP grandmaster to enter holdover mode when GPS is lost, allowing it to rely on its internal timing source.
- Configure Slow Syncing Algorithms: Adjust PTP settings to apply gradual frequency adjustments instead of immediate jumps.
- Monitor and Log Timing Performance: Regularly check synchronization accuracy and adjust settings as needed to optimize performance.
Conclusion
Slow syncing Blackburst/ Tri-Level Sync, AES, Timecode, Test Patterns and Precision Time Protocol (PTP) during GPS reference loss is a crucial technique for maintaining broadcast network stability and ensuring reliable time distribution. By gradually steering the clock and avoiding abrupt corrections, broadcasters can prevent video and audio disruptions, improve synchronization accuracy, and enhance overall system resilience. Implementing best practices for slow syncing can help safeguard live broadcasts, post-production workflows, and seamless playout operations even in challenging conditions.