A sync group does two jobs: the schedule starts every layout on time, and a start signal locks video frame to frame. That signal only reaches players whose operating system the sync feature supports. Check the platform support table before scheduling video to two screens; an unlisted player starts alone and drifts.
Two screens joined to the same sync group, one layout with a fullscreen video scheduled to both, and the video still plays out of step. The usual suspect is the network or a hidden setting, and the usual suspect is wrong. A sync group does two different jobs. The schedule starts every layout on time on every player, whatever it runs. The frame-accurate start signal only reaches players whose operating system the sync feature supports. If one screen in the group runs a system the support table does not list, it plays the schedule alone and starts the video whenever its own startup finishes, seconds behind the rest. This page puts numbers on that drift and lists what to check before you schedule video to two screens.
The schedule and the start signal are two different jobs
The schedule is a calendar. Every player in the group downloads the plan, downloads the layout and its video, and starts the layout at the set time. Any player on any operating system does this; it is ordinary scheduled playback.
The start signal is a separate command. At the agreed instant, one player acts as the leader, or timekeeper, and the followers start against it and take timing corrections from it while the video runs. That handshake lives in player software, and the sync feature's platform support table names the operating systems that implement it. Where this symptom was documented, the vendor's table listed Android and one smart-TV platform as supported and marked Windows players as unsupported.
An unlisted player still fetches the schedule, still shows the layout on time, still looks correctly configured. What never arrives is the start command, so it begins the video when its own app startup, file open and decode finish, and nothing corrects the gap afterwards. Group membership is not the handshake: the CMS accepted the display into the group either way.
| Job | Who gets it |
|---|---|
| Fetch the schedule, the layout and the video files | Every player in the group |
| Keep the clock close, with NTP or PTP | Every player on the network |
| Receive the frame-accurate start signal | Only players on operating systems the support table lists |
| Follow the leader's timing corrections during playback | Only those same players |
Why matching clocks are not enough
Sync groups sit on top of clock sync, and clock sync is the easy part. NTP over a local network typically holds two computers within single-digit milliseconds; PTP, standardized as IEEE 1588, gets under 1 millisecond on the same network. Those figures describe the time references, not the playback. Starting a video on an agreed millisecond takes a delivered command plus a player that opens and decodes the file so the first frame lands on that instant. Supported players do that and then hold step with the leader during playback; our video wall sync architecture page explains how they share that timing. Unsupported players never hear the command, so a correct clock changes nothing about when their video starts.
What the drift looks like in frames
Frame time is 1 divided by frame rate: 33.3 ms per frame at 30 fps, 16.7 ms at 60 fps. Two independent starts differ by whatever separates their app startup, file open and decode, which on ordinary hardware lands anywhere from a few hundred milliseconds to a few seconds and changes from boot to boot. A 1.0 s gap at 30 fps is 30 frames. Nobody watching two side-by-side screens 30 frames apart reads that as slightly off; the second screen replays the action. That is why the symptom feels like a broken setting: everything in the CMS looks right. For what shifts frame by frame while video runs, see our page on synchronizing video frame data during playback.
Example: the 10:00:00 start
A cafe hangs two portrait screens behind the bar and schedules one fullscreen MP4 to a sync group starting 10:00:00. Both players sit on the shop LAN, and NTP holds their clocks within 10 ms.
Screen A runs a player the support table lists. Screen B does not, so it plays the schedule alone. At 10:00:00 the leader sends the start signal and screen A shows its first frame 0.8 s later, after file open and decode. Screen B was still loading the same 1920 x 1080 file and shows its first frame 2.5 s after the hour.
Drift = 2.5 s − 0.8 s = 1.7 s. At 30 fps that is 1700 ms ÷ 33.3 ms ≈ 51 frames. A customer standing between the screens sees the same shot play twice.
Swap screen B for a listed player: the command reaches both, first frames land within one frame of each other, 33.3 ms at 30 fps, and follower corrections hold that gap while the video runs. Same schedule, same network, same file; the only change is that the start signal now has somewhere to land.
To measure a live pair, film both screens with a phone at 30 fps and step frames. Each recorded step is 33.3 ms, so 51 steps between the same content is the 1.7 s case.
What to check before you schedule video to two screens
- Open the sync feature's platform support table first. It names the operating systems that receive the start signal, and it decides the job. If your player's OS has no entry, no setting will pull the group into step.
- Put listed players in the group and keep them on one app version, so every screen runs the same sync software.
- Keep every player on the same local network; the start signalling travels across the LAN.
- Schedule the video, then film both screens before opening time and count frames between the same content. More than a handful of frames means the group is not locked. Fix it then, not in front of customers.
- If the content is one picture across two panels, stop and use one player for both screens. No group is needed.
When one player beats a sync group
One player driving both displays keeps one clock by construction, which is why it is the straightforward answer when a single picture spans two panels. Our pages on one Android signage player driving multiple displays and on choosing an HDMI splitter or video wall processor for two screens cover the hardware side.
When each screen shows its own content and nothing has to lock, skip the group: independent schedules work on every player. When two screens only need the same files at roughly the same time, the player included with IMD Signage displays pulls a playlist from your own web server on a timer; that matches files, not frames.
Choosing hardware for a multi-screen job starts with the guide to choosing a digital signage player and our display range.
Recommendation: before you schedule video to any sync group, open the feature's platform support table and confirm every player's operating system is listed. Move an unlisted player to a supported one, or take it out of the group and schedule it on its own; leaving it in expects a start signal that never arrives. When one picture has to span two panels, run one player into both screens.
Questions
Can the clocks match perfectly and the video still play out of step?
Yes. NTP over a local network keeps player clocks within single-digit milliseconds, and PTP brings that under 1 millisecond, yet the videos still start seconds apart. Clock sync aligns the time references. A frame-accurate start needs a command delivered to player software built to act on it. A player on an operating system that never receives that command starts the video whenever its own loading finishes, and a correct clock does not change that.
How far apart can two starts be before viewers notice?
Frame time sets the scale: 33.3 ms per frame at 30 fps and 16.7 ms at 60 fps. Side-by-side screens showing the same footage a few hundred milliseconds apart already read as a stagger, and a full second looks like a replay. Film both screens with a phone at 30 fps and step frames; each step is 33.3 ms, so a gap of ten steps is about 330 ms of drift.
Does the player included with IMD Signage displays join a sync group?
No frame-sync group feature is published for it. Its sync option pulls a playlist from your own web server on a timer, so two screens keep the same files on the same rotation; that matches content, not frames. For one picture across two panels, run one player into both screens. Set up as the home app, the player brings the show up full screen at power-on and plays images and MP4 video from microSD or USB with no network.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.