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Every screen opens its own copy of the same file, joins a room with a short code, and plays, pauses and seeks together, to within a few milliseconds on a good network.
To play a video in sync on several devices, give each device its own copy of the same file, open this page on every one of them, start a room on one and join it from the others with the six-character code. From then on, play, pause and seek on the host happen on every screen at once, and each device keeps correcting itself so the picture stays together.
Nothing is streamed. The video never travels between the devices; only small play, pause and clock messages do, normally straight from device to device. That is why it works with a 4K film over a modest connection, and why you need the file on each device first: copy it across, share it from a cloud folder or hand round a USB stick before you start.
People use it for a watch party with friends in other homes, to split one picture over a wall of screens, for a silent disco on everyone’s phones, and in classrooms, churches and galleries where several screens should show the same thing. If a device’s sound is late because of Bluetooth or a soundbar, measure its delay first and set it once.
Sync player
On each device, drop the file onto Drop the video to play in sync (or Open your copy of the video on a device that is joining). Phones pick it from Files or Photos. It is never uploaded.
Press Start a room, choose Who controls playback (Only me or Everyone) and the Sound (Every device or Only the host), then Create room. The code, a QR code and Copy link appear.
Scan the QR code, open the link, or press Join a room, type the Room code and press Join room. Each device appears under Devices with Same file or Different file and its sync quality.
Press Play for everyone (Space or K). Seek with ←/→ (Shift for 30 s), go Full screen (F), and use Pause for everyone or Start again for everyone as needed.
Every device has its own clock, and no two agree exactly. So the first thing a guest does after joining is ask the host the time, many times over. Each question carries the guest’s own time when it left, the host stamps its time when the question arrives and again when it sends the answer, and the guest notes when the answer comes back. From those four times it can work out both how long the trip took and how far its clock differs from the host’s.
A single answer can be thrown off by a busy Wi-Fi network, so the guest keeps asking a few times a second and trusts only the quickest round trips, where there was no time for anything to get in the way. It also watches how the difference changes over minutes, because cheap clocks run slightly fast or slow.
When the host presses play, it doesn’t say “play now”; it says “the film was at this point at this moment on my clock, playing at normal speed”. Every device turns that into “where should I be right now?”, compares it with the frame actually on its screen, and corrects. Small errors are fixed by playing a fraction faster or slower (between 0.3 and 3 per cent, too little to hear); a jump of more than about 300 ms is fixed with a seek. Each device reports how far off it is, shown as Excellent, Good, Fair or Poor next to its name.
On a good home or office network, devices usually stay within a few milliseconds of each other, well under the length of one video frame. Over the internet, between homes, the clock is still accurate but each device has more to correct and the numbers move around more. What limits it in practice is rarely the network:
The fix for the first two is the Output delay slider on each device. Add the delay this browser reports fills in what the browser knows about its own audio path, and Remember this delay on this device keeps it for next time. For anything the browser doesn’t know, such as a Bluetooth speaker, the host’s Flash and beep test makes every screen flash and beep on the shared clock so you can see and hear which one is late and nudge it with [ and ].
Because the video is never sent between devices, every device plays the file it already has. It has to be the same file, not just the same film: a different edit, a different frame rate or a copy that was re-encoded by a messaging app can have a few seconds more or less at the start, and the devices would then be in step with the clock but out of step with each other.
To catch that, each device makes a fingerprint of its file (its size, its length and a quick hash of three small slices) and the room compares them. A device shows Same file when it matches the host, Different file when it doesn’t, and Same length, other file when the length matches but the bytes don’t, which usually means a re-encoded copy that may still line up.
| Use | Who controls playback | Sound | Useful extras |
|---|---|---|---|
| Watch party across homes | Everyone | Every device | Share the code in your chat; see the watch party guide |
| Classroom with several screens | Only me | Only the host | Full screen on each screen; one teacher controls |
| Church overflow rooms | Only me | One room’s speakers | Start At a set time so every room begins together |
| Gallery or shop window | Only me | As the space needs | Loop the video; keep screens awake with full screen |
| Video wall | Only me | Only the host | Video wall with a grid; see the video wall guide |
| Silent disco or dance class | Only me | Every device | Each phone’s Output delay for headphones; see silent disco |
Rooms hold up to 50 devices and last up to 12 hours. A device that drops off, or is reloaded, can rejoin the same room with the same code and picks up where everyone else is.
No. Each device plays its own copy from its own storage. Only short messages travel between devices: play, pause, the position, and the clock checks that keep them together. That is why every device needs the file before you start.
No. Devices in different homes or countries can share a room. On the same network the direct connection is usually quickest and the sync tightest. If a network blocks direct connections, as some school and office networks do, the messages are passed through our relay instead and the room still works.
Up to 50, including the host. Every guest connects to the host, so the host should be a reasonably capable device on a steady connection; a laptop on mains power is ideal for a big room.
No. It plays video files you have on each device. Streaming services protect their streams and don’t let other websites control or read their players, so they can’t be synced from here; most of them offer their own group-watch features instead.
Its sound goes through something that adds delay after the browser, usually Bluetooth headphones, a speaker or a TV’s sound processing. Set that device’s Output delay until the sound lines up, or measure the delay and enter it, then press Remember this delay on this device.
Everyone else carries on. The device that dropped out reconnects to the room by itself when it can, or you can rejoin with the same code after a reload, and it catches up with the others as soon as it hears the host again.
It depends on the TV’s browser. It needs to open a file from a USB stick or the TV’s storage and play it smoothly, which many built-in browsers can’t do. A laptop, tablet or small computer plugged into the TV by HDMI is the dependable route.