Synchronization / Technical guide
Genlock, Frame Lock, Timecode and Word Clock
Genlock, frame lock, timecode and word clock are often placed in the same sentence because they all concern synchronization. They do not synchronize the same thing. A stable system begins by naming the layer that must agree: timeline position, video refresh, application buffers or audio samples.
Timecode: where are we?
Timecode identifies a position on a timeline. A playback application can use that address to move a playhead to the corresponding frame. LTC carries the address as an audio-like signal; MIDI Timecode carries it as MIDI messages. Timecode is useful for cueing and positional alignment, but two devices reading the same address are not automatically genlocked.
Genlock: when does a video frame begin?
Genlock aligns a video device or graphics output to an external timing reference, often called house sync. It keeps the start of frames tied to the same reference so switchers, processors, cameras and outputs can work within a common timing environment.
Genlock does not tell a media server which frame of content to play. It controls output timing, not creative timeline position.
Frame lock: do multiple displays refresh together?
Frame lock uses hardware to align the frames produced by multiple display outputs or systems. It is important when one visual canvas spans multiple outputs and a boundary between displays must remain continuous. NVIDIA’s documentation distinguishes frame lock between connected systems from genlock to an external reference; both can be used together.
Swap sync: do the applications present together?
Even when display hardware is synchronized, applications rendering on multiple GPUs or computers may need their buffer swaps coordinated. Swap synchronization deals with that software presentation boundary. It belongs in the design of clustered real-time systems and should not be assumed merely because a genlock connector is present.
Word clock: when is an audio sample taken?
Word clock aligns the sampling clock of compatible digital-audio devices. It helps prevent clicks, dropouts and sample-rate disagreement in an audio chain. It does not carry song position and it does not align video frames.
One show can need several references
A timecoded media clip may follow an LTC source while the server’s outputs are genlocked to house sync. Several rendering machines may also use frame lock or swap sync, while the audio network maintains its own clock. These layers cooperate, but none is a universal replacement for the others.
Diagnose the symptom by layer
- Wrong moment: inspect cueing, timecode address, offset and frame rate.
- Tear between outputs: inspect frame lock, genlock and display timing.
- Cluster frames do not present together: inspect swap synchronization and application configuration.
- Audio clicks or sample drift: inspect the digital-audio clock and sample rate.
The fastest troubleshooting language is specific: “The playheads share timecode but the outputs are not frame locked” is more useful than “the sync is broken.”