In your setup, one player (“leader”) will send a clock to all others (“player”).
To configure a standard v6 image for sync leader, create a file called “sync-leader.txt”. It can be on the media partition, any external drive or the boot partition.
Creating a file with the name “sync-player.txt” will indicated that the sync version will run, and try to follow the clock of the leading player.
On all Raspberries, the file that will be played back in a loop is called “sync.mp4”. If they are not the identical file, they should have the same length. Also, try to avoid short loops, as de-syncing might happen after the loop point- copy and paste your content in a timeline to create a long file (maybe one hour or so).
The filesystem is case-sensitive, all letters should be lower case.
Also, the file has to be encoded with h264, as the player uses the hardware decoding of Raspberry Pi. If you encounter lots of de-sync, try re-encoding your Video with different settings (Handbrake: AAC 160kbit for audio, constant quality of 20-25 for video).
Getting a seamless Loop
Once the leader starts playback, the players will try to sync on it by jumping ahead and pause there until the leader reaches that timecode. It might take a few tries, depending on your hardware and the specific file. So if your content is very short, it might de-sync a lot. To get around the issue, edit a file of maybe an hour long with your content looped within.
Try different encoding profiles to figure out which ones is best- again, this is very specific to your content and the hardware you are using. I would suggest to use Pi3 or newer for sync.
Connect the players via ethernet
Two players can be connected with a regular network/ethernet cable. If you use several, you can use a switch or router to connect them- using a router will make it easier, as it will provide ip adresses for all players.
Connect the players via WiFi
To use WiFi, set up a wireless router with a network name and password. Then, as soon as the logo video starts, press CTRL-C to exit to console. Type “sudo raspi-config” and press enter. Go to Performance Options > Overlay Filesystems and in the following dialog, Choose “No” to deactivate Read-Only mode. Back in the main menu, exit and choose to reboot. When the logo video starts again, repeat the steps above to get back into raspi-config. First, go to “Localisation” menu and choose the country for the correct wireless lan standards. Back in the main menu, go to “System options” and choose “Wireless LAN”, a dialog comes up asking for network name and passwort. After choosing to quit the program with a reboot, your player should connect to your wifi network.